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J. Phys. Chem. C 2009, 113, 5568–5572
“Petal Effect” on Surfaces Based on Lycopodium: High-Stick Surfaces Demonstrating High Apparent Contact Angles Edward Bormashenko,*,† Tamir Stein,†,‡ Roman Pogreb,† and Doron Aurbach‡ The Research Institute, Applied Physics Faculty, and Department of Chemistry and Biotechnology Engineering, Ariel UniVersity Center of Samaria, 40700 Ariel, and Department of Chemistry, Bar-Ilan UniVersity, 52900 Ramat-Gan, Israel ReceiVed: January 20, 2009; ReVised Manuscript ReceiVed: February 9, 2009
Wetting properties of lycopodium-based surfaces are reported. The surfaces demonstrate high apparent contact angles accompanied by high adhesion of water droplets, similar to the recently reported biological interfaces (rosea Rehd) showing the “petal effect”. Apparent contact angles depended strongly on the droplet volume. Vertical vibration of droplets deposited on the lycopodium-based reliefs promoted wetting transition. Semiquantitative analysis of the observed wetting transition taking into account the hierarchical topography of the relief is presented. We relate the high adhesion of droplets to the partial filling of pores constituting the relief. 1. Introduction In spite of the fact that the theoretical foundations of the wetting of rough surfaces have been developed by Cassie and Wenzel 50 years ago, the problem remains attractive to investigators.1-26 According to the Cassie model, air can remain trapped below the droplet, forming “air pockets”, as depicted in Figure 1A. Thus, hydrophobicity is strengthened because the droplet sits partially on the air.2 On the other hand, according to the Wenzel model, the roughness increases the surface area of the solid, which also geometrically modifies hydrophobicity, as shown in Figure 1B.3 It was also demonstrated that at least one more wetting regime exists on the textured surfaces, namely, the Cassie impregnating wetting regime when a liquid film impregnates a texture ahead of the droplet (pores are filled with liquid, and solid “islands” ahead of the droplet are dry, as depicted in Figure 1C).1,14 It was also demonstrated that the Cassie impregnating wetting state corresponded to the lowest energetic state of a droplet and the Cassie air trapping state corresponded to the highest one.14 Multiple minima of the Gibbs free energy corresponding to observable wetting states of liquid droplets deposited on rough surfaces were discussed recently.12,14 It is also agreed that the Wenzel wetting regime is featured by strong pinning of the triple line, resulting in a strong adhesion of a droplet and high contact angle hysteresis.17 An interest in the wetting of rough surfaces has been strengthened by the discovery of unusual self-cleaning properties of lotus leaves.24 Lotus leaves demonstrate the apparent contact angle close to 180° promoting easy sliding of water droplets. It was shown lately that high apparent contact angles not necessarily imply low sliding angles. Jiang et al. reported that the petals of red rose (rosea Rehd) are featured by high contact angles together with high adhesion.27 Jiang coined the term “petal effect” describing coexistence of high contact angles with a strong adhesion to the substrate.27 We report that surfaces based on lycopodium particles also demonstrate the pronounced petal effect. * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. † Ariel University Center of Samaria. ‡ Bar-Ilan University.
Figure 1. Cassie air trapping (A) and Wenzel (B) and Cassie impregnating (C) wetting regimes.
