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Controlled droplet transport on a gradient adhesion surface† Shile Feng, Sijie Wang, Chengcheng Liu, Yongmei Zheng* and Yongping Hou*
Received 17th January 2015, Accepted 19th February 2015 DOI: 10.1039/c5cc00467e www.rsc.org/chemcomm
A surface with continuously changed adhesion from ultrahigh to ultralow is fabricated by an integrated method of anodic oxidation combined with octafluorocyclobutane (C4F8) plasma. The control of droplet transport along the direction of the adhesion gradient in length is achieved, as the surface is submitted to either tilted angle or vibration frequency.
Recently, solid surfaces with special liquid–solid adhesion have attracted great attention in the fields of microfluidic systems, materials science, and biotechnology.1–5 Actually, there are two kinds of extreme cases, i.e., low adhesion to water and high adhesion to water. Water droplets on the surface of a lotus leaf will spontaneously roll off and remove dust particles due to the hierarchical micro/nanostructures of the lotus leaf, exhibiting typical examples of low adhesion surfaces.6,7 On the other hand, the gecko’s attachment system and rose petals give us an essential model of high adhesion.8,9 On these surfaces, droplets are pinned to the surfaces at any tilted angles, performing some potential applications, such as no loss microdroplet transportation.3,6 In order to satisfy different requirements, surfaces with tunable adhesion have acquired intense interest.10–16 Moon Kyu Kwak et al.1 developed a technique to fabricate nano-hairs by using a rigiflex poly(urethane acrylate) mold material to generate different kinds of slanted nanohairs, which can tune the directional adhesion properties. Dai et al.17 prepared aligned multi-walled carbon nanotubes combined with a shape-memory polymer to tune the adhesion. ´ndez et al.18 fabricated a well-controlled, chemical graHerna dient surface of a graphene substrate to push the directional motion of a droplet. Furthermore, smart responsive materials Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, 100191, P. R. China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Fig. S1–S6. SEM and AFM images of a graphite plate at different areas, three typical EDS spectra, advancing CAs and receding CAs in the different areas of the AOP graphite plate along the wettability direction, sliding behaviors of water droplets at a tilted angle of 16.21. See DOI: 10.1039/c5cc00467e
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are also used to realize reversible transition between low and high adhesion under external stimuli,19 such as pressure,20 vibration,21,22 and an electrical field.23 In all of these studies, the fabrication of surfaces with special liquid–solid adhesion is always complicated and usually the homogeneous surface exhibits a single adhesion property.24 It remains a challenge to develop a simple approach to achieve controlled droplet transmission via an adhesion gradient. Here, we use a simple anodic oxidation combined with a plasma method to establish a large adhesion gradient on which the sliding angles (SAs) decrease from above 901 to 3.91 gradually. Great anisotropy of adhesion is also observed, i.e., maximum difference of SA reaches about 301 at the same point. We could realize the transfer of a droplet from one point to another accurately by adjusting the tilted angle or exerting vibration and predict the transmission distance of the water droplets on a tilted surface via calculation. Such surfaces with controlled continuous change of adhesion may be used for the construction of future generation smart devices. A graphite plate can be prepared by the improved anodic oxidation (AO) or additive plasma treatment (AOP), so as to realize a wettable gradient feature from hydrophobic to hydrophilic or hydrophobic to superhydrophobic (Scheme 1a, see Experimental section, ESI†). Fig. 1 shows the surface wettability of different areas that are characterized by the water contact angles (CAs). In our experiment, the original graphite plate has a CA of B1201 due to the roughness.25 For the sample (S-AO) treated by improved AO,5 the wettable gradient is formed and CAs change gradually from B121.01 to B39.41 along the direction from the top (T) part to the bottom (B) part, with a degree of wettable gradient of 3.21 mm1. For the sample (S-AOP) treated by AOP, the whole surface of the graphite plate exhibits hydrophobic properties and the direction of the wettable gradient is reverse, i.e., CAs change from B133.01 (at the T part) to B157.91 (at the B part), with a degree of wettable gradient of 1.01 mm1. Fig. S1 and S2 (ESI†) show the scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM) images of S-AO and S-AOP. Compared to the
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Scheme 1 Schematic models of surface changes of the graphite plates. (a) Explanation for the change of chemical composition. (b) Explanation for the change of surface morphology and adhesion gradient. Due to the gradient of anodic oxidation, the surface of the graphite plate has a gradient of chemical composition and surface morphology. After AOP treatment, the surface becomes hydrophobic and the contact state is changed from the Wenzel state with high adhesion to the Cassie state with low adhesion gradually (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process).
