4378
Langmuir 2007, 23, 4378-4382
Environmental Scanning Electron Microscopy Study of the Fine Structure of the Triple Line and Cassie-Wenzel Wetting Transition for Sessile Drops Deposited on Rough Polymer Substrates Edward Bormashenko,* Yelena Bormashenko, Tamir Stein, Gene Whyman, and Roman Pogreb The College of Judea and Samaria, The Research Institute, 44837 Ariel, Israel
Zahava Barkay Wolfson Applied Materials Research Center, Tel AViV UniVersity, Ramat-AViV 69978, Israel ReceiVed NoVember 30, 2006. In Final Form: February 8, 2007 The wetting of rough honeycomb micrometrically scaled polymer substrates was studied. A very strong dependence of the apparent contact angle on the drop volume has been established experimentally. The environmental scanning electron microscopy study of the fine structure of the triple line is reported first. The triple line is not smooth and prefers grasping the polymer matrix over air holes. The precursor rim surrounding the drop has been observed. The revealed dependence of the apparent contact angle on the drop volume is explained by the transition between the pure Cassie and combined Wenzel-Cassie wetting regimes, which is induced by capillarity penetration of water into the holes of relief.
1. Introduction The diversity of physical phenomena occurring in sessile drops brought them to the attention of both physicists and engineers. The nontrivial evaporation-induced hydrodynamics responsible for the formation of a “coffee-stain” deposit in the sessile solution drops has been studied recently by Deegan and Witten et al.1-6 Pinning of the contact line brings into existence various capillary effects including the well-known hysteresis of the contact angle.7-13 Understanding the effects occurring in sessile drops is important for various technological applications including inkjet printing14 and microlens manufacturing.15-16 The authors have demonstrated recently that sessile drops deposited on polymer honeycomb templates could be used for the template-assisted crystallization process.17 The same drops could be applied as microreactors allowing the template-assisted “chemical garden” reaction.18 At the same time a lot of peculiarities of the sessile drop behavior remain unclear. One of the most debated topics in the * To whom correspondence should be addressed. E-mail: Edward@ yosh.ac.il. (1) Deegan, R. D. Phys. ReV. E 2000, 61 (1), 475-485. (2) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten Th. A. Phys. ReV. E 2000, 62 (1), 756-765. (3) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, Th. A. Nature 1997, 389, 827-829. (4) Petsi, A. J.; Burganos, V. N. Phys. ReV. E 2005, 72, 047301-1-047301-4. (5) Kajiya, T.; Nishitani, E.; Yamaue, T.; Doi, M. Phys. ReV. E 2006, 73, 011601-1-011601-4. (6) Zheng, R.; Popov, Yu. O.; Witten, Th. A. Phys. ReV. E 2005, 72, 0463031-046303-15. (7) de Gennes, P. G.; Brochard-Wyart, F.; Que´re´, D. Capillarity and Wetting Phenomena; Springer: Berlin, 2003. (8) Gao, L.; McCarthy, Th. J. Langmuir 2006, 22, 2966-2967. (9) Gao, L.; McCarthy, Th. J. Langmuir 2006, 22, 6234-6237. (10) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457-460. (11) Bico, J.; Thiele, U.; Que´re´, D. Colloids Surf., A 2002, 206, 41-46. (12) Marmur, A. Langmuir 2003, 19, 8343-8348. (13) Marmur, A. Soft Matter 2006, 2, 12-17. (14) Socol, Y.; Guzman, I. S. J. Phys. Chem. B 2006, 110 (37), 18347-18350. (15) Karabasheva, S.; Baluschev S.; Graf, K. Appl. Phys. Lett. 2006, 89, 0311101-031110-3. (16) Bonaccurso, E.; Butt, H-J.; Graf, K. Eur. Polym. J. 2004, 40, 975-980.