2. Experimental Section 2.1. Preparation of Surfaces. Surfaces were prepared according to the method described in ref 13. Lycopodium particles supplied by Fluka were spread on the extruded polyethylene film and hot pressed with a crimped template. The press temperature was 95 °C, and the process duration was 1 h. Softened polyethylene trapped particles of lycopodium. Then the sample was left inside the press for cooling and detached from the crimped template. 2.2. Preparation of Water Marbles. Water marbles were prepared with use of superhydrophobic surfaces produced with a process described in ref 28. Droplets of 10 µL were deposited with a precise microdosing syringe onto the superhydrophobic surface covered with a layer of lycopodium particles. Slight tilting of the superhydrophobic surface caused rolling of the droplet and coating it with lycopodium. Afterward, marbles were rolled to the lycopodium-based surfaces under investigation. 2.3. Scanning Electron Microscopy and Environmental Scanning Electron Microscopy Study of the Surfaces. Prepared surfaces were studied with scanning electron microscopy
10.1021/jp900594k CCC: $40.75 2009 American Chemical Society Published on Web 03/18/2009
“Petal Effect” on Surfaces Based on Lycopodium
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(SEM) performed with Inspect S (FEI Co.). Single particles of lycopodium were investigated with environmental scanning electron microscopy (ESEM). ESEM imaging was performed with a Quanta 200 FEG (field emission gun) device. The particles were imaged with a GSED (gaseous secondary electron detector) in the ESEM wet mode under a pressure of 5.4 Torr and temperature of 2 °C. 2.4. Vibration of the Droplets. Droplets of distilled water of 5-50 µL volume were carefully deposited onto the studied lycopodium-based relief with a precise microdosing syringe. The substrate with the droplet was bound up with the moving part of the vibration generator, producing vertical vibrations.14 The horizontal laser beam illuminated all of the drop profile and gave its enlarged image on the screen using a system of lenses. The frequency was chosen far from the resonance region for the drop volumes used; namely, the frequency was 36 Hz. The amplitude was changed from 50 to 600 µm. The drop was exposed to a vertical vibration of increased amplitude at constant frequency until the wetting transition took place, and the amplitude corresponding to this transition was fixed. Vibration of the droplets was visualized with a rapid digital camera Casio EX-FH20 at a speed of 1000 fps. 3. Results and Discussion 3.1. Structure and Wetting Properties of Surfaces Based on Lycopodium. Hydrophobic properties of lycopodium particles has attracted the attention of investigators in the past few years.29-32 Aussillous, Que´re´, and McHale used lycopodium for producing nonstick “liquid marbles”.29-31 Surfaces prepared according to the method described in section 2.1 and particles of lycopodium are depicted in Figure 2. The hierarchical structure of the surface merits attention. It can be recognized from the SEM image that the lycopodium particles were spread uniformly, the particles’ sizes were quite similar, about 30 µm in diameter, and the minimal pore size was 5 µm (see Figure 2B). Surfaces depicted in Figure 2A demonstrated apparent contact angles (APCAs) as high as 144°, as shown in Figure 3. At the same time, the droplets deposited on the lycopodium-based surfaces are featured by high adhesion and high hysteresis of the apparent contact angle. Water droplets on the studied surface kept the spherical shape even when the surface was turned upside down, as depicted in Figure 4. This observation resembles the petal effect reported not long ago by Jiang et al.27 It is reasonable to relate the high adhesion of the droplets to the Wenzel wetting regime occurring on the lycopodium. However, we will show that actually the situation is more complicated. We have reported that rough surfaces demonstrate the strong dependence of the APCA on the droplet volume.33 It was instructive to establish this dependence experimentally for lycopodiumbased surfaces. The dependence presented in Figure 5 shows that the APCA grows with the droplet volume up to a certain saturation value of 144°. It is noteworthy that saturation occurs at a volume of 25 µL (this value is of importance for our future discussion). Moreover, droplets with a volume V > 25 µL began to slide on the tilted lycopodium surfaces. It is noteworthy that the sliding angles R remain relatively large (R ) 60° for V ) 30 µL). The dependence of the APCA on the drop volume, which is significant for defining the wetting properties of a rough substrate, was studied recently by several groups.10,33,34 The dependence can be explained if the mixed heterogeneous nature of wetting the relief under discussion is considered. The mixed
Figure 2. (A) SEM image of the lycopodium-based substrate. (B) ESEM image of the lycopodium particle before pressing.
Figure 3. Water droplet (40 µL) deposited on the lycopodium-based surface.
heterogeneous wetting illustrated with Figure 6 has been treated in much detail by Marmur.10 According to the Marmur model, the droplet sits partially on air pockets and partially on lycopodium particles. The apparent contact angle θ is predicted by the following equation:
5570 J. Phys. Chem. C, Vol. 113, No. 14, 2009
cos θ ) rff cos θY + f - 1
Bormashenko et al.
