pubs.acs.org/Langmuir © 2009 American Chemical Society
Evaporation Behavior of Microliter- and Sub-nanoliter-Scale Water Droplets on Two Different Fluoroalkylsilane Coatings Tsutomu Furuta,† Munetoshi Sakai,‡ Toshihiro Isobe,† and Akira Nakajima*,†,‡ †
Department of Metallurgy and Ceramic Science Graduate School of Science and Engineering, Tokyo Institute of Technology 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan, and ‡Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan Received August 3, 2009. Revised Manuscript Received September 8, 2009
The evaporation behavior of microliter (2.0 μL) and subnanoliter (0.8 nL) scale water droplets was investigated on two smooth hydrophobic and hydrophilic fluoroalkylsilane coatings prepared using chemical vapor deposition. The contact angle was constant in the second stage of evaporation for a 2.0 μL droplet on the hydrophobic coating, but it was slightly decreased in the case of a 0.8 nL droplet. The contact angle decreased gradually in the same stage of evaporation for a 2.0 μL droplet on the hydrophilic coating, but it was almost constant for a 0.8 nL droplet. These differences in evaporation behavior are expected to originate from the differences of their magnitudes and signs of line tension.
I. Introduction Evaporation is a fundamental phenomenon for liquids. Various researchers have examined this phenomenon for liquid droplets on solid surfaces.1-12 Evaporation of liquid droplets is typically classified into three categories:1,5-9 (i) constant contact area mode (pinning of the three-phase contact line), (ii) constant contact angle mode (the three-phase contact line changes with a nearly constant contact angle), and (iii) mixed mode (the threephase contact line changes with decreasing contact angle). The length and practical features of these modes are affected by surface roughness and chemical homogeneity of the solid surface.11,12 Moreover, these effects become remarkable for small droplets because the shape change by evaporation in the unit period is considerable.13 However, comparisons of evaporation behaviors of subnanoliter-scale water droplets on hydrophobic and hydrophilic coatings14 are scarce in the literature. Especially, comparisons of smooth hydrophobic and hydrophilic coatings have not been well conducted for widely various droplet volumes. For this study, we used two different fluoroalkylsilanes and prepared smooth hydrophobic and hydrophilic coatings. Then, evaporation behavior was *Corresponding author. Tel.: +81-3-5734-2525. Fax: +81-3-5734-3355. E-mail:
[email protected].
(1) Picknett, R. G.; Bexon, R. J. Colloid Interface Sci. 1977, 61, 336. (2) Birdi, K. S.; Vu, D. T.; Winter, A. J. Phys. Chem. 1989, 93, 3702. (3) Rowan, S. M.; Newton, M. I.; McHale, G. J. Phys. Chem. 1995, 99, 13268. (4) Rowan, S. M.; McHale, G.; Newton, M. I.; Toorneman, M. J. Phys. Chem. 1997, 101, 1265. (5) Shanahan, M. E. R.; Bourges, C. Int. J. Adhes. Adhes. 1994, 14, 201. (6) Bourges-Monnier, C.; Shanahan, M. E. R. Langmuir 1995, 11, 2820. (7) Uno, K.; Hayashi, K.; Hayashi, T.; Ito, K.; Kitano, H. Colloid Polym. Sci. 1998, 276, 810. (8) Fukai, J.; Ishizuka, H.; Sakai, Y.; Kaneda, M.; Morita, M.; Takanara, A. Int. J. Heat Mass Transfer 2006, 49, 3561. (9) McHale, G.; Rowan, S. M.; Newton, M. I.; Banerjee, M. K. J. Phys. Chem. B 1998, 102, 1964. (10) Erbil, H. Y.; McHale, G.; Newton, M. I. Langmuir 2002, 18, 2636. (11) Yu, H. Z.; Soolaman, D. M.; Rowe, A. W.; Banks, J. T. ChemPhysChem 2004, 5, 1035. (12) Soolaman, D. M.; Yu, H. Z. J. Phys. Chem. B 2005, 109, 17967. (13) Furuta, T.; Nakajima, A.; Sakai, M.; Isobe, T.; Kameshima, Y.; Okada, K. Langmuir 2009, 25, 5417. (14) Golovko, D. S.; Butt, H.-J.; Bonaccurso, E. Langmuir 2009, 25, 75.
