INVESTIGATION OF LIQUID SURFACE RHEOLOGY OF SURFACTANT SOLUTIONS BY DROPLET SHAPE OSCILLATIONS - EXPERIMENTS

The experimental results of droplet shape oscillations are reported an d applied to the analysis of surface rheological properties of surfact ant solutions. An acoustic levitation technique is used to suspend the test drop in air and excite it into quadrupole shape oscillations. Th e equilibrium surface tension, Gibbs elasticity, and surface dilatatio nal viscosity are determined from the measurements of droplet static s hape under different levitation sound pressure, oscillation frequency, and free damping constant. Aqueous solutions of sodium dodecyl sulfat e, dodecyltrimethylammonium bromide, and n-octyl beta-D-glucopyranosid e are tested with this system. The concentrations of the solutions are below the critical micelle concentration. For these solutions it is f ound that the surface Gibbs elasticity approaches a maximum at a moder ate concentration, and its value is less than that directly calculated from the state equation of a static liquid surface. The surface dilat ational viscosity is found to be in a range around 0.1 cps. (C) 1997 A cademic Press.