Field driven transition of artificial membraneless organells: from spheres to discoids to prolate ellipsoids

The shape of an object can give them a distinctive transport advantage, evolutionarily helpful in complex organisms. For example, the discoidal shape of red blood cells and their ability to deform allow them to pass through channels with a diameter smaller than their own. Inspired by such non-classical shapes in natural systems, we studied external field driven shape transitions in one of the ubiquitous biological materials: membraneless organelles (MLOs). MLOs are a class of cellular compartments formed by liquid-liquid phase separation of biopolymers that do not have, otherwise commonly found, the lipid membrane. We found that the artificial MLOs made by phase separation of polyelectrolytes (also known as coacervates) show peculiar shape transitions under an external electric field. These transform from their equilibrium spherical shape to short-lived discoid before elongating into prolate ellipsoid aligned orthogonal to the applied field direction. We attribute these peculiar shape transitions to electrohydrodynamic flows combined with exceptional dielectric properties of polyelectrolyte MLOs. Such field-induced non-classical shape transitions can be directly utilized in the de-jamming of microfluidic colloidal flows.