Dr. David Zwicker
MPI for Dynamics and Self-Organization, Göttingen

Phase separation has emerged as an essential concept for the spatial organization of biomolecules inside cells since it explains how spontaneously forming droplets serve as membrane-less subcellular compartments. Yet, traditional theories of phase separation cannot explain how cells control these droplets. To unveil part of this mystery, I present two mechanisms that cells use. First, I focus on the interactions of droplets with the surrounding cytoskeleton. Inspired by synthetic experiments based on oil droplets in PDMS, I show how monodisperse emulsions form when droplets grow in a mesh that can break and re-arrange.  I also demonstrate that stiffness gradients cause elastic ripening, which biases droplets toward softer regions. In the second part, I discuss chemical reactions that influence the physical properties of droplet components. When such reactions are driven away from equilibrium, they can control droplet size and position so that multiple droplets can stably coexist. These two examples demonstrate that heterogeneous, living cells can regulate the size, number, and location of their droplets. Similar mechanisms may allow controlling droplets in technical applications in the future.