  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.