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investigated for both microliter-scale and subnanoliter-scale droplets.
II. Experimental Section We prepared smooth coatings using 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS17, CF3(CF2)7(CH2)2Si(OCH3)3) and trifluoropropyltrimethoxysilane (FAS3, CF3(CH2)2Si(OCH3)3). Detailed sample preparation procedures are described in the Supporting Information. Using atomic force microscopy (AFM, JSPM-4200; JEOL, Tokyo, Japan) with a Si cantilever (NSC36-c; μ-masch, Narva Mtn., Estonia) we evaluated the surface roughness of these coatings in a 5-μm-square area. The static water contact angle (WCA) was measured using the sessile drop method with 3-μL distilled water droplets and a commercial contact angle meter (Dropmaster 500; Kyowa Interface Science Co. Ltd., Saitama, Japan). The sliding angle (SA) was also evaluated for 30-μL water droplets using the same system. We performed each measurement at five different points (WCA) or three points (SA) and then averaged the measurements. The evaporation behavior of sub-nanoliter-scale water droplets was evaluated using an automatic microscopic contact angle meter (MCA-3; Kyowa Interface Science Co. Ltd., Saitama, Japan). A 0.8-nL water droplet (droplet radius, about 60 μm) was placed on these silane coatings. Its evaporation behavior was recorded using a high-speed camera. The contact angle and droplet radius were obtained by analyzing the recorded image. All these evaluations were conducted in ambient air at room temperature (around 25 °C). The relative humidity was approximately 30%. To compare the droplet size dependence, the evaporation behavior of 2.0 μL water droplets (droplet radius, about 800 μm) was also evaluated under identical conditions using the commercial contact angle meter described above (Dropmaster 500).
III. Results and Discussion Figure 1 portrays AFM images of the FAS17 and FAS3 coatings. No agglomerates of particles were observed in these coatings; the surface roughness value (Ra) was less than 0.20 nm for both. The WCA and SA of water droplets were, respectively, 107 ( 1° (FAS17) and 79 ( 1° (FAS3), and 14 ( 1° (FAS17) and 16 ( 2° (FAS3). The chemical homogeneity of these coatings was
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Figure 1. AFM micrographs (5 μm square) of the coating surfaces using (a) FAS17 and (b) FAS3.
confirmed by measuring the surface potential distribution using Kelvin force microscopy15). Although both coatings possess high smoothness and homogeneity, FAS17 was hydrophobic and FAS3 was hydrophilic. For 0.8 nL water droplets, evaporation on each coating was completed in about 8 s, but evaporation of the 2.0 μL water droplets took 20 min. Figure 2 presents the time dependence of the contact angle (Figure 2a) and contact radius (radius of the threephase contact line, Figure 2b) during evaporation. The number of the x-axis for these plots is the time ratio (t/t0), which is the time normalized by the time when the evaporation was completed (t0). The data presented in Figure 2 are not averages of several runs: they are from single measurements. However, we repeated the measurements three times for all samples and droplet volumes, thereby confirming that the results were similar. Regarding 2.0 μL and 0.8 nL water droplets, the three stages of evaporation are visible on both coatings. Arrows in Figure 2 indicate the expected beginning and ending of the second stage during evaporation. The contact angle of 2.0 μL water droplets on the FAS17 coating in the second stage was almost constant. However, that of 0.8 nL water droplets at this stage was decreased slightly. In the case of the FAS3 coating, the contact angle of 2.0 μL water droplets in the second stage decreased gradually by a constant rate. However, that of 0.8 nL water droplets in this stage was almost constant. Because the shape change in a unit period is larger for a 0.8 nL water droplet than a 2.0 μL one, it can be considered that the contact angle approaches its equilibrium receding value over the course of a very short (8 s) measurement time in the case of the FAS17 coating. However, the reverse trend of contact angle change for a 0.8 nL droplet evaporation against a 2.0 μL droplet between the FAS3 and FAS17 coatings is impossible to explain solely by this reason. Results confirmed that apparent contact angles for 2.0 μL water droplets on the FAS3 coating did not change remarkably, even after soaking in water for 5 s or 20 min (before soaking, 80.2 ( 0.4°; after 5 s 80.6 ( 1.3°, after 20 min 79.8 ( 0.9°). On the basis of this result and the small length of the FAS3 molecule (0.4 nm),16 it is difficult to attribute the origin of the reverse trend to water sorption into the hydrophilic FAS3 surface over a long evaporation period (20 min) for a 2.0 μL water droplet. On the other hand, the existence of line tension derived from the curvature of the three-phase contact line for a liquid droplet on a solid surface is well-known. The wettability on a smooth solid surface is given by Young’s equation, as σ γLV cos θ ¼ γSV - γSL ð1Þ r (15) Suzuki, S.; Nakajima, A.; Yoshida, N.; Sakai, M.; Hashimoto, A.; Kameshima, Y.; Okada, K. Langmuir 2007, 23, 8674. (16) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600.
