Abstract
Frosted glass is a common, low cost material. Its roughness can be used to control how it is wet by water. In this paper, the wetting properties of silicone oil and water are investigated. For the oil, wetting is total since the oleophilic character of the glass is enhanced by its roughness. Due to the remarkable optical properties of frosted glass, the spreading of oil droplets on its surface was recorded over three months. Frosted glass is a parahydrophilic surface because of its large contact angle hysteresis (up to 80° ). The behaviour of oil and water droplets was compared on a long piece of inclined frosted glass. The trajectories (and the spreading) of the droplets were studied and phenomenological laws were deduced to describe the dependence of the droplet speed on the initial volume of the droplet and the angle of inclination. Such dependences of speed at long travel distances (100 times the capillary length) were deduced and rationalised with a simple model that takes into account the thickness of the wake. Moreover, we analysed the flow inside the wake of water droplets sliding on inclined frosted glass. Suggestions are given on how to exploit drainage of the water droplet wake and the high hysteresis of water within the framework of open microfluidics.
Highlights
Microfluidics is probably the most successful scientific story of recent decades [1,2]
The behaviour of oil and water droplets was compared on a long piece of inclined frosted glass
We analysed the flow inside the wake of water droplets sliding on inclined frosted glass
Summary
Microfluidics is probably the most successful scientific story of recent decades [1,2]. In the present paper a rough glass surface is considered This low cost material has interesting wetting properties, the high hysteresis of the contact line of a water droplet. The wetting properties of a commercial frosted glass were studied when the plate was horizontal or inclined In the latter case, since the droplet loses some liquid on the surface, the water droplet motion was studied in detail. Frosted glass Rough hydrophilic surfaces are known as parahydrophilic surfaces; as summarised by Marmur [10], we can decouple the contact angle due to chemical effects (the nature of the solids, liquids and gases) from those caused by the surface texture.
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