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liquid interfacial friction and slip behaviors on roughness surface under applied voltage
Tribology International 144 (2020) 106128
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Solid/liquid interfacial friction and slip behaviors on roughness surface under applied voltage Yongning Wang, Yafeng Zhang *, Cheng Tang, Jiaxin Yu **, Hongtu He, Huiming Qi Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, Southwest University of Science and Technology, Mianyang, 621010, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Direct voltage Friction behavior Droplet Solid/liquid interface
The solid/liquid interfacial friction and slip behaviors on roughness surface under applied voltage were studied. Results showed that the slip behaviors of droplets on roughness surface in the unstable region cannot fit by electrowetting equation. The slip distance and directions mainly depended on difference between right and left friction force at the triple contact line, which were caused by the asymmetric roughness of the solid surface. Moreover, microstructures with various heights were used to control the solid/liquid slip behaviors. Results indicated that when the ridges of scratch far higher than surface roughness of the hydrophobic surface, the slip distance increased and slip direction can be controlled. The findings have potential applications in oriented driving of droplets by electric field.
1. Introduction
one of the most potential technologies to actuate microdroplets. A typical EWOD system composed of a substrate, droplet and a power supply, as shown in Fig. 1a. When voltage was applied between the droplet and substrate, the shape of the droplet changed with the increase of the applied voltage. Therefore, droplets can be actuated by array electrodes by EWOD system [20]. In order to actuate droplet accurately, solid/liquid interfacial behaviors under applied voltage have attracted considerable attention [21–25]. Previous studies were mainly concen trated on the deformation process of droplet [26], low voltage actuation [19]. and sliding friction behaviors [27]. These studies indicated that the solid/liquid interfacial behaviors were associated with surface roughness, which cause additional friction force at the triple contact line [27]. For a real surface, surface defects and surface roughness are un avoidable. Consequently, it is necessary to study the interfacial friction of droplet on a roughness surface to actuate droplets efficiently. In this paper, the variations of contact angle and triple contact line before and after slipping in the unstable region were characterized by contact angle meter. Moreover, the slip distance and slip direction of droplet under direct voltage were studied. Next, a nano-scratch was utilized to make microstructures to control the slip behavior of droplet. In addition, the interfacial friction of droplet on roughness surface was discussed using balance of forces at the triple contact line. The findings in this paper may be helpful to extend the wetting theory of droplet under direct voltage and has potential applications in oriented driving of
Oriented driving microdroplet on a solid surface has attracted much attention in recent years because of its wide application in lab-on-a-chip [1,2], inkjet printing [3,4], and DNA microarrays [5,6]. Traditionally, oriented driving of microfluidics was done by combination of micro pumps, microvalves and microchannels [2]. By intermittently switching on and off the microvalves and micropumps, individual droplets can be obtained and actuated according to requirements [7–9]. However, because the actuation systems were composed of complex mechanical devices, the reliabilities were restricted by the development of minia turization and integration. Consequently, all kinds of new approaches were proposed to manipulate microdroplet, such as magnetic actuation technology [10,11], optofluidic technology [12,13] and electrowetting on dielectric (EWOD) technology [14,15]. For magnetic-induced and light-induced actuation technologies, magnetic-sensing and photosen sitive particles should be added to the droplets to control the movement [10,12]. These particles may contaminate droplets and were difficult to remove. Conversely, by changing the effective solid/liquid interface tension through electric field, EWOD technology can be used to generate and actuate droplets accurately [16,17]. Due to without mechanical devices, EWOD technology has several advantages, such as non-polluting, real-time actuation, fast response, long-term stability and reliability [18,19]. Consequently, EWOD technologies are considered as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (J. Yu). https://doi.org/10.1016/j.triboint.2019.106128 Received 4 October 2019; Received in revised form 2 December 2019; Accepted 16 December 2019 Available online 17 December 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
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Tribology International 144 (2020) 106128
of the hydrophobic layer were acquired by white light scanning profil ometry (MFT-3000, Rtec, USA). The Z axis resolution of the white light scanning profilometry is 0.1 nm, and X & Y axis resolution is 1 μm. To study the effect of roughness on solid/liquid friction, moreover, a nanoscratch tester (G200, Keysight, USA) was used to make microstructures on the hydrophobic surface. During processing, a conical diamond tip with a radius of 1.8 μm was used and scratches were performed under 5 μN–30 μN respectively. Moreover, the scratch length for each scratch was 5 mm at a velocity of 5 μm/s. The topographies of the scratch profiles were acquired by white light scanning profilometry. All exper iments were performed under atmospheric conditions at room temperature. 3. Results 3.1. Slip behaviors before saturation The variation of contact angle and the length of triple contact line with applied voltage were shown in Fig. 2. Obviously, the contact angle and triple contact line curves can be divided into four regions, namely, Istatic region, IIsliding region, III unstable region and Ⅳ saturated re gion. In the static region, the contact angle decreased slightly while triple contact line remained constant. In the sliding region, the contact angle decreased and contact line increased with increasing of applied voltage. In the saturated region, the contact angle and triple contact line almost kept constant with increasing of voltage. It was noted that an unstable region exist before saturation. In this region, the variation of contact angle and triple contact line cannot fit by electrowetting equa tion. As shown in Fig. 2, sharply stick-slip behaviors of droplet can be observed before saturation. As shown in the inset of Fig. 2, the contact angle suddenly increased and triple contact line decreased respectively when the droplet slid from point A to point B. The difference of contact angle (DCA) and triple contact line (DCL) between point A and point B were summarized in Table 1. It was found that the DCA and DCL were independent on the concentrations of NaCl.
