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Dynamic lateral adhesion force of water droplets on microstructured hydrophobic surfaces
Accepted Manuscript Title: Dynamic Lateral Adhesion Force of Water Droplets on Microstructured Hydrophobic Surfaces Author: Jae Min Lee Seung-hwan Lee Jong Soo Ko PII: DOI: Reference:
S0925-4005(15)00286-5 http://dx.doi.org/doi:10.1016/j.snb.2015.02.101 SNB 18165
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
10-10-2014 14-2-2015 21-2-2015
Please cite this article as: J.M. Lee, S.-h. Lee, J.S. Ko, Dynamic Lateral Adhesion Force of Water Droplets on Microstructured Hydrophobic Surfaces, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.02.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Dynamic Lateral Adhesion Force of Water Droplets
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on Microstructured Hydrophobic Surfaces
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Jae Min Lee, Seung-hwan Lee, and Jong Soo Ko*
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Graduate School of Mechanical Engineering,
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Pusan National University,
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Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735, Korea
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To whom all correspondence should be addressed.
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Tel : +82-51-510-2488, Fax : +82-51-514-0685 E-mail: [email protected]
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Abstract This study investigates the change of the dynamic lateral adhesion forces of
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water droplets on microstructured surfaces by varying their hydrophobicities. While
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hydrophobic surfaces showed significant asymmetry in advancing-to-receding profiles,
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superhydrophobic surfaces did not. As a result, hydrophobic surfaces required greater
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moving forces at the moment of change of the moving direction than superhydrophobic
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surfaces. In addition, lateral adhesion forces measured from hydrophobic surfaces were
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much higher than those from superhydrophobic surfaces. In the present experimental
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conditions, water droplets did not roll on the surfaces with lateral adhesion forces larger
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than 21 mgf. These results support the possibility that the rolling ability of a water
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droplet on a surface can be evaluated by measuring the lateral adhesion force.
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Keywords: lateral adhesion force, contact angle, hydrophobic, superhydrophobic,
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microstructure
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1. Introduction
2 Surface modification to achieve superhydrophobicity is a typical example of
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biomimetics [1-3]. The representative plant that has a superhydrophobic characteristic is
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the lotus. The surface of the lotus leaf is covered with microprotrusions, which are in
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turn covered in nanoprotrusions composed of hydrophobic epicuticular wax crystalloids,
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and the lotus leaf has superhydrophobic and self-cleaning characteristics [4]. These
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characteristics are also referred to as the lotus effect. Artificial realization of a nano-
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micro hierarchical topology is the key factor to achieve a superhydrophobic surface with
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a static contact angle greater than 150, and numerous fabrication methods have been
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developed to date [5-9]. The rose petal is also covered with nano-micro hierarchical
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structures, showing a superhydrophobic surface. However, a water droplet on a petal
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does not roll; rather it is suspended when the petal is inverted [10]. As a result, the rose
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petal does not have a self-cleaning capability [11,12]. In short, the lotus leaf and the
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rose petal both show very high contact angles greater than 150 (superhydrophobic), but
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they show antithetic behavior with regard to the rolling of water droplets [13]. This
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indicates that it is not sufficient to predict the dynamic behavior of the water droplet on
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a superhydrophobic surface simply by measuring the contact angle.
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To investigate the dynamic behavior of the water droplet on a surface, several
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methods for measuring dynamic contact angles have been developed, including the
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dynamic sessile drop method [14] and tilting plate method [15]. Through the
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measurement of dynamic contact angles, the contact angle hysteresis, defined as the
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difference between advancing and receding contact angles, can be obtained [16-18]. The
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contact angle hysteresis is an important physical parameter that affects the dynamic 3
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behavior of water droplets, such as rolling and collision. In practice, rolling-off of a
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water droplet and self-cleaning ability are determined by the level of contact angle
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hysteresis measured from a surface. The roll-off angle/contact angle hysteresis should
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be maintained at less than 10° to realize the lotus effect on a surface. [19] Assessing the
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contact angle hysteresis is an effective means to predict the dynamic behavior of a water
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droplet on a surface. However, this measurement method uses an optical subsystem to
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capture the profile of a moving water droplet. Because it relies on the captured image of
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a water droplet, the actual adhesion force between the surface and the water droplet
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cannot directly be obtained.
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Recently, several approaches to measure normal and lateral adhesion forces have
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been reported. Atomic force microscopy (AFM), a representative approach for
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measuring normal adhesion force, has been used to measure the capillary bridging force
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of a liquid between the AFM tip and the substrate surface [20-26]. Although this
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method provides a quantitative means of measuring the adhesion force, the
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measurement area is limited to the submicroscopic level due to the usage of a nanoscale
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tip. It is therefore difficult to use the method with topographically-modified
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hydrophobic surfaces including a microstructured surface [25]. The most important
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approach to investigate static lateral adhesion forces to date is the tilted plate method
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[27-29]. This method involves gradual tilting of the plate and measuring the roll-off
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angle, followed by calculation of the static lateral adhesion forces from the relation to
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the projection of the gravitational force needed to initiate droplet movement. Although a
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useful way to measure the lateral adhesion forces, this method has several limitations.
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First, only ‘static’ lateral adhesion forces can be obtained. Second, this method is valid
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only for sufficiently large droplets, because small droplets do not slide owing to
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insufficient gravity. Third, it does not directly measure the forces. R. Tadmor et al.
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proposed a centrifuge method in which gravitation is replaced by a centrifugal force.
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The static lateral adhesion force of the small droplets can be measured by using a
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centrifugal adhesion balance (CAB) system [30]. This method also measures the ‘static’
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lateral adhesion forces and does not directly measure the forces.
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The capability of measuring a dynamic lateral adhesion force when a water
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droplet moves in the lateral direction along a surface would be very helpful to predict
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the dynamic behavior of a water droplet on a surface. In this study, we introduce a new
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approach to characterize surface properties: measuring the dynamic adhesion force of a
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surface and a water droplet that moves laterally to the surface. To verify the proposed
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measurement scheme, microstructured surfaces with different hydrophobicity are
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fabricated and characterized. In addition, the relationship of the lateral adhesion force
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and the rolling ability of a water droplet is discussed.
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2. Measurement Concept The configuration of the measurement system is shown in Fig. 1. An H-shape
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probe has been devised to measure the dynamic lateral adhesion force between the
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water droplet and the substrate surface. The lower end of a rigid bar is attached to the
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center of the H-shape probe, while its upper end is locked onto the probe holder. The
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holder is connected to a very precise load cell, allowing the very small force acting at it
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to be measured. Two water droplets are attached to each probe disk. The probe disks are
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modified to be hydrophilic to give sufficient adhesion force between the water droplet 5
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and the probe disk surface. Two samples are attached to each moving plate, where the
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lateral movement is precisely controlled by the respective linear servo actuator. Two
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linear actuators are clamped onto the stage.
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After two water droplets are dispensed on each probe disk, the two sample holders
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simultaneously move to the center with the same velocity, and then stop when they
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reach a certain distance () from the probe disks. The water droplets are constrained
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between the sample surfaces and the probe disks. The stage then moves downward and
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upward. During the whole measurement, the probe holders and the probe disks do not
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move. Briefly, while the probe does not move, the two moving plates move horizontally
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and then the stage moves vertically.
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To continuously measure the dynamic lateral adhesion forces using the proposed
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measurement concept, the probe disk should hold the droplet and the droplet volume
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should remain constant during the measurement. Two necessary conditions should be
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satisfied to meet these requirements. First, the adhesion force of the probe disk and the
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suitable for measurement on hydrophobic surfaces.
