Deformable microswimmer in an external force field

External forces, such as gravity, play significant role in the swimming properties of autonomous biological microswimmers as well as artificial swimming robots. Here we have studied the influence of the external forces on the transport characteristics of the triangular bead-spring microswimmers. The microswimmer, formed by connecting three beads using three springs in an equilateral triangular arrangement, is capable of performing autonomous translational ("mover") and rotational ("rotor") motions. We show that for a mover triangle the application of a small external force results in the alignment of swimming direction with that of the external force, a phenomenon known as "gravitaxis." We demonstrate that this gravitactic nature of the active triangle is purely due to the hydrodynamic interaction among the beads. Under large external force, however, the gravitactic nature is lost. This transition from gravitactic to nongravitactic motion of the microswimmer is characterized by a saddle node or pitchfork bifurcations (depending on nature of active forces), where the strength of the critical external force scales linearly with the active force amplitude, f(e)(c) similar to f(a). However, for the rotor triangle only saddle node bifurcation is observed, which results in a vanishing angular velocity as the strength of the external force is increased. The critical value of the external force for the rotor, however, scales as f(e)(c) similar to f(a)(2/3). These findings will provide insights into the nature of biological swimming under gravity, especially the gravitactic microorganisms such as Chlamydomonas, as well as help in the design of underwater vehicles.