Fig. 1. Schematic of the electrowetting system and forces at the triple contact line. (a) schematic diagram of the experimental method. (b) schematic of the balance of forces at triple contact line on an inhomogeneous surface under applied voltage.
droplets by electric field. 2. Materials and methods 2.1. Sample preparation Conductive silicon wafers coated with a 300 nm silicon oxide layer were used as the ground electrode (SSPP, Siltronic, Germany). In order to minimize contact angle hysteresis, the silicon oxide surface was hy drophobic treated with a Teflon emulsion (Teflon AF1600, Dupont, USA). The Teflon emulsion was spinning on the silicon oxide surface using a spin processor (KW4A, Beijing Saidekai Co., China) at 500 rpm for 20 s and 3000 rpm for 30 s respectively. After heating at 200 � C for 2 h, the hydrophobic layer with ~2 μm thick was obtained. The water contact angles of the specimens were approximately 130� .
3.2. Slip distance and directions For a homogenous surface, the variation of contact angle and triple contact line were axisymmetric. Theoretically, droplets changed sym metrically with applied voltage along a symmetrical axis (green dotted line), as shown in Fig. 3. As mentioned above, droplet slipped from one equilibrium state to other equilibrium state before saturation for a rough surface. So slip toward left and right of the droplet can be observed. Moreover, the slip behaviors of droplets were overlooked by a camera, as shown in Fig. 4. The photo of the droplet before and after slip is shown in Fig. 4b. The schematic diagram of the droplet before and after sliding is shown in Fig. 4c. 50 repeated experiments were done on a batch of specimens. Results showed that the slip directions of droplets under applied voltage were uncertainly. As shown in Fig. 4d, slip toward east (E), south (S), west (W), north (N) and other directions can be observed. The number of times in different directions was summarized in Table 2. Where E, S, W, and N represent the direction of east, south, west and north respectively. And the NW, NE, SW, SE represents the direction northwest, northeast, southwest and southeast respectively. According to the data in Table 2, it can be deduced that the slip directions were uncertain. The distance between the axes before and after slipping was termed as slip distance (SD). Results showed that the average SD of droplet was approximately 0.4 mm for the surface with a roughness of Ra 53.9 � 16.9 nm, as shown in Table 1. Moreover, there were no sig nificant differences of the SD among three different solutions. The dynamic responses of droplet with increase of applied voltage are shown in Fig. 3. The average contact angle θ, left contact angle θL and right contact angle θR for a droplet were showed under the photo graph. It was found that the difference between θL and θR (DLR) was less than 1� for an homogenous surface under applied voltage. However, the
2.2. Electrowetting parameters measurement The schematic diagram of electrowetting is shown in Fig. 1. The silicon wafer of the specimen was connected to the negative electrode of a power supply (PSW250–4.5, GuWei, China). Next, a 10 μL droplet was placed on the surface of specimen and a copper wire with diamond of 100 μm was inserted inside the droplet. And then the copper wire was connected to the positive electrode of the power supply. An applied voltage ranged from 0 to 200 V with incremental velocity of 1 V/s was provided. The voltage resolution of the power supply is 0.1% � 10 mV. The variations of contact angles of droplet with applied voltage were obtained using a contact angle meter (DSA30E, KRUSS, Germany). For the contact angle meter, the measurement resolution is 0.01� and 0.01 mm. Consequently, the lengths of triple contact line under different applied voltages were recorded by DSA contact angle analysis software (DSA30E, KRUSS, Germany). In order to study the effect of ion con centration on the slip behaviors of droplet under applied voltage, so dium chloride solution (NaCl) with different concentrations (0 mol/l, 1 mol/l and 2 mol/l) were used. 2.3. Morphology of the hydrophobic layer Considering the solid/liquid friction behaviors of droplet were closely associated with the morphology of specimens, the topographies 2
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Fig. 2. Variation of contact angles and the length of contact line with applied voltage for different NaCl solutions, (a) the contact angle of droplet as functions of applied voltage. (b) the length of contact lines a function of applied voltage. An unstable region (III) exists before saturation. In this region, the contact angle and triple contact line varied sharply and randomly.
3.3. Morphology of the hydrophobic layer and microstructures
Table 1 Slip parameters of droplets under applied voltage before saturation. NaCl concentration/ mol⋅L 1
DCA/�
DCL/mm
SD/mm
DLR/�
0 1 2
6.7� �4.28 6.7� �5.19 6.7� �4.24
0.2 � 0.19 0.2 � 0.21 0.2 � 0.17
0.4 � 0.42 0.4 � 0.7 0.4 � 0.41
1.6� �1.41 2.2� �2.11 1.6� �1.55
The morphologies of the hydrophobic layer were showed in Fig. 6. Micro-pits and micro-bulges were observed on the specimen surface (Fig. 6a). The surface roughness (Ra) of the hydrophobic surface was 53.9 � 16.9 nm. Moreover, the ridges caused by scratches can be divided into two groups. One group is that the height of microstructures is lower or equal to the surface roughness and the other group is that the height of microstructures is significant higher than the surface roughness. It is found that the height of the ridges obtained at 5 μN was about 50 nm, which equal to Ra of the hydrophobic surface approximately, as shown in Figs. 6b and 7. Moreover, the height of the ridges obtained at 20 μN was about 450 nm, which was far higher than Ra of the hydrophobic surface (Figs. 6c and 7).
