S. A. McGlashan

A review of drainage and spontaneous rupture in free standing thin films with tangentially immobile interfaces.

A review of spontaneous rupture in thin films with tangentially immobile interfaces is presented that emphasizes the theoretical developments of film drainage and corrugation growth through the linearization of lubrication theory in a cylindrical geometry. Spontaneous rupture occurs when corrugations from adjacent interfaces become unstable and grow to a critical thickness. A corrugated interface is composed of a number of waveforms and each waveform becomes unstable at a unique transition thickness. The onset of instability occurs at the maximum transition thickness, and it is shown that only upper and lower bounds of this thickness can be predicted from linear stability analysis. The upper bound is equivalent to the Frenkel criterion and is obtained from the zeroth order approximation of the H3 term in the evolution equation. This criterion is determined solely by the film radius, interfacial tension and Hamaker constant. The lower bound is obtained from the first order approximation of the H3 term in the evolution equation and is dependent on the film thinning velocity. A semi-empirical equation, referred to as the MTR equation, is obtained by combining the drainage theory of Manev et al. [J. Dispersion Sci. Technol., 18 (1997) 769] and the experimental measurements of Radoev et al. [J. Colloid Interface Sci. 95 (1983) 254] and is shown to provide accurate predictions of film thinning velocity near the critical thickness of rupture. The MTR equation permits the prediction of the lower bound of the maximum transition thickness based entirely on film radius, Plateau border radius, interfacial tension, temperature and Hamaker constant. The MTR equation extrapolates to Reynolds equation under conditions when the Plateau border pressure is small, which provides a lower bound for the maximum transition thickness that is equivalent to the criterion of Gumerman and Homsy [Chem. Eng. Commun. 2 (1975) 27]. The relative accuracy of either bound is thought to be dependent on the amplitude of the hydrodynamic corrugations, and a semi-empirical correlation is also obtained that permits the amplitude to be calculated as a function of the upper and lower bound of the maximum transition thickness. The relationship between the evolving theoretical developments is demonstrated by three film thickness master curves, which reduce to simple analytical expressions under limiting conditions when the drainage pressure drop is controlled by either the Plateau border capillary pressure or the van der Waals disjoining pressure. The master curves simplify solution of the various theoretical predictions enormously over the entire range of the linear approximation. Finally, it is shown that when the Frenkel criterion is used to assess film stability, recent studies reach conclusions that are contrary to the relevance of spontaneous rupture as a cell-opening mechanism in foams.

Comparison of entry flow techniques for measuring elongation flow properties

Abstract The elongation viscosity of linear low density polyethylene and low density polyethylene were estimated using converging flow and stagnation flow devices. Results were obtained from the converging flow rheometer using the analyses proposed by Binding [1] and Cogswell [2] . These results were compared with an estimate of uniaxial elongation viscosity obtained with the stagnation flow rheometer similar to that proposed by Mackay et al. [3] . The results show that the elongation viscosities determined by both flow geometries are comparable. Also, the measured elongation viscosity is affected to a small degree by geometric properties for converging flow.

Comparison of shear stress and wall slip measurement techniques on a linear low density polyethylene

Abstract Small gap, torsional, parallel plate experiments allow the measurement of shear stress and slip velocity at shear rates comparable to those measured in slit and capillary rheometers. Using this technique we compare data from parallel plate measurements, where effects of pressure and temperature are negligible, to those from pressure driven flows where they are not. The shear stress for all techniques agrees within experimental error at low shear rates. At higher shear rates the slit and capillary measurements diverge from the parallel plate measurements highlighting the effect of pressure and temperature on viscosity. Also, breakdown of the Cox–Merz rule at slightly higher shear rates is apparent when comparing dynamic and steady shear data from the parallel plate experiments. Since the complex viscosity diverges in the same manner as the viscosity derived from pressure driven flow it seems as if agreement of the complex viscosity with the viscosity derived from pressure driven flow is fortuitous. With the capillary geometry, there is no sign of slip effects at low stress levels probably due to experimental limitations while the torsional rheometer shows slip at all shear stresses. There is no critical stress for the onset of slip but there is a critical stress where slip increases dramatically.