Fig. 1 Photographs of the CAs on (a) S-AO and (b) S-AOP. The volume of the water droplet is 5 mL. Clearly, the wettable gradient exists on the gradient surface after both AO and AOP (T: top area and B: bottom area of the graphite plate during the anodic oxidation process).
original morphology (Fig. S3, ESI†), the surface becomes rough and a gradient of roughness is formed from the top part (porous structure) to the bottom part (smooth, compact structure) on S-AO. The root-mean-square (RMS) roughness of S-AO and S-AOP is obtained from Fig. S2 (ESI†), i.e., the RMS roughness of S-AO changes from B73.2 nm to B52.3 nm and the value for S-AOP decreases from B66.3 nm to B49.4 nm along the direction from the top (T) part to the bottom (B) part. It is well known that the surface of graphite is occupied by chemically stable surfaces (basal planes) and easily oxidized surfaces (amorphous boundaries).26 During the AO process, the easily oxidized surfaces may be selectively oxidized and then peeled off, which can lead to the increase in roughness. After the amorphous boundaries are peeled off, edge sites are initially oxidized to form such oxygen-containing surface groups (C–OH, CQO, COOH and finally CO2) due to more reactivity,27,28 and therefore the surface becomes smooth gradually. In our case, the current gradient and oxidation time gradient lead to the formation of an oxidation gradient on the graphite plate surface, which results in the formation of a roughness gradient (Fig. S1a–e, ESI†). After AOP, although the roughness gradient is reversed on S-AOP, the whole surface becomes a little smooth due to the etch effect of plasma (Fig. S1a–e, ESI†). Three typical energy dispersive spectrometer (EDS) spectra (original graphite plate, S-AO and S-AOP) are
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depicted in Fig. S4 (ESI†). The EDS data indicate that carbon is the only detectable element on the original graphite plate. After AO and AOP, the O and F element are introduced on the surface of the graphite plate, respectively. Fig. S4d (ESI†) shows the corresponding atomic ratio of the different areas (from the T to B part) after AO and AOP. Clearly, the O and F element gradients are formed and the gradient of the O element content changes a little after plasma treatment. Water adhesion as an important property of a solid surface can be accurately assessed by the sliding behaviors of a water droplet. As for S-AO, the SAs are always above 901, the water droplet does not slide down even when the S-AO is vertical or turned upside down. As for S-AOP, the SAs decrease from above 901 to 3.91 gradually, along the direction from the T part to B part (Fig. 2a), which implies the gradual change of adhesion from ultrahigh to ultralow. Not only an adhesion gradient, but also an anisotropic adhesion was observed. The maximum difference of SAs reaches about 301 at the same point as on S-AOP, which is much larger than previously reported.4 The reason is that the droplet could slide more easily towards more smooth and hydrophobic areas. In addition, adhesion forces are also examined to measure the gradient. The tangential adhesion forces change from 60 mN, 45 mN, 34 mN, 25 mN, 16 mN, 10 mN to 8 mN for droplets (5 mL) at the points of 3 mm, 7 mm, 11 mm, 15 mm, 19 mm, 23 mm and 27 mm (from the T part to the B part), respectively (Fig. 2b), which is consistent with the results of the SAs. We investigate the sliding behaviors of droplets at different tilted angles (Fig. 3a). Clearly, the sliding distance along the wettable gradient could be controlled by the tilted angle. When the tilted angle is 51, the sliding distance is B5 mm and when the angle becomes 121, the sliding distance could reach B20 mm. There are three main forces influencing the motion of droplets: the component of gravity (FG) along the tilted surface, the wettable gradient force (FW) and the hysteresis force (FH).5 In our experiment, the direction of FG and FW is the same. Accordingly, when the sum of FG + FW is below FH (tilted angle is 21), the droplet would be pinned to the surface. Otherwise, the droplet could slide along the surface. When a droplet slips along the tilted surface (the direction of FG, FW), the work caused by gravity (WG) and wettable gradient force
Fig. 2 (a) The slide angles in the different areas of the AOP graphite plate along different directions (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process). The volume of the water droplet is 5 mL. (b) Curve of tangential adhesion force in different positions of the AOP graphite plate. Clearly, an adhesion gradient and an anisotropic adhesion are formed on S-AOP.
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Fig. 3 (a) Sliding behaviors of droplets on the S-AOP surface with different tilted angles. The scale bar is 5 mm and the volume of the water droplet is 10 mL. The results indicate that the sliding distance could be controlled via tilted angle (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process). (b) The relation between the work and sliding distance. By comparing the values of WH and WG + WW, we could predict the sliding distance at a given tilted angle (WG: the work caused by gravity (Fg), Ww: the work caused by the wettable gradient force (Fw), WH: the work caused by the hysteresis force (FH)).
