Measure of high contact angles

Abstract

Water repellency is often defined as the territory of contact angles θ larger than 150° and there is some paradox between the huge number of papers devoted to this effect and the lack of specific method for determining such angles. We could think of deducing directly their values from side views of water drops but this measurement is not precise at large θ (uncertainties on the order of 10°), in particular because gravity effects then tend to modify the base of these liquid pearls. We report here that measuring the base size for a drop of known volume allows us to determine high angles with a precision on the order of 1°. We illustrate this technique by questioning the contact angle of liquid marbles (non-wetting drops encapsulated by a powder) and that of water pearls on hot solids.