  The actin cytoskeleton is a semiflexible biopolymer network whose morphology
is controlled by a wide range of biochemical and physical factors. Actin is
known to undergo a phase transition from a single-filament state to a bundled
state by the addition of polyethylene glycol (PEG) molecules in sufficient
concentration. While the depletion interaction experienced by these biopolymers
is well-known, the effect of changing the molecular weight of the depletant is
less well understood. Here, we experimentally identify a phase transition in
solutions of actin from networks of filaments to networks of bundles by varying
the molecular weight of PEG polymers, while holding the concentration of these
PEG polymers constant. We examine the states straddling the phase transition in
terms of micro and macroscale properties. We find that the mesh size, bundle
diameter, persistence length, and intra-bundle spacing between filaments across
the line of criticality do not show significant differences, while the
relaxation time, storage modulus, and degree of bundling change between the two
states do show significant differences. Our results demonstrate the ability to
tune actin network morphology and mechanics by controlling depletant size, a
property which could be exploited to develop actin-based materials with
switchable rigidity.
