Motion on Surfaces Diagram
Crawling is a well-known mechanism for motion on surfaces but is ineffective for fast migration in tissues, where cells have to squeeze through tiny gaps. In contrast, tractionless self-propulsion is well suited for that purpose. (A. Loisy, J. Eggers, T. Liverpool/University of Bristol)

Cells are observed to “crawl” by attaching themselves to a surface and using these anchor points to push themselves forward (like crawling on the ground). Scientists have identified a different propulsion mechanism particularly suited for cell motion in tissues – one that doesn’t rely on force transmission through anchor points.

Self-propulsion without traction is possible for “active” matter such as cells. Active matter is a special kind of matter in which metabolic energy is constantly converted into mechanical energy. This ability to generate mechanical forces internally, in the bulk, is what allows the drop to move without exerting forces at its boundaries (the walls).