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Design methodology for nano-engineered surfaces to control adhesion: Application to the anti-adhesion of particles
Applied Surface Science 389 (2016) 889–893
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Design methodology for nano-engineered surfaces to control adhesion: Application to the anti-adhesion of particles Taekyung Kim a,b,1 , Cheongwan Min a,1 , Myungki Jung a,b , Jinhyung Lee a,b , Changsu Park a,b , Shinill Kang a,b,∗ a b
National Center for Optically-Assisted Ultra-High Precision Mechanical Systems, Yonsei University, Seoul 03722, Republic of Korea School of Mechanical Engineering, Yonsei University, Seoul 03722, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 June 2016 Received in revised form 19 July 2016 Accepted 4 August 2016 Available online 6 August 2016 Keywords: Control of adhesion force Derjaguin approximation Nano-engineered surfaces Particle adhesion
a b s t r a c t With increasing demand for means of controlling surface adhesion in various applications, including the semiconductor industry, optics, micro/nanoelectromechanical systems, and the medical industry, nano-engineered surfaces have attracted much attention. This study suggests a design methodology for nanostructures using the Derjaguin approximation in conjunction with finite element analysis for the control of adhesion forces. The suggested design methodology was applied for designing a nano-engineered surface with low-adhesion properties. To verify this, rectangular and sinusoidal nanostructures were fabricated and analyzed using force-distance curve measurements using atomic force microscopy and centrifugal detachment testing. For force-distance curve measurements, modified cantilevers with tips formed with atypical particles were used. Subsequently, centrifugal detachment tests were also conducted. The surface wettability of rectangular and sinusoidal nanostructures was measured and compared with the measured adhesion force and the number of particles remaining after centrifugal detachment tests. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With increasing demand for the means of controlling surface adhesion in various fields, including semiconductors, displays, optical, micro/nanoelectromechanical systems, and the medical industry, technologies for controlling adhesion forces have become increasingly important. For example, microparticles cannot be detached readily from a surface because the adhesion force is typically much greater than the detachment forces of inertia and drag. Surface contamination due to the presence of such fine particles has caused enormous yield losses and degradation of reliability [1–7]. To address these issues, various investigations have examined the adhesion properties of biomimetic surfaces with superhydrophobic properties. Bhushan et al. proposed self-cleaning surfaces with a low adhesion force, resembling the structure of lotus leaves [8–12]. Their research confirmed that biomimetic surfaces with superhydrophobicity can be used to modify surface adhesion properties. However, Autumn et al. demonstrated experimental evidence that van der Waals forces are a dominant factor in
∗ Corresponding author at: Yonsei University, Mechanical Engineering, 50, Yonseiro, Seodaemun-gu, Seoul, Republic of Korea. E-mail address: [email protected] (S. Kang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.apsusc.2016.08.015 0169-4332/© 2016 Elsevier B.V. All rights reserved.
determining adhesion forces between micro- and nanostructures and solid surfaces [13]. They provided experimental evidence for the dry adhesion of gecko setae by van der Waals forces, showing that the toes of hydrophobic Tokay geckos adhered equally well to hydrophobic and hydrophilic surfaces. Their experimental results indicated that the remarkable adhesive properties of gecko setae were merely a result of the size and shape of the tips, and were not strongly affected by surface chemistry. Several studies regarding the van der Waals attraction energy model in particles and semi-infinite media with surface roughness have been reported. Czarnecki and Dabros presented adhesion force models considering various particle sizes, thicknesses of the rough layer, and separation values. Throughout their mathematical calculation, roughness of particle surfaces decreased the van der Waals attraction energy markedly between a globular particle and semi-infinite medium system [14]. Rumpf also proposed a model that described surface asperities as small hemispheres [15]. Furthermore, to calculate the adhesion force more accurately, Rabinovich proposed a model containing large and small regions of surface roughness irregularities [16,17]. Their results show a clear correlation between the surface characteristics (i.e., the size of asperities or surface roughness) and the adhesion force. However, a heterogeneous surface with random roughness does not guarantee uniform adhesion forces over the entire surface [18].
