Kavli Affiliate: David A. Weitz
| First 5 Authors: Sijie Sun, Nan Xue, Stefano Aime, Hyoungsoo Kim, Jizhou Tang
| Summary:
Addition of particles to a viscoelastic suspension dramatically alters the
properties of the mixture, particularly when it is sheared or otherwise
processed. Shear-induced stretching of the polymers results in elastic stress
that causes a substantial increase in measured viscosity with increasing shear,
and an attractive interaction between particles, leading to their chaining. At
even higher shear rates, the flow becomes unstable, even in the absence of
particles. This instability makes it very difficult to determine the properties
of a particle suspension. Here we use a fully immersed parallel plate geometry
to measure the high-shear-rate behavior of a suspension of particles in a
viscoelastic fluid. We find an unexpected separation of the particles within
the suspension resulting in the formation of a layer of particles in the center
of the cell. Remarkably, monodisperse particles form a crystalline layer which
dramatically alters the shear instability. By combining measurements of the
velocity field and torque fluctuations, we show that this solid layer disrupts
the flow instability and introduces a new, single-frequency component to the
torque fluctuations that reflects a dominant velocity pattern in the flow.
These results highlight the interplay between particles and a suspending
viscoelastic fluid at very high shear rates.
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