field is the fine structure of the triple line.7,12,13 Marmur in his recent extended theoretical research demonstrated that actually there exists a distribution of local (intrinsic) contact angles at the rough surface.19 Moreover, the well-known Wenzel and CassieBaxter equations are approximations which become better when the drop size becomes larger with respect to the surface roughness.13 Thus, only the large drops follow the predictions of the Wenzel and Cassie-Baxter models. The lack of reproducible experimental data in the field has to be emphasized. Until now the only tool allowing the experimental study of the fine structure of the triple line was atomic force microscopy20,21 with its inherent characteristic of tip-sample convolution. Environmental scanning electron microscopy (ESEM) in the wet mode is shown here to allow the close inspection of the triple line in the real time regime. In our present study we performed an ESEM investigation of the fine structure of the triple line for sessile water drops deposited on polymer micrometrically scaled honeycomb reliefs. This investigation supplied somewhat surprising results calling for additional theoretical and experimental insights. 2. Experimental Section Preparation of polystyrene micrometrically scaled honeycomb templates was carried out with a fast dip-coating method described in great detail in our previous papers.22,23 Polystyrene (PS) (5 wt %) was dissolved in a mixture of chloroform (CHCl3; 7.6 wt %) and dichloromethane (CH2Cl2; 87.4 wt %). Thoroughly cleaned polypro(17) Bormashenko, E.; Bormashenko, Ye.; Stanevsky, O.; Pogreb, R.; Whyman, G.; Stein, T.; Itzhaq, M. H.; Barkay, Z. Colloids Surf., A 2006, 290, 273-279. (18) Bormashenko, E.; Bormashenko, Ye.; Stanevsky, O.; Pogreb, R.; Whyman, G.; Stein, T.; Barkay, Z. Colloids Surf., A 2006, 289, 245-249. (19) Brandon, S.; Haimovich, N.; Yeger, E.; Marmur, A. J. Colloid Interface Sci. 2003, 263, 237-243. (20) Wang, R.; Cong, L.; Kido, M. Appl. Surf. Sci. 2002, 191, 74-84. (21) Wang, R.; Takeda, M.; Kido, M. Mater. Lett. 2002, 54, 140-144. (22) Bormashenko, E.; Pogreb, R.; Stanevsky, O.; Bormashenko, Ye.; Socol, Y.; Gendelman, O. Polym. AdV. Technol. 2005, 16, 299-304. (23) Bormashenko, E.; Pogreb, R.; Stanevsky, O.; Bormashenko, Ye.; Stein, T.; Cohen, R.; Nunberg, M.; Gaisin, V.-Z.; Gorelik, M.; Gendelman, O. Mater. Lett. 2005, 59, 2461-2464.
10.1021/la0634802 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007
Sessile Drops Deposited on Rough Polymer Substrates
Figure 1. SEM image of the honeycomb PS surface used for the wetting experiments. pylene (PP) substrates were pulled with a high speed of V ) 20-50 cm/min from the polymer solution and dried immediately with infrared lamps at a temperature of 80 °C. Thus, 2D PS honeycomb structures (such as depicted in Figure 1) were formed. Droplets of distilled water of 0.5-5 µL volume were deposited on the PS relief carefully with a precise microdosing syringe. The apparent contact angle (APCA in terms of abbreviations introduced by Marmur13) was measured with a lab-made goniometer. The ESEM study of the fine structure of the triple line was carried out with a Quanta 200 FEG (field emission gun) ESEM microscope. A PP substrate coated with a honeycomb PS relief (such as depicted in Figure 1) was fixed on the Peltier stage held at a temperature of 2 °C. A water droplet was deposited carefully with a precise microdispenser (microsyringe) on top of the sample surface prior to the pump-down process and stabilized to 2 °C. After the pumpdown process, the pressure in the sample chamber was stabilized to just about the dew point. In addition, prior to the pump-down process a few droplets of water were added to regions around the stage held at room temperature. This thus minimized the possibility of fluctuations or evaporation of the water droplet from the cooled honeycomb region during the pump-down process. The relative humidity in the sample chamber was controlled by slowly varying the pressure in small increments of 0.1 Torr while imaging the sample. The droplet-sample boundary was imaged with a GSED (gaseous secondary electron detector) in the ESEM wet mode. Imaging at 2 °C and 5.4 Torr guaranteed observation under the water vapor equilibrium state with minimum vapor condensation.
