Tunable asymmetric swimming in biflagellate microswimmers.

Many biological microswimmers can modulate their swimming gait to achieve directional control of motility, especially when performing steering towards specific directional cues. This can be achieved without the need for obvious morphological or structural asymmetries in the form of the organism, or in the number or organization of propulsion-generating appendages such as cilia. In this work, we identify and validate a core principle of asymmetric planar turning in biflagellate microswimmers: cilia-induced forces may interact constructively to drive translation while interacting destructively to drive rotation. We explore the ramifications of this tunable biflagellar swimming mechanism across a range of systems, from a simple, back-of-the-envelope model to a detailed computational representation of an exemplar swimmer. This leads to a general quantitative relation between the key drivers of asymmetry, such as ciliary beat frequency, and the curvature of emergent trajectories. We discuss how the model green alga Chlamydomonas reinhardtii, which actuates its two cilia in a symmetric breaststroke for forward swimming, may exploit this feature for phototaxis. Finally, we validate our predictions in a C. reinhardtii-inspired robophysical model, implementing closed-loop control to achieve phototactic turning in the plane.