Microscale solute flow probed with rotating microbead trapped in optical vortex

TL;DR

This paper introduces a novel optical vortex-based microbead method to study microscale solute flow and viscosity changes, revealing unexpected reverse flow phenomena with potential applications in microfluidics.

Contribution

The study presents a new technique using rotating microbeads in optical vortices to measure local solute flow and viscosity at microscale with high spatial resolution.

Findings

Detected reverse solute flow during sucrose dissolution

Demonstrated sensitivity of microbead rotation to local viscosity changes

Proposed a versatile method applicable to various substances

Abstract

The dynamics of solute flow in the microscopic chamber can be studied with optical tweezers. A method based on the metallic microbeads trapped in the focused optical vortex beam is proposed. This annular beam of a twisted wavefront exerts torque on a reflective object placed inside the dark core of the vortex. The induced rotational movement of the bead is sensitive to local viscosity changes in the surrounding medium, for example during the ongoing dissolution process. Two experimental configurations are described, both relying on tracing the angular velocity of the bead in time. In one-bead configuration the dynamics of local solute concentration can be studied. In two-bead case the direction and speed of solute flow can be probed with a spatial resolution of single micrometers. We approach the elementary problem of sucrose dissolution and diffusion in water. The surprising impression of the reverse solute flow was observed. Further experimental investigation led to the discovery that this phenomenon originates from the sucrose stream-like diffusion in the mid-depth of the measurement chamber. The rotating microbead method applies for various solid and liquid substances and may become a useful technique for microfluidics research.