3. Results and Discussion 3.1. Macroscopic Parameters of Wetting. First, let us discuss the macroscopic behavior of the droplet deposited on the honeycomb relief. The APCA demonstrated a very strong dependence on the droplet volume illustrated in Figure 2. The contact angle varied in a broad range from 64° to 105° (when APCA approaches 100°, the dependence becomes saturated). The obvious tendency of APCA to grow with the droplet volume could be recognized. Acute APCAs are inherent for small droplets and obtuse angles are observed for large ones, which can be recognized distinctly from Figure 3. This situation has been discussed recently by Marmur,13 and the observed tendency coincides with the experimental findings reported by other investigators. However, peculiarities of this dependence call for further insights. Indeed, the local contact angle established for PS experimentally17 is 76 ( 2°, and the calculated value24 is within 74-86°. Thus, it seems that large drops adhere to the Cassie-Baxter model (the APCA for large drops is obtuse), whereas small droplets behave according to the Wenzel wetting hypothesis, where the increase of the surface underlying the liquid amplifies the wetting and the APCA becomes (24) van Krevelen D. W. Properties of polymers; Elsevier: Amsterdam, 1997.
Langmuir, Vol. 23, No. 8, 2007 4379
smaller than the local angle. We will discuss this Cassie-Wenzel transition in more detail below. 3.2. Fine Structure of the Triple Line. The fine structure of the triple line studied with the ESEM technique is illustrated with Figure 4. We present the ESEM images obtained with small drops (the drop volume is 1 µL), demonstrating an acute APCA. ESEM imaging of the triple line inherent for large drops requires high sample tilting due to the droplet “shadowing” effect caused by the obtuse APCA. Two peculiarities of the triple line fine structure engaged our attention primarily. First, it could be seen that the triple line is far from being even. It is ulcerated by protrusions with a length of 1-3 µm. The liquid “prefers” polymer over air holes displayed by black spots in Figure 4. The polymer matrix is inhomogeneous and includes imperforated polymer islands with an area of several square micrometers. The ESEM image gives the impression that the triple line of the drop grasps at polymer islands (see Figure 4B). Such behavior of the triple line on the chemically heterogeneous substrates has been discussed theoretically by Marmur et al.19 The second important peculiarity of the triple line is a thin precursor water film of 1-5 µm width formed in the nearest vicinity of the drop seen as the shadowed rim in Figure 4. A similar precursor rim was observed recently by Zhao et al. when water droplets were deposited on nanorod arrays.25,26 The formation of the precursor rim was related in these experiments to a capillary flow described by the classical Washburn model.25 The experimental situation discussed in refs 25 and 26 is quite different from our case because of the large difference in local contact angles (in refs 25 and 26 the local contact angle is small). However, in both cases the formation of the precursor rim is due to the surface roughness but not to long-range forces.7 It has to be emphasized that the ESEM image does not supply unambiguous evidence of water penetration in the holes; this problem will be discussed in detail below. 3.3. Discussion. It is reasonable to start our discussion from some of the scaling features of the experimental situation. The characteristic dimension of the drop, l1 ≈ 1 mm, is much larger than the characteristic dimension of the details of the relief (holes), l2 ≈ 1 µm: l1 . l2. This justifies the use of Cassie-Baxter and Wenzel approaches based on the surface averaging even for “small” drops.19 On the other hand, the capillarity length, λ, defined as λ ) (γ/Fg)1/2, where γ and F are the surface tension and density of water,7 could be estimated in our case as λ ≈ 2.5 mm. Thus, it could be concluded that λ . l2 and gravity could not be responsible for the wetting of the holes, which is crucial for APCA formation. We have demonstrated in our recent papers that the peculiarities of the surface topography of the honeycomb polymer relief do not promote water penetration into the holes, and actually air pockets are formed (the Cassie state is realized).17,27 The close ESEM inspection of the triple line illustrated in Figure 5 supplied additional evidence to this fact. Indeed at least partially empty air holes of the template could be seen through the thin water precursor film. Regrettably, the ESEM study supplied this kind of information in areas where the water film is extremely thin (see Figure 5), and the real situation along the triple line is not completely clear. In any case, the precursor film surface possesses zero curvature, which may explain the observed air trapping under it: the Laplace pressure promoting water penetration is absent. (25) Fan J.-G.; Zhao Y.-P. Langmuir 2006, 22, 3662-3671. (26) Fan J.-G.; Dyer, D.; Zhang, G.; Zhao Y.-P. Nano Lett. 2004, 4 (11), 2133-2138. (27) Bormashenko, E.; Bormashenko Ye.; Whyman, G.; Pogreb, R.; Stanevsky O. J. Colloid Interface Sci. 2006, 302, 308-311.
