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Freezing of water droplets on silicon surfaces coated with various silanes
Chemical Physics Letters 445 (2007) 37–41 www.elsevier.com/locate/cplett
Freezing of water droplets on silicon surfaces coated with various silanes Shunsuke Suzuki a,b, Akira Nakajima a,b,*, Naoya Yoshida b,c, Munetoshi Sakai b, Ayako Hashimoto b, Yoshikazu Kameshima a,b, Kiyoshi Okada a a
b
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 Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan c Research Center for Advanced Science and Technology, The University of Tokyo 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan Received 9 May 2007; in final form 24 July 2007 Available online 1 August 2007
Abstract The freezing behavior of a supercooled water droplet on a silicon surface treated with using various silanes was observed directly using a high-speed camera system. Freezing stability is best ensured by heterogeneous nucleation from a three-phase (solid–liquid–air) contact line. Differential scanning calorimetry measurements revealed that the freezing temperature of a supercooled water droplet on a fluoroalkylsilane monolayer was lower than that of alkylsilane. Interaction between the fluorocarbon and water molecule, in addition to structural characteristics of silanes, might play an important role in the heterogeneous nucleation of supercooled water. 2007 Published by Elsevier B.V.
1. Introduction Technologies related to hydrophobic coatings are important for suppressing chemical reactions and chemical bonding between water and solid surfaces. Such coatings have been applied to various industrial items for anti-ice or anti-snow-adherence [1–6]. Ice forms by the freezing of water, which occurs by decreasing temperature. Water is easily supercooled when it is cooled without vibration or contamination, such as that by adherent dust. The maximum supercooling is reported to be 73 C [7]. Freezing of water droplets on a solid surface is commonly governed by a heterogeneous nucleation mechanism of solid and water. On the other hand, continuity and length of the threephase (solid–air–water) contact line of a water droplet are known to play an important role in its stability such *
Corresponding author. Address: 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. Fax: +81 3 5734 3355. E-mail address: [email protected] (A. Nakajima). 0009-2614/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.cplett.2007.07.066
as its sliding angle [8–12]. Line tension also works on this line [13,14]. Based on results of previous studies, stability of the three-phase contact line and other parts in the contact area are inferred to be different. Seeley et al. pointed out the importance of reduced dimensionality in heterogeneous nucleation of ice [15]. Nevertheless, the nucleation (freezing) site of a water droplet on a hydrophobic solid surface has remained undefined so far. Groups headed respectively by Gavish and PopovitzBiro [16,17] have investigated ice nucleation properties of a water droplet covered by monolayers of aliphatic alcohols on a solid surface. However, the effect of the solid surface’s molecular structure of self-assembled monolayers (SAMs) of silanes on ice nucleation also remains unclear, although they are commonly used for hydrophobic coating [18] of inorganic materials such as glass or silicon. Very recently, we developed processing conditions for highly smooth and homogeneous SAM coatings on a silicon surface using various silanes [19]. For the present study, we used three different silanes and coated them under optimal conditions. We then used a high-speed camera system to observe freezing behavior of a supercooled water droplet on the coatings. Subsequently, the
38
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
dependence of the freezing temperature on those surfaces’ characteristics was investigated using differential scanning calorimetry (DSC).
