We investigate experimentally the generic microscopic dynamical
behavior of a dust Coulomb liquid confined in a narrow mesoscopic
channel and sheared by two parallel counter laser beams along the
opposite boundaries, at the low stress limit where the shear induced
average speed is comparable to the thermal speed. A liquid confined in
a narrow channel with width down to the molecular scale usually exhibits
anomalous behavior in structure and motion deviating from the
bulk liquid, under the e®ect of discreteness, finite boundary, and lager
thermal fluctuations. It is an interesting and important issue for nanoscience
and technology. Nevertheless, the rheology studies have been
mainly limited to the macroscopic force and velocity measurement on
the confining boundary because of the lack of microscopic measures
at the small atomic scale, which is insu±cient to construct an obvious
microscopic picture for the sheared flow in a narrow channel. A dust
Coulomb liquid formed by micrometer sized dust particles charged
and suspended in a low pressure weakly ionized discharge background
is an heuristic system to mimic and understand the generic dynamical
behaviors at the kinetic level because of the capability of directly
visualization. In this work, through monitoring the spatio-temporal
evolution of micro-motion and correlating with the local structure rearrangement,
we demonstrate the following findings:
1. The mean velocity profile exhibits shear bands with high mean
shear rates in the outer region sandwiching a center zone with
low shear rate.
2. Stress enhanced avalanche type cooperative topological rearrangement
associated with the vortex type cooperative hopping involvi
ing a cluster of particles are observed in the shear band, which
are responsible for the observed enhanced longitudinal and transverse
velocity fluctuations and the higher structural relaxation
rate in the shear bands. It screens the external drive through
relaxing the local stress and leaves a weakly perturbed center
3. We also point out that the nonlinear threshold type stick-slip
hopping after accumulation of local stress is the origin for shear
thinning and shear banding which have also been observed in
glassy systems under the slow dynamics. Unlike in the glassy
system, the finite temperature e®ect leads to the absence of a
finite yield stress.
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