[SMaLS] Precise experimental evaluation of coarsening of quasi-2D droplet patterns

Speaker: Michio Tateno (Postdoctoral Fellow, Saleh Group)

Title: Precise experimental evaluation of coarsening of quasi-2D droplet patterns

Abstract: 

Ostwald ripening and Brownian coagulation are dominant processes governing the dynamics of the size statistics of phase-separated droplets. Classical theories for both mechanisms predict that the average droplet radius follows a power-law growth with the common growth exponent 1/3, which has been often used in recent studies of biomolecular condensates. However, these laws presuppose droplet patterns embedded in three-dimensional (3D) space, whereas relevant bio-molecular experiments often rely on microscopy imaging of 2D droplet patterns; the impact of this 2D geometrical constraint has not yet been subjected to rigorous experimental study. Here, we examine this effect using the DNA nanoparticle platform, which allows us to control the material properties of DNA droplets as well as droplet-substrate adhesion. In cases where droplets were selectively fixed to a substrate, we observed the Ostwald ripening process, which showed a rough agreement with the Lifshitz-Slyozov-Wagner theory. We quantitatively demonstrated that this process is extremely slow due to low surface tension of biomolecular droplets, and effectively negligible unless the Brownian motion of the droplets is intentionally immobilized. In cases where the substrate is passivated, the Brownian coarsening with growth exponent of approximately 0.2 is observed. This behavior is quantitatively reproduced by a modified Smoluchowski coagulation equation that accounts for the quasi-2D nature of the common experimental condition, i.e. with droplets obeying 3D mass conservation while diffusing in 2D. Our results clarify the physical principles of droplet size statistics that universally appear in the long-time regime when droplet sedimentation is dominant, which is particularly relevant to common in vitro experiments, and so has implications to a broad range of fields that rely on such experiments.