Molecular dynamics simulations of droplet fusion reveals shear thinning

Recent experiments have shown that biomolecular condensates are viscoelastic rather than purely viscous. One consequence, from both experimental and theoretical studies of shape dynamics, is that condensates exhibit shear thickening (or thinning), i.e., with effective viscosity higher (or lower) than the zero-shear viscosity. To gain deeper physical insight into the effects of viscoelasticity on condensate dynamics, here we carried out molecular dynamics simulations of droplet fusion. Droplets formed from phase separation of Lennard-Jones particles or chains were brought together to fuse. Particle droplets have fast shear relaxation, and their fusion speeds agree with the inverse of the viscocapillary ratio (zero-shear viscosity over interfacial tension). In contrast, chain droplets have slow shear relaxation, and their fusion speeds are faster than predicted by the viscocapillary model, thereby exhibiting shear thinning. The conformations of chains indeed show the telltale sign of shear thinning, with a tendency to align with the velocity field. We hope that these experimental and simulation studies together will compel the biomolecular condensate community to abandon the viscocapillary model and to finally think condensates as viscoelastic systems with all their complexities.