Lift at low Reynolds number

TL;DR

This review explores how boundary effects in low Reynolds number flows can generate lift forces, challenging traditional views that associate lift mainly with inertia and high Reynolds numbers.

Contribution

It synthesizes recent research on lift forces in viscosity-dominated flows, emphasizing boundary effects and unifying diverse findings across soft and biological matter.

Findings

Boundary softness influences lift generation

Flow gradients and surface charges induce lift at low Reynolds numbers

Recent studies reveal novel lift mechanisms in biological and soft matter

Abstract

Lift forces are widespread in hydrodynamics. These are typically observed for big and fast objects, and are often associated with a combination of fluid inertia (i.e. large Reynolds numbers) and specific symmetry-breaking mechanisms. In contrast, the properties of viscosity-dominated (i.e. low Reynolds numbers) flows make it more difficult for such lift forces to emerge. However, the inclusion of boundary effects qualitatively changes this picture. Indeed, in the context of soft and biological matter, recent studies have revealed the emergence of novel lift forces generated by boundary softness, flow gradients and/or surface charges. The aim of the present review is to gather and analyse this corpus of literature, in order to identify and unify the questioning within the associated communities, and pave the way towards future research.