Irreversible hydrodynamic trapping by surface rollers

• Published in: Soft matter 2020-03, Vol.16 (1), p.2611-262
• Main Authors: Chamolly, Alexander, Lauga, Eric, Tottori, Soichiro
• Format: Article
• Language: English
• Subjects: Cargo; Computational fluid dynamics; Lateral displacement; Microfluidics; Parameters; Rollers; Rotation; Steady flow; Three dimensional models; Trapping; Two dimensional analysis; Two dimensional models
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c9sm02250c

A colloidal particle driven by externally actuated rotation can self-propel parallel to a rigid boundary by exploiting the hydrodynamic coupling that surfaces induce between translation and rotation. As such a roller moves along the boundary it generates local vortical flows, which can be used to trap and transport passive cargo particles. However, the details and conditions for this trapping mechanism have not yet been fully understood. Here, we show that the trapping of cargo is accomplished through time-irreversible interactions between the cargo and the boundary, leading to its migration across streamlines into a steady flow vortex next to the roller. The trapping mechanism is explained analytically with a two dimensional model, investigated numerically in three dimensions for a wide range of parameters and is shown to be analogous to the deterministic lateral displacement (DLD) technique used in microfluidics for the separation of differently sized particles. The several geometrical parameters of the problem are analysed and we predict that thin, disc-like rollers offer the most favourable trapping conditions. A microscopic colloidal roller propelling along a flat rigid surface can irreversibly trap and transport passive cargo through cross-streamline migration induced by steric interaction between the cargo and the interface.