43 research outputs found
Cross-talk between topological defects in different fields revealed by nematic microfluidics
Topological defects are singularities in material fields that play a vital
role across a range of systems: from cosmic microwave background polarization
to superconductors, and biological materials. Although topological defects and
their mutual interactions have been extensively studied, little is known about
the interplay between defects in different fields -- especially when they
co-evolve -- within the same physical system. Here, using nematic
microfluidics, we study the cross-talk of topological defects in two different
material fields -- the velocity field and the molecular orientational field.
Specifically, we generate hydrodynamic stagnation points of different
topological charges at the center of star-shaped microfluidic junctions, which
then interact with emergent topological defects in the orientational field of
the nematic director. We combine experiments, and analytical and numerical
calculations to demonstrate that a hydrodynamic singularity of given
topological charge can nucleate a nematic defect of equal topological charge,
and corroborate this by creating , and topological defects in
, , and arm junctions. Our work is an attempt toward understanding
materials that are governed by distinctly multi-field topology, where disparate
topology-carrying fields are coupled, and concertedly determine the material
properties and response.Comment: 18 pages, 9 figure
Three-dimensional active defect loops
We describe the flows and morphological dynamics of topological defect lines and loops in three-dimensional active nematics and show, using theory and numerical modeling, that they are governed by the local profile of the orientational order surrounding the defects. Analyzing a continuous span of defect loop profiles, ranging from radial and tangential twist to wedge
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profiles, we show that the distinct geometries can drive material flow perpendicular or along the local defect loop segment, whose variation around a closed loop can lead to net loop motion, elongation, or compression of shape, or buckling of the loops. We demonstrate a correlation between local curvature and the local orientational profile of the defect loop, indicating dynamic coupling between geometry and topology. To address the general formation of defect loops in three dimensions, we show their creation via bend instability from different initial elastic distortions
Microfluidic control over topological states in channel-confined nematic flows
Compared to isotropic liquids, orientational order of nematic liquid crystals
makes their rheological properties more involved, and thus requires fine
control of the flow parameters to govern the orientational patterns. In
microfluidic channels with perpendicular surface alignment, nematics
discontinuously transition from perpendicular structure at low flow rates to
flow-aligned structure at high flow rates. Here we show how precise tuning of
the driving pressure can be used to stabilize and manipulate a previously
unresearched topologically protected chiral intermediate state which arises
before the homeotropic to flow-aligned transition. We characterize the
mechanisms underlying the transition and construct a phenomenological model to
describe the critical behaviour and the phase diagram of the observed chiral
flow state, and evaluate the effect of a forced symmetry breaking by
introduction of a chiral dopant. Finally, we induce transitions on demand
through channel geometry, application of laser tweezers, and careful control of
the flow rate
Discharging dynamics of topological batteries
Topological constraints have long been known to provide efficient mechanisms
for localizing and storing energy across a range of length scales, from knots
in DNA to turbulent plasmas. Despite recent theoretical and experimental
progress on the preparation of topological states, the role of topology in the
discharging dynamics is not well understood. Here, we investigate robust
topological energy release protocols in two archetypal soft systems through
simulations of 238 knotted elastic fibers and 3D liquid crystals across a range
of different topologies. By breaking the elastic fiber or switching the liquid
crystal surface anchoring, such topological batteries can perform mechanical
work or drive fluid flows. Our study reveals topologically resonant states for
which energy release becomes superslow or superfast. Owing to their intrinsic
stability we expect such tunable topological batteries to have broad
applications to storage and directed release of energy in soft matter.Comment: 8 pages, 5 figures; references added, discussion extende
Topology of three-dimensional active nematic turbulence confined to droplets
Active nematics contain topological defects which under sufficient activity
move, create and annihilate in a chaotic quasi-steady state, called active
turbulence. However, understanding active defects under confinement is an open
challenge, especially in three-dimensions. Here, we demonstrate the topology of
three-dimensional active nematic turbulence under the spherical confinement,
using numerical modelling. In such spherical droplets, we show the
three-dimensional structure of the topological defects, which due to closed
confinement emerge in the form of closed loops or surface-to-surface spanning
line segments. In the turbulent regime, the defects are shown to be strongly
spatially and time varying, with ongoing transformations between positive
winding, negative winding and twisted profiles, and with defect loops of zero
and non-zero topological charge. The timeline of the active turbulence is
characterised by four types of bulk topology-linked events --- breakup,
annihilation, coalescence and cross-over of the defects --- which we discuss
could be used for the analysis of the active turbulence in different
three-dimensional geometries. The turbulent regime is separated by a first
order structural transition from a low activity regime of a steady-state vortex
structure and an offset single point defect. We also demonstrate coupling of
surface and bulk topological defect dynamics by changing from strong
perpendicular to inplane surface alignment. More generally, this work is aimed
to provide insight into three-dimensional active turbulence, distinctly from
the perspective of the topology of the emergent three-dimensional topological
defects.Comment: 7 figure
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Sculpting stable structures in pure liquids
Pure liquids in thermodynamic equilibrium are structurally homogeneous. In liquid crystals, flow and light pulses are used to create reconfigurable domains with polar order. Moreover, through careful engineering of concerted microfluidic flows and localized optothermal fields, it is possible to achieve complete control over the nucleation, growth, and shape of such domains. Experiments, theory, and simulations indicate that the resulting structures can be stabilized indefinitely, provided the liquids are maintained in a controlled nonequilibrium state. The resulting sculpted liquids could find applications in microfluidic devices for selective encapsulation of solutes and particles into optically active compartments that interact with external stimuli
Smernice za diagnostiko, zdravljenje in sledenje bolnikov s sarkomi mehkih tkiv in kosti
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