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Living droplets: cell spreading as a wetting problem

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The shape assumed by living cells plays a major role in its physiological function. As a cell spreads on a flat substrate, it takes a shape which is only partly reminiscent of the one of a liquid droplet. Indeed, the surface concavity reverses from convex to concave close to the triple line that it forms with the substrate. This change of curvature sign is a key information about the mechanical balance that governs cell shape.

It is known that this balance is mostly between forces exerted within a thin material layer at the cell surface, made of a polymeric network and called the actin cortex. Within this cortex, motor-molecules of myosin generate a mechanical tension that can be modelled as a surface tension. This brings forward the comparison with the case of liquid droplets. However, the reversal of curvature sign indicates that one or more additional phenomena are at play and modify cell balance. We have already acquired experimental observations and quantifications that open the way to tackling this problem, first in the quasistatic case and then in the dynamic case. Solving it will allow us to understand the mechanisms that determine cell shape.

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