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droplet must be higher than that of the sample surface and the droplet. The probe disk surface thus should have strong hydrophilicity to endow a strong adhesion force between the probe disk and the droplet. Second, residual drops of water should not remain on the sample surfaces. Because of the high possibility of residual drops remaining on a sample surface that has a hydrophilic nature, the proposed method is
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The Bond number (Bo) is a basic dimensionless number and is defined as the ratio
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of the gravitational force and the surface tension force acting on a liquid droplet [31,32]. 6
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A high Bond number indicates that the liquid droplet is relatively affected by
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gravitational force, while a low Bond number indicates that it is more affected by
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surface tension. As shown in Fig. 1, the gap () between two parallel solid surfaces is
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small; the Bond number is given as Bo=g2/ [33]. Here, , , and g denote the density
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(997.0 kg/m3) and the surface tension (71.97 mN/m) of water, and the gravitational
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constant, respectively. In this study, because the gap was set to be 0.5 mm, the Bond
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number was 0.034. Therefore, it is expected that the surface tension dominates in this
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measurement.
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3. Design and Fabrication
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3.1 Probe
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Fig. 2a shows the designed probe. Two probe disks of 4 mm diameter are
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connected at both ends of a carbon probe-disk-holding rod that is 5 mm in length (H-
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shape probe). Another 45-mm-long carbon rod is attached to the center of the probe-
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disk-holding rod. Figs. 3a-e show the probe disk fabrication process. First, a 100 nm-
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thick copper conducting seed layer was deposited followed by the first photolithography
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step using a THB 151N photoresist (JSR Micro, USA) (Fig. 3a). Next, the first nickel
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electroforming step was carried out using a sulfamic acid based nickel electroplating
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solution (E-Form, DisChem, USA) (Fig. 3b). A 50-µm-thick nickel layer was obtained
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with a current density of 20 mA/cm2 at 50 C for 2 h. In the next processing step, the
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photoresist was removed by acetone and the second photolithography step was
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performed (Fig. 3c). A 50-µm-thick nickel layer was then deposited by the second
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nickel electroforming step (Fig. 3d). The purpose of the second photolithography and
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electroforming process is to form a 1- mm-dia. indent at the center of the upper part of
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the probe disk. The end of a probe-disk-holding rod of 1 mm diameter will be inserted
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in the indent and fixed in the following assembly process. In the next processing step,
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the nickel plate was separated from the silicon substrate by wet silicon chemical etching
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(Fig. 3e). The silicon was etched for 8 h at 85 C using a 25 wt% potassium hydroxide
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(KOH) solution (J. T. Baker, USA). The fabrication process of the probe disk was
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completed by coating the flat surface of the plate with a liquefied TiO2 solution
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(Glaschon, NCI, Korea) of a hydrophilic nature. By application of a TiO2 coating, the
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plate surface was modified from a weak hydrophilic character (contact angle, CA=71,
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electroformed nickel surface) to a strong hydrophilic character (CA=5, TiO2-coated
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surface). Each end of the probe-disk-holding rod is inserted to the indents of the
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fabricated probe disks and then fixed with bonding glue (ALTECO F-05, ALPHA
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TECHNO, Japan). Finally, another 45-mm-long carbon rod is bonded to the center of
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the probe-disk-holding rod using the same bonding material. Fig. 3f shows a fabricated
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probe.
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3.2 Microstructured Surfaces with Different Hydrophobicities
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To measure the dynamic lateral adhesion force according to the level of
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hydrophobicity of the surface, microstructured surfaces with different contact angles
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have been designed and fabricated. The designed microstructure is illustrated in Fig. 2b. 8
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We assumed that the water droplet is supported by the protruding microstructures. The
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apparent contact angle () based on Cassie and Baxter’s model is expressed as
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cos=f(1+cos)-1 [34]. Here, f is the solid fraction, defined as the ratio of the wetting
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area of the rough surface to the projected area, and is the contact angle of a water
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drop on a smooth surface. The contact angle () of a smooth surface coated with a
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plasma polymerized fluorocarbon (PPFC) layer was measured to be 103 and this value
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was used for the calculation of contact angles in this study. The contact angle can be
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controlled by reducing the contact area between the top of the microstructures and the
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water droplet. The reduced contact area resulted in a reduction of the surface energy,
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enhancing the hydrophobicity of the surface. To obtain samples with five different
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contact angles (from 130 to 170, each step=10), the spacing (b) was increased from 3
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to 36.6 µm, whereas the square pillar size (aa) was fixed at 66 µm2, as shown in
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Table 1. The designed structure height was 15 µm.
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Fig. 4 shows schematic illustrations of the process sequence for the fabrication
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of square pillar microstructures. The fabrication process for the microstructures starts
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with photolithography using a AZ1512 photoresist (Clariant, Swiss) on a 6-inch silicon
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wafer (LG Siltron, Korea) (Fig. 4a). The top silicon surface was then vertically etched
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to a 15 µm depth using a deep reactive ion etching (DRIE) system (STS Multiplex ICP,
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UK) (Fig. 4b). The fabrication process was completed by removal of the photoresist and
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subsequent coating on silicon microstructures with a 3-nm-thick PPFC layer of a
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hydrophobic nature (Fig. 4c). The silicon wafer was then diced to sample pieces with a
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size of 16 mm8 mm.
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Fig. 4d shows a scanning electron microscopy (SEM; JEOL JSM-6700F) image 9
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of the fabricated sample (a=6 µm, b=5 µm). The contact angles measured from each
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microstructure are listed in Table 1. The contact angles were measured using a contact
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angle-meter (SEO, Korea). Deionized water was used for measurement of the contact
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angles and the volume of a single water droplet was 4 µl. The measured contact angle
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was the average obtained from five measurements from each sample. Three samples
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were selected and used for the measurement in this study. The measured contact angles
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gradually increased with increasing b/a in a range from 134 to 161. The data reveal
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that the measured contact angles tend to be lower than the calculated angles. In our
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experience, the measured contact angles of fabricated microstructures are typically
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lower than the calculated values. The reason for this has yet to be identified. Although
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the measured contact angles are different from the calculated angles, both gradually
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increase according to b/a. The contact angles measured from three samples (b= 3, 5, 8.4
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µm) are lower than 150 and those measured from the other two samples (b=15.4, 36.6
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µm) are higher than 150. The five samples with different contact angles are appropriate
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for investigating the change of the lateral adhesion force according to the
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hydrophobicity level of the sample surface.
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4. Measurements and Discussion
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4.1. Measurement Setup
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A tensiometer (DCAT21, Data Physics, Germany) was used to measure the
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lateral adhesion force of a water droplet in this study. Deionized (DI) water droplets of 6
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μl volume are dispensed on each probe disk using a micropipette (eTouch, Sorenson, 10
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USA). Movement of each sample is controlled by a linear servo actuator (LSA-3024SD,
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Potenit, Korea) and the distance between the substrate surfaces and the probe discs was
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0.5 mm for all measurements. The stage moves upward 8 mm at a velocity of 0.1 mm/s,
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and then the moving direction of the stage changes to the opposite direction and the
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stage moves downward 8 mm at the same velocity. A charge-coupled device (CCD)
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camera (Navitar, USA) was used to capture moving images of the water droplets. All
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measurements were conducted in a cleanroom with a humidity of 60% and temperature
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of 20 °C.
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4.2. Measurement of Lateral Adhesion Forces
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Fig. 5 shows measurement cycles of the lateral adhesion forces of the five
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fabricated microstructured surfaces. In the case of the b=3 and 5 µm samples, lateral
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adhesion forces were maximal at the points (* marked) where the water droplets started
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to move downward. This phenomenon is very similar to the maximum static frictional
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force. Water droplets placed on the samples displaying this phenomenon did not roll in
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the tilting test, as presented in Table 1 (also see Figs. 8a and 8b). However, this
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phenomenon was not observed for the b=8.4, 15.4, and 36.6 µm samples. This is
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ascribed to the different degrees of hydrophobicity.
