EPJ E Topical review - Lift at low Reynolds number

Details

Published on 14 November 2023

In hydrodynamics, a lift phenomenon arises when a force acts on an object perpendicularly to its initial motion. In everyday life, we are familiar with this effect allowing for instance planes to take off or soccer balls to follow bent trajectories. For such big and fast objects, inertia combines with symmetry breaking (wing shape profile or ball rotation) to give rise to lift. However, lift forces are also at play at low Reynolds numbers, i.e. for small objects or slow flows where fluid viscosity dominates over inertia.

Such forces stem from the key role played by the flow boundaries and the deformability of the objects involved: velocity gradients, elastic deformations or transport in boundary layers can lead to the emergence of lift forces. These are crucial in many soft matter and biophysics problems such as flows of suspensions, particle sorting, joint lubrication or blood circulation. In a new Topical review published in EPJE, Lionel Bureau and Gwennou Coupier (Univ. Grenoble Alpes, CNRS, France) and Thomas Salez (Univ. Bordeaux, CNRS, France) review three important mechanisms that give rise to lift and have initially been studied by separate research communities: (i) soft lubrication occurring when an object flows in the close vicinity of a deformable wall, (ii) elastohydrodynamic effects taking place when a deformable object is placed in a flow gradient, and (iii) electrokinetic lift arising from the transport of ions in the surface of an electrically charged object. They describe the main ingredients at the origin of such lift forces, discuss their respective magnitude and relevance, and point to other possible yet unexplored means of generating lift forces at zero Reynolds.