(Ww) is changed into friction heat (WH), caused by hysteresis force. The gravity work (WG) is determined from the mass (m) and the sliding distance (x) of the liquid drop, and the tilted angle (a) of the graphite plate, i.e., WG = mgx sin a, where g is the acceleration due to gravity. The wettable gradient work (Ww) Ðx could be described as:14 WW ¼ 0 pw2 gk sin y dx, where w is the contact length of the droplet and the graphite plate, g is the surface tension of water, y is the position-responsive sessile CA of the water droplet and k is the wettable gradient. The hysteresis work (WH) depends on some parameters, such as the position-dependent receding CAs (yr) and advancing CAs Ðx (ya), which is described as:14 WH ¼ 0 wgðcos yr cos ya Þdx. From the data in Fig. 1 and Fig. S5 (ESI†), we could obtain the values of WG, Ww and WH as a function of sliding distance (x) along the gradient direction and the tilted angle (a) (Fig. 3b). When the water droplet stops sliding, the work is balanced, i.e., WG + Ww = WH. By comparing the values of WH and WG + WW, we could predict the sliding behaviors and get the sliding distance at a given tilted angle, as shown in Table S1 (ESI†). The results indicate that the actual measured values are very close to the as-calculated values. The fine difference may be due to the fact that during the sliding process, the contact length (w) increases gradually, which induces the underestimate of the wettable gradient force (FW) and hysteresis force (FH). In addition, the calculated results indicate, when the tilted angle reaches B16.51, that the water droplet would slide off from the graphite plate, which is very close to the threshold (16.21) observed in
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the experiment (Fig. S6, ESI†). Clearly, we could predict the sliding distance at a given tilted angle via calculation. In other words, on the adhesion gradient surface, we could realize the transfer of a droplet from one point to another easily. Not only the sliding behaviors at different tilted angles, but also the movement behaviors of droplets at different points under low vibration amplitudes were surveyed (Fig. 4). For sample S-AOP, due to less driving force (FW o FH), the droplet is pinned to the surface when it is dripped. The vibration would induce a body force on the droplet.29–32 The body force during each impulse is FB. During the acceleration phase (the vibration direction is the same as that of the wettable gradient), the net driving force acting on the droplet could be written as: FAL = FB + FW, whereas during deceleration phase, the net driving force against the direction of the wettable gradient is FAG = FB FW (Fig. 4f).33 If FAL > FH and FAG o FH, i.e., FH FW o FB o FW + FH, the droplet would move along the direction of the wettable gradient during the acceleration phase and is pinned during the deceleration phase, which is confirmed in our experiment. Here, we use the frequency to adjust FB. The results indicate that during 25–125 Hz, droplets (volume of 10 mL) move only along the wettable gradient and the movement distance could be adjusted to be between 2–12 mm (Fig. 4a–e). With increasing vibration frequency, the movement distance increases greatly at the initial stage, after reaching a maximum value (12 mm) at 75 Hz, then decreases obviously. Koray Sekeroglu et al.29 suggested that a droplet has an appropriate forcing frequency to reach the resonance mode to get great body force. According to the equation, the appropriate frequency of the droplets (volume of 10 mL) is B78 Hz, which is consistent with our results above. As the droplet moves along the direction of the wettable
Fig. 4 The movement behaviors of droplets on a graphite plate with a gradient of adhesion under different vibration frequencies: (a) 25 Hz, (b) 50 Hz, (c) 75 Hz, (d) 100 Hz, and (e) 125 Hz. (f) The force analysis on the droplets at different vibration directions (VD). With an increasing vibration frequency, the movement distance increases greatly at the initial stage, after reaching a maximum value at 75 Hz, then decreases obviously. By controlling the vibration frequency, we could adjust the value of FB to make the droplet only move along the direction of the wettable gradient. Therefore, with the help of low vibration amplitudes, we could realize the transfer of droplets from one point to another by vibration frequency easily. The scale bar is 5 mm and the volume of the water droplet is 10 mL (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process, FH: hysteresis force, FW: wettable gradient force, FB: body force due to the vibration, FAL: the net driving force along the direction of the wettable gradient during the acceleration phase, FAG: the net driving force against the direction of the wettable gradient during the deceleration phase).