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Figs. 6a and 6b show captured images of the smallest spacing sample (b=3 μm)
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and the largest spacing sample (b=36.6 μm), respectively, during measurement. The left
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images of Figs 6a and 6b were captured during upward movement of the stage (and
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samples), while the right images were captured during downward movement. For the
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smallest spacing sample shown in Fig. 6a, the advancing profile of the water drop is
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highly sloped, whereas the receding profile is almost vertical to the substrate surface 11
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(receding contact angle=86±6). On the other hand, for the largest spacing sample
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shown in Fig. 6b, the receding profile is also highly sloped (receding contact angle=154
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±5). Receding profiles of each sample with different spacing are redrawn in Fig. 6c
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and the measured receding contact angles are listed in Table 1. Advancing contact
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angles of each sample were not significantly different within a range of 153-157,
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whereas receding angles were dramatically different, as shown in Table 1. Therefore,
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while the sample with the smallest spacing shows large asymmetry in the advancing-to-
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receding profile (contact angle difference between advancing and receding=69), the
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sample with the largest spacing is almost symmetric (contact angle difference=3). With
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an increase of the spacing, the contact area between the sample surface and the water
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droplet decreases, resulting in a decrease of surface energy. Low surface energy
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decreases the adhesion force between the sample surface and the water droplet; the two
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profiles (advancing and receding) of the water droplet in the largest spacing sample are
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consequently almost symmetric.
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Fig. 7 shows the average values of the lateral adhesion forces during downward
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and upward movements by varying b/a (see Table 1 for exact values). Lateral adhesion
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forces measured with a moving distance range of 1-7 mm were used to obtain the
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average values. The absolute magnitudes of the lateral adhesion forces during
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downward and upward movements gradually decreased with an increase of b/a. For all
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samples, however, the lateral adhesion force during downward movement is slightly
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larger than that during upward movement. It is believed that the difference between the
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two is caused by the weight of the water droplet. Although the difference is relatively
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small compared to the upward lateral adhesion force for the samples with a small 12
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spacing, it becomes significant for the samples with a large spacing. In the case of the
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largest spacing (b=36.6 μm), the difference (2.99 mgf) is 2.25 times larger than the
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upward lateral adhesion force (1.33 mgf). This shows that the influence of the weight of
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the water droplet is significant in the samples with superhydrophobic surfaces.
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Therefore, a cautious approach is necessary in adopting the Bond number to analyze
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dominant forces acting on a microscale system with a superhydrophobic surface.
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To investigate a water drop’s ability to roll on the sample surface, a 4-µl water
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droplet was dispensed on the sample surface and the sample was tilted. Rolling or
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pinning of water droplets and the rolling angles for each sample are listed in Table 1.
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The water droplets on the b= 3 and 5 µm samples did not roll when the samples were
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tilted up to 90, as shown in Figs. 8a and 8b. On the other hand, the water droplets on
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the b= 8.4, 15.4, and 36.6 µm samples rolled when the samples were tilted, as shown in
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Figs. 8c-8e. However, there is a significant difference in the roll-off angles by varying
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the spacing. A water droplet placed on the sample with b=8.4 μm rolled at a tilting angle
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of 39, whereas it rolled at 4 when placed on the sample with b=36.4 μm.
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In these experimental conditions, water droplets did not roll on the surfaces with
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lateral adhesion forces larger than 21 mgf. Although the lateral adhesion forces have
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been obtained from limited microstructure sizes, the results of this experiment have
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strong implications in the field of surface evaluation. It is possible to assess the rolling
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ability of a water droplet on a surface by measuring the lateral adhesion force. Even
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though the contact angle of a surface is higher than 150, a water droplet on the surface
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does not roll; instead it adheres to the surface even when the surface is turned over.
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Therefore, it is difficult to judge the rolling ability of a water droplet on a surface from
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the measured contact angle data. The approach suggested in this study is a meaningful 13
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attempt to discover a link between the surface adhesion force and the rolling ability of a
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water droplet on a surface.
5. Conclusion
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The authors studied the change of the dynamic lateral adhesion forces of water
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droplets on microstructured surfaces by varying their hydrophobicities. Hydrophobicity
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was adjusted by means of the spacing of rectangular micropillars (area=66 µm2).
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Advancing contact angles of each sample were not significantly different between the
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two samples with the smallest (b=3 µm) and largest spacing (b=36.6 μm), whereas the
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receding contact angles were dramatically different. Therefore, while the sample with
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the smallest spacing showed large asymmetry in the advancing-to-receding profile, the
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sample with the largest spacing was almost symmetric. The absolute magnitudes of the
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lateral adhesion forces during downward and upward movements gradually decreased
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with an increase of the spacing. With an increase of the spacing, the contact area
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between the sample surface and the water droplet decreases, resulting in a decrease of
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the surface energy. Because of the weight of the water droplet, the lateral adhesion force
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during downward movement is slightly larger than that during upward movement. The
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influence of the weight of the water droplet is significant in the samples with
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superhydrophobic surfaces. In these experimental conditions, water droplets did not roll
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on the surfaces with lateral adhesion forces larger than 21 mgf. These results support the
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possibility that the rolling ability of a water droplet on a surface can be evaluated by
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measurement of the lateral adhesion force.
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1 Acknowledgement
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This
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(www.nanotech2020.org) funded by the Ministry of Science, ICT and Future Planning
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(MSIP, Korea) and the Ministry of Trade, Industry and Energy (MOTIE, Korea)
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[Project Name: Development of an oil skimmer using nanotextured hydrophobic
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micromeshes].
was
supported
by
the
Nano-Convergence
Foundation
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[15] B.A. Bezuglyi, O.A. Tarasov, A.A. Fedorets, Modified tilting-plate method for measuring contact angles, Colloid Journal 63 (2001) 735-741.
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[16] A. Marmur, Thermodynamic aspects of contact-angle hysteresis, Advances in Colloid and Interface Science 50 (1994) 121–141. [17] E.L. Decker, S. Garoff, Contact angle hysteresis: the need for new theoretical and
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experimental models, The Journal of Adhesion 63 (1997) 159–185.
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[18] H. Gouin, The wetting problem of fluids on solid surfaces. Part 2: the contact angle hysteresis, Continuum Mechanics and Thermodynamics 15 (2003) 597–611.
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[19] S. Wang, L. Jiang, Definition of superhydrophobic states, Advanced Materials 19 (2007) 3423–3424.
[20] A. Torii, M. Sasaki, K. Hane, S. Okuma, Adhesion of microstructures investigated by atomic force microscopy, Sensors and Actuators A 40 (1994) 71-76.
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[21] D. Bachmann, S. Kühne, C. Hierold, Determination of the adhesion energy of
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MEMS structures by applying Weibull-type distribution function, Sensors and
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Actuators A 132 (2006) 407-414.
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[22] S. Cai, B. Bhushan, Meniscus and viscous forces during separation of hydrophilic
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and hydrophobic surfaces with liquid-mediated contacts, Materials Science and
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Engineering: R: Reports 61 (2008) 78-106. 17
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[23] S. Cai, B. Bhushan, Meniscus and viscous forces during normal separation of liquid-mediated contacts, Nanotechnology 18 (2007) 465704 (14pp). [24] S. Cai, B. Bhushan, Meniscus and viscous forces during separation of hydrophilic
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and hydrophobic smooth/rough surfaces with symmetric and asymmetric contact
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angles, Philosophical Transactions of the Royal Society A 366 (2008) 1627-1647.
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[25] E.J. De Souza, L. Gao, T.J. McCarthy, E. Arzt, A.J. Crosby, Effect of contact angle
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hysteresis on the measurement of capillary forces, Langmuir 24 (2008) 1391-1396.
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[26] H. Butt, M. Kappl, Normal capillary forces, Advances in Colloid and Interface
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Science 146 (2009) 48-60.
[27] C.G.L. Furmidge, Studies at phase interfaces. I. The sliding of liquid drops on solid
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surfaces and a theory for spray retention, Journal of Colloid Science17 (1962) 309-
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[28] C. Antonini, F.J. Carmona, E. Pierce, M. Marengo, A. Amirfazl, General
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methodology for evaluating the adhesion force of drops and bubbles on solid
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surfaces, Langmuir 25 (2009) 6143–6154
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[29] A.I. ElSherbini, A.M. Jacobi, Retention forces and contact angles for critical liquid
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drops on non-horizontal surfaces, Journal of Colloid and Interface Science 299
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(2006) 841–849
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[30] R. Tadmor, P. Bahadur, A. Leh, H.E. N’guessan, R. Jaini, L. Dang, Measurement
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of Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate,
21 22 23
Physical Review Letters 103 (2009) 266101 (4pp).
