Supplemental Data

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Fast Drop Movements Resulting from the Phase Change on a Gradient Surface
S. Daniels, M.K. Chaudhury, J.C. Chen

Supplementary Material

Preparation of Silicon Surface
Most of the studies described in this work were carried out using a cylindrical copper block because of its high thermal conductivity. The surface of copper is, however, not amenable to the standard silanization chemistry developed for the silicon surfaces; thus, the copper block needed to be modified with a thin film of silicon. This was achieved either by depositing a thin film of silicon on copper by thermal evaporation or by bonding a thin silicon wafer to the block using a thermally conductive adhesive (
<_5 font="font" face="Symbol">mm). For the most part of our studies, the former method was used. Although bonding a thin silicon wafer to the copper yields results that are nearly as good as those obtained from the first method, a knowledge of the heat transfer coefficient of the adhesive layer (30 kW/m2K) was required for precise heat transfer calculations. In terms of surface properties, both the methods yielded similar results, although a bonded silicon wafer survived repeated heating and cleaning cycles much longer than the vapor-deposited silicon film.

The copper block was prepared by polishing one of its flat ends according to the following standard methods. The surface was first polished with diamond (6 μm) paste followed by aqueous suspensions of alumina power (0.3 μm) and colloidal silica (0.05 μm), respectively, until a mirrorlike finish was obtained. Between the polishing steps, the copper surface was washed with water and ethanol. After the final polishing and rinsing steps, the block was stored in methanol to minimize oxidation. When the block was ready for use, it was first blow-dried with dry nitrogen and then a thin (100 nm) layer of silicon was deposited on it by thermal evaporation of silicon under high vacuum (106 torr). It was found that a silicon film deposited directly on the copper block did not always adhere strongly to it. Co-deposition of a thin film (20 to 30 nm) of copper and silicon before the deposition of pure silicon yielded the desired result. Once a well-adhered film of silicon was obtained, it was ready for the silanization procedures. The block was treated with plasma or simply by a flame (2 to 3 s of exposure) that removed any organic materials deposited on it. The cleanliness of the resultant surface was judged by complete and uniform wetting of water upon it.

Preparation of Radial Gradient of Surface Energy
The radial gradient of surface energy was prepared on silicon by diffusion-controlled silanization of alkyl trichlorosilanes. The methodology used here is a modification of a procedure published previously [(6) in the text]. A small ( ~2 μl) drop of neat silane was placed at the center of a perfluorinated glass slide (75 mm by 50 mm), which was then positioned above (~2 mm) the silicon surface as shown in Web Fig. 1. As the silane evaporated from the drop, it diffused radially while reacting with the silicon (Si/SiO2). The central part of the wafer, which was closest to the drop, became maximally hydrophobic, whereas the distant parts became progressively hydrophilic. In order to prepare good-quality surfaces, a low-humidity environment was an important requirement, because it minimized the hydrolysis of chlorosilanes by atmospheric water. This was particularly important on humid days, when the presence of a small amount of adsorbed water on the silicon surface or in the deposition chamber affected the size of the gradient zone. Atmospheric or adsorbed water is of particular concern, because it competes with the surface silanols in reacting with the chlorosilanes. Although we have not developed a very precise understanding of the effects of humidity, the molecular weights of silanes, and the treatment times on the quality of the wettability gradients formed on silicon, the following sets of conditions yielded results of considerable satisfaction. First, the humidity of the deposition chamber was maintained at <10% by flushing the deposition chamber ( 85 by 52 by 30 cm) with dry air produced by a Ballstone FTIR purge gas generator. Second, the air convection was minimized in order to prepare a radially symmetric gradient. This was accomplished by locally isolating the deposition components (silicon, the silane source, etc.) with a polystyrene petri dish while the silanization reaction was carried out inside the deposition chamber. A rather steep gradient (~1 cm in diameter) could be prepared by using a drop of dodecyltrichlorosilane as the source with the treatment time of 5 to 7 min. On the other hand, a low-molecular-weight silane (octyltrichlorosilane) with a treatment time of 3 to 5 min produced a much longer (4.8 cm) gradient (Web Fig. 2). The slight uncertainty of the treatment times needed to produce the gradients of above lengths arises because of some variability of the humidity in the deposition chamber and because the diffusion length is proportional to the square root of the exposure time. We focused on reproducing the actual length of the gradient, for which some minor adjustments of exposure time were necessary. All depositions were carried out at room temperature (~23°C).

Characterization of the Gradient Surface
The gradient surfaces were characterized by measuring the contact angles of water as a function of radial position. Usually, the center of the gradient zone is hydrophobic, with an advancing contact angle of 90° to 100°. The surface becomes progressively hydrophilic away from the center. The variation of the contact angle on the silicon surface is generally nonlinear, as to be expected from the diffusion-controlled silanization methods used to form these gradients. Web Fig. 3 illustrates the wettability properties of two types of gradients that were prepared using dodecyl- and octyltrichlorosilanes. The hysteresis of contact angle was rather low ( 6° to 8°) on the octyltrichlorosilane-modified surface, whereas higher hysteresis (~15° at the center and 25° to 30° toward the edge) was observed on the dodecyltrichlorosilane-treated surface.

Preparation of 1D Periodic Gradient
A one-dimensional periodic gradient of surface energy was prepared on a silicon surface according to a modification of the method described above. Here, instead of a droplet source, parallel cotton strings soaked in pure silane were used as line sources for diffusion-controlled silanization (Web Fig. 3). Eleven strings with 0.5-cm spacing between two strings were used to cover a silicon wafer 5 cm in diameter. The distance between these strings and the silicon wafer was about 2 mm. This arrangement periodically modulated the surface energy of the silicon wafer perpendicular to the strings. Either a dodecyl or a tetradecyl functional trichlorosilane can be used to produce a strong gradient within the channels using the treatment times of 45 s and 1.5 min, respectively.

Estimation of Droplet Speeds Using High-Speed Video
Some preliminary measurements of the speed of drop moving in the periodic gradients were obtained using high-speed video with a time resolution of 1 ms between two frames. In two consecutive frames, a drop of about 0.5 mm is found to travel a distance of 0.15 cm (Web Fig. 4). This leads to the estimation of the droplet speed at an amazing 1.5 m/s.

Supplemental Figure 1. Schematic of the method used to form the radial gradient of surface energy of a silicon surface.

Medium version

Supplemental Figure 2. Variation of the advancing contact angles of water on the two gradient surfaces prepared using dodecyltrichlorosilane (DDS, open circles) and octyltrichlorosilane (OTS, solid circles). At a radial position beyond 5 mm for the DDS-treated surface and 25 mm for the OTS-treated surface, the contact angle became less than 10°, which could not be reliably measured by the method of goniometry.

Medium version

Supplemental Figure 3. Schematic of the method used to form a 1D periodic gradient of surface energy on the silicon surface.

Medium version

Supplemental Figure 4. High-speed video micrographs of a condensed water drop moving on a 1D periodic gradient surface, prepared by using CH3(CH2)13SiCl3. Note that the particular drop (1) in the micrograph has grown in size by coalescence with other drops in its path. The drop moves with a speed of about 1.5 m/s.

Medium version