Simultaneous Effects of Thermal Cycling and Shear on Flow Instabilities of Phase-Change Nanoemulsions Measured by Rheo-NMR and MRI Velocimetry

Organic phase change materials (PCMs) for thermal energy storage can be emulsified in water in the presence of surfactants to enable their use as pumpable heat transfer fluids. However, PCM nanoemulsions often exhibit instabilities during thermal cycling and shear flow that limit their use. To investigate their combined effects, rheological nuclear magnetic resonance (rheo-NMR) spectroscopy and magnetic resonance imaging (MRI) velocimetry methods were applied on a model octadecane-water-stearic acid system. Rheology measurements indicated that the viscosity exhibited hysteresis during thermal cycling, which correlated with the solid fraction of octadecane. Fluid velocity profiles and concentration distributions of liquid octadecane were noninvasively measured in a Searle cell. Nonlinear fluid velocity profiles developed after the octadecane solid-to-liquid phase transition, which recovered to linear profiles after octadecane melting at lower shear rates but notably not at higher shear rates. Nonuniform concentrations of liquid octadecane were measured during thermal cycling, a result of shear-induced mass transport, which causes local viscosity gradients that can lead to hydrodynamic instabilities and nonlinear fluid velocity profiles. The results not only show how shear affects flow instabilities in PCM nanoemulsions during thermal cycling but also demonstrate that this NMR methodology is a powerful tool for noninvasively measuring flow and concentration profiles in complex fluids.