Mechanical reorganization of cross-linked F-actin networks at the air-buffer interface

The response of an F-actin network at the air-buffer interface to nonlinear compression is studied. We observe two distinct classes of behavior: a mechanically induced structural reorganization of the monolayer and a subsequent reversible, nonlinear mechanical response. Measurements of pressure area isotherms, bulk modulus, and the epifluorescence microscopy of tracer beads provide evidence that an initial structural reorganization occurs that corresponds to the mechanically induced formation of bundles of actin. Once a steady state structure is formed, subsequent compressions exhibit four distinct regimes as a function of decreasing total area: a low density fragile network, a relatively uniform film of cross-linked bundles, a region characterized by a buckling instability, and finally, a close packed region. Evidence for the distinct regions comes from the epifluorescence microscopy of tracer beads and measurements of the bulk modulus. Key features of the dynamics include domain rearrangements in the fragile network and an irregular, jerk-like motion during the buckling transition.