E4511ps10-txt - Engineering 45 The Structure and Properties...

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Page 1 Engineering 45 The Structure and Properties of Materials Spring, 2010 Problem Set No. 11 Problem 1: Assume that a solid surface of material S, in contact with a vapor phase, V, is to be wet by a liquid of phase L. (a) It is desirable that the L phase wet S as well as possible. Show that this can be accomplished by minimizing the interfacial tension, σ SL , between the S and L phases. (b) Show that the L phase cannot completely wet S unless σ LV is less than σ SV , where σ is the interfacial tension. (c) Suppose S is to resist wetting by the widest possible range of liquids. Show that S should be chosen so that σ SV is as small as possible. (d) A good car wax should form a tight, continuous film over the surface of the car, but should resist wetting by water, so that raindrops bead up and easily roll off. To achieve this behavior, should one seek high or low values of the interfacial tensions between the wax and the car body, the wax and water, and the wax and air?
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