Bacterial Pickering emulsion stability quantified using microfluidic droplet coalescence tests

Pickering emulsions are typically stabilized with passive colloids. By replacing these passive agents with active bacteria, Pickering emulsion functionality can be enhanced; however, the complexity of the bacterial surface, which depends on surface-expressed molecules, presents major challenges to their utilization. To develop a framework for understanding bacterial-interface interactions, we combine dynamic microfluidic droplet-stability tests with a theoretical model of cell-interface interaction potentials.

We describe a microfluidic assay to evaluate bacterial stabilization of droplets against coalescence, testing several species and mutants over a range of cell concentrations. We characterize the zeta-potential and three-phase contact angle of each strain to use as inputs for an extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) model, which we use to interpret our emulsion stability and cell coverage results.

Our microfluidic assay reveals two distinct patterns of emulsion stabilization. In one, emulsion stability increases as cell concentration increases, consistent with classical Pickering emulsion stabilization. In the other, we observe an unexpected decrease in stability with increasing cell concentration for bacteria with larger negative charge and lower hydrophobicity. We propose a mechanism that these bacteria electrostatically stabilize droplets at low concentrations while promoting coalescence at high concentrations through a droplet-bridging mechanism. In addition to our XDLVO model, we further extend our analysis to account for the influence of bacterial appendages, which we show play a dominant role in interface breaching. Our model and results demonstrate the potential for developing a framework for evaluating bacteria to use as Pickering emulsion stabilizers.