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T. Kim et al. / Applied Surface Science 389 (2016) 889–893
Crosby et al. showed experimentally how microscale cylindrical post arrays can regulate the van der Waals energy required for interfacial separation and the maximum force for separating an interface. Polymeric cylindrical post arrays with various heights (20–250 m) and spacing (50–500 m) were prepared using conventional fabrication technologies including photolithography, and nanoimprint lithography and the patterned materials were characterized immediately using a contact adhesion test [19]. While most research regarding the adhesion of fine structures has been based on bio-inspired or empirical methods, an “adhesion design map” was introduced for a thorough understanding of adhesion by Spolenak et al. They established convenient guidelines (adhesion design maps) using mathematical equations describing limiting criteria for the fracture of fine structures and ideal contact strength [20]. Although they theorized the balance of these design parameters in adhesion maps for patterned surfaces, no complete experimental verification has yet been established [21]. Here, we suggest a design methodology of nanostructures using the Derjaguin approximation in conjunction with finite element analysis (FEA) for the control of adhesion forces. The suggested simulation methodology shows the extent to which the fill factor, pitch, and height of nanostructures affect the adhesion force, and enables quantitative calculations of the adhesion force. In this study, this was applied to designing a nano-engineered surface with low-adhesion properties. To confirm the proposed simulations, experimental results, including measurements of adhesion forces and centrifugal detachment tests with pulverized micro glass particles, were demonstrated. We fabricated two master nanostructures which were rectangular line and sinusoidal nanostructures, based on the theoretical framework described here via laser interference lithography, followed by deep reactive ion etching and reactive ion etching processes, respectively. After replicating the metallic roll stamp from the master structures by an electrochemical deposition process, polymeric nanostructures were fabricated via a roll-toroll imprinting process. The adhesion force was characterized using atomic force microscopy (AFM) based on the force–distance (FD) curve. The AFM system used a laser diode, position-sensitive detector, and cantilever with an attached 30-m-diameter spherical particle to measure the interaction between the particle and patterned surface [22–25]. Moreover, a cantilever with an attached atypical particle was used to assess the adhesion force of spontaneous particles by considering practical usage. To confirm that particles detached readily from the designed fine structures, centrifugal detachment tests were carried out, and the residual particles were counted [26]. By comparing the results of centrifugal detachment tests with the surface wettability of two nanostructures, we also found that the hydrophobicity and anti-adhesion force have little interdependence [13]. 2. Experimental methods 2.1. Adhesion force model for nanostructures The forces between a surface and a particle include van der Waals forces, electrostatic forces, and surface tension; however, here we focus on the van der Waals forces because they are the most significant in nanostructures [27]. We used the Hamaker equation and Derjaguin approximation to calculate the adhesion force between a spherical particle and planar nanostructured surface [28,29]. The interaction energy between two particles containing q atoms per cm3 is given by
E−
dv1
v1
dv2 v2
q2 , r6
(1)
where dvl , dv2 , Vl , and V2 designate the volume elements and total volumes of the two particles, respectively; r is the separation between dvl and dv2; and is the London van der Waals constant. The force between a sphere and an infinite mass bounded by a flat surface can be expressed by differentiating Eq. (1) with respect to d; i.e., F=
A ∂Ey (x) A ∂E =− = − Fy (x) , D ∂x D ∂d
(2)
where A is the Hamaker constant, which is material-dependent; D is the particle diameter; x is the ratio of the shortest distance d to the diameter of the particle; and y is the ratio of the diameters of two particles [28]. As the force between the planar surface and a spherical particle can be calculated from Eq. (2), it is possible to calculate the force between a nanostructured surface and a spherical particle by considering the patterned surface as a set of small planar elements using the Derjaguin approximation [29]; i.e., 2a1 a2 Vsh (H) = a1 + a2
∞ Vpl (h) dh,
(3)
H
where Vsh is the interaction energy between two spheres; a1 and a2 are radii of the two spheres; H is the separation between the spheres; and Vpl is the interaction energy per unit area between two parallel plates [29]. Two assumptions are required to use the Hamaker equation and Derjaguin approximation. First, regardless of the horizontal position of the particle, the adhesion force is constant (i.e., we consider the average adhesion force). Second, the height of the structure is significantly smaller than the diameter of the particle [28]. If the particle diameter and the separation between the surface and particle are known, the adhesion force may be calculated as a function of the height and fill factor only. The separation between the surface and the particle was 0.3 nm; i.e., close to physical contact [30]. Fig. 1a shows the definition of the fill factor for