Traveling bands of liquid droplets from stoichiometrically skewed binary condensation*

We investigate the kinetics of liquid-liquid phase separation using two 4-armed DNA nanostars, P and Q, the sticky ends of which are not palindromic, but rather complementary to each other. Using a vinyl microfluidic Y-shaped channel, we generate long sharp interfaces between two solutions that are accessible to fluorescence microscopy. By observing the time evolution of the interface between a nanostar solution and buffer alone, we measure the diffusion coefficient of a single nanostar, D_* to be 36 ± 3 μm²/s.  By monitoring the interface between a P nanostar solution and a Q nanostar solution, we observe stoichiometry-dependent kinetics in the band of liquid condensate droplets that form. When the P:Q ratio is 1:1, the droplets gradually coarsen and eventually stabilize.  Departures from 1:1 stoichiometry result in asymmetric droplet size and spatial distributions that migrate towards the lower concentration region.  We extract estimates of association and dissociation rate constants by modeling the condensation as a diffusion-limited process. These findings demonstrate the potential to control condensate stability through stoichiometry, a possibly biologically relevant mechanism.