FIG. 4.

Torques experienced by objects translating along a surface at constant velocity (U). (A) A sphere experiences increased drag on the side closest to the solid, creating a torque (“surface torque”), which causes it to roll forward. (B) A prolate spheroid experiences the surface torque, as well as an additional torque (“form-drag torque”) due to the increased cross-sectional area presented to flow as the cell rotates. These torques balance at an equilibrium angle (θ).