Microfluidic angle of repose test for Pickering emulsions

Abstract

Pickering emulsions are appealing systems for droplet-based microfluidic technology. However, to date, their implementation in experiments is limited by their intrinsic characteristics: they present a strong gel-like behaviour and their flowability is lower than their surfactant-stabilized counterpart. Measuring the rheological properties of Pickering emulsions is therefore of interest but very few methods provide measurements using low volumes (1 ml). Here, we propose a microfluidic version of the angle of repose method to study the flow behaviour of fluorinated Pickering emulsions adapted to small volumes of the sample. We observed a strong link between the nanoparticle level of fluorination and the flow behaviour of the water-in-oil (w/o) emulsions, obtaining a dramatic change from an empirically very poor flow behaviour to a good one by increasing the particles wettability by the fluorinated phase. Our method combines the advantages of microfluidic rheometers, such as low sample consumption and visual monitoring of the complex fluid elements, and allows the study of emulsions with a strong level of aggregation between the droplets.