Light source
Water droplet
Stainless plate
Sample
2. Experimental 2.1. Sample preparation and characterization High-speed camera
In our experiments, 1H,1H,2H,2H-Perfluorodecyltrimethoxysilane (FAS-17, TSL8233; GE Toshiba Silicones, Tokyo, Japan), trifluoropropyltrimethoxysilane (FAS-3, KBM-7103; Shin-Etsu Chemical. Co. Ltd., Tokyo, Japan), and octadecyltrimethoxysilane (ODS; Sigma Fine Chemical Co., Milwaukee, WI, USA) were used as water-repellent agents. A Si (1 0 0) wafer (Aki Corp., Miyagi, Japan) was cut into plates (30 · 50 mm) and circular tips (2.5mm diameter). These surfaces were cleaned of organic contaminants using ultrasonication in acetone and water, with vacuum ultraviolet illumination (VUV, 172 nm wavelength, UER-20; Ushio Inc., Tokyo, Japan) for 10 min in air at room temperature. In this experiment, VUV illumination provided Si–OH terminated surfaces, which reacted with organosilanes [20]. The pre-cleaned plates were coated with organosilanes using CVD method by heating together with 0.02 cm3 of either FAS-17, ODS or FAS-3 in a Petri dish at 150 C for 1 h (FAS-17, ODS) or 100 C for 3 h (FAS-3) with flowing N2. Then sample surfaces were rinsed using acetone, toluene and distilled water, and dried at 80 C. Coating of Si tips with FAS-17 or ODS was performed by immersing them in FAS-17 solution (130 lM, bis(trifluoromethyl)benzene (Wako Pure Chemical Industries Ltd., Tokyo, Japan)) or ODS solution (25 mM, water saturated xylene (Wako Pure Chemical Industries Ltd.)). After soaking, sample surfaces were rinsed using methylene chloride, acetone, and water; they were then dried at 80 C. The SAM of FAS-3 on the Si tip was prepared using CVD method, as for plates. Surface roughness (Ra) was evaluated in a 5-lm-square area using an atomic force microscope (AFM, JSPM4200; JEOL, Tokyo, Japan) with a Si cantilever. The sessile drop method, using a contact-angle meter (Dropmaster DM-500; Kyowa Interface Science Co. Ltd., Saitama, Japan), was used to measure the contact angles. The measured water droplet weighed 3 mg. The surface was blown with ionized air before measurement to eliminate any electrostatic charge. The sliding angle of a 30-mg water droplet on plain surfaces was measured using the DM-500 automatic sliding system. 2.2. Direct observation of freezing behavior of a water droplet (see Supporting Info.) A water droplet (45 mg) was placed gently on the sample surface. Then, the coated Si plate was set on a black aluminum block (102 · 72 · 51 mm), set into the cooling medium (a mixture of dry ice and ethanol) in a glass vessel (138
Al block
Glass vessel
Cooling medium
Fig. 1. Schematic illustration of an assembly for observation of freezing behavior of a supercooled water droplet.
(diameter) · 80 (height) mm). The temperature was monitored using a thermocouple; the cooling rate was about 5 C/min. A high-speed C-MOS camera with 512 · 512 pixel resolution (512 PCI; Photron Ltd., Tokyo, Japan) was used to obtain sequential images of the freezing of a water droplet on the surface. A metal halide lamp (LSM350; Sumita Optical Glass Inc., Saitama, Japan) was chosen as a bright flat light source for this observation. The light illuminated the sample through a heat-absorbing filter and an opal diffuser. The dark stainless plate was laid down before the camera lens to prevent light reflection. The apparatus for this observation is illustrated in Fig. 1. This experiment was performed in clean conditions. 2.3. DSC measurement We used Si tips treated with FAS-17, FAS-3, and ODS for differential scanning calorimetry (DSC, Q100; TA, USA) A 2.5-mg water droplet was placed on the coated tip in the sample cell, which was then sealed with an aluminum lid. The sample was then cooled from room temperature to 35 C at 0.5 C/min and the onset temperature of freezing was measured. For comparison, we examined a Si tip that had been cleaned with ethanol and acetone. The tip surface was covered with a natural oxide layer (SiO2, SiOH). Temperature tracking of the sample cell interior was confirmed by the freezing point of Hg. 3. Results and discussion Surface roughness values (Ra) of the sample surface are ca. 0.1 nm, 0.1 nm, and 0.2 nm, respectively, for FAS-17, FAS-3, and ODS. All surfaces were highly smooth and homogeneous. Neither heterogeneous defects nor dust particles were observed by AFM. Consequently, a small sliding angle (9, 14, and 8, respectively) was obtained from these three coatings. Fig. 2 shows sequential photographs of the freezing behavior of a water droplet on the surface of a Si plate treated with FAS-17. Ice nucleation definitely occurs at the interface between solid and water, suggesting heterogeneous nucleation around 22 C. However, it initiates not from the inside of the solid–water but at the three-
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
39
