Freezing of water droplets on silicon surfaces coated with various silanes

doi:10.1016/j.cplett.2007.07.066

Chemical Physics Letters

Volume 445, Issues 1–3, 4 September 2007, Pages 37-41

https://doi.org/10.1016/j.cplett.2007.07.066 Get rights and content

Abstract

The freezing behavior of a supercooled water droplet on a silicon surface treated with 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.

Graphical abstract

Freezing behavior difference of a supercooled water droplet on two different self-assembled monolayer coatings.

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], [2], [3], [4], [5], [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 three-phase (solid–air–water) contact line of a water droplet are known to play an important role in its stability such as its sliding angle [8], [9], [10], [11], [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 Popovitz-Biro [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 dependence of the freezing temperature on those surfaces’ characteristics was investigated using differential scanning calorimetry (DSC).

Section snippets

Sample preparation and characterization

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 (2.5-mm diameter). These surfaces were cleaned of organic contaminants using

Results and discussion

Surface roughness values (R_a) 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

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

Acknowledgement

This work was supported by JSPS Research Fellowships for Young Scientists No. H17-08586.

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Freezing of water droplets on silicon surfaces coated with various silanes

Abstract

Graphical abstract

Introduction

Section snippets

Sample preparation and characterization

Results and discussion

Conclusion

Acknowledgement

J. Colloid Interf. Sci.

Colloid Surf. A

Chem. Phys. Lett.

J. Ceram. Soc. Jpn.

IEICE Trans. Electron

Monatshefte Chem.

J. Ceram. Soc. Jpn.

J. Mater. Sci.

J. Adhesion Soc. Jpn.

J. Phys. Chem.

Langmuir

Langmuir

Langmuir