67 research outputs found
Emergence of active nematic behaviour in monolayers of isotropic cells
There is now growing evidence of the emergence and biological functionality
of liquid crystal features, including nematic order and topological defects, in
cellular tissues. However, how such features that intrinsically rely on
particle elongation, emerge in monolayers of cells with isotropic shapes is an
outstanding question. In this article we present a minimal model of cellular
monolayers based on cell deformation and force transmission at the cell-cell
interface that explains the formation of topological defects and captures the
flow-field and stress patterns around them. By including mechanical properties
at the individual cell level, we further show that the instability that drives
the formation of topological defects and leads to active turbulence, emerges
from a feedback between shape deformation and active driving. The model allows
us to suggest new explanations for experimental observations in tissue
mechanics, and to propose designs for future experiments
Multi-scale statistics of turbulence motorized by active matter
A number of micro-scale biological flows are characterized by spatio-temporal
chaos. These include dense suspensions of swimming bacteria, microtubule
bundles driven by motor proteins, and dividing and migrating confluent layers
of cells. A characteristic common to all of these systems is that they are
laden with active matter, which transforms free energy in the fluid into
kinetic energy. Because of collective effects, the active matter induces
multi-scale flow motions that bear strong visual resemblance to turbulence. In
this study, multi-scale statistical tools are employed to analyze direct
numerical simulations (DNS) of periodic two- (2D) and three-dimensional (3D)
active flows and compare them to classic turbulent flows. Statistical
descriptions of the flows and their variations with activity levels are
provided in physical and spectral spaces. A scale-dependent intermittency
analysis is performed using wavelets. The results demonstrate fundamental
differences between active and high-Reynolds number turbulence; for instance,
the intermittency is smaller and less energetic in active flows, and the work
of the active stress is spectrally exerted near the integral scales and
dissipated mostly locally by viscosity, with convection playing a minor role in
momentum transport across scales.Comment: Accepted in Journal of Fluid Mechanics (2017
Binding self-propelled topological defects in active turbulence
We report on the emergence of stable self-propelled bound defects in
monolayers of active nematics, which form virtual full-integer topological
defects in the form of vortices and asters. Through numerical simulations and
analytical arguments, we identify the phase-space of the bound defect formation
in active nematic monolayers. It is shown that an intricate synergy between the
nature of active stresses and the flow-aligning behaviour of active particles
can stabilise the motion of self-propelled positive half-integer defects into
specific bound structures. Our findings uncover new complexities in active
nematics with potential for triggering new experiments and theories
Active Inter-cellular Forces in Collective Cell Motility
The collective behaviour of confluent cell sheets is strongly influenced both
by polar forces, arising through cytoskeletal propulsion and by active
inter-cellular forces, which are mediated by interactions across cell-cell
junctions. We use a phase-field model to explore the interplay between these
two contributions and compare the dynamics of a cell sheet when the polarity of
the cells aligns to (i) their main axis of elongation, (ii) their velocity, and
(iii) when the polarity direction executes a persistent random walk.In all
three cases, we observe a sharp transition from a jammed state (where cell
rearrangements are strongly suppressed) to a liquid state (where the cells can
move freely relative to each other) when either the polar or the inter-cellular
forces are increased. In addition, for case (ii) only, we observe an additional
dynamical state, flocking (solid or liquid), where the majority of the cells
move in the same direction. The flocking state is seen for strong polar forces,
but is destroyed as the strength of the inter-cellular activity is increased.Comment: 15 pages,22 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
Mesoscale modelling of polymer aggregate digestion
We use mesoscale simulations to gain insight into the digestion of
biopolymers by studying the break-up dynamics of polymer aggregates (boluses)
bound by physical cross-links. We investigate aggregate evolution, establishing
that the linking bead fraction and the interaction energy are the main
parameters controlling stability with respect to diffusion. We show
a simplified model that chemical breakdown of the constituent
molecules causes aggregates that would otherwise be stable to disperse. We
further investigate breakdown of biopolymer aggregates in the presence of fluid
flow. Shear flow in the absence of chemical breakdown induces three different
regimes depending on the flow Weissenberg number (). i) At ,
shear flow has a negligible effect on the aggregates. ii) At , the
aggregates behave approximately as solid bodies and move and rotate with the
flow. iii) At , the energy input due to shear overcomes the
attractive cross-linking interactions and the boluses are broken up. Finally,
we study bolus evolution under the combined action of shear flow and chemical
breakdown, demonstrating a synergistic effect between the two at high reaction
rates
Two-dimensional, blue phase tactoids
We use full nematohydrodynamic simulations to study the statics and dynamics
of monolayers of cholesteric liquid crystals. Using chirality and temperature
as control parameters we show that we can recover the two-dimensional blue
phases recently observed in chiral nematics, where hexagonal lattices of
half-skyrmion topological excitations are interleaved by lattices of trefoil
topological defects. Furthermore, we characterise the transient dynamics during
the quench from isotropic to blue phase. We then proceed by confining
cholesteric stripes and blue phases within finite-sized tactoids and show that
it is possible to access a wealth of reconfigurable droplet shapes including
disk-like, elongated, and star-shaped morphologies. Our results demonstrate a
potential for constructing controllable, stable structures of liquid crystals
by constraining 2D blue phases and varying the chirality, surface tension and
elastic constants.Comment: 13 pages, 5 figures, Molecular Physics, Frenkel Special Issue (2018
Periodic orbits, pair nucleation, and unbinding of active nematic defects on cones
Geometric confinement and topological constraints present promising means of
controlling active materials. By combining analytical arguments derived from
the Born-Oppenheimer approximation with numerical simulations, we investigate
the simultaneous impact of confinement together with curvature singularity by
characterizing the dynamics of an active nematic on a cone. Here, the
Born-Oppenheimer approximation means that textures can follow defect positions
rapidly on the time scales of interest. Upon imposing strong anchoring boundary
conditions at the base of a cone, we find a a rich phase diagram of
multi-defect dynamics including exotic periodic orbits of one or two
flank defects, depending on activity and non-quantized geometric charge at the
cone apex. By characterizing the transitions between these ordered dynamical
states, we can understand (i) defect unbinding, (ii) defect absorption and
(iii) defect pair nucleation at the apex. Numerical simulations confirm
theoretical predictions of not only the nature of the circular orbits but also
defect unbinding from the apex.Comment: 17 pages, 13 figures, 5 table
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