phase contact line. This trend is the same as that of surfaces treated with ODS or FAS-3, and its repeatability was confirmed. For comparison, we coated FAS-17 on the surface of TiO2 anatase polycrystalline film. A small (1-mmsquare) hydrophilic rectangular region was formed at the center part of FAS-17 coating by VUV irradiation through a stainless photomask (Fig. 3). Then, a water droplet was placed on the rectangular region and cooled with UV illumination using a Hg–Xe lamp (LA-310UV; Hayashi Watch Works, Tokyo, Japan). The water contact angle on the rectangular TiO2 part was almost 0 under this condition. Even when the superhydrophilic region was formed in the center part of the contact circle, freezing initiated at the three-phase contact line (Fig. 4). These results imply that the contact line of the droplet is more suitable than the inside of the contact area for nucleation of ice on a solid surface. Two plausible explanations of this result are: (1) excess energy (instability) of the three-phase line; and (2) a temperature difference between the three-phase line and inside of the contact area attributable to the difference of heat capacity and thermal conductivity. Although detailed analyses are required, the latter case will be less pronounced because of the higher thermal conductivity of Si and an aluminum block, and the much larger heat capacity of the heat sink than that of a 30-mg water droplet. Fig. 5 shows the freezing temperature evaluated from DSC measurement of a 2.5 mg water droplet on a plain surface and surfaces treated with various silanes for each water contact angle. Water contact angles of plain Si (no coating) and Si treated using ODS, FAS-3, and FAS-17 were, respectively, 68, 101, 77, and 106. Freezing temperatures of supercooled droplets were 16.3 C, 19.9 C, 22.4 C, and 22.7 C for the respective surfaces. The temperature error was ±0.1 C. All freezing points of supercooled water were greater than 73 C, which is assumed as the freezing point of nearly homoge-
UV
Rectangular pin hole (1-mm-square) Photomask FAS-17 TiO2 film Si
Three-phase contact line
Fig. 2. Sequential photographs of freezing behavior of a supercooled water droplet on a silicon surface treated with FAS-17. Ice nucleation occurs at the bottom left of the droplet.
Hydrophilic part (inside of droplet) Fig. 3. Formation of small hydrophilic region on the FAS-17 surface.
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
Freezing temperature / ºC
40
plain Si FAS-3 ODS FAS-17
-12
-16
-20
-24 60
80
100
120
Water contact angle / deg. Fig. 5. Freezing temperature evaluated from DSC measurement of a 2.5 mg water droplet on various surfaces against those for the water contact angle.
Fig. 4. Sequential photographs of freezing behavior of a supercooled water droplet on a FAS-17 coating with a rectangular superhydrophilic region in the center of the contact area. The interval of the images was 32 ms. Ice nucleation occurs at the bottom right of the droplet.
neous nucleation [7]. The freezing temperatures of water droplets are almost equivalent to the results of DSC measurements. Apparently, this freezing behavior occurs as a result of a heterogeneous nucleation at the interface between the solid and the supercooled water. In this experiment, we confirmed the droplet mass dependence of the onset temperature on ODS coating by changing the water droplet mass to 3–30 mg. The change of onset temperature was within 0.8 C, which indicates that the difference in freezing temperature results from fundamental nucleation behavior, rather than the difference in number of nucle-
ation sites attributable to contact-angle differences. Although the FAS-3 surface exhibited a lower contact angle than the ODS surface, the FAS-3 freezing point was lower than that of ODS (see Supporting Information 1). We repeated experiments more than three times and confirmed that this freezing order was not changed. The activation energy of heterogeneous nucleation is well known to increase with increasing contact angle [21]. The order of the freezing points follows that of the water contact angle, except on FAS-3, suggesting that heterogeneous ice nucleation in supercooled water is suppressed on a fluorocarbon surface. Defects and heterogeneity in coating are not a reason for the low freezing temperature because they should enhance freezing. Existence of strong interaction between water molecules and the fluorocarbon surface has been predicted from computational energy calculations presented in several studies [22,23]. The results obtained from this study are attributable to the strong interaction between fluorocarbon and water molecules and to the resultant low mobility of water molecules on the surface. Another possibility is the difference in the molecular structure of silanes. Water molecules must rearrange their structure for freezing at the interface in heterogeneous ice nucleation. The ODS length is greater than that of FAS3 and the rigidity of the hydrocarbon group is lower than that of the fluorocarbon group [24], meaning that water molecules on ODS possess greater freedom than those on a FAS-3 surface. Very recently, we used a high-speed camera system with particle image velocimetry to observe the internal fluidity of water droplets during sliding on silicon surfaces treated with ODS and FAS-3. The droplets’ velocity during sliding was controlled by slipping and rolling motions, and their contributions were different between ODS and FAS-3 [25]. The slipping mode contribution against overall sliding is greater in ODS (48%) than in FAS-3 (37%). Consequently, the molecular structure of ODS might provide greater freedom to water molecules at the interface than FAS-3 at the interface, thereby making ice nucleation more feasible.
