Shear-induced coalescence of oil-in-water Pickering emulsions

Abstract

This work reports on coalescence in oil-in-water Pickering emulsions subjected to simple shear flow. The emulsions were stabilized by silanized fumed silica particles forming layers a few hundred nanometers thick around drops that are tens of micrometers in size. The drop size and particle concentration in the emulsions were fixed, while the salt concentration was varied to adjust the colloidal interactions between the drops and particles. At rest the oil drops do not coalesce. The susceptibility of the drops to orthokinetic coalescence was found to depend on the extent of particle flocculation in the attached particle layer. The evolution of the drop size with time and shear rate was consistent with phenomenological models used to describe the behavior of emulsions under shear.

Rheological responses of Pickering emulsions prepared using colloidal hydrophilic silica particles in the presence of NaCl

“…Both quantities increase steeply with increasing NaCl concentration. Such an increase in the yield stress with increasing salt concentration near the cfc was observed for the Pickering emulsions prepared by partially hydrophobized fumed silica particles in the presence of NaCl [24,25]; the resulting yield stresses at the oil volume faction of 0.5 were less than 10 Pa. The larger yield stresses observed in this study are attributed to the higher oil volume fraction (i.e., 0.70).…”

“…Silica aggregates prepared by changes in salt concentrations have frequently been used to prepare Pickering emulsions using various silica particles such as fumed silica [21], precipitated silica [22], Stober-type silica [23], Ludox colloidal silica [23], and hydrophobic fumed silica [24,25]. Moreover, their stabilities, interfacial properties, and rheological responses have been discussed as a function of salt concentration around the cfc [21,22,24,25].…”

“…Moreover, their stabilities, interfacial properties, and rheological responses have been discussed as a function of salt concentration around the cfc [21,22,24,25]. Few studies, however, have systematically investigated the stability of Pickering emulsions prepared using hydrophilic silica particles as a function of salt concentration below and above the cfc in terms of measurements of the state of aggregated silica particles, droplet size, fraction of droplets in the emulsified layer, the amount of adsorbed emulsifier, and the rheological behavior of the emulsion at equilibrium.…”

Controlling Pickering Emulsion Destabilisation: A Route to Fabricating New Materials by Phase Inversion

“…Whitby et al [95] investigated coalescence in dilute oil-in-water Pickering emulsions (at a drop volume fraction, ϕ = 0.5 under shear applied by a rotational rheometer. The bromohexadecane drops ( r d = 35 μm) were stabilised by silanised fumed silica particles ( r p ~ 10 nm) that formed layers a few hundred nanometres thick around the drops.…”

Multi-body coalescence in Pickering emulsions

“…Little is known, however, about the coalescence of a collection of hundreds and thousands of Pickering droplets as in real emulsions. This is particularly important when particle stabilizers are used to produce near-monodispersed droplets 17 , during which the distribution of droplet size can be significantly broadened by coalescence under gravity, floatation and shear 18,19 .…”