(1)
In this equation θY is the equilibrium (Young) contact angle, f is the fraction of the projected area of the solid surface that is wet by the liquid, and rf is the roughness ratio of the wet area.10 It should be mentioned that the contact radius of a droplet (∼1
Figure 6. Scheme of the wetting regime occurring on the lycopodiumbased surfaces (the scheme ignores the process occurring on the micrometric scale).
Figure 4. Water droplets (10 µL) attached to vertical (A) and horizontal (B) lycopodium-based surfaces.
Figure 5. Dependence of the apparent contact angle on the droplet volume.
mm) is much larger than the average diameter of a lycopodium particle (∼30 µm). This scaling argument justifies the use of eq 1. The Young contact angle has to be established on the atomically flat surfaces; this is obviously impossible for lycopodium. However, certain semiqualitative analysis allowing explanation of the distinct dependence of the APCA on the droplet volume is possible. Indeed, the pressure exerted by the droplet on the substrate can be estimated very roughly as P(r) ≈ 2γ/r + 2Fgr; γ is the surface tension at the water/vapor interface, F is the density of the droplet, r is the characteristic dimension of the droplet, if the deformation of the droplet due to gravity is neglected, and r is the droplet’s radius. It can be readily seen that P(r) takes the minimum, when r ) r* ) (γ/ Fg)1/2 ) lca ) 2.7 mm, where lca is the well-known capillary length.1 The maximal radius of droplets used in our experiments corresponding to V ) 50 µL was 2.3 mm. Thus, we conclude that the pressure exerted on the lycopodium-based substrate grows when the drop becomes smaller. The growth of the pressure leads to deeper penetration of the water into the grooves of the relief depicted in Figure 6. As a consequence, parameters rf and f in eq 1 are varied and the apparent contact angle θ changes in turn.10,33,35 It was also instructive to study the mobility of liquid marbles deposited on the lycopodium-based surfaces. A liquid marble is a nonstick droplet wrapped with a hydrophobic powder. Such nonstick droplets have attracted the attention of investigators lately.28-31,36 A typical lycopodium-based marble deposited on the lycopodium-based surface is depicted in Figure 7. In contrast to water drops, the marbles demonstrated extremely low adhesion to the lycopodium-based surface and rolled when the surfaces were tilted at an inclination as small as 5.5 ( 1°. This observation validates the hypothesis put forward in refs 28-31, supposing that liquid confined in a marble is separated from the substrate by an air layer. 3.2. Vibration of Droplets and Nature of Wetting Transitions. We have reported already that vibration of droplets can promote wetting transitions, i.e., changes in the apparent contact angle.14,37-39 Wetting transitions were subjected to intensive theoretical and experimental research recently.17,34,35,37-39 Vibrations of droplets on lycopodium surfaces were carried out at 36 Hz frequency with an increasing amplitude up to the wetting transition, when the apparent contact angle changed abruptly as depicted in Figure 8. Visualization of vibrations with a high-
“Petal Effect” on Surfaces Based on Lycopodium
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Figure 9. Dependence of the pressure corresponding to the wetting transition on the droplet volume.
Figure 7. Lycopodium-coated water marble (50 µL) deposited on the lycopodium-based surface.
Figure 10. Dependence of the force per unit length acting on the triple line corresponding to the wetting transition on the droplet volume.
Figure 8. Photograph of a 40 µL droplet after the change of the wetting regime stipulated by vibrations.