nanostructures. The normalized adhesion force was calculated as a function of the height and fill factor of a sinusoidal nanostructure, as shown in Fig. 1b. The adhesion force decreased rapidly as the height increased to several nanometers and then decreased more gradually. The adhesion force increased almost linearly as a function of the fill factor. It was impossible to assess the effect of pitch using Eqs. (1)–(3). For this reason, FEA simulations were carried out to determine the distribution of the adhesion force on the structures, and hence the influence of the pitch [31]. The nanostructures were described using a set of finite elements with an equivalent volume of nanostructures, and the adhesion force was calculated by summing the adhesion forces between a spherical particle and these finite elements using the Hamaker equation. The adhesion force was determined under the assumption that the vertical position of the particle varied as a function of the in-plane position. Fig. 1b shows the simulated normalized adhesion force as a function of the pitch and shape of the nanostructures. The normalized adhesion force was less than 1 when the pitch of the nanostructures was 400 nm. 2.2. Fabrication of designed nanostructures Two types of nanostructure with submicron pitch were fabricated based on the proposed theoretical framework; i.e., rectangular and sinusoidal nanostructures [32,33]. Silicon masters of the rectangular and sinusoidal nanostructures were fabricated using laser interference lithography, followed by deep reactive ion
T. Kim et al. / Applied Surface Science 389 (2016) 889–893
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Fig. 1. (a) Definition of fill factor for nanostructures (b) Normalized adhesion force of a sinusoidal nanostructure as a function of the height and fill factor, represented in log scale (c) The normalized adhesion force of rectangular line nanostructure and sinusoidal nanostructure as a function of the pitch of the nanostructure calculated using finite element analysis (FEA), represented in log scale. The fill factor was 0.5 and the height of the nanostructures was 100 nm.
etching and reactive ion etching processes, respectively. Following deposition of the seed layer on the silicon master, a 100-m-thick flexible metal stamp was replicated from the silicon master. It was then wrapped mechanically around a roll base to fabricate a metallic roll stamp [34,35]. A self-assembled monolayer was formed on the metallic roll stamp for ease of release. Then, polymeric nanostructures were replicated using the metallic roll master via a roll-to-roll imprinting process.
3. Results and discussion 3.1. AFM FD measurements Fig. 3a shows the adhesion force measured via the force–distance (FD) curve using AFM with a cantilever with tips consisting of an atypical particle (Fig. 2a). The adhesion force was found to depend on the horizontal location of the particle,
Fig. 2. (a) Schematic diagram of the force-distance (FD) curve measurement process using an atypical cantilever tip for the characterization of the adhesion force. A confocal microscopic image of the AFM cantilever with an attached atypical glass particle. Atomic force microscopy (AFM) FD curve measurement results of (b) bare surface, (c) rectangular line nanostructures, and (d) sinusoidal nanostructures.
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Fig. 3. Comparisons of measured adhesion forces for different nanostructured surfaces and surface wettability. The rectangular nanostructures and sinusoidal nanostructures showed similar contact angles. This result is consistent with Autumn et al. [13].
with local maxima in the central region between structures and at the center of the structure. As expected, we found that the adhesion force (i.e., the pull-off values shown in Fig. 2b–d) for the sinusoidal nanostructures was much smaller than on the bare surface. These results showed that the reduction in adhesion force was of the same order of magnitude as the theoretical predictions. The surface wettability was also measured and compared with the measured adhesion force (Fig. 3). Although the rectangular nanostructures and sinusoidal nanostructures had similar contact angles; the adhesion force of the sinusoidal nanostructures was about 3.1 times smaller than that of the rectangular nanostructures. This result indicates that hydrophobicity does not guarantee low adhesion.
Fig. 4. Number of particles attached to samples following centrifugal detachment tests at 1000 rpm, and the measured contact angle. The mean diameter of the atypical particle was 38.5 m.
ion etching, electrochemical deposition, and roll-to-roll imprinting processes. The adhesion force was characterized by measuring the FD curve using AFM with a modified cantilever tip, consisting of an atypical particle, considering practical usage. The adhesion force of the engineered nanostructures was significantly smaller than that of the bare surface, as measured using both tips. Particles with a mean diameter of 38.5 m were found to detach easily from the nanostructured surface with a sinusoidal surface profile and a lattice constant of 300 nm in centrifugal detachment tests, which is consistent with the reduction in the adhesion force measured using the AFM FD curve. In addition, we confirmed that a hydrophobic surface does not guarantee low adhesion [13]. Application of this method to various practical examples, including semiconductors, displays, optics, micro/nanoelectromechanical systems, and biomedical devices, is the subject of ongoing work.