[31] W.H. Hager, Wilfrid Noel Bond and the Bond number, Journal of Hydraulic Research 50 (2012) 3-9.
24
[32] P.G. de Gennes, F. Brochard-Wyard, D. Quéré, A. Reisinger, Capillarity and
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wetting phenomena : drops, bubbles, pearls, waves, first ed., Springer, New York,
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2004.
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[33] J. Berthier, Microdrops and Digital Microfluidics, first ed., William Andrew Pub., Norwich, 2008. 18
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[34] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces. Transactions of the Faraday Society 40 (1945) 546-551.
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1
Table and Figure Captions
2 Table 1. Geometric parameter of the microstructures; measured and calculated contact
4
angle, water roll-off angle, upward and downward sliding adhesive force.
5
Fig. 1. Measurement scheme of dynamic lateral adhesion force of water droplets on
6
microstructured sample surfaces. While the probe disk is fixed, the stage moves up and
7
down.
8
Fig. 2 Geometries of the designed probe disk (a) and microstructures that will be
9
formed on a sample surface (b). a, b, and h denote pillar width, spacing between
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neighboring pillars, and height of the microstructures, respectively.
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Fig. 3 Fabrication process of the probe disk: (a) Cu seed layer deposition on a Si wafer
12
and first photolithography, (b) first Ni electroforming, (c) second photolithography, (d)
13
second Ni electroforming, (f) separation of the nickel probe disk from the silicon
14
substrate with PPFC (top and side faces) and a TiO2 coating (bottom). The two
15
fabricated probe disks are fixed at each end of a horizontal short carbon rod with epoxy;
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the carbon rod is then bonded with a vertical long carbon rod. (f) Photo of a probe that
17
is picked up with the probe holder.
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Fig. 4 Fabrication process of the microstructures: (a) photolithography, (b) silicon dry
19
etching by DRIE, and (c) removal of the photoresist and PPFC deposition. (d) SEM
20
image of fabricated microstructures with dimensions of a=6 μm, b=8.4 μm, h=15 μm.
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Fig. 5 Graphs of the change in the sliding adhesive force according to moving distance
22
of the droplet: spacing 3 μm; spacing 5 μm; spacing 8.4 μm; spacing 15.4
23
μm; spacing 36.6 μm. All pillar sizes are 6 μm.
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Fig. 6 Images of droplet meniscus profiles when the droplets move downward (left
25
image) or upward (right image) on microstructured hydrophobic surfaces: (a) spacing 3
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μm; (b) spacing 36.6 μm; (c) enlarged image of the box area of figure b. Dotted lines are
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the receding profiles. Numbers 1, 2, 3, 4, and 5 correspond to samples with different
2
spacing (3, 5, 8.4, 15.4, and 36.6 μm, respectively).
3
Fig. 7 Lateral adhesion forces for upward and downward sliding by varying b/a.
4
Fig. 8 Photo images of the water roll-off tilt angle of a water droplet on each sample: (a)
5
spacing 3 μm, no sliding at 90 tilting angle; (b) spacing 5 μm, no sliding at 90; (c)
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spacing 8.4 μm, sliding at 39; (d) spacing 15.4 μm, sliding at 18; (e) spacing 36.6 μm,
7
sliding at 4.
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1
Table 1. Geometric parameter of the microstructures; measured and calculated contact
2
angle, water roll-off angle, upward and downward lateral adhesion forces.
3 Water roll-off angle [deg]
3
130
134 ± 2
Pinning
5 8.4
140 150
139 ± 1 145 ± 3
Pinning 39 ± 4
15.4 36.6
160 170
153 ± 1 161 ± 1
18 ± 2 4±1
downward lateral adhesion force [mgf]
86 ± 6
Upward lateral adhesion force [mgf] 31.36 ± 5.12
102 ± 6 115 ± 9
19.07 ± 4.83 15.05 ± 1.74
-20.96 ± 3.84 -16.38 ± 1.97
128 ± 8 154 ± 5
6.90 ± 1.26 1.33 ± 1.30
-8.56 ± 1.48 -4.32 ± 1.27
Receding contact angle [deg]
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Measured contact angle [deg]
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Calculated contact angle [deg]
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-36.65 ± 3.25
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Fig. 1. Measurement scheme of dynamic lateral adhesion force of water droplets on
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microstructured sample surfaces. While the probe disk is fixed, the stage moves up and
3
down.
5
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Fig. 2 Geometries of the designed probe disk (a) and microstructures that will be
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formed on a sample surface (b). a, b, and h denote pillar width, spacing between
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neighboring pillars, and height of the microstructures, respectively.
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Fig. 3 Fabrication process of the probe disk: (a) Cu seed layer deposition on a Si wafer
2
and first photolithography, (b) first Ni electroforming, (c) second photolithography, (d)
3
second Ni electroforming, (f) separation of the nickel probe disk from the silicon
4
substrate with PPFC (top and side faces) and a TiO2 coating (bottom). The two
5
fabricated probe disks are fixed at each end of a horizontal short carbon rod with epoxy;
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the carbon rod is then bonded with a vertical long carbon rod. (f) Photo of a probe that
7
is picked up with the probe holder.
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Fig. 4 Fabrication process of the microstructures: (a) photolithography, (b) silicon dry
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etching by DRIE, and (c) removal of the photoresist and PPFC deposition. (d) SEM
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image of fabricated microstructures with dimensions of a=6 μm, b=8.4 μm, h=15 μm.
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Fig. 5 Graphs of the change in the sliding adhesive force according to moving distance
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of the droplet: spacing 3 μm; spacing 5 μm; spacing 8.4 μm; spacing 15.4
3
μm; spacing 36.6 μm. All pillar sizes are 6 μm.
5 6
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Fig. 6 Images of droplet meniscus profiles when the droplets move downward (left
2
image) or upward (right image) on microstructured hydrophobic surfaces: (a) spacing 3
3
μm; (b) spacing 36.6 μm; (c) enlarged image of the box area of figure b. Dotted lines are
4
the receding profiles. Numbers 1, 2, 3, 4, and 5 correspond to samples with different
5
spacing (3, 5, 8.4, 15.4, and 36.6 μm, respectively).
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Fig. 7 Lateral adhesion forces for upward and downward sliding by varying b/a.
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Downward force
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Lateral adhesion force [gf]
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Fig. 8 Photo images of the water roll-off tilt angle of a water droplet on each sample: (a)
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spacing 3 μm, no sliding at 90 tilting angle; (b) spacing 5 μm, no sliding at 90; (c)
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spacing 8.4 μm, sliding at 39; (d) spacing 15.4 μm, sliding at 18; (e) spacing 36.6 μm,
4
sliding at 4.
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Ms. Ref. No.: SNB-D-14-02788
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Seung-hwan Lee received the B.S. degree and M.S. degree in mechanical engineering from Pusan National University, Korea in 2011 and 2013, respectively. He joined the KT&G corporation, South Korea, where he is currently an R&D staff member.
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Jong Soo Ko received B.S. degree in mechanical engineering from Pusan National University in 1991. He received M.S. and Ph.D. degrees in mechanical engineering from KAIST, in 1994 and 2000, respectively. His doctoral research involved the investigation of IR sensor arrays. Beginning in 2000, he worked at the ETRI in Korea, where he was responsible for the development of a biochip. Since 2003, he has been working at Pusan National University. His research interests include nano/microfabrication, biomimetics, and sensors and actuators.
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Jae Min Lee received the B.S. degree in mechanical engineeringfrom Pusan National University, Korea in 2008 respectively. In September 2008, he joined the Micro Electro Mechanical System group in department of mechanicalengineering. He is currently working towards the Ph.D. His current research activities are MEMS, electrodeposition and sensors and actuators.
