  The fluid-structure interactions between flexible fibers and viscous flows
play an essential role in various biological phenomena, medical problems, and
industrial processes. Of particular interest is the case of particles freely
transported in time-dependent flows. This work elucidates the dynamics and
morphologies of actin filaments under oscillatory shear flows by combining
microfluidic experiments, numerical simulations, and theoretical modeling. Our
work reveals that, in contrast to steady shear flows, in which small
orientational fluctuations from a flow-aligned state initiate tumbling and
deformations, the periodic flow reversal allows the filament to explore many
different configurations at the beginning of each cycle. Investigation of
filament motion during half time periods of oscillation highlights the critical
role of the initial filament orientation on the emergent dynamics. This strong
coupling between orientation and deformation results in new deformation regimes
and novel higher-order buckling modes absent in steady shear flows. The primary
outcome of our analysis is the possibility of suppression of buckling
instabilities for certain combinations of the oscillation frequency and initial
filament orientation, even in very strong flows. We explain this unusual
behavior through a weakly nonlinear Landau theory of buckling, in which we
treat the filaments as inextensible Brownian Euler-Bernoulli rods whose
hydrodynamics are described by local slender-body theory.
