Elastic Films and Reversibility in a Microfluidic Pore

Thin films of water in a single pore act as elastic membranes under tension that increase capillary pressures without increasing the dissipated mechanical work. A single wedge-shaped microfluidic pore was fabricated using two-photon laser machining of the photoresist SU-8 to have sharp corners at the pore entrance from the nonwetting reservoir. The singular geometry acts as a strong pinning defect that nucleates wetting films during drainage. The films are measured using confocal fluorescent microscopy on the transparent micromodel. The tension in the film behaves like an elastic tether that rigidly shifts the Pc-Sw scanning loop higher in pressure, while approximately conserving the dissipated mechanical work around the loop. This contribution of elastic forces to hysteretic dissipation has important consequences for the interpretation of Pc-Sw measurements in porous media.