Measurement of tack of Newtonian liquids on porous substrates

Probe-tack experiments of Tirumkudulu [Phys. Fluids 15, 1588 (2003)] have shown that squeeze flow of Newtonian liquids on flat, impermeable substrates can be successfully modeled using the lubrication approximation. Here, we present a model for squeeze flow of Newtonian liquids on porous substrates where the flow in the gap is coupled to the fluid flow in the porous media. The competition of spreading and imbibition of liquid on a partially saturated porous substrate determines the force versus gap profile in both the squeeze (compression) and pull-off (tension) modes. The finite difference method was used to discretize the lubrication equation in the gap while boundary element method was employed to solve for flow in the porous substrate. The model predicts a lower magnitude of force for porous substrates in both compression and tension modes compared to that for impermeable substrates. Experiments on porous alumina substrates with Newtonian liquids show close agreement with the model predictions in both compression and tension modes when the gap is corrected for the obliqueness of the confining surfaces. Cavitation is predicted for some cases in the tension mode when the pressure in the gap reduced below the vapor pressure of liquid. (c) 200