3.2. Centrifugal detachment tests We investigated the particle adhesion on the fabricated nanostructures using centrifugal detachment tests, where the number of particles remaining on the surface was counted using an optical microscope. Compared with the bare surface, the number of particles remaining on the surface was reduced by 88% with the rectangular nanostructures and by 98% with sinusoidal nanostructures, as shown in Fig. 4. This is consistent with the trends in the measured adhesion force shown in Fig. 3: i.e., as the adhesion force decreased, particles were more readily detached from the surface. 4. Conclusions We have described a design method for nanostructures using the Derjaguin approximation in conjunction with FEA simulations for controlling surface adhesion using nano-engineered surfaces. To confirm the simulation, a nano-engineered surface for the anti-adhesion of particles was demonstrated as an example of controlling surface adhesion. To reduce adhesion force, the pitch of the structure should be much smaller than the particle size. The adhesion force was found to decrease markedly with the height of the structure up to a few nanometers, and decreased more gradually thereafter. The adhesion force increased almost linearly as a function of the fill factor. Several engineered nanostructures were fabricated using laser interference lithography, deep reactive
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government(MSIP) (No.2015R1A5A1037668). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.08. 015. References [1] Ralf Blossey, Self-cleaning surfaces-virtual realities, Nat. Mater. 2 (2003) 301–306. [2] R. Kohli, K.L. Mittal, Developments in Surface Contamination and Cleaning: Fundamentals and Applied Aspects, Elsevier, Amsterdam, 2008. [3] R. Young, Low-scatter mirror degradation by particle contamination, Opt. Eng. 15 (1976), 156516–156516. [4] K.P. Schmid, R. Silye, G. Palmisano, S. Doppler, M. Winkler, Efficacy of microscopic surface patterning for reducing hospital environmental contamination, ThGOT Zeulenroda Annual Conference (2011). [5] M.L. Carman, T.G. Estes, A.W. Feinberg, J.F. Schumacher, W. Wilkerson, L.H. Wilson, M.E. Callow, J.A. Callow, A.B. Brennan, Engineered antifouling microtopographies—correlating wettability with cell attachment, Biofouling 22 (2006) 11–21.
T. Kim et al. / Applied Surface Science 389 (2016) 889–893 [6] B. Bhushan, Tribology issues and opportunities in MEMS, in: Proceedings of the Nsf/Afosr/Asme Workshop on Tribology Issues and Opportunities in Mems, Colombus, Ohio, USA, November, Springer: 1997, 1998, pp. 9–11. [7] J. Visser, Particle adhesion and removal: a review, Part. Sci. Technol. 13 (1995) 169–196. [8] B. Bhushan, Y.C. Jung, Micro-and nanoscale characterization of hydrophobic and hydrophilic leaf surfaces, Nanotechnology 17 (2006) 2758. [9] B. Bhushan, Y.C. Jung, Wetting, adhesion and friction of superhydrophobic and hydrophilic leaves and fabricated micro/nanopatterned surfaces, J. Phys. Condens. Matter 20 (2008) 225010. [10] B. Bhushan, Y.C. Jung, K. Koch, Micro- nano-and hierarchical structures for superhydrophobicity, self-cleaning and low adhesion, Philos. Trans. A Math. Phys. Eng. Sci. 367 (2009) 1631–1672. [11] B. Bhushan, Y.C. Jung, K. Koch, Self-cleaning efficiency of artificial superhydrophobic surfaces, Langmuir 25 (2009) 3240–3248. [12] B. Bhushan, Y.C. Jung, Natural and biomimetic artificial surfaces for superhydrophobicity self-cleaning, low adhesion, and drag reduction, Prog. Mater. Sci. 56 (2011) 1–108. [13] K. Autumn, M. Sitti, Y.A. Liang, A.M. Peattie, W.R. Hansen, S. Sponberg, T.W. Kenny, R. Fearing, J.N. Israelachvili, R.J. Full, Evidence for van der Waals adhesion in gecko setae, Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 12252–12256. [14] J. Czarnecki, T. Dabro´s, Attenuation of the van der Waals attraction energy in the particlesemi-infinite medium system due to the roughness of the particle surface, J. Colloid Interface Sci. 78 (1980) 25–30. [15] H. Rumpf, F. Bull, Particle Technology, vol. 116, Chapman and Hall, London, 1990. [16] Y.I. Rabinovich, J.J. Adler, A. Ata, R.K. Singh, B.M. Moudgil, Adhesion between nanoscale rough surfaces: I. Role of asperity geometry, J. Colloid Interface Sci. 232 (2000) 10–16. [17] Y.I. Rabinovich, J.J. Adler, A. Ata, R.K. Singh, B.M. Moudgil, Adhesion between nanoscale rough surfaces: II. Measurement and comparison with theory, J. Colloid Interface Sci. 232 (2000) 17–24. [18] D. Mijatovic, J.C.T. Eijkel, A. van den Berg, Technologies for nanofluidic systems: top-down vs. bottom-up—a review, Lab Chip 5 (2005) 492–500. [19] A.J. Crosby, M. Hageman, A. Duncan, Controlling polymer adhesion with pancakes, Langmuir 21 (2005) 11738–11743. [20] R. Spolenak, S. Gorb, E. Arzt, Adhesion design maps for bio-inspired attachment systems, Acta Biomater. 1 (2005) 5–13.
