Coalescence or fusion of cellular clusters is essential in embryonic development and tissue engineering. Previous works in this field often use a highly approximate theory of viscous sintering from the '40s as the basis for measurement and comparison. However, there are numerous unique features that distinguish cellular aggregates from other squishy soft matter systems, as shown in Fig.1.
For example, will the unique interactions independent of distance but dependent on neighboring relations, which can be simulated with the vertex model (shown in Fig.2) or Voronoi model, influence the dynamics of coalescence? We need new theories for cellular clusters!
But, before we dig into these specific features of cellular clusters, one question arises, and surprisingly, with no answer yet: How about the difference in microscopic dynamics? Will this influence the coalescence? Cellular aggregates share a common feature with many other complex liquid systems, such as colloids and polymers: a strong coupling to a dissipative background.
We run simulations with standard Kob-Andersen liquids and evolve two contacting droplets according to standard molecular dynamics and Brownian dynamics. To our surprise, the coalescence dynamics are very different! The difference can be directly told by eyes in Fig. 3. We also find lurking evidence in existing experiments on the coalescence of nucleoli for this qualitative difference. Moreover, there are new power laws governing the time evolution of geometric observables and new characteristic time- and length- scales. Besides simulations, we also explain the new behavior with Darcy's law and a generalized Navier-Stokes-like Equation. Check our preprint Coalescing Clusters Unveil New Regimes of Frictional Fluid Mechanics for more details. What is more exciting is that we are still at the starting line and there are a bunch of new things to try, including the vertex model that we have started.