Non-liquid dynamics during the coalescence of colloidal and cellular aggregates

The coalescence of liquid-like aggregates made of mesoscopically large units – such as colloids or epithelial cells – is ubiquitous in industrial applications dealing with emulsions, and in studies of living biological tissues. In these systems inertia plays little role, thus we numerically study the coalescence of both particulate and biological aggregates via Brownian dynamics. We find that the physics of aggregate coalescence is different from liquid droplet coalescence, both at early stages (where thermal capillary effects are important) and at intermediate and late stages (where we can compare with direct solutions of the Navier-Stokes equation). We also find quantitatively different scaling laws for the neck growth in our two microscopic models, which we attribute to the importance of “metric” vs “topological” interaction potentials. We use these discrete models to connect both the microscopic force laws and the equations of motion to the macroscopic behavior of these unconventional soft matter systems. By comparison with the well-understood properties of conventional liquid drops, we uncover a range of previously unidentified possibilities for how meso-scale droplets can coalesce.