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[21] E.P. Chan, C. Greiner, E. Arzt, A.J. Crosby, Designing model systems for enhanced adhesion, MRS Bull. 32 (2007) 496–503. [22] W.C. Hinds, Aerosol Technology: Properties, Behavior and Measurement of Airborne Particles, 2nd ed., John Wiley & Sons, Inc., Hoboken, 2012. [23] H.-J. Butt, B. Cappella, M. Kappl, Force measurements with the atomic force microscope: technique, interpretation and applications, Surf. Sci. Rep. 59 (2005) 1–152. [24] D. Schaefer, J. Gomez, Atomic force microscope techniques for adhesion measurements, J. Adhes. 74 (2000) 341–359. [25] R.F. Tabor, D.Y. Chan, F. Grieser, R.R. Dagastine, Structural forces in soft matter systems, J. Phys. Chem. Lett. 2 (2011) 434–437. [26] P. Sinha, I. Szilagyi, F.J. Montes Ruiz-Cabello, P. Maroni, M. Borkovec, Attractive forces between charged colloidal particles induced by multivalent ions revealed by confronting aggregation and direct force measurements, J. Phys. Chem. Lett. 4 (2013) 648–652. [27] H. Mizes, M. Ott, E. Eklund, D. Hays, Small particle adhesion: measurement and control, Colloids Surf. A: Physicochem. Eng. Asp. 165 (2000) 11–23. [28] B.V. Derjaguin, Untersuchungen über die Reibung und Adhäsion, IV, Colloid Polym. Sci. 69 (1934) 155–164. [29] H. Hamaker, The London—van der Waals attraction between spherical particles, Physica 4 (1937) 1058–1072. [30] H. Krupp, Particle adhesion, theory and experiment, Adv. Colloid Interface Sci. 1 (1967) 111–239. [31] G. Dhatt, E. Lefranc¸ois, G. Touzot, Finite Element Method, John Wiley & Sons, Inc., Hoboken, 2012. [32] S. Kang, Micro/nano Replication: Processes and Applications, John Wiley & Sons, Inc., Hoboken, 2012. [33] S. Je, J. Shim, J. Kim, M. Kim, J. Lee, H. Nho, J. Han, S.-m. Kim, S. Kang, Development of direct deep reactive ion etching process using laser interference lithographed etch barrier without intermediate layer, Jpn. J. Appl. Phys. 52 (2013), 10MC04-1-10MC04-5. [34] J. Han, S. Choi, J. Lim, B.S. Lee, S. Kang, Fabrication of transparent conductive tracks and patterns on flexible substrate using a continuous UV roll imprint lithography, J. Phys. D: Appl. Phys. 42 (2009) 115503. [35] Joongeok Kim, Jungjin Han, Taekyung Kim, Shinill Kang, Fabrication of a metallic roll stamp with low internal stress and high hardness for large area display applications by a pulse reverse current electroforming process, J. Micromech. Microeng. 24 (2014) 125004.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Design methodology for nano-engineered surfaces to control adhesion: Application to the anti-adhesion of particles Taekyung Kim a,b,1 , Cheongwan Min a,1 , Myungki Jung a,b , Jinhyung Lee a,b , Changsu Park a,b , Shinill Kang a,b,∗ a b
National Center for Optically-Assisted Ultra-High Precision Mechanical Systems, Yonsei University, Seoul 03722, Republic of Korea School of Mechanical Engineering, Yonsei University, Seoul 03722, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 June 2016 Received in revised form 19 July 2016 Accepted 4 August 2016 Available online 6 August 2016 Keywords: Control of adhesion force Derjaguin approximation Nano-engineered surfaces Particle adhesion
