Viscous tweezers: controlling particle orientation with viscosity

TL;DR

This paper introduces a novel method called viscous tweezers that manipulates particle orientation by dynamically altering fluid viscosity anisotropy, enabling control over particle sinking angles without direct external forces.

Contribution

The study presents a new approach to control particle orientation through global viscosity anisotropy changes, demonstrated with explicit calculations for a pushpin-shaped particle in an anisotropic fluid.

Findings

Particle tilt angle depends on viscosity anisotropy axis.

Control over particle orientation is achievable via feedback loop.

Method applicable to various complex fluid systems.

Abstract

Control of particle motion is generally achieved by applying an external field that acts directly on each particle. Here, we propose a global way to manipulate the motion of a particle by dynamically changing the properties of the fluid in which it is immersed. We exemplify this principle by considering a small particle sinking in an anisotropic fluid whose viscosity depends on the shear axis. In the Stokes regime, the motion of an immersed object is fully determined by the viscosity of the fluid through the mobility matrix, which we explicitly compute for a pushpin-shaped particle. Rather than falling upright under the force of gravity, as in an isotropic fluid, the pushpin tilts to the side, sedimenting at an angle determined by the viscosity anisotropy axis. By changing this axis, we demonstrate control over the pushpin orientation as it sinks, even in the presence of noise, using a closed feedback loop. This strategy to control particle motion, that we dub viscous tweezers, could be experimentally realized in systems ranging from polyatomic fluids under external fields to chiral active fluids of spinning particles by suitably changing their direction of global alignment or anisotropy.