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gradient (also the direction of the adhesion gradient), FH increases gradually. As FB + FW r FH, the droplet begins to decelerate and cease motion ultimately. Therefore, with the help of low vibration amplitudes, we could adjust the value of FB via the vibration frequency to realize the transfer of droplets from one point to another easily. As for adhesion, it is well known that the distinct contact mode (Wenzel state, Cassie state) is a key factor. In general, the rough microstructure would cause the intrusion pressure (DP) to prevent the liquid spontaneous transition from Cassie to Wenzel state, which could be described as the following equation:2,4 DP = 2g/R = cg(cos ya)/A, where g is the interface tension, R is the radius of meniscus, c is the circumference of the pore, A is the cross-sectional area of the pore, and ya is the advancing CA. In our experiment, due to large pores on the surface of the original graphite plate, DP is small and the water droplet could fully penetrate the pores to form a continuous (three-phase contact line) TCL, exhibiting a Wenzel state. Thus, the surface possesses high adhesion that could pin the water droplet to the surface without any movement. After AO, the surface becomes hydrophilic and the water droplet could easily penetrate the pores (DP becomes smaller, even negative), still exhibiting a Wenzel state and high adhesion. After AOP, from the top areas to the bottom ones, the radius of the pore (R) becomes small (Fig. S1 and S2, ESI†) and the value of ya changes a little (Fig. S5, ESI†), which could induce the increase in penetration pressure.34,35 Finally, the penetration of the water droplet is limited and a water droplet is suspended by the gas layers trapped at the micro-scales, exhibiting a Cassie state. The water droplet only contacts the peak of the hierarchical surface, leading an extremely discontinuous TCL. The adhesion of the surface is extremely low and the water droplet easily rolls off with the surface slightly tilted or vibrating. Therefore, after AOP, the contact state can be tuned between the Wenzel state, with high adhesion, to the Cassie state, with low adhesion, via adjusting the microstructures and the chemical composition, showing an adhesion gradient from ultrahigh to ultralow (Scheme 1b). We could adjust the gravity (tilted angle) or body force (vibrating frequency) to control the sliding behaviors of the droplet along the direction from B to T and realize the transmission of the droplet from point to point. In summary, a graphite plate surface with a gradient of adhesion from ultrahigh to ultralow and large anisotropic adhesion was prepared by a simply improved anodic oxidation progress as well as a plasma treatment. The present approach could provide a new strategy to prepare surfaces with continual tunable water adhesion and controlled transmission of a droplet from point to point. Such a surface has promising application in dynamic water-repellency and no loss microdroplet transportation.36–38 This work was supported by the National Key Basic Research Program of China (2013CB933001), the National Natural Science Foundation of China (21234001, 51203006, 21204002,
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21004002, and 21473007), and the Doctoral Fund of Ministry of Education of China (20121102110035).
Notes and references 1 M. K. Kwak, H. E. Jeong, T. I. Kim, H. Yoon and K. Y. Suh, Soft Matter, 2010, 6, 1849. 2 H. Mei, D. Luo, P. Guo, C. Song, C. Liu, Y. Zheng and L. Jiang, Soft Matter, 2011, 7, 10569. 3 T. Sun, G. Wang, L. Feng, B. Liu, Y. Ma, L. Jiang and D. Zhu, Angew. Chem., Int. Ed., 2004, 43, 357. 4 Y. Zheng, X. Gao and L. Jiang, Soft Matter, 2007, 3, 178. 5 S. Feng, S. Wang, L. Gao, G. Li, Y. Hou and Y. Zheng, Angew. Chem., Int. Ed., 2014, 53, 6163. 6 R. Blossey, Nat. Mater., 2003, 2, 301. 7 X. Zhang, F. Shi, J. Niu, Y. Jiang and Z. Wang, J. Mater. Chem., 2008, 18, 621. 8 M. Jin, X. Feng, L. Feng, T. Sun, J. Zhai, T. Li and L. Jiang, Adv. Mater., 2005, 17, 1977. 9 L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia and L. Jiang, Langmuir, 2008, 24, 4114. 10 F. Xia and L. Jiang, Adv. Mater., 2008, 20, 2842. 11 B. Balu, A. D. Berry, K. T. Patel, V. Breedveld and D. W. Hess, J. Adhes. Sci. Technol., 2011, 25, 627. 12 M. Prakash and J. W. M. Bush, Int. J. Nonl. Mech., 2011, 46, 607. 13 X. Sheng and J. Zhang, Appl. Surf. Sci., 2011, 257, 6811. 14 H. Bai, X. Tian, Y. Zheng, J. Ju, Y. Zhao and L. Jiang, Adv. Mater., 2010, 22, 5521. 15 D. Xia, L. M. Johnson and G. P. Lopez, Adv. Mater., 2012, 24, 1287. 16 K. Jin, J. C. Cremaldi, J. S. Erickson, Y. Tian, J. N. Israelachvili and N. S. Pesika, Adv. Funct. Mater., 2014, 24, 573. 17 L. Qu, Y. Li and L. Dai, Bioinspiration and Biomimicry in Chemistry: Reverse-Engineering Nature, 2012, p. 251. 18 C. H. Sandra, J. C. Charlee, E. J. Chad, D. T. Stanislav, R. Stine, T. R. Jeremy, H. L. Evgeniya, R. B. David, D. P. Brian, D. C. Joshua, L. R. Thomas, E. S. Paul and G. W. Scott, ACS Nano, 2013, 7, 4746. 19 X. J. Huang, D. H. Kim, M. Im, J. H. Lee, J. B. Yoon and Y. K. Choi, Small, 2009, 5, 90. 