A basic swimmer at low Reynolds number

TL;DR

This paper demonstrates a simple, controllable fluid pumping mechanism at low Reynolds numbers using three colloidal beads manipulated by optical tweezers, breaking time-reversal symmetry to induce flow.

Contribution

It introduces a minimal model of low-Reynolds-number swimming using soft optical traps, expanding on previous theoretical models with experimental and numerical validation.

Findings

The system can generate controlled fluid flow.

Soft optical traps provide a new control parameter.

Experimental and numerical results agree well.

Abstract

Swimming and pumping at low Reynolds numbers are subject to the "Scallop theorem", which states that there will be no net fluid flow for time reversible motions. Living organisms such as bacteria and cells are subject to this constraint, and so are existing and future artificial "nano-bots" or microfluidic pumps. We study a very simple mechanism to induce fluid pumping, based on the forced motion of three colloidal beads through a cycle that breaks time-reversal symmetry. Optical tweezers are used to vary the inter-bead distance. This model is inspired by a strut-based theoretical swimmer proposed by Najafi and Golestanian [Phys.Rev. E, 69, 062901, 2004], but in this work the relative softness of the optical trapping potential introduces a new control parameter. We show that this system is able to generate flow in a controlled fashion, characterizing the model experimentally and numerically.