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where r signifies the radius of the solid-liquid contact area, θ denotes a contact angle, and γSL, γSV, and γLV respectively signify the interfacial free energies per unit area of solid-liquid, solidgas, and liquid-gas interfaces. The line tension is described as σ in the equation. As might be readily apparent from Young’s equation, the effect of line tension increases concomitantly with decreasing contact radius, namely the droplet volume. Precise line tension values are obtainable when a droplet is sufficiently small, e.g., a subnanoliter-scale droplet.17 Recent studies have revealed that the sign of the line tension is not always positive; it is sometimes negative.18-20 Marmur et al. used a theoretical approach to show that the line tension sign changes from positive to negative with increasing contact angle from hydrophilic (θ < 90°) to hydrophobic (θ > 90°).18 Therefore, the line tension direction might contribute to this reverse trend on the volume dependence of evaporation behavior between FAS3 and FAS17. Because the contact angle changes during evaporation, it can be assumed that some excess energy (Δf) exists at the three-phase contact line as ð2Þ Δf ¼ γSV - γSL - γLV cos θt where θt represents the contact angle at the time t. This relation is described using line tension (σ), as in eqs 3 and 4.21 Δf 3 dðπr2 Þ ¼ σ 3 dð2πrÞ
ð3Þ
ðγSV - γSL - γLV cos θt Þ 3 2πrt dr ¼ σ 3 2πdr
ð4Þ
Therein, rt represents the contact radius at time t. By subtracting eq 4 for two different times (t1, t2), we can obtain eq 5 as shown below. σ 1 Δ Δ cos θt ¼ ð5Þ γLV rt In this report, we describe the left part of eq 5 as the hysteresis term and the right part as the line tension term. Table 1 presents calculation results of these terms for the beginning and ending of the second stage during evaporation of 0.8 nL and 2.0 μL water droplets on FAS17 and FAS3 coatings from the contact angle and contact radius. In this calculation, the surface energy of water (γLV) was assumed as 72 mJ 3 m-2. The line tension value obtained for subnanoliter-scale water droplets on plasma-polymerized hexane coating (1 10-7 J 3 m-1)22 was used. We discuss only the order of the value for the calculation in this study because the accuracy of the contact angle measurement for this line tension value under rapid evaporation is unclear. Regarding the FAS3 coating, a positive value was used because it is hydrophilic; for the FAS17 coating, a negative value was applied for this calculation. For the FAS17 coating, orders of the line tension terms for 0.8 nL and 2.0 μL water droplets were almost equivalent to the values of hysteresis terms. When the sign of line tension is negative on the FAS17 coating, its direction is inversely normal from the center of (17) Furuta, T.; Nakajima, A.; Sakai, M.; Isobe, T.; Kameshima, Y.; Okada, K. Chem. Lett. 2009, 38, 580. (18) Marmur, A. J. Colloid Interface Sci. 1997, 186, 462. (19) Marmur, A.; Krasovitski, B. Langmuir 2002, 18, 8919. (20) Szleifer, I.; Widom, B. Mol. Phys. 1992, 75, 925. (21) de Gennes, P.-G., Brochard-Wyartet, F. Quere, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer: New York, 2004. Japanese translation version by Okumura, K.; Hyoumen-chouryoku no butsurigaku; Yoshioka Shoten Press: Tokyo, 2003; Chapter 3, p 70. (22) Yang, J.; Rose, F. R. A. J.; Gadegaard, N.; Alexander, M. R. Langmuir 2009, 25, 2567.
