HOLE FORMATION AND SHEETING IN THE DRAINAGE OF THIN LIQUID-FILMS

Foam, emulsion, and pseudoemulsion films stabilized by surfactants above the critical micelle concentration exhibit pronounced stepwise thinning or stratification. Dynamic removal of a layer is initiated by the formation and subsequent sheeting of a hole (or holes) at each step of the stratified thinning. We outline a nonlinear hydrodynamic stability analysis which successfully models the dynamics of hole formation and sheeting by accounting for an equilibrium oscillatory structural component in the disjoining pressure isotherm. Initiation of a hole is attributed to the nonlinear growth of a hydrodynamic instability during film thinning under the action of a constant capillary pressure and an oscillatory disjoining pressure. Subsequent growth or sheeting of the hole is explained by outward fluid flow within the inhomogeneous thin film as radial pressure gradients develop from curvature variations and viscous resistance to flow. Qualitative comparison is made between the proposed hydrodynamic theory and experimental observations of thinning events and rates.