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Biographies
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S0925-4005(15)00286-5 http://dx.doi.org/doi:10.1016/j.snb.2015.02.101 SNB 18165
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
10-10-2014 14-2-2015 21-2-2015
Please cite this article as: J.M. Lee, S.-h. Lee, J.S. Ko, Dynamic Lateral Adhesion Force of Water Droplets on Microstructured Hydrophobic Surfaces, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.02.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Dynamic Lateral Adhesion Force of Water Droplets
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on Microstructured Hydrophobic Surfaces
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Jae Min Lee, Seung-hwan Lee, and Jong Soo Ko*
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Graduate School of Mechanical Engineering,
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Pusan National University,
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Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735, Korea
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*
To whom all correspondence should be addressed.
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Tel : +82-51-510-2488, Fax : +82-51-514-0685 E-mail: [email protected]
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Abstract This study investigates the change of the dynamic lateral adhesion forces of
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water droplets on microstructured surfaces by varying their hydrophobicities. While
4
hydrophobic surfaces showed significant asymmetry in advancing-to-receding profiles,
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superhydrophobic surfaces did not. As a result, hydrophobic surfaces required greater
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moving forces at the moment of change of the moving direction than superhydrophobic
7
surfaces. In addition, lateral adhesion forces measured from hydrophobic surfaces were
8
much higher than those from superhydrophobic surfaces. In the present experimental
9
conditions, water droplets did not roll on the surfaces with lateral adhesion forces larger
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than 21 mgf. These results support the possibility that the rolling ability of a water
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droplet on a surface can be evaluated by measuring the lateral adhesion force.
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Keywords: lateral adhesion force, contact angle, hydrophobic, superhydrophobic,
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microstructure
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1. Introduction
2 Surface modification to achieve superhydrophobicity is a typical example of
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biomimetics [1-3]. The representative plant that has a superhydrophobic characteristic is
5
the lotus. The surface of the lotus leaf is covered with microprotrusions, which are in
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turn covered in nanoprotrusions composed of hydrophobic epicuticular wax crystalloids,
7
and the lotus leaf has superhydrophobic and self-cleaning characteristics [4]. These
8
characteristics are also referred to as the lotus effect. Artificial realization of a nano-
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micro hierarchical topology is the key factor to achieve a superhydrophobic surface with
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a static contact angle greater than 150, and numerous fabrication methods have been
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developed to date [5-9]. The rose petal is also covered with nano-micro hierarchical
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structures, showing a superhydrophobic surface. However, a water droplet on a petal
13
does not roll; rather it is suspended when the petal is inverted [10]. As a result, the rose
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petal does not have a self-cleaning capability [11,12]. In short, the lotus leaf and the
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rose petal both show very high contact angles greater than 150 (superhydrophobic), but
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they show antithetic behavior with regard to the rolling of water droplets [13]. This
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indicates that it is not sufficient to predict the dynamic behavior of the water droplet on
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a superhydrophobic surface simply by measuring the contact angle.
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To investigate the dynamic behavior of the water droplet on a surface, several
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methods for measuring dynamic contact angles have been developed, including the
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dynamic sessile drop method [14] and tilting plate method [15]. Through the
22
measurement of dynamic contact angles, the contact angle hysteresis, defined as the
23
difference between advancing and receding contact angles, can be obtained [16-18]. The
24
contact angle hysteresis is an important physical parameter that affects the dynamic 3
Page 3 of 31
behavior of water droplets, such as rolling and collision. In practice, rolling-off of a
2
water droplet and self-cleaning ability are determined by the level of contact angle
3
hysteresis measured from a surface. The roll-off angle/contact angle hysteresis should
4
be maintained at less than 10° to realize the lotus effect on a surface. [19] Assessing the
5
contact angle hysteresis is an effective means to predict the dynamic behavior of a water
6
droplet on a surface. However, this measurement method uses an optical subsystem to
7
capture the profile of a moving water droplet. Because it relies on the captured image of
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a water droplet, the actual adhesion force between the surface and the water droplet
9
cannot directly be obtained.
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Recently, several approaches to measure normal and lateral adhesion forces have
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been reported. Atomic force microscopy (AFM), a representative approach for
12
measuring normal adhesion force, has been used to measure the capillary bridging force
13
of a liquid between the AFM tip and the substrate surface [20-26]. Although this
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method provides a quantitative means of measuring the adhesion force, the
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measurement area is limited to the submicroscopic level due to the usage of a nanoscale
16
tip. It is therefore difficult to use the method with topographically-modified
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hydrophobic surfaces including a microstructured surface [25]. The most important
18
approach to investigate static lateral adhesion forces to date is the tilted plate method
19
[27-29]. This method involves gradual tilting of the plate and measuring the roll-off
20
angle, followed by calculation of the static lateral adhesion forces from the relation to
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the projection of the gravitational force needed to initiate droplet movement. Although a
22
useful way to measure the lateral adhesion forces, this method has several limitations.
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First, only ‘static’ lateral adhesion forces can be obtained. Second, this method is valid
24
only for sufficiently large droplets, because small droplets do not slide owing to
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insufficient gravity. Third, it does not directly measure the forces. R. Tadmor et al.
2
proposed a centrifuge method in which gravitation is replaced by a centrifugal force.
3
The static lateral adhesion force of the small droplets can be measured by using a
4
centrifugal adhesion balance (CAB) system [30]. This method also measures the ‘static’
5
lateral adhesion forces and does not directly measure the forces.
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The capability of measuring a dynamic lateral adhesion force when a water
7
droplet moves in the lateral direction along a surface would be very helpful to predict
8
the dynamic behavior of a water droplet on a surface. In this study, we introduce a new
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approach to characterize surface properties: measuring the dynamic adhesion force of a
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surface and a water droplet that moves laterally to the surface. To verify the proposed
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measurement scheme, microstructured surfaces with different hydrophobicity are
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fabricated and characterized. In addition, the relationship of the lateral adhesion force
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and the rolling ability of a water droplet is discussed.
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2. Measurement Concept The configuration of the measurement system is shown in Fig. 1. An H-shape
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probe has been devised to measure the dynamic lateral adhesion force between the
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water droplet and the substrate surface. The lower end of a rigid bar is attached to the
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center of the H-shape probe, while its upper end is locked onto the probe holder. The
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holder is connected to a very precise load cell, allowing the very small force acting at it
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to be measured. Two water droplets are attached to each probe disk. The probe disks are
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modified to be hydrophilic to give sufficient adhesion force between the water droplet 5
Page 5 of 31
and the probe disk surface. Two samples are attached to each moving plate, where the
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lateral movement is precisely controlled by the respective linear servo actuator. Two
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linear actuators are clamped onto the stage.
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After two water droplets are dispensed on each probe disk, the two sample holders
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simultaneously move to the center with the same velocity, and then stop when they
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reach a certain distance () from the probe disks. The water droplets are constrained
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between the sample surfaces and the probe disks. The stage then moves downward and
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upward. During the whole measurement, the probe holders and the probe disks do not
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move. Briefly, while the probe does not move, the two moving plates move horizontally
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and then the stage moves vertically.
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To continuously measure the dynamic lateral adhesion forces using the proposed
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measurement concept, the probe disk should hold the droplet and the droplet volume
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should remain constant during the measurement. Two necessary conditions should be
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satisfied to meet these requirements. First, the adhesion force of the probe disk and the
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suitable for measurement on hydrophobic surfaces.
15 16 17 18
droplet must be higher than that of the sample surface and the droplet. The probe disk surface thus should have strong hydrophilicity to endow a strong adhesion force between the probe disk and the droplet. Second, residual drops of water should not remain on the sample surfaces. Because of the high possibility of residual drops remaining on a sample surface that has a hydrophilic nature, the proposed method is
21
The Bond number (Bo) is a basic dimensionless number and is defined as the ratio
22
of the gravitational force and the surface tension force acting on a liquid droplet [31,32]. 6
Page 6 of 31
A high Bond number indicates that the liquid droplet is relatively affected by
2
gravitational force, while a low Bond number indicates that it is more affected by
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surface tension. As shown in Fig. 1, the gap () between two parallel solid surfaces is
4
small; the Bond number is given as Bo=g2/ [33]. Here, , , and g denote the density
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(997.0 kg/m3) and the surface tension (71.97 mN/m) of water, and the gravitational
6
constant, respectively. In this study, because the gap was set to be 0.5 mm, the Bond
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number was 0.034. Therefore, it is expected that the surface tension dominates in this
8
measurement.
