Microscale solute flow probed with rotating microbead trapped in optical vortex

Weronika Lamperska,Sławomir Drobczyński,Jan Masajada

doi:10.1007/s00348-021-03223-6

Weronika Lamperska, Sławomir Drobczyński + Show 1 more

Open Access

https://doi.org/10.1007/s00348-021-03223-6

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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.Graphic abstract

Highlights

Optical vortex is a singular beam of a helical wavefront, which means that the phase of the vortex beam circulates around the optical axis (Vasnetsov 1999, Soskin 2001)
It has been speculated that the surprising diffusion dynamics in the bicircular chamber is a heat-related effect stemming from the halogen lamp used as an external white light illumination
Two-laser optical tweezers were used for trapping metallic microbeads in optical vortex traps

Summary

Introduction

Optical vortex is a singular beam of a helical wavefront, which means that the phase of the vortex beam circulates around the optical axis (Vasnetsov 1999, Soskin 2001). The cross section of the optical vortex is a bright ring with a dark disk in the center (Fig. 1b). Transparent particles of the radius smaller than the radius of the vortex beam (e.g., submicron-sized particles) are typically trapped by the bright ring and move along it (Simpson 2010, Bacia 2015). The transparent particles large enough to cover the bright ring of the optical vortex beam are trapped at the beam center (Simpson 2010) and so are opaque particles of any size. The MB placed inside the dark disk is surrounded by the bright ring of high light intensity and gets repelled from the ring toward its center due to radiation pressure. The MB is trapped in the dark disk in x–y plane and partially along the optical axis z

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