DLR varied with applied voltage for the rough surface. As shown in Fig. 5, the DLR increased with applied voltage and reached maximum value before saturation and then sharply decreased after slipping. Finally, it kept constant due to saturation. The average DLR for droplets ranged from 1.6� to 2.2� , as shown in Table 1. Moreover, there were no significant differences for DLR among three different NaCl solutions.
Fig. 3. Dynamic responses of droplets with in crease of applied voltage. The dotted green lines indicated the center of symmetry and the solid red lines represented the length of contact line. The slip distance was termed as the distance be tween the axes before and after slipping. For an homogenous surface, the variation of contact angle and triple contact line was axisymmetric. For a rough surface, the slip directions of droplet were random. So slip toward left and right of the droplets can be observed under applied voltage. The direction of red arrow means the increasing of applied voltage. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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with applied voltage can be divided into three typical regions, namely, static region, sliding region and saturated region [29]. However, it was noted that an unstable region exist before saturation for a roughness surface. Consequently, the balance of forces at the triple contact line should be further discussed. When a droplet is rested on a homogeneous surface, the droplet will eventually reach an equilibriums state under the action of surface ten sions and the contact angle between solid/liquid interface is termed as Young’s angle (θY) [30]. cosθY ¼
γ sv
γ sl
(1)
γ lv
where γ is the surface tension, subscripts s, v, l represent solid, vapor and liquid phase respectively. The contact angle (θ) of droplet will gradually decrease under applied voltage in a electrowetting system due to the insulating layer at solid/liquid interface can be considered as a capacitance which can eff
reduce the effective interfacial tension (γsl ) [31]. γ eff sl ¼ γ sl
Table 2 Number of slip times in different directions.
cosθ ¼
Slip direction
W
E
N
S
NW
NE
SW
SE
8
3
10
8
9
3
5
4
2d
(2)
U2
where U is the applied voltage, d is the thickness of dielectric t, ε0 and εd is the permittivity of free space and dielectric constant respectively. Moreover, for a rough surface, interfacial friction forces caused by chemical and topographic inhomogeneities of the solid surface should be considered [32,33]. Consequently, balance forces on the triple con tact line under applied voltage can be obtained (Fig. 1b) and the contact angle (θ) was determined by combined actions between surface tension and friction force (f).
Fig. 4. The slip behaviors of droplet were overlooked by a camera. Slip toward east (E), south (S), west (W), north (N) and other directions can be observed.
Number of times
ε0 ε1
γsv
γeff sl γlv
f
(3)
It should be noted that the contact angles were different along the perimeter of the droplet due to topographic inhomogeneities. As shown in Fig. 3, θL and θR for a droplet were difference. Consequently, ac cording to equation (3), the friction forces at right (fR) were different eff
from that at left (fL) (Fig. 1b). Moreover, γsl decreased with increasing applied voltage. In sliding region, the wetting area of droplet increased and more micro-pits and micro-bulges were covered by droplets. As a result, the DLR was continually amplified. Therefore, the DLR increased with applied voltage (Fig. 5). With increasing applied voltage further, eff
γ sl tended to zero eventually and the contact angle and contact line curves kept constant due to saturation [34–36]. Consequently, the DLR eff
reached maximum value before saturation (Fig. 5). Once γ sl equal to zero, the droplets reached saturated region. The contact line and contact eff
angle were not affected by applied voltage. Due to absence of γsl , the forces at triple contact line were out of balance. If the friction force on the left (fL) was greater than that on the right, the droplet slipped toward right to find an equilibrious state. Consequently, the DCA and DCL changed before and after slipping (Table 1). At this equilibrious state, the triple line reached equilibrium renewedly under the action of ten sion force and friction force. Conversely, droplet slip toward left if the friction force on the left (fR) was greater than that on the right. Conse quently, the slip directions of droplet were uncertain and mainly depended on the difference between fR and fL. Moreover, results indicated that no significant differences in DCA and DCL among different solutions. Sedev et al. [37] pointed that the ionic concentration had no effect on the electrowetting behaviors because the interfacial tension and viscosity cannot be significant changed by ionic concentration. Varagnolo [38] et al. and Zhang et al. [39] suggested that solid/liquid interfacial behaviors were mainly depended on chemical and topographic heterogeneity. In this paper, the
Fig. 5. Variation of the difference between θL and θR (DLR) with applied voltage. The DLR increased with applied voltage and reached maximum value before saturation and then sharply decreased after slipping. Finally, it kept constant due to saturation.
4. Discussion The variations of contact line under applied voltage were a stick-slip process rather than continuously due to the existence of friction force [28]. Generally, for a homogenous surface, the variation of contact angle
eff
hydrophobic layers were chemical homogeneity. Furthermore, the γsl
4
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Fig. 6. White light scanning profilometry images of the Teflon surface, (a) hydrophobic surface, (b) the profile of scratch under normal load of 5 μN, (c) the profile of scratch under normal load of 20 μN.
