Tractionless self-propulsion of an active drop: a mechanism for cell motion without adhesion

The autonomous motion exhibited by living cells, called motility, is fundamental to many physiological and pathological processes. Because it is easily observable, most studies of cell motility have focused on crawling, a mode of motion exhibited by cells on surfaces. Crawling crucially relies on focal adhesions with the surface, which allow a cell to exert traction and thereby propel itself forward. This implies that crawling is not suited for motion on nonadhesive surfaces or for moving rapidly through three dimensional environments such as tissues. How cells achieve self-propulsion in such environments remains a debated question. In this talk, we will report on a new mode of motility whose signature is the absence of traction exerted on the surrounding environment. We show, both analytically and by numerical simulations, that the equations of motion for a thin drop of active nematic (a minimal model of the cytoskeleton) on a substrate possess a simple self-propelling solution with no traction imparted on the solid surface. This mode of self-propusion is driven in the bulk (rather than at the boundaries in crawling) and provides a robust physical mechanism for adhesion-independent cell migration in crowded environments like tissues.