a b s t r a c t With increasing demand for means of controlling surface adhesion in various applications, including the semiconductor industry, optics, micro/nanoelectromechanical systems, and the medical industry, nano-engineered surfaces have attracted much attention. This study suggests a design methodology for nanostructures using the Derjaguin approximation in conjunction with finite element analysis for the control of adhesion forces. The suggested design methodology was applied for designing a nano-engineered surface with low-adhesion properties. To verify this, rectangular and sinusoidal nanostructures were fabricated and analyzed using force-distance curve measurements using atomic force microscopy and centrifugal detachment testing. For force-distance curve measurements, modified cantilevers with tips formed with atypical particles were used. Subsequently, centrifugal detachment tests were also conducted. The surface wettability of rectangular and sinusoidal nanostructures was measured and compared with the measured adhesion force and the number of particles remaining after centrifugal detachment tests. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With increasing demand for the means of controlling surface adhesion in various fields, including semiconductors, displays, optical, micro/nanoelectromechanical systems, and the medical industry, technologies for controlling adhesion forces have become increasingly important. For example, microparticles cannot be detached readily from a surface because the adhesion force is typically much greater than the detachment forces of inertia and drag. Surface contamination due to the presence of such fine particles has caused enormous yield losses and degradation of reliability [1–7]. To address these issues, various investigations have examined the adhesion properties of biomimetic surfaces with superhydrophobic properties. Bhushan et al. proposed self-cleaning surfaces with a low adhesion force, resembling the structure of lotus leaves [8–12]. Their research confirmed that biomimetic surfaces with superhydrophobicity can be used to modify surface adhesion properties. However, Autumn et al. demonstrated experimental evidence that van der Waals forces are a dominant factor in
∗ Corresponding author at: Yonsei University, Mechanical Engineering, 50, Yonseiro, Seodaemun-gu, Seoul, Republic of Korea. E-mail address: [email protected] (S. Kang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.apsusc.2016.08.015 0169-4332/© 2016 Elsevier B.V. All rights reserved.
determining adhesion forces between micro- and nanostructures and solid surfaces [13]. They provided experimental evidence for the dry adhesion of gecko setae by van der Waals forces, showing that the toes of hydrophobic Tokay geckos adhered equally well to hydrophobic and hydrophilic surfaces. Their experimental results indicated that the remarkable adhesive properties of gecko setae were merely a result of the size and shape of the tips, and were not strongly affected by surface chemistry. Several studies regarding the van der Waals attraction energy model in particles and semi-infinite media with surface roughness have been reported. Czarnecki and Dabros presented adhesion force models considering various particle sizes, thicknesses of the rough layer, and separation values. Throughout their mathematical calculation, roughness of particle surfaces decreased the van der Waals attraction energy markedly between a globular particle and semi-infinite medium system [14]. Rumpf also proposed a model that described surface asperities as small hemispheres [15]. Furthermore, to calculate the adhesion force more accurately, Rabinovich proposed a model containing large and small regions of surface roughness irregularities [16,17]. Their results show a clear correlation between the surface characteristics (i.e., the size of asperities or surface roughness) and the adhesion force. However, a heterogeneous surface with random roughness does not guarantee uniform adhesion forces over the entire surface [18].
