Figure 1.

Actin network growth from a surface. (A) Sketch of an actin network (red mesh) growing from a surface (gray) where actin nucleators (blue) are located. The surface is locally subject to an external normal stress σ_nn and the network grows at velocity v_p. The actin filaments interacting with the surface can be either associated to nucleators or dissociated from them. Dissociated filaments, which polymerize toward the surface and generate an average pushing force f_d, associate to nucleators at a rate k_a. Filaments associated to nucleators resist the growth with an average force f_a per filament and dissociate from nucleators at a rate k_d. (B) Nature of the resistive force: After their initial attachment to a nucleator at the surface, attached filaments are continuously loaded by network growth during their lifetime. The initial attachment of a detached filament to a surface nucleator at t = 0 does not impose any instantaneous stretch on either the actin network or the new filament-nucleator link. Once attached to the surface, the new attached filament resists an increasing load over time as a result of network elongation. After a time t, the network has advanced a distance $v_{p}t$, and the combination of this network elongation with the attachment of the filament at the surface generates deformations $\delta \left(t\right)$ and ${\delta }_{\ell }\left(t\right)$ of the actin network and filament-nucleator link, respectively. (In color online.)