43 research outputs found
Velocity Correlations in an Active Nematic
The flow properties of a continuum model for an active nematic is studied and
compared with recent experiments on suspensions of microtubule bundles and
molecular motors. The velocity correlation length is found to be independent of
the strength of the activity while the characteristic velocity scale increases
monotonically as the activity is increased, both in agreement with the
experimental observations. We interpret our results in terms of the creation
and annihilation dynamics of a gas of topological defects
Instabilities and Topological Defects in Active Nematics
We study a continuum model of an extensile active nematic to show that
mesoscale turbulence develops in two stages: (i) ordered regions undergo an
intrinsic hydrodynamic instability generating walls, lines of stong bend
deformations, (ii) the walls relax by forming oppositely charged pairs of
defects. Both creation and annihilation of defect pairs reinstate nematic
regions which undergo further instabilities, leading to a dynamic steady state.
We compare this with the development of active turbulence in a contractile
active nematic
Driven active and passive nematics
We investigate similarities in the micro-structural dynamics between
externally driven and actively driven nematics. Walls, lines of strong
deformations in the director field, and topological defects are characteristic
features of an active nematic. Similar structures form in driven passive
nematics when there are inhomogeneities in imposed velocity gradients due to
non-linear flow fields or geometrical constraints. Specifically, pressure
driven flow of a tumbling passive nematic in an expanding-contracting channel
produces walls and defects similar to those seen in active nematics. We also
study the response of active nematics to external driving, confirming that
imposed shear suppresses the hydrodynamic instabilities. We show that shear
fields can lead to wall alignments and the localisation of active turbulence.Comment: Molecular Physic
Active nematic materials with substrate friction
Active turbulence in dense active systems is characterized by high vorticity
on a length scale that is large compared to that of individual entities. We
describe the properties of active turbulence as momentum propagation is
screened by frictional damping. As friction is increased, the spacing between
the walls in the nematic director field decreases as a consequence of the more
rapid velocity decays. This leads to, first, a regime with more walls and an
increased number of topological defects, and then to a jammed state in which
the walls deliminate bands of opposing flow, analogous to the shear bands
observed in passive complex fluids
Biphasic, Lyotropic, Active Nematics
We perform dynamical simulations of a two-dimensional active nematic fluid in
coexistence with an isotropic fluid. Drops of active nematic become elongated,
and an effective anchoring develops at the nematic-isotropic interface. The
activity also causes an undulatory instability of the interface. This results
in defects of positive topological charge being ejected into the nematic,
leaving the interface with a diffuse negative charge. Quenching the active
lyotropic fluid results in a steady state in which phase-separating domains are
elongated and then torn apart by active stirring.Comment: 7 pages, 8 figure
Active transport in a channel: stabilisation by flow or thermodynamics
Recent experiments on active materials, such as dense bacterial suspensions
and microtubule-kinesin motor mixtures, show a promising potential for
achieving self-sustained flows. However, to develop active microfluidics it is
necessary to understand the behaviour of active systems confined to channels.
Therefore here we use continuum simulations to investigate the behaviour of
active fluids in a two-dimensional channel. Motivated by the fact that most
experimental systems show no ordering in the absence of activity, we
concentrate on temperatures where there is no nematic order in the passive
system, so that any nematic order is induced by the active flow. We
systematically analyze the results, identify several different stable flow
states, provide a phase diagram and show that the key parameters controlling
the flow are the ratio of channel width to the length scale of active flow
vortices, and whether the system is flow aligning or flow tumbling