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T. Kim et al. / Applied Surface Science 389 (2016) 889–893
Crosby et al. showed experimentally how microscale cylindrical post arrays can regulate the van der Waals energy required for interfacial separation and the maximum force for separating an interface. Polymeric cylindrical post arrays with various heights (20–250 m) and spacing (50–500 m) were prepared using conventional fabrication technologies including photolithography, and nanoimprint lithography and the patterned materials were characterized immediately using a contact adhesion test [19]. While most research regarding the adhesion of fine structures has been based on bio-inspired or empirical methods, an “adhesion design map” was introduced for a thorough understanding of adhesion by Spolenak et al. They established convenient guidelines (adhesion design maps) using mathematical equations describing limiting criteria for the fracture of fine structures and ideal contact strength [20]. Although they theorized the balance of these design parameters in adhesion maps for patterned surfaces, no complete experimental verification has yet been established [21]. Here, we suggest a design methodology of nanostructures using the Derjaguin approximation in conjunction with finite element analysis (FEA) for the control of adhesion forces. The suggested simulation methodology shows the extent to which the fill factor, pitch, and height of nanostructures affect the adhesion force, and enables quantitative calculations of the adhesion force. In this study, this was applied to designing a nano-engineered surface with low-adhesion properties. To confirm the proposed simulations, experimental results, including measurements of adhesion forces and centrifugal detachment tests with pulverized micro glass particles, were demonstrated. We fabricated two master nanostructures which were rectangular line and sinusoidal nanostructures, based on the theoretical framework described here via laser interference lithography, followed by deep reactive ion etching and reactive ion etching processes, respectively. After replicating the metallic roll stamp from the master structures by an electrochemical deposition process, polymeric nanostructures were fabricated via a roll-toroll imprinting process. The adhesion force was characterized using atomic force microscopy (AFM) based on the force–distance (FD) curve. The AFM system used a laser diode, position-sensitive detector, and cantilever with an attached 30-m-diameter spherical particle to measure the interaction between the particle and patterned surface [22–25]. Moreover, a cantilever with an attached atypical particle was used to assess the adhesion force of spontaneous particles by considering practical usage. To confirm that particles detached readily from the designed fine structures, centrifugal detachment tests were carried out, and the residual particles were counted [26]. By comparing the results of centrifugal detachment tests with the surface wettability of two nanostructures, we also found that the hydrophobicity and anti-adhesion force have little interdependence [13]. 2. Experimental methods 2.1. Adhesion force model for nanostructures The forces between a surface and a particle include van der Waals forces, electrostatic forces, and surface tension; however, here we focus on the van der Waals forces because they are the most significant in nanostructures [27]. We used the Hamaker equation and Derjaguin approximation to calculate the adhesion force between a spherical particle and planar nanostructured surface [28,29]. The interaction energy between two particles containing q atoms per cm3 is given by
E−
dv1
v1
dv2 v2
q2 , r6
(1)
where dvl , dv2 , Vl , and V2 designate the volume elements and total volumes of the two particles, respectively; r is the separation between dvl and dv2; and is the London van der Waals constant. The force between a sphere and an infinite mass bounded by a flat surface can be expressed by differentiating Eq. (1) with respect to d; i.e., F=
A ∂Ey (x) A ∂E =− = − Fy (x) , D ∂x D ∂d
(2)
where A is the Hamaker constant, which is material-dependent; D is the particle diameter; x is the ratio of the shortest distance d to the diameter of the particle; and y is the ratio of the diameters of two particles [28]. As the force between the planar surface and a spherical particle can be calculated from Eq. (2), it is possible to calculate the force between a nanostructured surface and a spherical particle by considering the patterned surface as a set of small planar elements using the Derjaguin approximation [29]; i.e., 2a1 a2 Vsh (H) = a1 + a2
∞ Vpl (h) dh,
(3)
H
where Vsh is the interaction energy between two spheres; a1 and a2 are radii of the two spheres; H is the separation between the spheres; and Vpl is the interaction energy per unit area between two parallel plates [29]. Two assumptions are required to use the Hamaker equation and Derjaguin approximation. First, regardless of the horizontal position of the particle, the adhesion force is constant (i.e., we consider the average adhesion force). Second, the height of the structure is significantly smaller than the diameter of the particle [28]. If the particle diameter and the separation between the surface and particle are known, the adhesion force may be calculated as a function of the height and fill factor only. The separation between the surface and the particle was 0.3 nm; i.e., close to physical contact [30]. Fig. 1a shows the definition of the fill factor for nanostructures. The normalized adhesion force was calculated as a function of the height and fill factor of a sinusoidal nanostructure, as shown in Fig. 1b. The adhesion force decreased rapidly as the height increased to several nanometers and then decreased more gradually. The adhesion force increased almost linearly as a function of the fill factor. It was impossible to assess the effect of pitch using Eqs. (1)–(3). For this reason, FEA simulations were carried out to determine the distribution of the adhesion force on the structures, and hence the influence of the pitch [31]. The nanostructures were described using a set of finite elements with an equivalent volume of nanostructures, and the adhesion force was calculated by summing the adhesion forces between a spherical particle and these finite elements using the Hamaker equation. The adhesion force was determined under the assumption that the vertical position of the particle varied as a function of the in-plane position. Fig. 1b shows the simulated normalized adhesion force as a function of the pitch and shape of the nanostructures. The normalized adhesion force was less than 1 when the pitch of the nanostructures was 400 nm. 2.2. Fabrication of designed nanostructures Two types of nanostructure with submicron pitch were fabricated based on the proposed theoretical framework; i.e., rectangular and sinusoidal nanostructures [32,33]. Silicon masters of the rectangular and sinusoidal nanostructures were fabricated using laser interference lithography, followed by deep reactive ion
T. Kim et al. / Applied Surface Science 389 (2016) 889–893
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Fig. 1. (a) Definition of fill factor for nanostructures (b) Normalized adhesion force of a sinusoidal nanostructure as a function of the height and fill factor, represented in log scale (c) The normalized adhesion force of rectangular line nanostructure and sinusoidal nanostructure as a function of the pitch of the nanostructure calculated using finite element analysis (FEA), represented in log scale. The fill factor was 0.5 and the height of the nanostructures was 100 nm.