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3. Design and Fabrication
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3.1 Probe
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Fig. 2a shows the designed probe. Two probe disks of 4 mm diameter are
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connected at both ends of a carbon probe-disk-holding rod that is 5 mm in length (H-
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shape probe). Another 45-mm-long carbon rod is attached to the center of the probe-
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disk-holding rod. Figs. 3a-e show the probe disk fabrication process. First, a 100 nm-
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thick copper conducting seed layer was deposited followed by the first photolithography
17
step using a THB 151N photoresist (JSR Micro, USA) (Fig. 3a). Next, the first nickel
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electroforming step was carried out using a sulfamic acid based nickel electroplating
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solution (E-Form, DisChem, USA) (Fig. 3b). A 50-µm-thick nickel layer was obtained
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with a current density of 20 mA/cm2 at 50 C for 2 h. In the next processing step, the
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photoresist was removed by acetone and the second photolithography step was
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performed (Fig. 3c). A 50-µm-thick nickel layer was then deposited by the second
2
nickel electroforming step (Fig. 3d). The purpose of the second photolithography and
3
electroforming process is to form a 1- mm-dia. indent at the center of the upper part of
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the probe disk. The end of a probe-disk-holding rod of 1 mm diameter will be inserted
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in the indent and fixed in the following assembly process. In the next processing step,
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the nickel plate was separated from the silicon substrate by wet silicon chemical etching
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(Fig. 3e). The silicon was etched for 8 h at 85 C using a 25 wt% potassium hydroxide
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(KOH) solution (J. T. Baker, USA). The fabrication process of the probe disk was
9
completed by coating the flat surface of the plate with a liquefied TiO2 solution
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(Glaschon, NCI, Korea) of a hydrophilic nature. By application of a TiO2 coating, the
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plate surface was modified from a weak hydrophilic character (contact angle, CA=71,
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electroformed nickel surface) to a strong hydrophilic character (CA=5, TiO2-coated
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surface). Each end of the probe-disk-holding rod is inserted to the indents of the
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fabricated probe disks and then fixed with bonding glue (ALTECO F-05, ALPHA
15
TECHNO, Japan). Finally, another 45-mm-long carbon rod is bonded to the center of
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the probe-disk-holding rod using the same bonding material. Fig. 3f shows a fabricated
17
probe.
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3.2 Microstructured Surfaces with Different Hydrophobicities
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To measure the dynamic lateral adhesion force according to the level of
21
hydrophobicity of the surface, microstructured surfaces with different contact angles
22
have been designed and fabricated. The designed microstructure is illustrated in Fig. 2b. 8
Page 8 of 31
We assumed that the water droplet is supported by the protruding microstructures. The
2
apparent contact angle () based on Cassie and Baxter’s model is expressed as
3
cos=f(1+cos)-1 [34]. Here, f is the solid fraction, defined as the ratio of the wetting
4
area of the rough surface to the projected area, and is the contact angle of a water
5
drop on a smooth surface. The contact angle () of a smooth surface coated with a
6
plasma polymerized fluorocarbon (PPFC) layer was measured to be 103 and this value
7
was used for the calculation of contact angles in this study. The contact angle can be
8
controlled by reducing the contact area between the top of the microstructures and the
9
water droplet. The reduced contact area resulted in a reduction of the surface energy,
10
enhancing the hydrophobicity of the surface. To obtain samples with five different
11
contact angles (from 130 to 170, each step=10), the spacing (b) was increased from 3
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to 36.6 µm, whereas the square pillar size (aa) was fixed at 66 µm2, as shown in
13
Table 1. The designed structure height was 15 µm.
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Fig. 4 shows schematic illustrations of the process sequence for the fabrication
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of square pillar microstructures. The fabrication process for the microstructures starts
16
with photolithography using a AZ1512 photoresist (Clariant, Swiss) on a 6-inch silicon
17
wafer (LG Siltron, Korea) (Fig. 4a). The top silicon surface was then vertically etched
18
to a 15 µm depth using a deep reactive ion etching (DRIE) system (STS Multiplex ICP,
19
UK) (Fig. 4b). The fabrication process was completed by removal of the photoresist and
20
subsequent coating on silicon microstructures with a 3-nm-thick PPFC layer of a
21
hydrophobic nature (Fig. 4c). The silicon wafer was then diced to sample pieces with a
22
size of 16 mm8 mm.
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Fig. 4d shows a scanning electron microscopy (SEM; JEOL JSM-6700F) image 9
Page 9 of 31
of the fabricated sample (a=6 µm, b=5 µm). The contact angles measured from each
2
microstructure are listed in Table 1. The contact angles were measured using a contact
3
angle-meter (SEO, Korea). Deionized water was used for measurement of the contact
4
angles and the volume of a single water droplet was 4 µl. The measured contact angle
5
was the average obtained from five measurements from each sample. Three samples
6
were selected and used for the measurement in this study. The measured contact angles
7
gradually increased with increasing b/a in a range from 134 to 161. The data reveal
8
that the measured contact angles tend to be lower than the calculated angles. In our
9
experience, the measured contact angles of fabricated microstructures are typically
10
lower than the calculated values. The reason for this has yet to be identified. Although
11
the measured contact angles are different from the calculated angles, both gradually
12
increase according to b/a. The contact angles measured from three samples (b= 3, 5, 8.4
13
µm) are lower than 150 and those measured from the other two samples (b=15.4, 36.6
14
µm) are higher than 150. The five samples with different contact angles are appropriate
15
for investigating the change of the lateral adhesion force according to the
16
hydrophobicity level of the sample surface.
cr
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4. Measurements and Discussion
19
4.1. Measurement Setup
20
A tensiometer (DCAT21, Data Physics, Germany) was used to measure the
21
lateral adhesion force of a water droplet in this study. Deionized (DI) water droplets of 6
22
μl volume are dispensed on each probe disk using a micropipette (eTouch, Sorenson, 10
Page 10 of 31
USA). Movement of each sample is controlled by a linear servo actuator (LSA-3024SD,
2
Potenit, Korea) and the distance between the substrate surfaces and the probe discs was
3
0.5 mm for all measurements. The stage moves upward 8 mm at a velocity of 0.1 mm/s,
4
and then the moving direction of the stage changes to the opposite direction and the
5
stage moves downward 8 mm at the same velocity. A charge-coupled device (CCD)
6
camera (Navitar, USA) was used to capture moving images of the water droplets. All
7
measurements were conducted in a cleanroom with a humidity of 60% and temperature
8
of 20 °C.
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4.2. Measurement of Lateral Adhesion Forces
M
10
Fig. 5 shows measurement cycles of the lateral adhesion forces of the five
12
fabricated microstructured surfaces. In the case of the b=3 and 5 µm samples, lateral
13
adhesion forces were maximal at the points (* marked) where the water droplets started
14
to move downward. This phenomenon is very similar to the maximum static frictional
15
force. Water droplets placed on the samples displaying this phenomenon did not roll in
16
the tilting test, as presented in Table 1 (also see Figs. 8a and 8b). However, this
17
phenomenon was not observed for the b=8.4, 15.4, and 36.6 µm samples. This is
18
ascribed to the different degrees of hydrophobicity.
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Figs. 6a and 6b show captured images of the smallest spacing sample (b=3 μm)
20
and the largest spacing sample (b=36.6 μm), respectively, during measurement. The left
21
images of Figs 6a and 6b were captured during upward movement of the stage (and
22
samples), while the right images were captured during downward movement. For the
23
smallest spacing sample shown in Fig. 6a, the advancing profile of the water drop is
24
highly sloped, whereas the receding profile is almost vertical to the substrate surface 11
Page 11 of 31
(receding contact angle=86±6). On the other hand, for the largest spacing sample
2
shown in Fig. 6b, the receding profile is also highly sloped (receding contact angle=154
3
±5). Receding profiles of each sample with different spacing are redrawn in Fig. 6c
4
and the measured receding contact angles are listed in Table 1. Advancing contact
5
angles of each sample were not significantly different within a range of 153-157,
6
whereas receding angles were dramatically different, as shown in Table 1. Therefore,
7
while the sample with the smallest spacing shows large asymmetry in the advancing-to-
8
receding profile (contact angle difference between advancing and receding=69), the
9
sample with the largest spacing is almost symmetric (contact angle difference=3). With
10
an increase of the spacing, the contact area between the sample surface and the water
11
droplet decreases, resulting in a decrease of surface energy. Low surface energy
12
decreases the adhesion force between the sample surface and the water droplet; the two
13
profiles (advancing and receding) of the water droplet in the largest spacing sample are
14
consequently almost symmetric.
