# Calculation of Force of Interaction

Two hydrophobic glass slides measuring 1 cm x 1 cm x 1 mm are floating on water with their sides parallel to each other. Estimate the force of interaction between the solids as a function of their separation distance. Assume that the intrinsic contact angle of the water is 110 degrees.

What I mean by the intrinsic contact angle is that if you put a drop of water on the slides, the contact angle of the water against the slide would be 110 degrees.

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#### Solution Summary

Let us consider that d be the distance between the plates, alpha be the area of the ?lm between the plates, p be the circumference of the ?lm and theta be the contact angle. Then the plates will be pressed together with a force: F = (2S*alpha*cos(theta))/ d + pS*sin(theta). Solution shows full calculations and answer.