20 A. Lafuma and D. Quere, Nat. Mater., 2003, 2, 457. 21 E. Bormashenko, R. Pogreb, G. Whyman and M. Erlich, Langmuir, 2007, 23, 6501. 22 E. Bormashenko, R. Pogreb, G. Whyman and M. Erlich, Langmuir, 2007, 23, 12217. 23 T. N. Krupenkin, J. A. Taylor, T. M. Schneider and S. Yang, Langmuir, 2004, 20, 3824. 24 M. Liu and L. Jiang, Adv. Funct. Mater., 2010, 20, 3753. 25 F. Taherian, V. Marcon, N. F. A. Van Der Vegt and F. Leroy, Langmuir, 2013, 29, 1457. 26 A. Fukunaga and S. Ueda, Compos. Sci. Technol., 2000, 60, 249. 27 A. G. Pandolfo and A. F. Hollenkamp, J. Power Sources, 2006, 157, 11. 28 Z. R. Yue, W. Jiang, L. Wang, S. D. Gardner and C. U. Pittman Jr, Carbon, 1999, 37, 1785. 29 S. Koray, A. G. Umut, D. Utkan and C. D. Melik, Appl. Phys. Lett., 2011, 99, 063703. 30 H. Bai, X. Tian, Y. Zheng, J. Ju, Y. Zhao and L. Jiang, Adv. Mater., 2010, 22, 5521. ´re ´, J. Fluid Mech., 2004, 510, 29. 31 E. Lorenceau and D. Que 32 D. Susan and K. C. Manoj, Langmuir, 2002, 18, 3404. 33 T. Xu, J. Yang, J. Liu and Q. Fu, Appl. Phys. A: Mater. Sci. Process., 2008, 90, 431. 34 E. Bormashenko, R. Pogreb, G. Whyman and Y. Bormashenko, Appl. Phys. Lett., 2007, 90, 201917. 35 D. Elsworth, Int. J. Numer. Anal. Methods Geomech., 1992, 16, 45. 36 N. R. Chiou, C. Lu, J. Guan, L. J. Lee and A. J. Epstein, Nat. Nanotechnol., 2007, 2, 354. 37 Y. Hou, Y. Chen, Y. Xue, L. Wang, Y. Zheng and L. Jiang, Soft Matter, 2012, 8, 11236. 38 B. Xue, L. Gao, Y. Hou, Z. Liu and L. Jiang, Adv. Mater., 2013, 25, 273.
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Cite this: DOI: 10.1039/c5cc00467e
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Controlled droplet transport on a gradient adhesion surface† Shile Feng, Sijie Wang, Chengcheng Liu, Yongmei Zheng* and Yongping Hou*
Received 17th January 2015, Accepted 19th February 2015 DOI: 10.1039/c5cc00467e www.rsc.org/chemcomm
A surface with continuously changed adhesion from ultrahigh to ultralow is fabricated by an integrated method of anodic oxidation combined with octafluorocyclobutane (C4F8) plasma. The control of droplet transport along the direction of the adhesion gradient in length is achieved, as the surface is submitted to either tilted angle or vibration frequency.
Recently, solid surfaces with special liquid–solid adhesion have attracted great attention in the fields of microfluidic systems, materials science, and biotechnology.1–5 Actually, there are two kinds of extreme cases, i.e., low adhesion to water and high adhesion to water. Water droplets on the surface of a lotus leaf will spontaneously roll off and remove dust particles due to the hierarchical micro/nanostructures of the lotus leaf, exhibiting typical examples of low adhesion surfaces.6,7 On the other hand, the gecko’s attachment system and rose petals give us an essential model of high adhesion.8,9 On these surfaces, droplets are pinned to the surfaces at any tilted angles, performing some potential applications, such as no loss microdroplet transportation.3,6 In order to satisfy different requirements, surfaces with tunable adhesion have acquired intense interest.10–16 Moon Kyu Kwak et al.1 developed a technique to fabricate nano-hairs by using a rigiflex poly(urethane acrylate) mold material to generate different kinds of slanted nanohairs, which can tune the directional adhesion properties. Dai et al.17 prepared aligned multi-walled carbon nanotubes combined with a shape-memory polymer to tune the adhesion. ´ndez et al.18 fabricated a well-controlled, chemical graHerna dient surface of a graphene substrate to push the directional motion of a droplet. Furthermore, smart responsive materials Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, 100191, P. R. China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Fig. S1–S6. SEM and AFM images of a graphite plate at different areas, three typical EDS spectra, advancing CAs and receding CAs in the different areas of the AOP graphite plate along the wettability direction, sliding behaviors of water droplets at a tilted angle of 16.21. See DOI: 10.1039/c5cc00467e
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are also used to realize reversible transition between low and high adhesion under external stimuli,19 such as pressure,20 vibration,21,22 and an electrical field.23 In all of these studies, the fabrication of surfaces with special liquid–solid adhesion is always complicated and usually the homogeneous surface exhibits a single adhesion property.24 It remains a challenge to develop a simple approach to achieve controlled droplet transmission via an adhesion gradient. Here, we use a simple anodic oxidation combined with a plasma method to establish a large adhesion gradient on which the sliding angles (SAs) decrease from above 901 to 3.91 gradually. Great anisotropy of adhesion is also observed, i.e., maximum difference of SA reaches about 301 at the same point. We could realize the transfer of a droplet from one point to another accurately by adjusting the tilted angle or exerting vibration and predict the transmission distance of the water droplets on a tilted surface via calculation. Such surfaces with controlled continuous change of