arXiv subjects

Colin Denniston

Publications and source records attributed to Colin Denniston.

31 records · Page 2Linked to original sources

arXiv2001

On molecular and continuum boundary conditions for a miscible binary fluid

We show that molecular dynamics simulations can furnish useful boundary conditions at a solid surface bounding a two-component fluid. In contrast to some previous reports, convective-diffusive flow is consistent with continuum equations down to atomic scales. However, concentration gradients can produce flow without viscous dissipation that is inconsistent with the commonly used Navier slip condition. Also, differential wetting of the two components coupled to a concentration gradient can drive convective flows that could be used to make nano-pumps or motors.

cond-mat.soft↗

arXiv2001

Flexoelectric surface switching of bistable nematic devices

We report on a novel method of dynamically controlling the boundary conditions at the surface of a nematic liquid crystal using a surface flexoelectric effect. By moving the surface directors we show that one is able to manipulate defects which lie near the surface. This can be used to produce switching of a nematic liquid crystal device between two states with very similar free energies. This results in a bistable device that can retain either state with no applied voltage. Switching between the states occurs when the movement of the surface directors rotates those in the bulk which are then able to create or annihilate defects which lie near the surface of the device.

cond-mat.soft↗

arXiv2001

Phase Ordering in Nematic Liquid Crystals

We study the kinetics of the nematic-isotropic transition in a two-dimensional liquid crystal by using a lattice Boltzmann scheme that couples the tensor order parameter and the flow consistently. Unlike in previous studies, we find the time dependences of the correlation function, energy density, and the number of topological defects obey dynamic scaling laws with growth exponents that, within the numerical uncertainties, agree with the value 1/2 expected from simple dimensional analysis. We find that these values are not altered by the hydrodynamic flow. In addition, by examining shallow quenches, we find that the presence of orientational disorder can inhibit amplitude ordering.

cond-mat.soft↗

arXiv2000

Simulations of liquid crystals in Poiseuille flow

Lattice Boltzmann simulations are used to explore the behavior of liquid crystals subject to Poiseuille flow. In the nematic regime at low shear rates we find two possible steady state configurations of the director field. The selected state depends on both the shear rate and the history of the sample. For both director configurations there is clear evidence of shear-thinning, a decrease in the viscosity with increasing shear rate. Moreover, at very high shear rates or when the order parameter is large, the system transforms to a log-rolling state with boundary layers that may exhibit oscillatory behavior.

cond-mat.soft↗

arXiv2000

Lattice Boltzmann Simulations of Liquid Crystal Hydrodynamics

We describe a lattice Boltzmann algorithm to simulate liquid crystal hydrodynamics. The equations of motion are written in terms of a tensor order parameter. This allows both the isotropic and the nematic phases to be considered. Backflow effects and the hydrodynamics of topological defects are naturally included in the simulations, as are viscoelastic properties such as shear-thinning and shear-banding.

cond-mat.soft↗

arXiv1999

Simulations of liquid crystal hydrodynamics

We present a lattice Boltzmann algorithm for liquid crystal hydrodynamics. The coupling between the tensor order parameter and the flow is treated consistently allowing investigation of a wide range of non-Newtonian flow behavior. We present results for the effect of hydrodynamics on defect coalescence; of the appearance of the log-rolling and kayaking states in Poiseuille flow; and for banding and coexistence of isotropic and nematic phases under shear.

cond-mat.soft↗

arXiv1998

Dynamics and stress in gravity driven granular flow

We study, using simulations, the steady-state flow of dry sand driven by gravity in two-dimensions. An investigation of the microscopic grain dynamics reveals that grains remain separated but with a power-law distribution of distances and times between collisions. While there are large random grain velocities, many of these fluctuations are correlated across the system and local rearrangements are very slow. Stresses in the system are almost entirely transfered by collisions and the structure of the stress tensor comes almost entirely from a bias in the directions in which collisions occur.

cond-mat.soft↗

arXiv1997

Low Energy Excitations and Phase Transitions in the Frustrated Two-Dimensional XY Model

We study the critical properties of the two-dimensional (2D) XY model in a transverse magnetic field with filling factors f=1/3 and 2/5. To obtain a comparison with recent experiments, we investigate the effect of weak quenched bond disorder for f=2/5. A finite-size scaling analysis of extensive Monte Carlo simulations strongly suggests that the critical exponents of the phase transition for f=1/3 and for f=2/5 with disorder are those of the pure 2D Ising model. The relevant low energy excitations are domain walls, and we show that their properties determine the nature of the phase transition.

cond-mat.stat-mech↗

arXiv1997

Commensurability in One Dimension and the Josephson Junction Ladder

We study a Josephson junction ladder in a magnetic field in the absence of charging effects via a transfer matrix formalism. The eigenvalues of the transfer matrix are found numerically, giving a determination of the different phases of the ladder. The spatial periodicity of the ground state exhibits a devil's staircase as a function of the magnetic flux filling factor f. If the transverse Josephson coupling is varied a continuous superconducting-normal transition in the transverse direction is observed, analogous to the breakdown of the KAM trajectories in dynamical systems. We also examine how these properties may be affected by a current injected along the ladder.

cond-mat.stat-mech↗

arXiv1997

Incommensurability in the Frustrated Two-Dimensional XY Model

To examine the properties of the frustrated XY model at an incommensurate field we have examined a sequence of magnetic filling factors f which approach the irrational value of one minus the golden mean. At all f studied, the system undergoes a finite-temperature weak first-order transition involving the freezing out of Ising-like domain walls. As one approaches incommensurability, the low temperature phase of the system changes from the staircase states found by Halsey to a striped phase consisting of a superlattice of parallel shift (Pott's-like) domain walls.

cond-mat.stat-mech↗

arXiv1997

Domain Walls and Phase Transitions in the Frustrated Two-Dimensional XY Model

We study and compare the critical properties of the two-dimensional (2D) XY model in a transverse magnetic field with magnetic filling factors f=1/3 and f=2/5. In addition to the spin waves, the low energy excitations of the system consist of various domain walls between degenerate ground states. The lowest energy domain wall has a similar structure for both f=1/3 and f=2/5 and its properties dictate the nature of the phase transition. For f=2/5 these lowest energy walls have a negative energy for binding to each other, giving rise to a branching domain-wall structure and leading to a first order phase transition. For f=1/3 this binding energy is positive, resulting in a linear critical interface. In order to make a comparison to recent experiments, we investigate the effect of small quenched bond disorder for f=2/5. A finite-size scaling analysis of extensive Monte Carlo simulations strongly suggests that the critical exponents of the phase transition for f=1/3, and for f=2/5 with disorder, fall into the universality class of the two-dimensional Ising model.

cond-mat.stat-mech↗

arXiv1995

Phases of Josephson Junction Ladders

We study a Josephson junction ladder in a magnetic field in the absence of charging effects via a transfer matrix formalism. The eigenvalues of the transfer matrix are found numerically, giving a determination of the different phases of the ladder. The spatial periodicity of the ground state exhibits a devil's staircase as a function of the magnetic flux filling factor $f$. If the transverse Josephson coupling is varied a continuous superconducting-normal transition in the transverse direction is observed, analogous to the breakdown of the KAM trajectories in dynamical systems.

cond-mat↗

arXiv1994

Dynamics of a Driven Single Flux Line in Superconductors

We study the low temperature dynamics of a single flux line in a bulk type-II superconductor, driven by a surface current, both near and above the onset of an instability which sets in at a critical driving. We found that above the critical driving, the velocity profile of the flux line develops a discontinuity.

cond-mat↗