Spontaneous wall suction in stratified fluids: a new phenomenon exhibiting Aristotle’s ancient view of motion

Fluids with gravitationally stable density stratification caused by diffusing solutes are ubiquitous in nature and may spontaneously generate flows in the destabilizing presence of immersed bodies. In this paper we document the discovery of a counterintuitive phenomenon: objects in such fluids may self-induce suction forces producing near constant accelerations towards nearby walls, in the absence of any external forces. Intriguingly, the fundamental mechanism is akin to Aristotle’s ancient idea whereby motion is sustained through flows abhorring vacuum generation. Here we present an experimental, computational, and theoretical study to fully explore this new phenomenon. First, experiments exhibiting wall collapse are presented. Next, flow and density structures are measured and compared quantitatively to computational simulations with spheres and cylinders, both in free space and near symmetry-disrupting vertical walls. Further computations reveal a competition between the pressure and viscous stress forces that enable a “lubrication screening,” overcoming the resistance of a thin lubricating layer. In particular, a low pressure region in the gap develops and the particle spontaneously moves to fill the vacuum by being pushed along by ensuing flows. The resulting unexpected motion in a viscous dominated flow propels the particle almost all the way to the wall within a distance scale set by the stratified fluid properties, ultimately decelerating with a soft-landing. Lastly, extensions of these new phenomena to thin and porous geometries are discussed with theoretical and computational predictions showing how the wall-induced motion can be reversed by porosity, pushing the body away from the wall.