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Figure 2. Evaporation behaviors of 0.8 nL and 2.0 μL water droplets against the normalized time ratio (time, t/time for complete evaporation, t0): (a) contact angle change, and (b) contact radius change (9, FAS17-0.8 nL; 0, FAS17-2.0 μL; 2, FAS3-0.8 nL; 4, FAS32.0 μL).
Figure 3. Schematic illustration of evaporation behavior for 0.8 nL and 2.0 μL water droplets on FAS17 and FAS3 coatings: (a) FAS17-0.8 nL, (b) FAS17-2.0 μL, (c) FAS3-0.8 nL, and (d) FAS3-2.0 μL.
Table 1. Calculated Values of Hysteresis Terms and Line Tension Terms for 0.8 nL and 2.0 μL Water Droplets in the Second Stage of Evaporation on FAS17 and FAS3 Coatings FAS17
hysteresis term line tension term
FAS3
0.8 nL
2.0 μL
0.8 nL
2.0 μL
0.063 0.020
0.009 0.003
- 0.002 - 0.055
0.165 - 0.002
contact radius. This effect is expected to be more remarkable for a 0.8 nL water droplet than for a 2.0 μL droplet. The slight decrease of contact angle in the second stage of evaporation for a 0.8 nL water droplet is rationalized by this effect (Figure 3a,b). However, for FAS3, orders of line tension terms for 0.8 nL and 2.0 μL water droplets differed greatly from values of hysteresis terms. As described above, the contact angle of a 2.0 μL water droplet in the second stage of evaporation decreased gradually on the FAS3 coating. This trend is commonly observed on hydrophilic coatings.7 Although the practical reason for this phenomenon remains unclear, it is deduced that the small mobility of three-phase contact line by the good affinity between solid surface 12000 DOI: 10.1021/la902848s
and water molecules plays an important role in this result. The reason for the discrepancy of values between the line tension term and hysteresis term on the FAS3 coating is attributable to the trend of a gradual contact angle decrease in the second stage of evaporation. When the sign of line tension is positive on the FAS3 coating, its direction is normal to the center of the contact radius. As was true for the FAS17 coating, this effect is more remarkable for a 0.8 nL water droplet than for a 2.0 μL droplet. It can be inferred, therefore, that the contact angle remained almost constant in the second stage of evaporation for 0.8 nL water droplets (Figure 3c, d). Detailed calculations revealed that hysteresis terms for a 0.8 nL droplet scatter and are sometimes positive at the end of the second stage of evaporation. However, they are sufficiently smaller than those for 2.0 μL water droplets (0.165), which suggests that the line tension inhibits the contact angle decrease. These results indicate that the sign of line tension depends on wettability, and that the line tension affects the evaporation behavior of sub-nanoliter-scale water droplets. The line tension value from the result of a 0.8 nL droplet on FAS17 by direct calculation using eq 5 was -3.1 10-7 J 3 m-1; Langmuir 2009, 25(20), 11998–12001
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the order of this value is consistent with that of ref 22. On the other hand, it was 3.1 10-9 J 3 m-1 for the FAS3 coating. The calculated line tension value on FAS3 will be affected by the trend of gradual contact angle decrease during evaporation on the coating. The model described in this manuscript is a simplified one. We believe that additional experiments performed by changing the solid surface and droplet mass, and detailed modeling with considering thermodynamics and kinetics of evaporation are necessary for additional understanding of this phenomenon. These investigations should be addressed in future work.
coating, the contact angle of 0.8 nL water droplets in the second stage of evaporation decreased slightly, although those of 2.0 μL droplets were constant. For the FAS3 coating, the contact angle of 0.8 nL water droplets in the second stage of evaporation was almost constant. However, that of 2.0 μL droplets decreased gradually. These trend differences are attributable to the differences of signs for line tension and the magnitude of their effect against water droplets. Evaporation behavior of sub-nanoliterscale droplets is affected by the line tension more remarkably than that of microliter-scale droplets.
IV. Conclusion
Supporting Information Available: Details of the sample preparation. This material is available free of charge via the Internet at http://pubs.acs.org.
Two smooth hydrophobic and hydrophilic coatings were prepared using two different fluoroalkyl silanes. On the FAS17
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