Preparation of monodisperse water-in-oil emulsions using microchannel homogenization

The droplet size and uniformity of water-in-oil (W/O) emulsions are important properties governing their stability in diverse applications. Monodisperse emulsions are preferred over polydisperse emulsions because their droplet size and distribution are easier to control. Here, we prepared (relatively) monodisperse W/O emulsions using microchannel homogenization (MCH) with an asymmetric straight-through microchannel (MC) array chip. Numerous asymmetric straight-through MCs, each comprising a microslot and circular microhole (diameter: 10 µm), were microfabricated on a silicon-on-insulator chip. We investigated the effects of emulsifier concentration, continuous-phase viscosity, dispersed-phase volume fraction, and the operating pressure of coarse W/O emulsions on the preparation of W/O emulsions, droplet size, and size distribution. The continuous phase was composed of soybean or medium-chain triglyceride oil solutions with tetraglycerin monolaurate condensed ricinoleic acid ester as the emulsifier. The dispersed phase was a Milli-Q water solution containing NaCl and/or polyethylene glycol (m.w. 20,000). The results demonstrate the suitability of MCH for the preparation of monodisperse W/O emulsions with an average droplet diameter of 12–15 µm and 8% lowest coefficient of variation using hydrophobically surface-treated asymmetric straight-through MCs with appropriately selected emulsion compositions and operating conditions.