4380 Langmuir, Vol. 23, No. 8, 2007
Bormashenko et al.
Figure 2. Dependence of the APCA on the droplet volume.
Figure 3. Acute (A) and obtuse (B) APCAs formed by drops of different volumes (0.5 µL, image A; 5 µL, image B) on the same rough interface depicted in Figure 1.
The maximal APCA θ was calculated according to the Cassie-Baxter formula7
cos θ ) -1 + f(cos θE + 1)
Figure 4. ESEM images of the triple line, drop volume 1 µL, inclination of the substrate -15°.
(1)
where θE is the local (Young) contact angle and f is the fraction of the liquid-polymer interface in the underlying substrate surface. The computer treatment of the SEM image, such as presented in Figure 1, gives 0.5 < f < 0.55, which corresponds, according to eq 1, to θ ) 107-118° for 74° < θE e 86°. The measured value17 θE ) 76° gives the calculated value θ ) 108°, in good correspondence with the experimental value of the
saturated APCA, 105°, which strongly supports the idea that water does not penetrate into the holes when the drops are large. Thus, acute angles observed for small drops call for an explanation. We propose a simple mechanism explaining the Cassie-Wenzel transition when the drop becomes small. Indeed, when a drop turns small, the Laplace pressure under the droplet surface grows according to P ≈ 2γ/r, where r is the radius of the drop and γ is the water surface tension, which is ∼70 mJ/m2.
Sessile Drops Deposited on Rough Polymer Substrates
Langmuir, Vol. 23, No. 8, 2007 4381
Figure 7. Different wetting states for water droplets of various volumes on the patterned polymer substrate (see eq 2).
Figure 5. Detailed ESEM image of the precursor (no substrate inclination, drop volume 1 µL). Air holes (black spots) are seen through the thin water film.
Figure 8. Scheme presenting evaporation of a water droplet deposited on rough polymer surfaces: θ1, θ2, advancing and receding angles, respectively, observed at the initial stage of the evaporation; θ3, acute APCA inherent for the final stage of evaporation.
Liu and Lange; however, their extended theoretical research has not been accompanied by experimental data.29 When the Laplace-pressure-induced wetting transition occurs, the water intrusions in the polymer microstructure (Figure 6) promote the inhomogeneous wetting regime treated recently by Marmur12 (see also ref 30), in which APCA is given by Figure 6. Scheme illustrating the wetting regime transition. Laplacepressure-induced water intrusions into the holes are shown.