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
Recently, Yoshida et al. investigated sliding behavior of a water droplet on the surface of various methyl methacrylate (MMA)–fluoroalkyl methacrylate (FMA) copolymers [26]. Their results revealed that, in the longer and shorter fluoroalkyl chains (e.g., FAS-17, FAS-3), the dynamic contact angle hysteresis decreased with increased FMA concentration. They attributed this result to the aggregation effect of the longer fluoroalkyl chain and to the slight orientation freedom of the shorter fluoroalkyl chain. Their result implies that structural features of the molecule also play an important role for dynamics of water molecules at the solid–liquid interface. Consequently, the slight difference in freezing points of FAS-17 and FAS-3 might be attributable to this molecule-length effect. 4. Conclusion For this study, we used a high-speed camera to observe the freezing behavior of a water droplet on silicon surfaces treated with various silanes. Freezing preferably occurs from the three-phase contact line of the solid–water contact area. The DSC measurements taken in this study revealed that fluoroalkylsilane suppresses freezing of the water droplet. Molecules’ structural characteristics, in addition to interaction between the fluorocarbon and water molecules, might play important roles in this result. Acknowledgement This work was supported by JSPS Research Fellowships for Young Scientists No. H17-08586. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett. 2007.07.066.
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References [1] A. Nakajima, J. Ceram. Soc. Jpn. 112 (2004) 533. [2] G. Yamaguchi, K. Takai, H. Saito, IEICE Trans. Electron E83-C (2000) 1139. [3] A. Nakajima, K. Hashimoto, T. Watanabe, Monatshefte Chem. 132 (2001) 31. [4] T. Kako, A. Nakajima, Z. Kato, K. Uematsu, T. Watanabe, K. Hashimoto, J. Ceram. Soc. Jpn. 110 (2000) 186. [5] T. Kako, A. Nakajima, H. Irie, Z. Kato, K. Uematsu, T. Watanabe, K. Hashimoto, J. Mater. Sci. 39 (2004) 547. [6] M. Yoshida, J. Adhesion Soc. Jpn. 30 (1994) 418. [7] L.S. Bartell, J. Huang, J. Phys. Chem. 98 (1994) 7455. [8] Z. Yoshimitsu, A. Nakajima, T. Watanabe, K. Hashimoto, Langmuir 18 (2002) 5818. ¨ ner, J. Youngblood, T.J. [9] W. Chen, A.Y. Fadeev, M.C. Hsieh, D. O McCathy, Langmuir 15 (1999) 3395. ¨ ner, T.J. McCarthy, Langmuir 16 (2000) 7777. [10] D. O [11] J.P. Youngblood, T.J. McCarthy, Macromolecules 32 (1999) 6800. [12] C.W. Extrand, Y. Kumagai, J. Colloid Interf. Sci. 170 (1995) 515. [13] J. Drelich, Colloid Surf. A 116 (1996) 43. [14] A. Amirfazli, S. Ha¨nig, A. Mu¨ller, A.W. Neumann, Langmuir 16 (2000) 2024. [15] L.H. Seeley, G.T. Seidler, Phys. Rev. Lett. 87 (2001) 055702. [16] M. Gavish, R. Popovitz-Biro, M. Lahav, L. Leiserowitz, Science 250 (1990) 973. [17] R. Popovitz-Biro, J.L. Wang, J. Majewski, E. Shavit, L. Leiserowitz, M. Lahav, J. Am. Chem. Soc. 116 (1994) 1179. [18] A.F. Heneghan, H.J. Moore, T.R. Lee, A.D.J. Haymet, Chem. Phys. Lett. 385 (2004) 441. [19] Final Proceedings of the Research of Super Hydro philic/phobic Surface Project, Kanagawa Academy of Science and Technology, Kanagawa, Japan, 2007 (in Japanese). [20] A. Hozumi, K. Ushiyama, H. Sugimura, O. Takai, Langmuir 15 (1999) 7600. [21] D. Turnbull, B. Vonnegut, Ind. Eng. Chem. 44 (1952) 1292. [22] H. Umeyama, K. Morokuma, J. Am. Chem. Soc. 99 (1977) 1316. [23] P.M. Claesson, H.K. Christenson, J. Phys. Chem. 92 (1988) 1650. [24] H. Murase, Ph.D. Thesis, The University of Tokyo, Tokyo, Japan, 1998. [25] M. Sakai, J.H. Song, N. Yoshida, S. Suzuki, Y. Kameshima, A. Nakajima, Langmuir 22 (2006) 4906. [26] N. Yoshida, Y. Abe, H. Shigeta, A. Nakajima, H. Ohsaki, K. Hashimoto, T. Watanabe, J. Am. Chem. Soc. 128 (2006) 743.
