Manuscript on Elastic Ripening

Together with our collaborators in Eric Dufresne’s group at ETH Zürich, we published a manuscript on the ripening dynamics of droplets embedded in an elastic environment on arxiv this morning. In this work, we watch the dynamics of oil droplets in elastic PDMS gels with stiffness gradients. In an earlier publication in PRX, they have shown that the PDMS gel restricts droplet growth, leading to mono-disperse emulsions. Here, the idea is that a growing droplet has to work against the gel’s elasticity, implying slower growth. Accordingly, we observe that droplets in stiffer regions shrink while droplets in softer regions grow when there are stiffness gradients. We recapitulate this elastic ripening in a simple simulation by assuming that the elastic gel adds to the internal pressure inside the droplet and that droplet monomers are transported by diffusion in the dilute phase. This simple theory, even neglecting surface tensions, qualitatively matches the experimental data without any fit parameters. This illustrates that stiffness gradients can affect droplet dynamics much more strongly than Ostwald ripening, in particular, if the surface tension is low. Since low surface tensions are typical for biomolecular condensates, we believe that elastic ripening is significant for placing condensates inside cells. Indeed, Cliff Brangwynne’s lab has recently shown a qualitatively similar effect in the nucleus.