cos θ ) -1 + f + rf f cos θE
It can be verified that for a radius r ≈ 1 mm (Figure 2, V ≈ 2 µL, θ ≈ 80°) corresponding to the transition between the Cassie and Wenzel states the Laplace pressure is 140 Pa. Lafuma and Que´re´ have stressed that the Laplace pressure is exerted on a substrate.10 In addition, they applied an external pressure on the drop, and it was demonstrated in their experimental research performed on the rough unwetted polymer substrates that when an external pressure of 100-200 Pa is imposed on the drop, the Cassie-Wenzel transition occurs.10 Such small pressures sufficient for a wetting transition are not surprising; there is much experimental evidence that even the weight of the droplet can impose an influence on the wetting regime of the droplet (of course, this is true for drops sufficiently larger than those discussed in our paper).28 Lafuma and Que´re´ did not observe capillarityinduced Cassie-Wenzel transition and resorted to external pressure for a simple reason: the fluorinated polymers used in their research are intrinsically hydrophobic (θE ) 110°), whereas our polystyrene reliefs are characterized by a smaller Young angle; hence, lower pressures are necessary for water intrusion into the interface texture. The pressure-induced transition between different wetting regimes was also studied recently in detail by
where rf is the roughness of the wet area. Figure 7 presents the scheme of transition between the pure Cassie-Baxter and the Wenzel regimes through the inhomogeneous wetting states with partial penetration of water into polymer caves. For large drops, if f equals the structure factor 0.55 (all holes filled with air), it follows from eq 2 that r ≈ 1 for experimental values θ ) 105° and θE ) 76°. In small drops water penetrates substrate holes, but a limited number of them remain filled with air (see Figure 5). If we arbitrarily assign f ) 0.9, then it follows from eq 2 that the r value changes to 2.5 for the experimental APCA θ ) 64°. It should be noted that for the pattern used the situation is quite complicated because holes forming the honeycomb topography of the substrate are far from being cylindrical or of other regular form.27 Thus, the straightforward calculation of rf is hindered, and eq 2 may be exploited in this case as a reliable estimation of APCA, with f and rf being semiempirical parameters. Equation 2 has been exploited by Marmur12 for the description of equilibrium between the Wenzel and Cassie regimes on hydrophobic rough surfaces. Note that the roughness of the wetted surface, rf, in eq 2 depends on the fraction, f, of the wetted surface itself. Real values of f and rf are determined by the condition of the free energy minimum,12 and some solutions
(28) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818-5822.
(2)
(29) Liu, B.; Lange, F. F. J. Colloid Interface Sci. 2006, 298 (2), 899-909. (30) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Langmuir 2006, 22, 1640-1645.
4382 Langmuir, Vol. 23, No. 8, 2007
Figure 9. NaCl crystals located in the holes of the polymer template, resulting from the evaporation of the NaCl water solution drop.
obtained in ref 12 correspond to APCA θ ) π. However, the equilibrium conditions from ref 12 can also be satisfied for APCA θ ) 0, which is more natural for our intrinsically wetted material (polystyrene) with a local (Young) angle lower than 90°. Therefore, we can suppose that the mentioned equilibrium also takes place in our case of formation of a thin precursor film with APCA close to zero. We suggest that the mechanism of the capillarity-induced wetting transition explains a broad range of experimentally observed phenomena including very different APCAs inherent for evaporated drops deposited on rough surfaces.27,30,31 In particular, obtuse advancing, θ1, and receding, θ2, angles inherent for the sessile drops at the initial stage of evaporation were changed to acute APCA θ3 at the final stage (see Figure 8). Indeed, when the drop evaporates, it becomes finally small enough for the Laplace-pressure-induced Cassie-Wenzel transition to occur, (31) Bormashenko, E.; Stein, T.; Whyman, G.; Bormashenko, Ye.; Pogreb, R. Langmuir 2006, 22, 9982-9985.
Bormashenko et al.
as is described in our paper. The formation of the precursor rim displayed in Figure 4 also becomes clear; it could be related to capillarity suction inherent for small drops (see also refs 25 and 26). Finally, the phenomenon of the template-assisted growth of crystals on the honeycomb templates reported recently by our group17 receives a natural explanation. Let the 1-3 wt % NaCl water solution drop be evaporated on the surface depicted in Figure 1. The process results in the appearance of NaCl crystals in the holes of the template, such as depicted in Figure 9. This can occur only if the solution penetrates into the holes of the polymer template. We suggest that the capillarity suction induced by the Laplace pressure at the final stage of the evaporation explains the phenomenon. It has to be emphasized that the CassieWenzel transition occurs in our case with no external pressure, in contrast to the situations already discussed.10,29
Conclusions The fine structure of the triple line was studied with ESEM for drops deposited on rough polymer substrates. The triple line is found to meander with the tendency to be hooked by the polymer template. The water precursor rim has been observed in the vicinity of the drop edge. A very strong dependence of the apparent contact angle on the drop size has been observed. For an explanation of this phenomenon we suggested that the Cassie-Wenzel wetting transition occurs for small droplets and also for intrinsically wetted substrates. This transition is due to the Laplace pressure forcing the liquid to partially penetrate the pores when the drop is sufficiently small. The mechanism of the capillarity-suction-induced wetting transition explains a broad range of wetting phenomena observed on rough polymer substrates. LA0634802