Fig. 7. The height of ridges on Teflon surface at various normal loads.
mainly depended on the electrical properties of silicon dioxide layer rather than onic concentration of droplet. Consequently, the DCA and DCL between deionized water and NaCl solution have no significant difference as shown in Table 1. As mentions above, the slip behaviors of droplet before saturation depended on the difference between fR and fL. Moreover, the fR and fL were closely associated with the topography of the surface. Conse quently, it is inferred that the slip behaviors of droplet might be controlled by microstructure. Therefore, microstructures were made on the hydrophobic surface using nano-scratch. According to the height of microstructures, those microstructures were divided into two groups. One group is that the height of microstructures is lower or equal to the surface roughness and the other group is that the height of microstruc tures is significant higher than the surface roughness. As shown in Fig. 7, one of the ridges obtained at 5 μN were about 50 nm, which equal to Ra of the hydrophobic surface approximately and the other ridges obtained at 20 μN were about 450 nm, which were far higher than Ra of the hydrophobic surface. Results showed that slip behaviors were observed on the microstructured surface also. The slip behaviors of droplet were not changed when the height of the ridges lower or equal to Ra. How ever, the slip direction and slip distance of droplet completely changed by the ridges which higher than Ra. As shown in Fig. 8, the red line represented where the ridge of scratch was located. When the droplet was rest on the right of the microstructure, the droplets slipped right under applied voltage before saturation. Conversely, it slipped left. The SD and DRL of droplet on the Teflon surface with different height of ridges were summarized in Table 3. When the ridges of scratch were equal to Ra of the hydrophobic surface, the SD and DRL were not changed significantly. However, SD and DRL increased significantly when the ridges of scratch were far higher than Ra of the hydrophobic
Fig. 8. The slip direction of droplet on the Teflon surface with different mi crostructures. The red dotted line represent where the scratch was located. When the droplet was rest on the right of the ridge of scratch, the droplets slipped right under applied voltage before saturation. Conversely, it slipped left. The direction of red arrow means the increasing of applied voltage. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
surface. The dynamic behaviors of droplet under applied voltage on a surface with ridges higher than Ra can be explained in Fig. 9. The Micropits on the surface can prevent the deformation of contact line. As a result, the contact angle continually increased because the triple contact line cannot move under applied voltage. When the ridges of scratch were greater than Ra of the hydrophobic surface, the contact angles adjoining to the higher ridges were larger than that adjoining to the lower ridges. As mentioned above, the forces at triple contact line were out of balance eff
due to absence of γsl . When the droplet was rest on the right of the higher ridge of scratch, the droplet slipped right under applied voltage before saturation. Consequently, the SD of droplet was mainly depended on the DRL. However, the SD cannot increase indefinitely with increase Table 3 The slip parameters of droplet on the Teflon surface with different height of ridges.
5
Height of ridge/nm
SD/mm
DLR/�
50 450
0.4 � 0.42 0.7 � 0.94
1.6� �1.41 3.5� �3.48
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51605402), Scientific Research Fund of Sichuan Provincial Education Department (Grant No. 17ZA0409) and the Longshan Academic Talent Research Supporting Program of SWUST (Grant No. 18LZX555). References
Fig. 9. Schematic diagram of the slip behaviors controlled by ridge of scratch. When the droplet was rest on the right of the higher ridge of scratch, the droplets slipped right under applied voltage before saturation.
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of the height of ridges. It was noted that although the contact angles can be increased by the height of the ridges, the maximum value can’t exceed 180� . Therefore, according to eq. (3), the friction forces caused by ridges can be obtained by γ sv þ γlv � f � γsv
(4)
It was found that the friction force ranged from γsv and γsv þ γlv . However, it should be noted that the friction term between solid/liquid may cause by chemical and topographic inhomogeneities [33]. The in fluences of chemical homogeneities on the friction were not involved in Equation (4). Consequently, Formula 4 is applicable to estimate the ef fect of surface topography on solid/liquid friction. In summary, it can be found that the slip behaviors can be controlled by designing surface morphology. The findings have potential for directional driving of droplets. 5. Conclusion The solid/liquid interfacial friction and slip behaviors on roughness surface under applied voltage were studied. It was found that inhomo geneous surface will cause the difference of contact angle between left and right of the droplets. Moreover, the slip directions and distance of droplet were uncertain and mainly depended on the friction force at the triple contact line, which were caused by the asymmetric roughness of solid surface. So slip toward left and right of the droplet can be observed on inhomogeneous surface. Moreover, there were no significant differ ences of the slip distance among three different solutions. Based on the observations, two type of microstructures were made on the hydro phobic surface using nano-scratch to control the friction force at the triple contact line to change the slip behaviors of droplet. When the ridges of scratch equal to Ra of the hydrophobic surface, the slip be haviors cannot be significant changed. When the ridges of scratch far higher than Ra of the hydrophobic surface, the slip distance increased and slip direction can be controlled. Moreover, the solid/liquid inter facial friction force caused by roughness ranged from γ sv and γ sv þ γlv . The results have potential applications in oriented driving of droplets by electric field. Author contributions Yafeng Zhang and Jiaxin Yu conceived and designed this experiment; Yongning Wang, Cheng Tang conducted the experiments; Y.F. Zhang and Yongning Wang wrote the paper; and Hongtu He, Huiming Qi contributed to the analysis and discussion. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Contents lists available at ScienceDirect
Tribology International journal homepage: http://www.elsevier.com/locate/triboint
Solid/liquid interfacial friction and slip behaviors on roughness surface under applied voltage Yongning Wang, Yafeng Zhang *, Cheng Tang, Jiaxin Yu **, Hongtu He, Huiming Qi Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, Southwest University of Science and Technology, Mianyang, 621010, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Direct voltage Friction behavior Droplet Solid/liquid interface
The solid/liquid interfacial friction and slip behaviors on roughness surface under applied voltage were studied. Results showed that the slip behaviors of droplets on roughness surface in the unstable region cannot fit by electrowetting equation. The slip distance and directions mainly depended on difference between right and left friction force at the triple contact line, which were caused by the asymmetric roughness of the solid surface. Moreover, microstructures with various heights were used to control the solid/liquid slip behaviors. Results indicated that when the ridges of scratch far higher than surface roughness of the hydrophobic surface, the slip distance increased and slip direction can be controlled. The findings have potential applications in oriented driving of droplets by electric field.
