Rheology
of cholesteric blue phases
Mathematics Institute, University of Warwick, Coventry CV4 7AL, England
School of Physics, James Clerk Maxwell Building, Edinburgh, EH9 3JZ, Scotland
Computational Science Institute - ETH, CAB F84, CH-8092 Zurich, Switzerland
Dipartimento di Fisica and
Sezione INFN, Universita di Padova, Via Marzolo 8, 35131 Padova, Italy
The Rudolf Peierls Centre for Theoretical Physics, 1
Keble Road, Oxford OX1 3NP, England
Blue phases of cholesteric liquid crystals offer a
spectacular example of aturally occurring disclination line network. Here we
use a lattice Boltzmann algorithm to investigate the response of three types of
blue phases to an imposed Poiseuille flow. Our algorithm solves the Beris-
Edwards equations of motion, which comprise an equation for the order parameter
evolution and the Navier-Stokes equation. Our simulations show that the
disclination network and director field configuration couple together to yield
a very rich phenomenology as far as the rheological properties of the system
are concerned. We show that shear forces bend and twist and can unzip the
disclination lines. Under gentle forcing the network opposes the flow and the
apparent viscosity is significantly higher than that of an isotropic liquid. In
this regime we also observe a mild shear thickening regime, whose magnitude
depends on the topology of the blue phase under consideration. With increased
forcing we find strong shear thinning corresponding to the disruption of the
defect network. As the viscosity starts to drop, the imposed flow sets the
network into motion. Disclinations breakup and re-form with their neighbours in
the flow direction. This gives rise to oscillations in the time-dependent measurement
of the average stress. We compare our results with experimental observations on
the rheological properties of blue phases and of disclination line networks in
colloidal intrusions in cholesteric liquid crystals.