Deformation and motion by gravity and magnetic field of a droplet of water-based magnetic fluid on a hydrophobic surface
Introduction
Control of liquid droplets on solid surfaces has attracted much attention as an indispensable technology for various industrial items, especially for hydrophobic coatings. Recently, not only for water-repellent treatment, this technology has also become important for microscale chemical processing systems. To date, various investigations have been carried out to examine the motion of a liquid droplet on a solid surface using external fields such as gravity, electric fields, photo-illumination, and a heat gradient [1], [2], [3], [4], [5]. Motions of water droplets on several hydrophobic surfaces under dc and ac electric fields were reported by Higashiyama et al.: the water droplet was elongated or deformed under a dc field, whereas those under an ac field were deformed and synchronized [2], [3]. Ichimura et al. controlled an oil droplet on an azobenzene coating using UV and VIS illumination with photochemical isomerization and the resultant surface free energy gradient [4]. Brzoska et al. demonstrated that a droplet of silicone oil moves on a silicon wafer treated with n-hexadecyltrichlorosilane by the surface energy gradient because of the heat gradient [5], [6].
All these investigations were conducted using liquid droplets. However, investigations on the control of a solid and liquid mixture using an external field are limited. Recently, Kako et al. investigated sliding behavior of a water and glass–bead mixture [7], [8] using gravity on a tilted solid surface. Their results revealed that the dominant factor for governing sliding behavior switches from a three-phase (solid–liquid–water) contact line to viscous flow in an internal mixture droplet by increasing solid concentration. Moreover, when the surface is hydrophilic, solid–liquid separation is induced under certain conditions. Those conditions strongly affect the mixture's sliding behavior. The motion of the solid and liquid mixture is complex and is difficult to understand solely from that of the liquid.
Magnetic fluids are very common solid–liquid mixtures. They are used for various industrial applications such as magnetic seals and acceleration sensors. Such fluids mainly comprise three components: a solvent, magnetic particles such as magnetite, and a surfactant for dispersing magnetic particles within the solvent [9]. Into commercial magnetic fluids, excess surfactant for dispersing magnetic particles is added to obtain high stability. The present study used a commercial water-based magnetic fluid. We investigated its deformation and motion behavior on a hydrophobic surface using gravity and an external magnetic field.
Section snippets
Experimental procedure
Heptadecafluorodecyltrimethoxysilane (CF3(CF2)7CH2CH2Si(OCH3)3), TSL8233, a product of GE Toshiba Silicones, Tokyo, Japan (hereafter referred to as FAS-17), was used as a water-repellent agent in our experiment. Pieces (30 mm × 50 mm × 1 mm) cut from a Pyrex glass plate were sonicated in ethanol and dried. The FAS-17 coating was applied using chemical vapor deposition (CVD). A glass container with a cleaned Pyrex glass substrate and 40 μl of FAS-17 were heated in a furnace that was maintained at 473 K
Results and discussion
The respective surface roughness and water contact angle on the surface coated with FAS-17 are 0.2 nm and 107.0 ± 1.2°. Contact angles of tetrabromoethane (C2H2Br2, 48 mJ m−2; Wako Pure Chemicals Industries Ltd.) and n-hexadecane (CH3(CH2)14CH3, 28 mJ m−2; Wako Pure Chemicals Industries Ltd.) on the surface were evaluated to calculate the surface energy using extended Fowkes method [10]. The corresponding contact angles are 79.5 ± 0.7°, 67.9 ± 0.8°, respectively; the obtained surface energy value was 18 mJ m
Conclusions
In this study, we investigated motion and deformation of water-based magnetic fluids on a hydrophobic surface under an external field. The surface energy and the resultant contact angle of the magnetic fluid depend on the LAS concentration. However, viscosity is governed by magnetite. The front edge of the droplet moves under a weak external field, and the rear edge moves under a higher external field. The force that is necessary for moving the front edge is almost equivalent to that of gravity
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