Surface tensions of viscous and anisotropic fluids

Abstract

The surface tensions against air and the drop shape behavior of various viscous and anisotropic fluids are studied. These tensions are important in many industrial applications and they are of particular fundamental interest in understanding the mechanical significance of tensions of uncommon fluids. Viscous liquid-crystalline phases or biphasic dispersions ( c=8–20 wt% AOT in water) of aqueous AOT surfactant (sodium bis(2-ethylhexyl) sulfosuccinate) and the gel-like aqueous polymer solutions ( c=0.1–0.6 wt%) of charged Carbopol 934 (sodium carboxy-polymethylene) are examined at 25°C in air. They are found to have smooth, convex, and axisymmetric drop shapes which satisfy various equilibrium criteria but do not obey the Laplace-Young (L-Y) equation for a uniform value of surface tension γ. A novel computational method of pendant drop profile matching in sections successfully matches these drop profiles to a semi-empirical equation, which is based on the L-Y equation (Δ P=2 Hγ where 2 H is the mean curvature and Δ P is the pressure difference across the interface) with a non-uniform tension γ. Very low γ app-values are obtained, ranging from 3 to 25 mN m −1 for AOT/water and from 6 to 70 mN m −1 for Carbopol/ water. Average surface tension gradients of 3.5 to 7 N m −2 for AOT/water and over 16 N m −2 for Carbopol/water appear to be sustainable at equilibrium by the independently determined yield stress of these fluids.