1. Introduction
one of the most potential technologies to actuate microdroplets. A typical EWOD system composed of a substrate, droplet and a power supply, as shown in Fig. 1a. When voltage was applied between the droplet and substrate, the shape of the droplet changed with the increase of the applied voltage. Therefore, droplets can be actuated by array electrodes by EWOD system [20]. In order to actuate droplet accurately, solid/liquid interfacial behaviors under applied voltage have attracted considerable attention [21–25]. Previous studies were mainly concen trated on the deformation process of droplet [26], low voltage actuation [19]. and sliding friction behaviors [27]. These studies indicated that the solid/liquid interfacial behaviors were associated with surface roughness, which cause additional friction force at the triple contact line [27]. For a real surface, surface defects and surface roughness are un avoidable. Consequently, it is necessary to study the interfacial friction of droplet on a roughness surface to actuate droplets efficiently. In this paper, the variations of contact angle and triple contact line before and after slipping in the unstable region were characterized by contact angle meter. Moreover, the slip distance and slip direction of droplet under direct voltage were studied. Next, a nano-scratch was utilized to make microstructures to control the slip behavior of droplet. In addition, the interfacial friction of droplet on roughness surface was discussed using balance of forces at the triple contact line. The findings in this paper may be helpful to extend the wetting theory of droplet under direct voltage and has potential applications in oriented driving of
Oriented driving microdroplet on a solid surface has attracted much attention in recent years because of its wide application in lab-on-a-chip [1,2], inkjet printing [3,4], and DNA microarrays [5,6]. Traditionally, oriented driving of microfluidics was done by combination of micro pumps, microvalves and microchannels [2]. By intermittently switching on and off the microvalves and micropumps, individual droplets can be obtained and actuated according to requirements [7–9]. However, because the actuation systems were composed of complex mechanical devices, the reliabilities were restricted by the development of minia turization and integration. Consequently, all kinds of new approaches were proposed to manipulate microdroplet, such as magnetic actuation technology [10,11], optofluidic technology [12,13] and electrowetting on dielectric (EWOD) technology [14,15]. For magnetic-induced and light-induced actuation technologies, magnetic-sensing and photosen sitive particles should be added to the droplets to control the movement [10,12]. These particles may contaminate droplets and were difficult to remove. Conversely, by changing the effective solid/liquid interface tension through electric field, EWOD technology can be used to generate and actuate droplets accurately [16,17]. Due to without mechanical devices, EWOD technology has several advantages, such as non-polluting, real-time actuation, fast response, long-term stability and reliability [18,19]. Consequently, EWOD technologies are considered as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (J. Yu). https://doi.org/10.1016/j.triboint.2019.106128 Received 4 October 2019; Received in revised form 2 December 2019; Accepted 16 December 2019 Available online 17 December 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
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of the hydrophobic layer were acquired by white light scanning profil ometry (MFT-3000, Rtec, USA). The Z axis resolution of the white light scanning profilometry is 0.1 nm, and X & Y axis resolution is 1 μm. To study the effect of roughness on solid/liquid friction, moreover, a nanoscratch tester (G200, Keysight, USA) was used to make microstructures on the hydrophobic surface. During processing, a conical diamond tip with a radius of 1.8 μm was used and scratches were performed under 5 μN–30 μN respectively. Moreover, the scratch length for each scratch was 5 mm at a velocity of 5 μm/s. The topographies of the scratch profiles were acquired by white light scanning profilometry. All exper iments were performed under atmospheric conditions at room temperature. 3. Results 3.1. Slip behaviors before saturation The variation of contact angle and the length of triple contact line with applied voltage were shown in Fig. 2. Obviously, the contact angle and triple contact line curves can be divided into four regions, namely, Istatic region, IIsliding region, III unstable region and Ⅳ saturated re gion. In the static region, the contact angle decreased slightly while triple contact line remained constant. In the sliding region, the contact angle decreased and contact line increased with increasing of applied voltage. In the saturated region, the contact angle and triple contact line almost kept constant with increasing of voltage. It was noted that an unstable region exist before saturation. In this region, the variation of contact angle and triple contact line cannot fit by electrowetting equa tion. As shown in Fig. 2, sharply stick-slip behaviors of droplet can be observed before saturation. As shown in the inset of Fig. 2, the contact angle suddenly increased and triple contact line decreased respectively when the droplet slid from point A to point B. The difference of contact angle (DCA) and triple contact line (DCL) between point A and point B were summarized in Table 1. It was found that the DCA and DCL were independent on the concentrations of NaCl.
