Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics

The aim of the paper is to examine the adsorption kinetics of soft microgels and to understand the role of fundamental parameters such as electrostatics and deformability on the process. This knowledge is further exploited to produce microgel-stabilized emulsions using a co-flow microfluidic device. Uncharged microgels made of poly(N-isopropylacrylamide) are synthesized with variable cross-linker contents, and charged ones are produced by introducing pH sensitive co-monomers during the synthesis. The study is carried out by measuring the microgels adsorption kinetics by means of the pendant drop method. The surface pressure is derived from the previous results as a function of time and is measured as a function of the area compression using a Langmuir trough. Emulsions are produced using a microfluidic device varying the microgels concentration and their stability is visually assessed. The microgels deformability as well as higher particle concentrations favour their adsorption. The adsorption is not governed by diffusion, it is cooperative and irreversible. Conversely, the kinetics is slowed down for increasing cross-linking density. The presence of charges slows down the kinetics of adsorption. In the presence of electrolyte, the kinetics accelerates and becomes similar to the one of neutral microgels. The original features of microgel adsorption is highlighted and the differences with adsorption of polymers, star polymers, proteins, and polyelectrolytes are emphasized. Taking benefit from the adsorption kinetics, the required formulation conditions for producing microgel-stabilized emulsions using a co-flow microfluidic device are derived. There exists a critical concentration above which microgels spontaneously adsorb in a sufficient way to decrease the interfacial tension. This critical microgel concentration increases with the cross-linking density and is higher for charged microgels. Whatever the kinetics, the same surface pressure is finally reached. This peculiar behaviour is likely a consequence of the presence of dangling chains in the as-prepared microgels. Consequently, a microgel excess is required to produce emulsions using microfluidics where adsorption has to be spontaneous.