Freezing of water droplets on silicon surfaces coated with various silanes Shunsuke Suzuki a,b, Akira Nakajima a,b,*, Naoya Yoshida b,c, Munetoshi Sakai b, Ayako Hashimoto b, Yoshikazu Kameshima a,b, Kiyoshi Okada a a
b
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 Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan c Research Center for Advanced Science and Technology, The University of Tokyo 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan Received 9 May 2007; in final form 24 July 2007 Available online 1 August 2007
Abstract The freezing behavior of a supercooled water droplet on a silicon surface treated with using various silanes was observed directly using a high-speed camera system. Freezing stability is best ensured by heterogeneous nucleation from a three-phase (solid–liquid–air) contact line. Differential scanning calorimetry measurements revealed that the freezing temperature of a supercooled water droplet on a fluoroalkylsilane monolayer was lower than that of alkylsilane. Interaction between the fluorocarbon and water molecule, in addition to structural characteristics of silanes, might play an important role in the heterogeneous nucleation of supercooled water. 2007 Published by Elsevier B.V.
1. Introduction Technologies related to hydrophobic coatings are important for suppressing chemical reactions and chemical bonding between water and solid surfaces. Such coatings have been applied to various industrial items for anti-ice or anti-snow-adherence [1–6]. Ice forms by the freezing of water, which occurs by decreasing temperature. Water is easily supercooled when it is cooled without vibration or contamination, such as that by adherent dust. The maximum supercooling is reported to be 73 C [7]. Freezing of water droplets on a solid surface is commonly governed by a heterogeneous nucleation mechanism of solid and water. On the other hand, continuity and length of the threephase (solid–air–water) contact line of a water droplet are known to play an important role in its stability such *
Corresponding author. Address: 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. Fax: +81 3 5734 3355. E-mail address: [email protected] (A. Nakajima). 0009-2614/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.cplett.2007.07.066
as its sliding angle [8–12]. Line tension also works on this line [13,14]. Based on results of previous studies, stability of the three-phase contact line and other parts in the contact area are inferred to be different. Seeley et al. pointed out the importance of reduced dimensionality in heterogeneous nucleation of ice [15]. Nevertheless, the nucleation (freezing) site of a water droplet on a hydrophobic solid surface has remained undefined so far. Groups headed respectively by Gavish and PopovitzBiro [16,17] have investigated ice nucleation properties of a water droplet covered by monolayers of aliphatic alcohols on a solid surface. However, the effect of the solid surface’s molecular structure of self-assembled monolayers (SAMs) of silanes on ice nucleation also remains unclear, although they are commonly used for hydrophobic coating [18] of inorganic materials such as glass or silicon. Very recently, we developed processing conditions for highly smooth and homogeneous SAM coatings on a silicon surface using various silanes [19]. For the present study, we used three different silanes and coated them under optimal conditions. We then used a high-speed camera system to observe freezing behavior of a supercooled water droplet on the coatings. Subsequently, the
38
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
dependence of the freezing temperature on those surfaces’ characteristics was investigated using differential scanning calorimetry (DSC).