speed camera shows that the contact (i.e., triple) line is pinned to a substrate up to the threshold amplitude corresponding to the wetting transition. Vibration-based experiments supplied valuable quantitative information characterizing the wetting transition, i.e., the critical pressure and force acting on a unit length of the triple line corresponding to the wetting transition. The pressure p in the droplet calculated in refs 14 and 38 is given by the following formula:
p ) pi +
2γ + Fgl R(θ)
(2) F)
where pi is the pressure increase coming from oscillation (see ref 18), l is the droplet height, and R is the radius of the droplet; the dependence R(θ) was established experimentally (see Figure 5; the radius is expressed in terms of the volume by a wellknown formula, supplied in ref 38). The pressure pi is calculated according to
pi )
FVAω2 π(R(θ))2 sin2 θ
to P* ≈ 70 Pa for V > 25 µL. This observation gives additional evidence to the change in the wetting regime occurring at V ) 25 µL already discussed in the previous section. We supposed in the previous section that up to V ) 25 µL the pores of the relief are partially filled. It is reasonable to suggest that the further penetration of the water needs larger pressures when compared with those necessary for filling initially empty pores. The hierarchical structure of the relief has also to be considered, and it stands to reason that filling of pores related to different scales of the relief calls for different pressures. It is appropriate to suggest that at P* ≈ 140 Pa small micrometrically scaled pores (see Figure 2) are filled. This suggestion is supported by additional experimental observations, giving evidence that pressures of 100-150 Pa are sufficient for filling micrometric pores.17,37 The force acting on the unit length of the triple line calculated in ref 38 is given by the following formula:
(3)
where A and ω are the amplitude and frequency of vibration.37,38 The dependence of the pressure p corresponding to the wetting transition on the droplet volume V is depicted in Figure 9. It can be recognized that the wetting transition occurs at a constant pressure until a volume of 25 µL, when the pressure necessary for transition falls abruptly, from P* ≈ 140 Pa for V < 25 µL
pR (2θ - sin 2θ) 4 sin θ
(4)
The dependence of the critical force per unit length of the triple line corresponding to the wetting transition on the droplet volume is presented in Figure 10. It is clearly seen that the force grows up to a volume of 25 µL, and afterward comes saturation. It has to be stressed that the nature of the wetting transition on lycopodium surfaces is quite different from already reported wetting transitions observed on a broad range of textured surfaces, when the transition occurred under the constant value of the resulting force acting on the unit length of the triple line.14,38,39 This discrepancy can be explained if we consider simple scaling arguments and the approach already discussed in the previous paragraph. Indeed, the depinning force F ≈ pR, and it grows with the droplet size up to a volume of 25 µL (see eq 4 and Figure 9). It also can be supposed that at large volumes the small pores are empty and the depinning force comes to saturation (see Figure 10). We already noted that the sliding
5572 J. Phys. Chem. C, Vol. 113, No. 14, 2009 angles kept large values even for large drops (V > 25 µL). We suppose that large drops are retained by the large-scale (∼30 µm) details of the relief, i.e., particles of lycopodium as depicted in Figure 6. The saturation value of the depinning force as high as 1000 mN/m deserves consideration. It is much larger than corresponding values of the depinning force (∼200 mN/m) established on the micrometrically scaled surfaces reported in ref 14. The high adhesion of droplets to lycopodium-based surfaces supplies the natural explanation of this observation. We conclude that under vibrations we observed a wetting transition from the heterogeneous wetting state described by eq 1 to the Wenzel state. It also can be suggested that the final wetting regime could be the Cassie impregnating state, illustrated with Figure 1C and described in much detail in refs 1 and 14. For the clear distinction between Wenzel and Cassie impregnating regimes it is necessary to establish the equilibrium (Young) contact angle.1,14 This experimental task remains unsolved for lycopodium particles. Conclusions Lycopodium-based surfaces demonstrate high apparent contact angles (θ ) 144°). Hand in hand with high apparent contact angles the droplets deposited on the lycopodium-based surfaces demonstrate high adhesion. In contrast, liquid marbles keep high mobility on the lycopodium substrates. The apparent contact angle depended strongly on the droplet volume. Partial filling of the pores explains this dependence and high adhesion of the droplets. Vertical vibration of the droplets promoted a change in the wetting regime. Calculation of the pressure exerted on the substrate and the force acting on a unit length of the triple line explains semiquantitatively the origin of the wetting transition occurring under vibrations. Acknowledgment. We are thankful to Dr. G. Whyman for extremely fruitful discussions, grateful to Mrs. Al. Musin and Mrs. Ye. Bormashenko for their kind help in preparing the manuscript, and indebted to Dr. Z. Barkay for ESEM imaging. This work was supported by the Israel Ministry of Immigrant Absorption. References and Notes (1) de Gennes, P. G.; Brochard-Wyart, F.; Que˙re˙, D. Capillarity and Wetting Phenomena; Springer: Berlin, 2003. (2) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11–16. (3) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (4) Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495–2532. (5) Que˙re˙, D.; Reyssat, M. Philos. Trans. R. Soc. London, A 2008, 366, 1539–1556.
