Spreading of Liquid Drops over Thick Porous Layers: Complete Wetting Case

Abstract

The spreading of small silicone oil drops (capillary regime of spreading) over various dry thick porous substrates (permeable in both normal and tangential directions) was experimentally investigated. The time evolution of the radii of both the drop base and the wetted region on the surface of the porous substrate was monitored. It was observed that the total duration of the spreading process can be divided into two stages: a first stage, when the drop base expands until its maximum value is reached, and a subsequent second stage, when the drop base shrinks. It was found that the dynamic contact angle remains constant during the second stage of spreading. The latter fact has nothing to do with the contact angle hysteresis, as there is no hysteresis in the system. Appropriate scales are used, with a dimensionless time, to plot the dimensionless radii of the drop base and of the wetted circle on the surface of the porous substrate, the relative dynamic contact angle, and the effective contact angle inside the porous substrate. All these experimental data fell onto universal curves when the spreading of different silicone oils is done on porous substrates of similar pore size and porosity.