Spatio-temporally resolved dynamical transitions in flow of Pickering emulsions through porous media

Pickering emulsions, when formulated using suitable particles, can exhibit both droplet-droplet and droplet- grain attraction while flowing through porous media. Despite their potential impact on the flow behavior of Pickering emulsions, the underlying mechanisms and dynamics of these attractions are not yet fully understood. To address this, we developed a custom-built micro-sandpack apparatus coupled with confocal microscopy to simultaneously measure the pressure gradient and visualize the flow patterns of emulsions. Our results reveal that the flow of attractive Pickering emulsions displays large-amplitude oscillations in the pressure gradient which cannot be explained using traditional models (e.g. geometric straining, filtration, or continuum). We have identified localized flow patterns characterized by particle attachment and deposition to nearby grains, droplet deposition, jamming of droplets within the throat, and droplet release. These patterns occur cyclically, corresponding to the gradual and rapid changes in the overall pressure gradient. Photonic force microscopy confirmed the strength of droplet-droplet attraction and revealed the underlying cause of this flow dynamics: the formation of chains of nanoparticles that tether droplets together. Our work provides new insights into the mechanism for self-regulated morphological evolution of Pickering emulsion during flow through porous media, resulting from the combined effects of droplet jamming/straining (in line with prior observations of emulsion flow through porous media) and particle deposition (typically observed in the flow of particulate suspensions).