Bormashenko et al. (6) Nosonovsky, M.; Bhushan, B. AdV. Funct. Mater. 2008, 18, 843– 855. (7) Nosonovsky, M.; Bhushan, B. J. Phys.: Condens. Matter 2008, 20, 225009. (8) Nosonovsky, M.; Bhushan, B. J. Phys.: Condens. Matter 2008, 20, 395005. (9) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 2, 224–240. (10) Marmur, A. Langmuir 2003, 19, 8343–8348. (11) Marmur, A. Langmuir 2004, 20, 3517–3519. (12) Marmur, A. Soft Matter 2006, 2, 12–17. (13) Bormashenko, E.; Stein, T.; Whyman, G.; Bormashenko, Ye.; Pogreb, R Langmuir 2006, 22, 9982–9985. (14) Bormashenko, E.; Pogreb, R.; Stein, T.; Whyman, G.; Erlich, M.; Musin, A.; Machavariani, V.; Aurbach, D. Phys. Chem. Chem. Phys. 2008, 27, 4056–4061. (15) Gao, L.; McCarthy, Th. J. Langmuir 2006, 22, 2966–2967. (16) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22, 3125–3130. (17) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457–460. (18) Bico, J.; Thiele, U.; Que´re´, D. Colloids Surf., A 2002, 206, 41–46. (19) Jopp, J.; Gru¨ll, H.; Yerushalmi-Rozen, R. Langmuir 2004, 20, 10015–10019. (20) Thiele, U.; Brusch, L.; Bestehorn, M.; Ba¨r, M. Eur. Phys. J. E 2003, 11, 255–271. (21) Herminghaus, S. Europhys. Lett. 2000, 52, 165–170. (22) Shibuichi, A.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512–19517. (23) He, B.; Lee, J.; Patankar, N. A. Colloids Surf., A 2004, 248, 101– 104. (24) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (25) Patankar, N. A. Langmuir 2004, 20, 7097–7102. (26) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Langmuir 2006, 22, 1640–1645. (27) Feng, L.; Zhang, Ya.; Xi, J.; Zhu, Y.; Wang, N.; Fan Xia, F.; Jiang, L. Langmuir 2008, 24, 4114–4119. (28) Bormashenko, E; Bormashenko, Y.; Musin, A. J. Colloid Interface Sci. [Online early access]. DOI: 10.1016/j.jcis.2008.09.079. (29) Aussillous, P.; Que´re´, D. Proc. R. Soc. London, A 2006, 462, 973– 999. (30) Aussillous, P.; Que´re´, D. Nature 2001, 411, 924–927. (31) McHale, G.; Elliott, S. J.; Newton, M. I.; Herbertson, D. L.; Esmer, K. Langmuir 2009, 25, 529–533. (32) Paunov, V. N.; Mackenzie, G.; Stoyanov, S. D. J. Mater. Chem. 2007, 17, 609–612. (33) Bormashenko, E.; Bormashenko, Ye.; Stein, T.; Whyman, G.; Pogreb, R Langmuir 2007, 23, 4378–4382. (34) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K Langmuir 2002, 18, 5818–5822. (35) Ran, Ch.; Ding, G.; Liu, W.; Deng, Y.; Hou, W. Langmuir 2008, 24, 9952–9955. (36) Mahadevan, L. Nature 2001, 411, 895–896. (37) Bormashenko, E.; Pogreb, R.; Whyman, G.; Bormashenko, Ye.; Erlich, M. Appl. Phys. Lett. 2007, 90, 201917. (38) Bormashenko, E.; Pogreb, R.; Whyman, G.; Bormashenko, Ye.; Erlich, M. Langmuir 2007, 23, 6501–6503. (39) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23, 12217–12221.
JP900594K
