Spontaneous diffusion induced suction between bodies in stratified fluid

We present an experimental and computational study of attractive forces that emerge in stable stratification between bodies and walls. Direct numerical simulation of fluid flow and density fields for fixed bodies show the suction producing asymmetric deformation of density isolines resulting in a self-induced propulsive force. In turn, computational simulations for a moving sphere reveal a surprising balance between the pressure and viscous stresses allowing particles to approach nearby boundaries with a steady acceleration. The particle effectively screens the tremendous resistive lubrication forces in a low Reynolds number flow by significantly reducing the scale on which viscous drag dominates and slows the approach. We further explore the universality of this mechanism across geometries of cylinders, ellipsoids, and arrows, and demonstrate that porosity of the particulate material can switch the interaction to being repulsive. Simulations are validated against fixed body PIV experiments and dynamic experiments of spheres approaching a wall.