Shear-induced emulsion droplet diffusion studies using NMR

Shear-induced droplet diffusion of flowing hard spheres is relatively well understood and has been extensively studied both experimentally and via simulations. The same however is not true of soft spheres, specifically emulsions, despite their broad and extensive industrial relevance. Here we seek to demonstrate that appropriate NMR techniques can be used to quantitatively measure shear-induced droplet diffusion. Limited literature indicates that dilute dispersions of soft spheres experience significantly larger shear-induced droplet diffusion relative to otherwise equivalent hard sphere suspensions. Here we explore whether this effect persists to high concentrations.

Nuclear Magnetic Resonance (NMR) pulsed field gradient (PFG) techniques were used to measure shear-induced droplet diffusion for capillary flow of various water-in-oil (w/o) emulsions in a direction transverse to flow. Two adaptations were necessary - the acquired signal was analyzed so as to quantitatively distinguish restricted molecular diffusion within the emulsion droplets from shear-induced diffusion of the droplets, whilst flow-compensated PFG pulse sequences were shown to be necessary to account for any erroneous effects due to flow. A range of w/o emulsions were considered to enable measurement of shear-induced droplet diffusion as a function of both water content and mean shear rate. The surfactant content of these emulsions was adjusted such that they presented similar (stationary) emulsion droplet size distributions (DSD) which were also measured using NMR PFG techniques.

The droplet shear-induced diffusion data for the emulsion systems were compared against relevant results from the literature. Consistent with predictions for dilute systems, significantly greater droplet diffusion was measured relative to hard sphere suspensions at all concentrations, and a quadratic dependence was found between droplet diffusion and mean droplet size. For more concentrated emulsions, a peak in the droplet diffusion-concentration relationship was observed for the first time in emulsions, prior to the onset of emulsion inversion.