Adhesion differentials control the rheology of biomimetic emulsions

Animal morphogenesis involves complex tissue deformation processes, which require tight control over tissue rheology. Yet, it remains insufficiently understood how tissue rheology results from the interplay between cellular packing and cellular forces, such as cortical tension, cell pressure, and cell-cell adhesion. Here, we follow a biomimetic approach to study this interplay. We mimic adhesive cells with oil droplets whose adhesion strength and specificity can be flexibly tuned. Using microfluidics, we expose 2D emulsions to an oscillatory geometry imposing cyclic pure shear, and we develop a geometric method to quantify their rheology using only imaging data. We find that some of the emulsions made of two droplet types progressively change their yielding behavior across subsequent shear cycles. Combining this with vertex model simulations, we show that the observed shift in yielding behavior is due to a progressive compaction, which only occurs in emulsions with a high adhesion differential and only when exposed to oscillatory shear. Gradients of cell compaction have been observed during animal development. Our work demonstrates how such gradients can be used to control gradients of tissue rheological properties. Moreover, the progressive compaction suggests the emergence of a pumping mechanism, which potentially acts in many cellular materials, from foams to tissues.