Fig. 1. Schematic of the electrowetting system and forces at the triple contact line. (a) schematic diagram of the experimental method. (b) schematic of the balance of forces at triple contact line on an inhomogeneous surface under applied voltage.
droplets by electric field. 2. Materials and methods 2.1. Sample preparation Conductive silicon wafers coated with a 300 nm silicon oxide layer were used as the ground electrode (SSPP, Siltronic, Germany). In order to minimize contact angle hysteresis, the silicon oxide surface was hy drophobic treated with a Teflon emulsion (Teflon AF1600, Dupont, USA). The Teflon emulsion was spinning on the silicon oxide surface using a spin processor (KW4A, Beijing Saidekai Co., China) at 500 rpm for 20 s and 3000 rpm for 30 s respectively. After heating at 200 � C for 2 h, the hydrophobic layer with ~2 μm thick was obtained. The water contact angles of the specimens were approximately 130� .
3.2. Slip distance and directions For a homogenous surface, the variation of contact angle and triple contact line were axisymmetric. Theoretically, droplets changed sym metrically with applied voltage along a symmetrical axis (green dotted line), as shown in Fig. 3. As mentioned above, droplet slipped from one equilibrium state to other equilibrium state before saturation for a rough surface. So slip toward left and right of the droplet can be observed. Moreover, the slip behaviors of droplets were overlooked by a camera, as shown in Fig. 4. The photo of the droplet before and after slip is shown in Fig. 4b. The schematic diagram of the droplet before and after sliding is shown in Fig. 4c. 50 repeated experiments were done on a batch of specimens. Results showed that the slip directions of droplets under applied voltage were uncertainly. As shown in Fig. 4d, slip toward east (E), south (S), west (W), north (N) and other directions can be observed. The number of times in different directions was summarized in Table 2. Where E, S, W, and N represent the direction of east, south, west and north respectively. And the NW, NE, SW, SE represents the direction northwest, northeast, southwest and southeast respectively. According to the data in Table 2, it can be deduced that the slip directions were uncertain. The distance between the axes before and after slipping was termed as slip distance (SD). Results showed that the average SD of droplet was approximately 0.4 mm for the surface with a roughness of Ra 53.9 � 16.9 nm, as shown in Table 1. Moreover, there were no sig nificant differences of the SD among three different solutions. The dynamic responses of droplet with increase of applied voltage are shown in Fig. 3. The average contact angle θ, left contact angle θL and right contact angle θR for a droplet were showed under the photo graph. It was found that the difference between θL and θR (DLR) was less than 1� for an homogenous surface under applied voltage. However, the
2.2. Electrowetting parameters measurement The schematic diagram of electrowetting is shown in Fig. 1. The silicon wafer of the specimen was connected to the negative electrode of a power supply (PSW250–4.5, GuWei, China). Next, a 10 μL droplet was placed on the surface of specimen and a copper wire with diamond of 100 μm was inserted inside the droplet. And then the copper wire was connected to the positive electrode of the power supply. An applied voltage ranged from 0 to 200 V with incremental velocity of 1 V/s was provided. The voltage resolution of the power supply is 0.1% � 10 mV. The variations of contact angles of droplet with applied voltage were obtained using a contact angle meter (DSA30E, KRUSS, Germany). For the contact angle meter, the measurement resolution is 0.01� and 0.01 mm. Consequently, the lengths of triple contact line under different applied voltages were recorded by DSA contact angle analysis software (DSA30E, KRUSS, Germany). In order to study the effect of ion con centration on the slip behaviors of droplet under applied voltage, so dium chloride solution (NaCl) with different concentrations (0 mol/l, 1 mol/l and 2 mol/l) were used. 2.3. Morphology of the hydrophobic layer Considering the solid/liquid friction behaviors of droplet were closely associated with the morphology of specimens, the topographies 2
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Fig. 2. Variation of contact angles and the length of contact line with applied voltage for different NaCl solutions, (a) the contact angle of droplet as functions of applied voltage. (b) the length of contact lines a function of applied voltage. An unstable region (III) exists before saturation. In this region, the contact angle and triple contact line varied sharply and randomly.
3.3. Morphology of the hydrophobic layer and microstructures
Table 1 Slip parameters of droplets under applied voltage before saturation. NaCl concentration/ mol⋅L 1
DCA/�
DCL/mm
SD/mm
DLR/�
0 1 2
6.7� �4.28 6.7� �5.19 6.7� �4.24
0.2 � 0.19 0.2 � 0.21 0.2 � 0.17
0.4 � 0.42 0.4 � 0.7 0.4 � 0.41
1.6� �1.41 2.2� �2.11 1.6� �1.55
The morphologies of the hydrophobic layer were showed in Fig. 6. Micro-pits and micro-bulges were observed on the specimen surface (Fig. 6a). The surface roughness (Ra) of the hydrophobic surface was 53.9 � 16.9 nm. Moreover, the ridges caused by scratches can be divided into two groups. One group is that the height of microstructures is lower or equal to the surface roughness and the other group is that the height of microstructures is significant higher than the surface roughness. It is found that the height of the ridges obtained at 5 μN was about 50 nm, which equal to Ra of the hydrophobic surface approximately, as shown in Figs. 6b and 7. Moreover, the height of the ridges obtained at 20 μN was about 450 nm, which was far higher than Ra of the hydrophobic surface (Figs. 6c and 7).