adhesion may be used for the construction of future generation smart devices. A graphite plate can be prepared by the improved anodic oxidation (AO) or additive plasma treatment (AOP), so as to realize a wettable gradient feature from hydrophobic to hydrophilic or hydrophobic to superhydrophobic (Scheme 1a, see Experimental section, ESI†). Fig. 1 shows the surface wettability of different areas that are characterized by the water contact angles (CAs). In our experiment, the original graphite plate has a CA of B1201 due to the roughness.25 For the sample (S-AO) treated by improved AO,5 the wettable gradient is formed and CAs change gradually from B121.01 to B39.41 along the direction from the top (T) part to the bottom (B) part, with a degree of wettable gradient of 3.21 mm1. For the sample (S-AOP) treated by AOP, the whole surface of the graphite plate exhibits hydrophobic properties and the direction of the wettable gradient is reverse, i.e., CAs change from B133.01 (at the T part) to B157.91 (at the B part), with a degree of wettable gradient of 1.01 mm1. Fig. S1 and S2 (ESI†) show the scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM) images of S-AO and S-AOP. Compared to the
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Scheme 1 Schematic models of surface changes of the graphite plates. (a) Explanation for the change of chemical composition. (b) Explanation for the change of surface morphology and adhesion gradient. Due to the gradient of anodic oxidation, the surface of the graphite plate has a gradient of chemical composition and surface morphology. After AOP treatment, the surface becomes hydrophobic and the contact state is changed from the Wenzel state with high adhesion to the Cassie state with low adhesion gradually (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process).
Fig. 1 Photographs of the CAs on (a) S-AO and (b) S-AOP. The volume of the water droplet is 5 mL. Clearly, the wettable gradient exists on the gradient surface after both AO and AOP (T: top area and B: bottom area of the graphite plate during the anodic oxidation process).
original morphology (Fig. S3, ESI†), the surface becomes rough and a gradient of roughness is formed from the top part (porous structure) to the bottom part (smooth, compact structure) on S-AO. The root-mean-square (RMS) roughness of S-AO and S-AOP is obtained from Fig. S2 (ESI†), i.e., the RMS roughness of S-AO changes from B73.2 nm to B52.3 nm and the value for S-AOP decreases from B66.3 nm to B49.4 nm along the direction from the top (T) part to the bottom (B) part. It is well known that the surface of graphite is occupied by chemically stable surfaces (basal planes) and easily oxidized surfaces (amorphous boundaries).26 During the AO process, the easily oxidized surfaces may be selectively oxidized and then peeled off, which can lead to the increase in roughness. After the amorphous boundaries are peeled off, edge sites are initially oxidized to form such oxygen-containing surface groups (C–OH, CQO, COOH and finally CO2) due to more reactivity,27,28 and therefore the surface becomes smooth gradually. In our case, the current gradient and oxidation time gradient lead to the formation of an oxidation gradient on the graphite plate surface, which results in the formation of a roughness gradient (Fig. S1a–e, ESI†). After AOP, although the roughness gradient is reversed on S-AOP, the whole surface becomes a little smooth due to the etch effect of plasma (Fig. S1a–e, ESI†). Three typical energy dispersive spectrometer (EDS) spectra (original graphite plate, S-AO and S-AOP) are
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depicted in Fig. S4 (ESI†). The EDS data indicate that carbon is the only detectable element on the original graphite plate. After AO and AOP, the O and F element are introduced on the surface of the graphite plate, respectively. Fig. S4d (ESI†) shows the corresponding atomic ratio of the different areas (from the T to B part) after AO and AOP. Clearly, the O and F element gradients are formed and the gradient of the O element content changes a little after plasma treatment. Water adhesion as an important property of a solid surface can be accurately assessed by the sliding behaviors of a water droplet. As for S-AO, the SAs are always above 901, the water droplet does not slide down even when the S-AO is vertical or turned upside down. As for S-AOP, the SAs decrease from above 901 to 3.91 gradually, along the direction from the T part to B part (Fig. 2a), which implies the gradual change of adhesion from ultrahigh to ultralow. Not only an adhesion gradient, but also an anisotropic adhesion was observed. The maximum difference of SAs reaches about 301 at the same point as on S-AOP, which is much larger than previously reported.4 The reason is that the droplet could slide more easily towards more smooth and hydrophobic areas. In addition, adhesion forces are also examined to measure the gradient. The tangential adhesion forces change from 60 mN, 45 mN, 34 mN, 25 mN, 16 mN, 10 mN to 8 mN for droplets (5 mL) at the points of 3 mm, 7 mm, 11 mm, 15 mm, 19 mm, 23 mm and 27 mm (from the T