Interfacial tension coefficient from the retraction of ellipsoidal drops

Abstract

A new method for the determination of the interfacial tension coefficient between two immiscible fluids is proposed. The method is particularly useful for the binary polymer blends. The deformed drop retraction method, DDRM, makes it possible to determine the dynamic interfacial tension coefficient, ν, from the time evolution of a distorted fluid drop toward its equilibrium form. Analysis of this interfacial tension-driven process led to a theoretical relation between the shape retraction rate and the system's geometrical and rheological characteristics. Measurements of either low viscosity model systems or high viscosity industrial polymer mixtures led to a good agreement with values obtained from the widely used breaking thread method. DDRM enables to measure ν in polymeric blends of commercial interest—the high viscosity systems that would be very difficult to characterize by other techniques. Furthermore, for the first time it is possible to follow the time dependence of the interfacial tension coefficient, thus unambiguously determine the dynamic and equilibrium values of ν12. For example, in low density polyethylene blends with polystyrene, LDPE/PS, ν decreased with the polymer-polymer contact time, tc, from ν = 6.9 mN/m at tc = 12 min, to ν = 5.2 mN/m at tc ≥ 75 min—the latter may represent the true thermodynamic equilibrium value, ν12. However, it is not clear whether such a reduction is exclusively due to the thermodynamically driven migration of chain-ends, low molecular weight fractions and additives, or by the thermal degradation as well. The contact time dependence of ν explained some of the differences reported for the data obtained using different measurement techniques, viz. pendant drop, capillary breakup, or ellipsoid retraction techniques. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1393–1403, 1997