Formation, flow and dynamic stability of foams generated from viscous shear-thinning fluids

Foams are complex, multi-component, structures that are present in a wide range of industrial applications such as foods, pharmaceuticals, mineral transport, oil and gas. Specifically, in the food industry, foaming is ubiquitous since many foamed products, for example ice cream, whipped cream and chocolate mousse contain air in the form of microscopic bubbles. These bubbles reduce the number of calories, impart better texture and improve organoleptic properties. Therefore, a foaming operation which can achieve a good degree of control of the air volume fraction and bubble size distribution is of paramount importance. In this research study, an advanced, non-invasive, X-ray micro-Computed Tomography technique is adopted to probe the three-dimensional microstructure of a range of wet foams generated from viscous pseudoplastic fluids using a pilot-scale continuous rotor-stator device. For the first time ever, an extensive study is conducted to elucidate the combined influence of processing parameters (rotor speed and gas-liquid volumetric flowrate (G/L) ratio) and liquid properties (surfactant content and xanthan gum concentration) on foam formation from shear-thinning fluids using a multi-stage rotor-stator unit. Rotor speed, residence time and G/L ratio were the dominant factors responsible for achieving fine-textured and highly statically stable foams. In addition, operating at N > 2000 rpm is undesirable both in terms of energy efficiency and product microstructure. The dynamic foam stability is investigated by passing wet foams through narrow orifice constrictions. It is established that the microstructure of wet foams is preserved when allowed to flow through a narrow orifice constriction provided there is a minimal pressure drop ( 36000 Pa), however, a combination of foam expansion and bubble coalescence is responsible for the loss of air volume and bubble coalescence. Furthermore, by increasing the rotor speed (i.e. energy input), surfactant concentration and continuous phase apparent viscosity, the rate of bubble coalescence can be significantly reduced. The effects of rotor speed, however, became only significant at a rotor speed of N = 2000 rpm. Such a wet foam, with a more uniform bubble size distribution, was able to pass through a narrow orifice constriction with a relatively less bubble coalescence. An extensive study of steady shear and dynamic oscillatory rheometry of wet foams generated from viscous shear-thinning fluids is also conducted as part of this research. At air volume fractions of 0.60, the apparent viscosity and storage moduli increase significantly as bubble size becomes smaller and more uniform with increasing rotor speed. Moreover, a novel simultaneous in-situ foam microstructure visualisation and rheometry are conducted to explicate the flow complexities that can arise when such structured fluids are imposed to higher shear rates. Simultaneous in-situ visualisation of the foams under shear confirmed the absence of bubble breakage and, for the first time, unravelled the existence of inward radial shear-induced migration of liquid, which is responsible for the time-dependence of the foams