part to the B part), respectively (Fig. 2b), which is consistent with the results of the SAs. We investigate the sliding behaviors of droplets at different tilted angles (Fig. 3a). Clearly, the sliding distance along the wettable gradient could be controlled by the tilted angle. When the tilted angle is 51, the sliding distance is B5 mm and when the angle becomes 121, the sliding distance could reach B20 mm. There are three main forces influencing the motion of droplets: the component of gravity (FG) along the tilted surface, the wettable gradient force (FW) and the hysteresis force (FH).5 In our experiment, the direction of FG and FW is the same. Accordingly, when the sum of FG + FW is below FH (tilted angle is 21), the droplet would be pinned to the surface. Otherwise, the droplet could slide along the surface. When a droplet slips along the tilted surface (the direction of FG, FW), the work caused by gravity (WG) and wettable gradient force
Fig. 2 (a) The slide angles in the different areas of the AOP graphite plate along different directions (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process). The volume of the water droplet is 5 mL. (b) Curve of tangential adhesion force in different positions of the AOP graphite plate. Clearly, an adhesion gradient and an anisotropic adhesion are formed on S-AOP.
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Fig. 3 (a) Sliding behaviors of droplets on the S-AOP surface with different tilted angles. The scale bar is 5 mm and the volume of the water droplet is 10 mL. The results indicate that the sliding distance could be controlled via tilted angle (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process). (b) The relation between the work and sliding distance. By comparing the values of WH and WG + WW, we could predict the sliding distance at a given tilted angle (WG: the work caused by gravity (Fg), Ww: the work caused by the wettable gradient force (Fw), WH: the work caused by the hysteresis force (FH)).
(Ww) is changed into friction heat (WH), caused by hysteresis force. The gravity work (WG) is determined from the mass (m) and the sliding distance (x) of the liquid drop, and the tilted angle (a) of the graphite plate, i.e., WG = mgx sin a, where g is the acceleration due to gravity. The wettable gradient work (Ww) Ðx could be described as:14 WW ¼ 0 pw2 gk sin y dx, where w is the contact length of the droplet and the graphite plate, g is the surface tension of water, y is the position-responsive sessile CA of the water droplet and k is the wettable gradient. The hysteresis work (WH) depends on some parameters, such as the position-dependent receding CAs (yr) and advancing CAs Ðx (ya), which is described as:14 WH ¼ 0 wgðcos yr cos ya Þdx. From the data in Fig. 1 and Fig. S5 (ESI†), we could obtain the values of WG, Ww and WH as a function of sliding distance (x) along the gradient direction and the tilted angle (a) (Fig. 3b). When the water droplet stops sliding, the work is balanced, i.e., WG + Ww = WH. By comparing the values of WH and WG + WW, we could predict the sliding behaviors and get the sliding distance at a given tilted angle, as shown in Table S1 (ESI†). The results indicate that the actual measured values are very close to the as-calculated values. The fine difference may be due to the fact that during the sliding process, the contact length (w) increases gradually, which induces the underestimate of the wettable gradient force (FW) and hysteresis force (FH). In addition, the calculated results indicate, when the tilted angle reaches B16.51, that the water droplet would slide off from the graphite plate, which is very close to the threshold (16.21) observed in
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the experiment (Fig. S6, ESI†). Clearly, we could predict the sliding distance at a given tilted angle via calculation. In other words, on the adhesion gradient surface, we could realize the transfer of a droplet from one point to another easily. Not only the sliding behaviors at different tilted angles, but also the movement behaviors of droplets at different points under low vibration amplitudes were surveyed (Fig. 4). For sample S-AOP, due to less driving force (FW o FH), the droplet is pinned to the surface when it is dripped. The vibration would induce a body force on the droplet.29–32 The body force during each impulse is FB. During the acceleration phase (the vibration direction is the same as that of the wettable gradient), the net driving force acting on the droplet could be written as: FAL = FB + FW, whereas during deceleration phase, the net driving force against the direction of the wettable gradient is FAG = FB FW (Fig. 4f).33 If FAL > FH and FAG o FH, i.e., FH FW o FB o FW + FH, the droplet would move along the direction of the wettable gradient during the acceleration phase and is pinned during the deceleration phase, which is confirmed in our experiment. Here, we use the frequency to adjust FB. The results indicate that during 25–125 Hz, droplets (volume of 10 mL) move only along the wettable gradient and the movement distance could be adjusted to be between 2–12 mm (Fig. 4a–e). With increasing vibration frequency, the movement distance increases greatly at the initial stage, after reaching a maximum value (12 mm) at 75 Hz, then decreases obviously. Koray Sekeroglu et al.29 suggested that a droplet has an appropriate forcing frequency to reach the resonance mode to get great body force. According to the equation, the appropriate frequency of the droplets (volume of 10 mL) is B78 Hz, which is consistent with our results above. As the droplet moves along the direction of the wettable
