Pair interaction between sedimenting polar objects in the Stokes regime

We study triangles placed in a quasi-two-dimensional container and sedimented at low Reynolds number as a model of the stokesian sedimentation of polar objects. Individual polar objects, unlike non-polar axisymmetric objects, rotate as they sediment due to coupling between the orientational and translational degrees of freedom and ultimately reorient into stable orientations [Jayaweera, Mason, J. Fluid Mech. 22 (1965)]. In order to study the pair interaction between polar objects, we sediment pairs of triangles with initial conditions that include both stable and unstable orientations of the individual triangles. Surprisingly, none of the periodic motions observed [Chajwa, Menon, Phys. Rev. Lett. 122 (2019)] with non-polar object pairs are seen with pairs of triangles. Instead, we observe that triangles reorient and translate away from each other until both objects achieve a stable orientation. As a special case, we also observe that equilateral triangle pairs (which have three-fold, rather than polar, symmetry) continuously rotate in their steady state.