Screening, hyperuniformity, and instability in the sedimentation of irregular objects

Sedimentation, the settling of colloidal objects under gravity, involves long-ranged flow-mediated interactions among the objects. Consequently, sedimenting suspensions exhibit a complex many-body dynamics with strong fluctuations, even in the case of athermal (non-Brownian) objects at zero Reynolds number. We study the sedimentation of objects of irregular shape by employing a fluctuating hydrodynamic model. We find that the anisotropic coupling between the external drive, hydrodynamic interactions, and objects' orientations, directly suppresses concentration and velocity fluctuations. This allows the suspension to avoid the anomalous fluctuations predicted for suspensions of spheres or symmetric spheroids. The suppression of concentration fluctuations leads to a dynamic, hyperuniform structure. For certain object shapes, the anisotropic response may act in the opposite direction, destabilizing uniform sedimentation.