DLR varied with applied voltage for the rough surface. As shown in Fig. 5, the DLR increased with applied voltage and reached maximum value before saturation and then sharply decreased after slipping. Finally, it kept constant due to saturation. The average DLR for droplets ranged from 1.6� to 2.2� , as shown in Table 1. Moreover, there were no significant differences for DLR among three different NaCl solutions.
Fig. 3. Dynamic responses of droplets with in crease of applied voltage. The dotted green lines indicated the center of symmetry and the solid red lines represented the length of contact line. The slip distance was termed as the distance be tween the axes before and after slipping. For an homogenous surface, the variation of contact angle and triple contact line was axisymmetric. For a rough surface, the slip directions of droplet were random. So slip toward left and right of the droplets can be observed under applied voltage. The direction of red arrow means the increasing of applied voltage. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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with applied voltage can be divided into three typical regions, namely, static region, sliding region and saturated region [29]. However, it was noted that an unstable region exist before saturation for a roughness surface. Consequently, the balance of forces at the triple contact line should be further discussed. When a droplet is rested on a homogeneous surface, the droplet will eventually reach an equilibriums state under the action of surface ten sions and the contact angle between solid/liquid interface is termed as Young’s angle (θY) [30]. cosθY ¼
γ sv
γ sl
(1)
γ lv
where γ is the surface tension, subscripts s, v, l represent solid, vapor and liquid phase respectively. The contact angle (θ) of droplet will gradually decrease under applied voltage in a electrowetting system due to the insulating layer at solid/liquid interface can be considered as a capacitance which can eff
reduce the effective interfacial tension (γsl ) [31]. γ eff sl ¼ γ sl
Table 2 Number of slip times in different directions.
cosθ ¼
Slip direction
W
E
N
S
NW
NE
SW
SE
8
3
10
8
9
3
5
4
2d
(2)
U2
where U is the applied voltage, d is the thickness of dielectric t, ε0 and εd is the permittivity of free space and dielectric constant respectively. Moreover, for a rough surface, interfacial friction forces caused by chemical and topographic inhomogeneities of the solid surface should be considered [32,33]. Consequently, balance forces on the triple con tact line under applied voltage can be obtained (Fig. 1b) and the contact angle (θ) was determined by combined actions between surface tension and friction force (f).
Fig. 4. The slip behaviors of droplet were overlooked by a camera. Slip toward east (E), south (S), west (W), north (N) and other directions can be observed.
Number of times
ε0 ε1
γsv
γeff sl γlv
f
(3)
It should be noted that the contact angles were different along the perimeter of the droplet due to topographic inhomogeneities. As shown in Fig. 3, θL and θR for a droplet were difference. Consequently, ac cording to equation (3), the friction forces at right (fR) were different eff
from that at left (fL) (Fig. 1b). Moreover, γsl decreased with increasing applied voltage. In sliding region, the wetting area of droplet increased and more micro-pits and micro-bulges were covered by droplets. As a result, the DLR was continually amplified. Therefore, the DLR increased with applied voltage (Fig. 5). With increasing applied voltage further, eff
γ sl tended to zero eventually and the contact angle and contact line curves kept constant due to saturation [34–36]. Consequently, the DLR eff
reached maximum value before saturation (Fig. 5). Once γ sl equal to zero, the droplets reached saturated region. The contact line and contact eff
angle were not affected by applied voltage. Due to absence of γsl , the forces at triple contact line were out of balance. If the friction force on the left (fL) was greater than that on the right, the droplet slipped toward right to find an equilibrious state. Consequently, the DCA and DCL changed before and after slipping (Table 1). At this equilibrious state, the triple line reached equilibrium renewedly under the action of ten sion force and friction force. Conversely, droplet slip toward left if the friction force on the left (fR) was greater than that on the right. Conse quently, the slip directions of droplet were uncertain and mainly depended on the difference between fR and fL. Moreover, results indicated that no significant differences in DCA and DCL among different solutions. Sedev et al. [37] pointed that the ionic concentration had no effect on the electrowetting behaviors because the interfacial tension and viscosity cannot be significant changed by ionic concentration. Varagnolo [38] et al. and Zhang et al. [39] suggested that solid/liquid interfacial behaviors were mainly depended on chemical and topographic heterogeneity. In this paper, the
Fig. 5. Variation of the difference between θL and θR (DLR) with applied voltage. The DLR increased with applied voltage and reached maximum value before saturation and then sharply decreased after slipping. Finally, it kept constant due to saturation.
4. Discussion The variations of contact line under applied voltage were a stick-slip process rather than continuously due to the existence of friction force [28]. Generally, for a homogenous surface, the variation of contact angle
eff
hydrophobic layers were chemical homogeneity. Furthermore, the γsl
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Fig. 6. White light scanning profilometry images of the Teflon surface, (a) hydrophobic surface, (b) the profile of scratch under normal load of 5 μN, (c) the profile of scratch under normal load of 20 μN.