15
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Fig. 7 shows the average values of the lateral adhesion forces during downward
16
and upward movements by varying b/a (see Table 1 for exact values). Lateral adhesion
17
forces measured with a moving distance range of 1-7 mm were used to obtain the
18
average values. The absolute magnitudes of the lateral adhesion forces during
19
downward and upward movements gradually decreased with an increase of b/a. For all
20
samples, however, the lateral adhesion force during downward movement is slightly
21
larger than that during upward movement. It is believed that the difference between the
22
two is caused by the weight of the water droplet. Although the difference is relatively
23
small compared to the upward lateral adhesion force for the samples with a small 12
Page 12 of 31
spacing, it becomes significant for the samples with a large spacing. In the case of the
2
largest spacing (b=36.6 μm), the difference (2.99 mgf) is 2.25 times larger than the
3
upward lateral adhesion force (1.33 mgf). This shows that the influence of the weight of
4
the water droplet is significant in the samples with superhydrophobic surfaces.
5
Therefore, a cautious approach is necessary in adopting the Bond number to analyze
6
dominant forces acting on a microscale system with a superhydrophobic surface.
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To investigate a water drop’s ability to roll on the sample surface, a 4-µl water
8
droplet was dispensed on the sample surface and the sample was tilted. Rolling or
9
pinning of water droplets and the rolling angles for each sample are listed in Table 1.
10
The water droplets on the b= 3 and 5 µm samples did not roll when the samples were
11
tilted up to 90, as shown in Figs. 8a and 8b. On the other hand, the water droplets on
12
the b= 8.4, 15.4, and 36.6 µm samples rolled when the samples were tilted, as shown in
13
Figs. 8c-8e. However, there is a significant difference in the roll-off angles by varying
14
the spacing. A water droplet placed on the sample with b=8.4 μm rolled at a tilting angle
15
of 39, whereas it rolled at 4 when placed on the sample with b=36.4 μm.
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In these experimental conditions, water droplets did not roll on the surfaces with
17
lateral adhesion forces larger than 21 mgf. Although the lateral adhesion forces have
18
been obtained from limited microstructure sizes, the results of this experiment have
19
strong implications in the field of surface evaluation. It is possible to assess the rolling
20
ability of a water droplet on a surface by measuring the lateral adhesion force. Even
21
though the contact angle of a surface is higher than 150, a water droplet on the surface
22
does not roll; instead it adheres to the surface even when the surface is turned over.
23
Therefore, it is difficult to judge the rolling ability of a water droplet on a surface from
24
the measured contact angle data. The approach suggested in this study is a meaningful 13
Page 13 of 31
1
attempt to discover a link between the surface adhesion force and the rolling ability of a
2
water droplet on a surface.
5. Conclusion
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The authors studied the change of the dynamic lateral adhesion forces of water
6
droplets on microstructured surfaces by varying their hydrophobicities. Hydrophobicity
7
was adjusted by means of the spacing of rectangular micropillars (area=66 µm2).
8
Advancing contact angles of each sample were not significantly different between the
9
two samples with the smallest (b=3 µm) and largest spacing (b=36.6 μm), whereas the
10
receding contact angles were dramatically different. Therefore, while the sample with
11
the smallest spacing showed large asymmetry in the advancing-to-receding profile, the
12
sample with the largest spacing was almost symmetric. The absolute magnitudes of the
13
lateral adhesion forces during downward and upward movements gradually decreased
14
with an increase of the spacing. With an increase of the spacing, the contact area
15
between the sample surface and the water droplet decreases, resulting in a decrease of
16
the surface energy. Because of the weight of the water droplet, the lateral adhesion force
17
during downward movement is slightly larger than that during upward movement. The
18
influence of the weight of the water droplet is significant in the samples with
19
superhydrophobic surfaces. In these experimental conditions, water droplets did not roll
20
on the surfaces with lateral adhesion forces larger than 21 mgf. These results support the
21
possibility that the rolling ability of a water droplet on a surface can be evaluated by
22
measurement of the lateral adhesion force.
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Page 14 of 31
1 Acknowledgement
3
This
4
(www.nanotech2020.org) funded by the Ministry of Science, ICT and Future Planning
5
(MSIP, Korea) and the Ministry of Trade, Industry and Energy (MOTIE, Korea)
6
[Project Name: Development of an oil skimmer using nanotextured hydrophobic
7
micromeshes].
was
supported
by
the
Nano-Convergence
Foundation
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1
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superhydrophobic state with high adhesive force, Langmuir 24 (2008) 4114-4119. [12] E. Bormashenko, T. Stein, R. Pogreb, D. Aurbach, “Petal effect” on surfaces based
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[13] M. Nosonovsky and B. Bhushan, Green Tribology, (Chapter 2) Lotus Versus Rose:
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Biomimetic Surface Effects, Green Energy and Technology, Springer-Verlag
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Berlin Heidelberg, 2012.
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[15] B.A. Bezuglyi, O.A. Tarasov, A.A. Fedorets, Modified tilting-plate method for measuring contact angles, Colloid Journal 63 (2001) 735-741.
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[16] A. Marmur, Thermodynamic aspects of contact-angle hysteresis, Advances in Colloid and Interface Science 50 (1994) 121–141. [17] E.L. Decker, S. Garoff, Contact angle hysteresis: the need for new theoretical and
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experimental models, The Journal of Adhesion 63 (1997) 159–185.
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[19] S. Wang, L. Jiang, Definition of superhydrophobic states, Advanced Materials 19 (2007) 3423–3424.
[20] A. Torii, M. Sasaki, K. Hane, S. Okuma, Adhesion of microstructures investigated by atomic force microscopy, Sensors and Actuators A 40 (1994) 71-76.
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[21] D. Bachmann, S. Kühne, C. Hierold, Determination of the adhesion energy of
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MEMS structures by applying Weibull-type distribution function, Sensors and
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[22] S. Cai, B. Bhushan, Meniscus and viscous forces during separation of hydrophilic
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and hydrophobic smooth/rough surfaces with symmetric and asymmetric contact
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angles, Philosophical Transactions of the Royal Society A 366 (2008) 1627-1647.
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[25] E.J. De Souza, L. Gao, T.J. McCarthy, E. Arzt, A.J. Crosby, Effect of contact angle
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hysteresis on the measurement of capillary forces, Langmuir 24 (2008) 1391-1396.
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[26] H. Butt, M. Kappl, Normal capillary forces, Advances in Colloid and Interface
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[27] C.G.L. Furmidge, Studies at phase interfaces. I. The sliding of liquid drops on solid
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surfaces and a theory for spray retention, Journal of Colloid Science17 (1962) 309-
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[28] C. Antonini, F.J. Carmona, E. Pierce, M. Marengo, A. Amirfazl, General
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methodology for evaluating the adhesion force of drops and bubbles on solid
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surfaces, Langmuir 25 (2009) 6143–6154
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[29] A.I. ElSherbini, A.M. Jacobi, Retention forces and contact angles for critical liquid
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drops on non-horizontal surfaces, Journal of Colloid and Interface Science 299
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[30] R. Tadmor, P. Bahadur, A. Leh, H.E. N’guessan, R. Jaini, L. Dang, Measurement
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of Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate,
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Physical Review Letters 103 (2009) 266101 (4pp).
[31] W.H. Hager, Wilfrid Noel Bond and the Bond number, Journal of Hydraulic Research 50 (2012) 3-9.
