Microscopic shape and contact angle measurement at a superhydrophobic surface

Abstract

We have studied the microscopic shape, contact angle and Laplace law behavior of the liquid–gas interfaces at a superhydrophobic surface. A superhydrophobic surface is immersed in water, and the radius of liquid gas menicsi that span between adjacent ridges of the surface texture is measured. The surface pattern consists of rectangular grooves, such that the sample is simultaneously an optical grating. The diffraction properties encode the shape of the menisci. The shape of the menisci is determined by measuring the intensity of several diffraction orders as a function of the incident angle, and fitting the data to numerical calculations of the diffraction. The uncertainty of the determined menisci deflections is a few nanometres. Observing the deflection as a function of externally controlled hydrostatic pressure, Laplace's law is probed for the menisci on the micrometre scale. The microscopic contact angle is determined by measuring the radius of the menisci prior to collapse. Close agreement with the macroscopic Young angle is found. A stability limit for the superhydrophobic-to-impregnated transition is given. The measurement is a microscopic analogue of ‘bubble’ and ‘sessile drop’ type methods.