Light source
Water droplet
Stainless plate
Sample
2. Experimental 2.1. Sample preparation and characterization High-speed camera
In our experiments, 1H,1H,2H,2H-Perfluorodecyltrimethoxysilane (FAS-17, TSL8233; GE Toshiba Silicones, Tokyo, Japan), trifluoropropyltrimethoxysilane (FAS-3, KBM-7103; Shin-Etsu Chemical. Co. Ltd., Tokyo, Japan), and octadecyltrimethoxysilane (ODS; Sigma Fine Chemical Co., Milwaukee, WI, USA) were used as water-repellent agents. A Si (1 0 0) wafer (Aki Corp., Miyagi, Japan) was cut into plates (30 · 50 mm) and circular tips (2.5mm diameter). These surfaces were cleaned of organic contaminants using ultrasonication in acetone and water, with vacuum ultraviolet illumination (VUV, 172 nm wavelength, UER-20; Ushio Inc., Tokyo, Japan) for 10 min in air at room temperature. In this experiment, VUV illumination provided Si–OH terminated surfaces, which reacted with organosilanes [20]. The pre-cleaned plates were coated with organosilanes using CVD method by heating together with 0.02 cm3 of either FAS-17, ODS or FAS-3 in a Petri dish at 150 C for 1 h (FAS-17, ODS) or 100 C for 3 h (FAS-3) with flowing N2. Then sample surfaces were rinsed using acetone, toluene and distilled water, and dried at 80 C. Coating of Si tips with FAS-17 or ODS was performed by immersing them in FAS-17 solution (130 lM, bis(trifluoromethyl)benzene (Wako Pure Chemical Industries Ltd., Tokyo, Japan)) or ODS solution (25 mM, water saturated xylene (Wako Pure Chemical Industries Ltd.)). After soaking, sample surfaces were rinsed using methylene chloride, acetone, and water; they were then dried at 80 C. The SAM of FAS-3 on the Si tip was prepared using CVD method, as for plates. Surface roughness (Ra) was evaluated in a 5-lm-square area using an atomic force microscope (AFM, JSPM4200; JEOL, Tokyo, Japan) with a Si cantilever. The sessile drop method, using a contact-angle meter (Dropmaster DM-500; Kyowa Interface Science Co. Ltd., Saitama, Japan), was used to measure the contact angles. The measured water droplet weighed 3 mg. The surface was blown with ionized air before measurement to eliminate any electrostatic charge. The sliding angle of a 30-mg water droplet on plain surfaces was measured using the DM-500 automatic sliding system. 2.2. Direct observation of freezing behavior of a water droplet (see Supporting Info.) A water droplet (45 mg) was placed gently on the sample surface. Then, the coated Si plate was set on a black aluminum block (102 · 72 · 51 mm), set into the cooling medium (a mixture of dry ice and ethanol) in a glass vessel (138
Al block
Glass vessel
Cooling medium
Fig. 1. Schematic illustration of an assembly for observation of freezing behavior of a supercooled water droplet.