24
[32] P.G. de Gennes, F. Brochard-Wyard, D. Quéré, A. Reisinger, Capillarity and
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wetting phenomena : drops, bubbles, pearls, waves, first ed., Springer, New York,
26
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[33] J. Berthier, Microdrops and Digital Microfluidics, first ed., William Andrew Pub., Norwich, 2008. 18
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[34] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces. Transactions of the Faraday Society 40 (1945) 546-551.
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1
Table and Figure Captions
2 Table 1. Geometric parameter of the microstructures; measured and calculated contact
4
angle, water roll-off angle, upward and downward sliding adhesive force.
5
Fig. 1. Measurement scheme of dynamic lateral adhesion force of water droplets on
6
microstructured sample surfaces. While the probe disk is fixed, the stage moves up and
7
down.
8
Fig. 2 Geometries of the designed probe disk (a) and microstructures that will be
9
formed on a sample surface (b). a, b, and h denote pillar width, spacing between
an
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3
neighboring pillars, and height of the microstructures, respectively.
11
Fig. 3 Fabrication process of the probe disk: (a) Cu seed layer deposition on a Si wafer
12
and first photolithography, (b) first Ni electroforming, (c) second photolithography, (d)
13
second Ni electroforming, (f) separation of the nickel probe disk from the silicon
14
substrate with PPFC (top and side faces) and a TiO2 coating (bottom). The two
15
fabricated probe disks are fixed at each end of a horizontal short carbon rod with epoxy;
16
the carbon rod is then bonded with a vertical long carbon rod. (f) Photo of a probe that
17
is picked up with the probe holder.
18
Fig. 4 Fabrication process of the microstructures: (a) photolithography, (b) silicon dry
19
etching by DRIE, and (c) removal of the photoresist and PPFC deposition. (d) SEM
20
image of fabricated microstructures with dimensions of a=6 μm, b=8.4 μm, h=15 μm.
21
Fig. 5 Graphs of the change in the sliding adhesive force according to moving distance
22
of the droplet: spacing 3 μm; spacing 5 μm; spacing 8.4 μm; spacing 15.4
23
μm; spacing 36.6 μm. All pillar sizes are 6 μm.
24
Fig. 6 Images of droplet meniscus profiles when the droplets move downward (left
25
image) or upward (right image) on microstructured hydrophobic surfaces: (a) spacing 3
26
μm; (b) spacing 36.6 μm; (c) enlarged image of the box area of figure b. Dotted lines are
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Page 20 of 31
the receding profiles. Numbers 1, 2, 3, 4, and 5 correspond to samples with different
2
spacing (3, 5, 8.4, 15.4, and 36.6 μm, respectively).
3
Fig. 7 Lateral adhesion forces for upward and downward sliding by varying b/a.
4
Fig. 8 Photo images of the water roll-off tilt angle of a water droplet on each sample: (a)
5
spacing 3 μm, no sliding at 90 tilting angle; (b) spacing 5 μm, no sliding at 90; (c)
6
spacing 8.4 μm, sliding at 39; (d) spacing 15.4 μm, sliding at 18; (e) spacing 36.6 μm,
7
sliding at 4.
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Page 21 of 31
1
Table 1. Geometric parameter of the microstructures; measured and calculated contact
2
angle, water roll-off angle, upward and downward lateral adhesion forces.
3 Water roll-off angle [deg]
3
130
134 ± 2
Pinning
5 8.4
140 150
139 ± 1 145 ± 3
Pinning 39 ± 4
15.4 36.6
160 170
153 ± 1 161 ± 1
18 ± 2 4±1
downward lateral adhesion force [mgf]
86 ± 6
Upward lateral adhesion force [mgf] 31.36 ± 5.12
102 ± 6 115 ± 9
19.07 ± 4.83 15.05 ± 1.74
-20.96 ± 3.84 -16.38 ± 1.97
128 ± 8 154 ± 5
6.90 ± 1.26 1.33 ± 1.30
-8.56 ± 1.48 -4.32 ± 1.27
Receding contact angle [deg]
ip t
Measured contact angle [deg]
cr
Calculated contact angle [deg]
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Spacing, b [µm]
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-36.65 ± 3.25
22
Page 22 of 31
Fig. 1. Measurement scheme of dynamic lateral adhesion force of water droplets on
2
microstructured sample surfaces. While the probe disk is fixed, the stage moves up and
3
down.
5
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23
Page 23 of 31
Fig. 2 Geometries of the designed probe disk (a) and microstructures that will be
2
formed on a sample surface (b). a, b, and h denote pillar width, spacing between
3
neighboring pillars, and height of the microstructures, respectively.
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1
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5 6 24
Page 24 of 31
Fig. 3 Fabrication process of the probe disk: (a) Cu seed layer deposition on a Si wafer
2
and first photolithography, (b) first Ni electroforming, (c) second photolithography, (d)
3
second Ni electroforming, (f) separation of the nickel probe disk from the silicon
4
substrate with PPFC (top and side faces) and a TiO2 coating (bottom). The two
5
fabricated probe disks are fixed at each end of a horizontal short carbon rod with epoxy;
6
the carbon rod is then bonded with a vertical long carbon rod. (f) Photo of a probe that
7
is picked up with the probe holder.
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9 10 25
Page 25 of 31
Fig. 4 Fabrication process of the microstructures: (a) photolithography, (b) silicon dry
2
etching by DRIE, and (c) removal of the photoresist and PPFC deposition. (d) SEM
3
image of fabricated microstructures with dimensions of a=6 μm, b=8.4 μm, h=15 μm.
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Page 26 of 31
Fig. 5 Graphs of the change in the sliding adhesive force according to moving distance
2
of the droplet: spacing 3 μm; spacing 5 μm; spacing 8.4 μm; spacing 15.4
3
μm; spacing 36.6 μm. All pillar sizes are 6 μm.
5 6
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Page 27 of 31
Fig. 6 Images of droplet meniscus profiles when the droplets move downward (left
2
image) or upward (right image) on microstructured hydrophobic surfaces: (a) spacing 3
3
μm; (b) spacing 36.6 μm; (c) enlarged image of the box area of figure b. Dotted lines are
4
the receding profiles. Numbers 1, 2, 3, 4, and 5 correspond to samples with different
5
spacing (3, 5, 8.4, 15.4, and 36.6 μm, respectively).
cr
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Page 28 of 31
1
Fig. 7 Lateral adhesion forces for upward and downward sliding by varying b/a.
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Upward force
b/a
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Downward force
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Lateral adhesion force [gf]
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Page 29 of 31
Fig. 8 Photo images of the water roll-off tilt angle of a water droplet on each sample: (a)
2
spacing 3 μm, no sliding at 90 tilting angle; (b) spacing 5 μm, no sliding at 90; (c)
3
spacing 8.4 μm, sliding at 39; (d) spacing 15.4 μm, sliding at 18; (e) spacing 36.6 μm,
4
sliding at 4.
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Page 30 of 31
1
Ms. Ref. No.: SNB-D-14-02788
2 3 4
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Seung-hwan Lee received the B.S. degree and M.S. degree in mechanical engineering from Pusan National University, Korea in 2011 and 2013, respectively. He joined the KT&G corporation, South Korea, where he is currently an R&D staff member.
M
Jong Soo Ko received B.S. degree in mechanical engineering from Pusan National University in 1991. He received M.S. and Ph.D. degrees in mechanical engineering from KAIST, in 1994 and 2000, respectively. His doctoral research involved the investigation of IR sensor arrays. Beginning in 2000, he worked at the ETRI in Korea, where he was responsible for the development of a biochip. Since 2003, he has been working at Pusan National University. His research interests include nano/microfabrication, biomimetics, and sensors and actuators.
d
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Jae Min Lee received the B.S. degree in mechanical engineeringfrom Pusan National University, Korea in 2008 respectively. In September 2008, he joined the Micro Electro Mechanical System group in department of mechanicalengineering. He is currently working towards the Ph.D. His current research activities are MEMS, electrodeposition and sensors and actuators.
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Biographies
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