Cavitation controls droplet size in elastic media
Importance
Biological cells use liquid-like droplets to distribute their proteins in space. These droplets are usually surrounded by an elastic matrix, such as cytoskeleton or chromatin, but how the matrix affects the droplets is unclear. To elucidate this, we introduce a theoretical model describing experiments in a synthetic system. We show that regular arrangements of large droplets of controlled sizes coexist with droplets on the mesh size when the elastic matrix can rupture under stress. We thus show how elastic matrices can control droplets. Similar effects could play a role in biological cells, and our theory forms the basis for further investigation.
Abstract
Biological cells use droplets to separate components and spatially control their interior. Experiments show that the complex and crowded cellular environment affects the arrangement of droplets and their size. To understand this behavior, here we construct a theoretical description of the growth of droplets in an elastic matrix, which is motivated by experiments in synthetic systems where monodisperse emulsions are formed upon a drop in temperature. We show that large droplets only form when they break up the surrounding matrix during a cavitation event. The energy barrier associated with cavitation stabilizes small droplets of the order of the size of the meshes and decreases the stochastic effects of nucleation. Therefore, the cavity droplets have similar sizes and strongly correlated positions. In particular, we predict the density of cavity droplets, which increases with faster cooling, as in the experiments. Our model also suggests how the adjustment of the cooling protocol and the density of nucleation sites affects the droplet size distribution. In summary, our theory explains how elastic matrices affect droplets in the synthetic system, and it provides a framework for understanding the biological case.