  We describe simulations of a microscopic elastic filament immersed in a fluid
and subject to a uniform external force. Our method accounts for the
hydrodynamic coupling between the flow generated by the filament and the
friction force it experiences. While models that neglect this coupling predict
a drift in a straight configuration, our findings are very different. Notably,
a force with a component perpendicular to the filament axis induces bending and
perpendicular alignment. Moreover, with increasing force we observe four shape
regimes, ranging from slight distortion to a state of tumbling motion that
lacks a steady state. We also identify the appearance of marginally stable
structures. Both the instability of these shapes and the observed alignment can
be explained by the combined action of induced bending and non-local
hydrodynamic interactions. Most of these effects should be experimentally
relevant for stiff micro-filaments, such as microtubules.