(diameter) · 80 (height) mm). The temperature was monitored using a thermocouple; the cooling rate was about 5 C/min. A high-speed C-MOS camera with 512 · 512 pixel resolution (512 PCI; Photron Ltd., Tokyo, Japan) was used to obtain sequential images of the freezing of a water droplet on the surface. A metal halide lamp (LSM350; Sumita Optical Glass Inc., Saitama, Japan) was chosen as a bright flat light source for this observation. The light illuminated the sample through a heat-absorbing filter and an opal diffuser. The dark stainless plate was laid down before the camera lens to prevent light reflection. The apparatus for this observation is illustrated in Fig. 1. This experiment was performed in clean conditions. 2.3. DSC measurement We used Si tips treated with FAS-17, FAS-3, and ODS for differential scanning calorimetry (DSC, Q100; TA, USA) A 2.5-mg water droplet was placed on the coated tip in the sample cell, which was then sealed with an aluminum lid. The sample was then cooled from room temperature to 35 C at 0.5 C/min and the onset temperature of freezing was measured. For comparison, we examined a Si tip that had been cleaned with ethanol and acetone. The tip surface was covered with a natural oxide layer (SiO2, SiOH). Temperature tracking of the sample cell interior was confirmed by the freezing point of Hg. 3. Results and discussion Surface roughness values (Ra) of the sample surface are ca. 0.1 nm, 0.1 nm, and 0.2 nm, respectively, for FAS-17, FAS-3, and ODS. All surfaces were highly smooth and homogeneous. Neither heterogeneous defects nor dust particles were observed by AFM. Consequently, a small sliding angle (9, 14, and 8, respectively) was obtained from these three coatings. Fig. 2 shows sequential photographs of the freezing behavior of a water droplet on the surface of a Si plate treated with FAS-17. Ice nucleation definitely occurs at the interface between solid and water, suggesting heterogeneous nucleation around 22 C. However, it initiates not from the inside of the solid–water but at the three-
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
39
phase contact line. This trend is the same as that of surfaces treated with ODS or FAS-3, and its repeatability was confirmed. For comparison, we coated FAS-17 on the surface of TiO2 anatase polycrystalline film. A small (1-mmsquare) hydrophilic rectangular region was formed at the center part of FAS-17 coating by VUV irradiation through a stainless photomask (Fig. 3). Then, a water droplet was placed on the rectangular region and cooled with UV illumination using a Hg–Xe lamp (LA-310UV; Hayashi Watch Works, Tokyo, Japan). The water contact angle on the rectangular TiO2 part was almost 0 under this condition. Even when the superhydrophilic region was formed in the center part of the contact circle, freezing initiated at the three-phase contact line (Fig. 4). These results imply that the contact line of the droplet is more suitable than the inside of the contact area for nucleation of ice on a solid surface. Two plausible explanations of this result are: (1) excess energy (instability) of the three-phase line; and (2) a temperature difference between the three-phase line and inside of the contact area attributable to the difference of heat capacity and thermal conductivity. Although detailed analyses are required, the latter case will be less pronounced because of the higher thermal conductivity of Si and an aluminum block, and the much larger heat capacity of the heat sink than that of a 30-mg water droplet. Fig. 5 shows the freezing temperature evaluated from DSC measurement of a 2.5 mg water droplet on a plain surface and surfaces treated with various silanes for each water contact angle. Water contact angles of plain Si (no coating) and Si treated using ODS, FAS-3, and FAS-17 were, respectively, 68, 101, 77, and 106. Freezing temperatures of supercooled droplets were 16.3 C, 19.9 C, 22.4 C, and 22.7 C for the respective surfaces. The temperature error was ±0.1 C. All freezing points of supercooled water were greater than 73 C, which is assumed as the freezing point of nearly homoge-
UV
Rectangular pin hole (1-mm-square) Photomask FAS-17 TiO2 film Si
Three-phase contact line
Fig. 2. Sequential photographs of freezing behavior of a supercooled water droplet on a silicon surface treated with FAS-17. Ice nucleation occurs at the bottom left of the droplet.
Hydrophilic part (inside of droplet) Fig. 3. Formation of small hydrophilic region on the FAS-17 surface.