Fig. 7. The height of ridges on Teflon surface at various normal loads.
mainly depended on the electrical properties of silicon dioxide layer rather than onic concentration of droplet. Consequently, the DCA and DCL between deionized water and NaCl solution have no significant difference as shown in Table 1. As mentions above, the slip behaviors of droplet before saturation depended on the difference between fR and fL. Moreover, the fR and fL were closely associated with the topography of the surface. Conse quently, it is inferred that the slip behaviors of droplet might be controlled by microstructure. Therefore, microstructures were made on the hydrophobic surface using nano-scratch. According to the height of microstructures, those microstructures were divided into two groups. One group is that the height of microstructures is lower or equal to the surface roughness and the other group is that the height of microstruc tures is significant higher than the surface roughness. As shown in Fig. 7, one of the ridges obtained at 5 μN were about 50 nm, which equal to Ra of the hydrophobic surface approximately and the other ridges obtained at 20 μN were about 450 nm, which were far higher than Ra of the hydrophobic surface. Results showed that slip behaviors were observed on the microstructured surface also. The slip behaviors of droplet were not changed when the height of the ridges lower or equal to Ra. How ever, the slip direction and slip distance of droplet completely changed by the ridges which higher than Ra. As shown in Fig. 8, the red line represented where the ridge of scratch was located. When the droplet was rest on the right of the microstructure, the droplets slipped right under applied voltage before saturation. Conversely, it slipped left. The SD and DRL of droplet on the Teflon surface with different height of ridges were summarized in Table 3. When the ridges of scratch were equal to Ra of the hydrophobic surface, the SD and DRL were not changed significantly. However, SD and DRL increased significantly when the ridges of scratch were far higher than Ra of the hydrophobic
Fig. 8. The slip direction of droplet on the Teflon surface with different mi crostructures. The red dotted line represent where the scratch was located. When the droplet was rest on the right of the ridge of scratch, the droplets slipped right under applied voltage before saturation. Conversely, it slipped left. The direction of red arrow means the increasing of applied voltage. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
surface. The dynamic behaviors of droplet under applied voltage on a surface with ridges higher than Ra can be explained in Fig. 9. The Micropits on the surface can prevent the deformation of contact line. As a result, the contact angle continually increased because the triple contact line cannot move under applied voltage. When the ridges of scratch were greater than Ra of the hydrophobic surface, the contact angles adjoining to the higher ridges were larger than that adjoining to the lower ridges. As mentioned above, the forces at triple contact line were out of balance eff
due to absence of γsl . When the droplet was rest on the right of the higher ridge of scratch, the droplet slipped right under applied voltage before saturation. Consequently, the SD of droplet was mainly depended on the DRL. However, the SD cannot increase indefinitely with increase Table 3 The slip parameters of droplet on the Teflon surface with different height of ridges.
5
Height of ridge/nm
SD/mm
DLR/�
50 450
0.4 � 0.42 0.7 � 0.94
1.6� �1.41 3.5� �3.48
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51605402), Scientific Research Fund of Sichuan Provincial Education Department (Grant No. 17ZA0409) and the Longshan Academic Talent Research Supporting Program of SWUST (Grant No. 18LZX555). References
Fig. 9. Schematic diagram of the slip behaviors controlled by ridge of scratch. When the droplet was rest on the right of the higher ridge of scratch, the droplets slipped right under applied voltage before saturation.
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of the height of ridges. It was noted that although the contact angles can be increased by the height of the ridges, the maximum value can’t exceed 180� . Therefore, according to eq. (3), the friction forces caused by ridges can be obtained by γ sv þ γlv � f � γsv
(4)
It was found that the friction force ranged from γsv and γsv þ γlv . However, it should be noted that the friction term between solid/liquid may cause by chemical and topographic inhomogeneities [33]. The in fluences of chemical homogeneities on the friction were not involved in Equation (4). Consequently, Formula 4 is applicable to estimate the ef fect of surface topography on solid/liquid friction. In summary, it can be found that the slip behaviors can be controlled by designing surface morphology. The findings have potential for directional driving of droplets. 5. Conclusion The solid/liquid interfacial friction and slip behaviors on roughness surface under applied voltage were studied. It was found that inhomo geneous surface will cause the difference of contact angle between left and right of the droplets. Moreover, the slip directions and distance of droplet were uncertain and mainly depended on the friction force at the triple contact line, which were caused by the asymmetric roughness of solid surface. So slip toward left and right of the droplet can be observed on inhomogeneous surface. Moreover, there were no significant differ ences of the slip distance among three different solutions. Based on the observations, two type of microstructures were made on the hydro phobic surface using nano-scratch to control the friction force at the triple contact line to change the slip behaviors of droplet. When the ridges of scratch equal to Ra of the hydrophobic surface, the slip be haviors cannot be significant changed. When the ridges of scratch far higher than Ra of the hydrophobic surface, the slip distance increased and slip direction can be controlled. Moreover, the solid/liquid inter facial friction force caused by roughness ranged from γ sv and γ sv þ γlv . The results have potential applications in oriented driving of droplets by electric field. Author contributions Yafeng Zhang and Jiaxin Yu conceived and designed this experiment; Yongning Wang, Cheng Tang conducted the experiments; Y.F. Zhang and Yongning Wang wrote the paper; and Hongtu He, Huiming Qi contributed to the analysis and discussion. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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