Fig. 4 The movement behaviors of droplets on a graphite plate with a gradient of adhesion under different vibration frequencies: (a) 25 Hz, (b) 50 Hz, (c) 75 Hz, (d) 100 Hz, and (e) 125 Hz. (f) The force analysis on the droplets at different vibration directions (VD). With an increasing vibration frequency, the movement distance increases greatly at the initial stage, after reaching a maximum value at 75 Hz, then decreases obviously. By controlling the vibration frequency, we could adjust the value of FB to make the droplet only move along the direction of the wettable gradient. Therefore, with the help of low vibration amplitudes, we could realize the transfer of droplets from one point to another by vibration frequency easily. The scale bar is 5 mm and the volume of the water droplet is 10 mL (T: top area and B: bottom area of the graphite plate during the perpendicularly anodic oxidation process, FH: hysteresis force, FW: wettable gradient force, FB: body force due to the vibration, FAL: the net driving force along the direction of the wettable gradient during the acceleration phase, FAG: the net driving force against the direction of the wettable gradient during the deceleration phase).
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gradient (also the direction of the adhesion gradient), FH increases gradually. As FB + FW r FH, the droplet begins to decelerate and cease motion ultimately. Therefore, with the help of low vibration amplitudes, we could adjust the value of FB via the vibration frequency to realize the transfer of droplets from one point to another easily. As for adhesion, it is well known that the distinct contact mode (Wenzel state, Cassie state) is a key factor. In general, the rough microstructure would cause the intrusion pressure (DP) to prevent the liquid spontaneous transition from Cassie to Wenzel state, which could be described as the following equation:2,4 DP = 2g/R = cg(cos ya)/A, where g is the interface tension, R is the radius of meniscus, c is the circumference of the pore, A is the cross-sectional area of the pore, and ya is the advancing CA. In our experiment, due to large pores on the surface of the original graphite plate, DP is small and the water droplet could fully penetrate the pores to form a continuous (three-phase contact line) TCL, exhibiting a Wenzel state. Thus, the surface possesses high adhesion that could pin the water droplet to the surface without any movement. After AO, the surface becomes hydrophilic and the water droplet could easily penetrate the pores (DP becomes smaller, even negative), still exhibiting a Wenzel state and high adhesion. After AOP, from the top areas to the bottom ones, the radius of the pore (R) becomes small (Fig. S1 and S2, ESI†) and the value of ya changes a little (Fig. S5, ESI†), which could induce the increase in penetration pressure.34,35 Finally, the penetration of the water droplet is limited and a water droplet is suspended by the gas layers trapped at the micro-scales, exhibiting a Cassie state. The water droplet only contacts the peak of the hierarchical surface, leading an extremely discontinuous TCL. The adhesion of the surface is extremely low and the water droplet easily rolls off with the surface slightly tilted or vibrating. Therefore, after AOP, the contact state can be tuned between the Wenzel state, with high adhesion, to the Cassie state, with low adhesion, via adjusting the microstructures and the chemical composition, showing an adhesion gradient from ultrahigh to ultralow (Scheme 1b). We could adjust the gravity (tilted angle) or body force (vibrating frequency) to control the sliding behaviors of the droplet along the direction from B to T and realize the transmission of the droplet from point to point. In summary, a graphite plate surface with a gradient of adhesion from ultrahigh to ultralow and large anisotropic adhesion was prepared by a simply improved anodic oxidation progress as well as a plasma treatment. The present approach could provide a new strategy to prepare surfaces with continual tunable water adhesion and controlled transmission of a droplet from point to point. Such a surface has promising application in dynamic water-repellency and no loss microdroplet transportation.36–38 This work was supported by the National Key Basic Research Program of China (2013CB933001), the National Natural Science Foundation of China (21234001, 51203006, 21204002,
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21004002, and 21473007), and the Doctoral Fund of Ministry of Education of China (20121102110035).
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