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
Freezing temperature / ºC
40
plain Si FAS-3 ODS FAS-17
-12
-16
-20
-24 60
80
100
120
Water contact angle / deg. Fig. 5. Freezing temperature evaluated from DSC measurement of a 2.5 mg water droplet on various surfaces against those for the water contact angle.
Fig. 4. Sequential photographs of freezing behavior of a supercooled water droplet on a FAS-17 coating with a rectangular superhydrophilic region in the center of the contact area. The interval of the images was 32 ms. Ice nucleation occurs at the bottom right of the droplet.
neous nucleation [7]. The freezing temperatures of water droplets are almost equivalent to the results of DSC measurements. Apparently, this freezing behavior occurs as a result of a heterogeneous nucleation at the interface between the solid and the supercooled water. In this experiment, we confirmed the droplet mass dependence of the onset temperature on ODS coating by changing the water droplet mass to 3–30 mg. The change of onset temperature was within 0.8 C, which indicates that the difference in freezing temperature results from fundamental nucleation behavior, rather than the difference in number of nucle-
ation sites attributable to contact-angle differences. Although the FAS-3 surface exhibited a lower contact angle than the ODS surface, the FAS-3 freezing point was lower than that of ODS (see Supporting Information 1). We repeated experiments more than three times and confirmed that this freezing order was not changed. The activation energy of heterogeneous nucleation is well known to increase with increasing contact angle [21]. The order of the freezing points follows that of the water contact angle, except on FAS-3, suggesting that heterogeneous ice nucleation in supercooled water is suppressed on a fluorocarbon surface. Defects and heterogeneity in coating are not a reason for the low freezing temperature because they should enhance freezing. Existence of strong interaction between water molecules and the fluorocarbon surface has been predicted from computational energy calculations presented in several studies [22,23]. The results obtained from this study are attributable to the strong interaction between fluorocarbon and water molecules and to the resultant low mobility of water molecules on the surface. Another possibility is the difference in the molecular structure of silanes. Water molecules must rearrange their structure for freezing at the interface in heterogeneous ice nucleation. The ODS length is greater than that of FAS3 and the rigidity of the hydrocarbon group is lower than that of the fluorocarbon group [24], meaning that water molecules on ODS possess greater freedom than those on a FAS-3 surface. Very recently, we used a high-speed camera system with particle image velocimetry to observe the internal fluidity of water droplets during sliding on silicon surfaces treated with ODS and FAS-3. The droplets’ velocity during sliding was controlled by slipping and rolling motions, and their contributions were different between ODS and FAS-3 [25]. The slipping mode contribution against overall sliding is greater in ODS (48%) than in FAS-3 (37%). Consequently, the molecular structure of ODS might provide greater freedom to water molecules at the interface than FAS-3 at the interface, thereby making ice nucleation more feasible.
S. Suzuki et al. / Chemical Physics Letters 445 (2007) 37–41
Recently, Yoshida et al. investigated sliding behavior of a water droplet on the surface of various methyl methacrylate (MMA)–fluoroalkyl methacrylate (FMA) copolymers [26]. Their results revealed that, in the longer and shorter fluoroalkyl chains (e.g., FAS-17, FAS-3), the dynamic contact angle hysteresis decreased with increased FMA concentration. They attributed this result to the aggregation effect of the longer fluoroalkyl chain and to the slight orientation freedom of the shorter fluoroalkyl chain. Their result implies that structural features of the molecule also play an important role for dynamics of water molecules at the solid–liquid interface. Consequently, the slight difference in freezing points of FAS-17 and FAS-3 might be attributable to this molecule-length effect. 4. Conclusion For this study, we used a high-speed camera to observe the freezing behavior of a water droplet on silicon surfaces treated with various silanes. Freezing preferably occurs from the three-phase contact line of the solid–water contact area. The DSC measurements taken in this study revealed that fluoroalkylsilane suppresses freezing of the water droplet. Molecules’ structural characteristics, in addition to interaction between the fluorocarbon and water molecules, might play important roles in this result. Acknowledgement This work was supported by JSPS Research Fellowships for Young Scientists No. H17-08586. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett. 2007.07.066.
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