etching and reactive ion etching processes, respectively. Following deposition of the seed layer on the silicon master, a 100-m-thick flexible metal stamp was replicated from the silicon master. It was then wrapped mechanically around a roll base to fabricate a metallic roll stamp [34,35]. A self-assembled monolayer was formed on the metallic roll stamp for ease of release. Then, polymeric nanostructures were replicated using the metallic roll master via a roll-to-roll imprinting process.
3. Results and discussion 3.1. AFM FD measurements Fig. 3a shows the adhesion force measured via the force–distance (FD) curve using AFM with a cantilever with tips consisting of an atypical particle (Fig. 2a). The adhesion force was found to depend on the horizontal location of the particle,
Fig. 2. (a) Schematic diagram of the force-distance (FD) curve measurement process using an atypical cantilever tip for the characterization of the adhesion force. A confocal microscopic image of the AFM cantilever with an attached atypical glass particle. Atomic force microscopy (AFM) FD curve measurement results of (b) bare surface, (c) rectangular line nanostructures, and (d) sinusoidal nanostructures.
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Fig. 3. Comparisons of measured adhesion forces for different nanostructured surfaces and surface wettability. The rectangular nanostructures and sinusoidal nanostructures showed similar contact angles. This result is consistent with Autumn et al. [13].
with local maxima in the central region between structures and at the center of the structure. As expected, we found that the adhesion force (i.e., the pull-off values shown in Fig. 2b–d) for the sinusoidal nanostructures was much smaller than on the bare surface. These results showed that the reduction in adhesion force was of the same order of magnitude as the theoretical predictions. The surface wettability was also measured and compared with the measured adhesion force (Fig. 3). Although the rectangular nanostructures and sinusoidal nanostructures had similar contact angles; the adhesion force of the sinusoidal nanostructures was about 3.1 times smaller than that of the rectangular nanostructures. This result indicates that hydrophobicity does not guarantee low adhesion.
Fig. 4. Number of particles attached to samples following centrifugal detachment tests at 1000 rpm, and the measured contact angle. The mean diameter of the atypical particle was 38.5 m.
ion etching, electrochemical deposition, and roll-to-roll imprinting processes. The adhesion force was characterized by measuring the FD curve using AFM with a modified cantilever tip, consisting of an atypical particle, considering practical usage. The adhesion force of the engineered nanostructures was significantly smaller than that of the bare surface, as measured using both tips. Particles with a mean diameter of 38.5 m were found to detach easily from the nanostructured surface with a sinusoidal surface profile and a lattice constant of 300 nm in centrifugal detachment tests, which is consistent with the reduction in the adhesion force measured using the AFM FD curve. In addition, we confirmed that a hydrophobic surface does not guarantee low adhesion [13]. Application of this method to various practical examples, including semiconductors, displays, optics, micro/nanoelectromechanical systems, and biomedical devices, is the subject of ongoing work.
3.2. Centrifugal detachment tests We investigated the particle adhesion on the fabricated nanostructures using centrifugal detachment tests, where the number of particles remaining on the surface was counted using an optical microscope. Compared with the bare surface, the number of particles remaining on the surface was reduced by 88% with the rectangular nanostructures and by 98% with sinusoidal nanostructures, as shown in Fig. 4. This is consistent with the trends in the measured adhesion force shown in Fig. 3: i.e., as the adhesion force decreased, particles were more readily detached from the surface. 4. Conclusions We have described a design method for nanostructures using the Derjaguin approximation in conjunction with FEA simulations for controlling surface adhesion using nano-engineered surfaces. To confirm the simulation, a nano-engineered surface for the anti-adhesion of particles was demonstrated as an example of controlling surface adhesion. To reduce adhesion force, the pitch of the structure should be much smaller than the particle size. The adhesion force was found to decrease markedly with the height of the structure up to a few nanometers, and decreased more gradually thereafter. The adhesion force increased almost linearly as a function of the fill factor. Several engineered nanostructures were fabricated using laser interference lithography, deep reactive
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