Superhydrophobic coatings possess a strong water-repellent characteristic, which, among several other potential applications, enhances the mobility of water droplets over the surface. The coating traps air within its micropores, such that a submerged moving body experiences shear-free and no-slip regions over, respectively, the air pockets and the solid surface. This, in turn, may lead to significant skin-friction reduction. The coating maintains its superhydrophobicity as long as the air remains entrapped. It is therefore of great interest to precisely measure the amount of trapped air, which is particularly difficult to estimate for coatings with disordered microstructures. A novel method to measure the effective thickness and gas volume fraction of superhydrophobic coatings with either ordered or random microroughness is advanced. The technique is applied to both aerogel and electrospun fibrous coatings. The experiments utilize a sensitive weighing scale (down to 10(-4) gm) and height gauge (down to 10 μm) to determine the buoyancy force on an immersed, coated glass-slide substrate. The measured force is used to calculate the volume fraction of entrapped air. The coating's effective thickness also follows from the same calculations. The sensitivity of our particular scale enables the measuring of thicknesses down to 3 μm, which is not readily possible with conventional thickness gauges. Smaller thicknesses could be measured using more sensitive scales.
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