High Internal Phase Emulsions: Interfacial, Rheological and Transport Studies

In this work, we report parametric experimental and modeling studies to explore the interfacial, rheological and pipeline transport properties of high internal phase emulsions (HIPEs). We use a model composition of HIPE formulation to imitate the interfacial and rheological properties of emulsion explosives. These HIPEs are typically stabilized by poly(isobutylene) succinic anhydride (PiBSA)-based emulsifiers, and have complex interfacial and rheological properties. We measure the dynamic interfacial tension for different concentrations of PiBSA based emulsifiers using pendant drop tensiometry. The adsorption behavior of four types of emulsifiers are explored at oil--water and toluene--water interfaces. It was noticed that the adsorption kinetics of these emulsifiers follow a non-standard adsorption process. Modeling studies of typical two rate adsorption processes are also discussed to explain this data. Further, the rheological properties of high internal phase emulsions (HIPEs), comprising polydisperse aqueous droplets in oil, have been characterized as a function of emulsification time, salt concentration, phase mass fractions, and aging. The droplet size distribution and structural details of the emulsion samples were obtained using cryogenic-scanning electron microscopy (cryo-SEM) and optical microscopy. The amplitude sweep tests performed on HIPE samples with a high mass fraction of dispersed phase (93.5 wt%) show that the strain behavior, especially the yield strain and crossover strain, are almost independent of the droplet size and polydispersity. However, emulsions with smaller droplets have higher yield stress and storage moduli values; explanations for these observations, based on the physical properties of the systems are suggested. Furthermore, it is observed that, for constant mass fractions of oil and aqueous phases, the strain behavior is also independent of the salt concentration in the dispersed phase. Our findings indicate that, independent of the salt concentration, the energy requirement for the emulsion to start flowing is greater when smaller droplets are present. Aging studies, performed over a period of 6 months, show no significant change in the rheological properties of the HIPEs. Experimental rheology data is compared to the Princen and Kiss model, and with a modified Mougel model, giving insight into the critical effects of non-ideality induced by polydispersity in thickened emulsions. Further, we report experimental studies of pipeline transportation of highly viscous fluids as a water-lubricated core-annular flow (CAF). The pressure drop and flow properties in horizontal CAF are reported for two such fluids: i) furnace oil, and ii) a high internal phase emulsion (HIPE). In furnace oil--water CAF, the height profiles (with respect to the centerline of the pipe) of the upper and lower interfaces of the core are obtained using a high-speed camera and image analysis. Time series of the interface height are used to calculate the average holdup of the oil phase and speed of the interface, as well as the power spectra of the interface profile. We find that the ratio of the effective velocity of the annular fluid to the core velocity, alpha=Ua/Uc, shows a large scatter. Using the average value of this ratio (0.74) yields a good estimate of the measured holdup for the whole range of flow rate ratios, mainly due to the low sensitivity of the holdup ratio to the velocity ratio. Dimensional analysis implies that, if the thickness of the annular fluid is much smaller than the pipe radius, then, for the given range of parameters in our experiments, the nondimensional interface shape, as well as the nondimensional wall shear stress, can depend only on the shear Reynolds number and the velocity ratio. The spectral analysis of the interfacial waves indicates that, at low values of shear Reynolds number, the energy in the spectrum peaks at lower wavenumber (near k*~0), while at high Re, it peaks at around k*~0.6. A bimodal behavior is seen at the intermediate values of Re. Furthermore, our experimental data show that the effective wall shear stress is, to a large extent, proportional to the square of the core velocity. Using the implied scalings for holdup ratio and wall shear stress, we can derive an expression for the pressure drop across the pipe in terms of the flow rates, which agrees well with our experimental measurements. Similar studies on HIPE--water CAF indicate that alpha has a larger scatter than oil--water CAF, and thus the holdup model (with average alpha) underpredicts the holdup at lower Qa/Qc, and overpredicts at higher Qa/Qc. In CAF, the surface deformation of the HIPE was found to depend on its rheological properties. The HIPE with higher yield stress and storage modulus values had negligible surface evolution, and a relatively smooth (HIPE--water) interface, compared to the interface of oil--water CAF. It was also observed that the coefficient of friction in oil--water CAF was higher than that with HIPE--water CAF. The major effect of rheology of the HIPE, with continuous phase mass fraction of 15 wt%, appears to be a thinning of the core, which in turn leads to slightly higher core velocity, compared to HIPE with continuous phase mass fraction of 6.5 wt%.