78 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
Topological-defect-induced surface charge heterogeneities in nematic electrolytes
We show that topological defects in an ion-doped nematic liquid crystal can
be used to manipulate the surface charge distribution on chemically
homogeneous, charge-regulating external surfaces, using a minimal theoretical
model. In particular, the location and type of the defect encodes the precise
distribution of surface charges and the effect is enhanced when the liquid
crystal is flexoelectric. We demonstrate the principle for patterned surfaces
and charged colloidal spheres. More generally, our results indicate an
interesting approach to control surface charges on external surfaces without
changing the surface chemistry.Comment: 6 pages, 4 figures, Supplemental Information can be found under
ancillary files. V2: Extra discussion, added additional images to the S
Reconfigurable knots and links in chiral nematic colloids
Tying knots and linking microscopic loops of polymers, macromolecules, or
defect lines in complex materials is a challenging task for material
scientists. We demonstrate the knotting of microscopic topological defect lines
in chiral nematic liquid crystal colloids into knots and links of arbitrary
complexity by using laser tweezers as a micromanipulation tool. All knots and
links with up to six crossings, including the Hopf link, the Star of David and
the Borromean rings are demonstrated, stabilizing colloidal particles into an
unusual soft matter. The knots in chiral nematic colloids are classified by the
quantized self-linking number, a direct measure of the geometric, or Berry's,
phase. Forming arbitrary microscopic knots and links in chiral nematic colloids
is a demonstration of how relevant the topology can be for the material
engineering of soft matter.Comment: 6 pages, 3 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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2
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
Vector beams generated by microlasers based on topological liquid-crystal structures
Structured light with designable intensity, polarization and phase fields is
today of high relevance, with application ranging from imaging, metrology,
optical trapping, ultracold atoms, classical and quantum communications and
memory. Specifically, vortex and vector beams can be generated directly in the
laser cavity, however, a controllable, geometrically simple and easy to
manufacture laser microcavity that generates structured light on demand,
especially tailored polarization, is still an open challenge. Here we show that
tunable laser vector beams can be generated from self-assembled liquid-crystal
(LC) micro-structures with topological defects inside a thin Fabry-P\'erot
microcavity. The LC superstructure provides complex three dimensional
birefringent refractive index profiles with order parameter singularities. The
topology of the LC structures is transferred into the topology of the light
polarization. The oriented fluorescent dye emission dipoles enable the
selection of optical modes with a particular polarization, as enabled by the
birefringence profile in the laser cavity. The proposed lasers have no
principal limitation for realizing structured light with arbitrarily tailored
intensity and polarization fields
Anisotropic electrostatic screening of charged colloids in nematic solvents
The physical behaviour of anisotropic charged colloids is determined by their
material dielectric anisotropy, affecting colloidal self-assembly, biological
function and even out-of-equilibrium behaviour. However, little is known about
anisotropic electrostatic screening, which underlies all electrostatic
effective interactions in such soft or biological materials. In this work, we
demonstrate anisotropic electrostatic screening for charged colloidal particles
in a nematic electrolyte. We show that material anisotropy behaves markedly
different from particle anisotropy: The electrostatic potential and pair
interactions decay with an anisotropic Debye screening length, contrasting the
constant screening length for isotropic electrolytes. Charged dumpling-shaped
near-spherical colloidal particles in a nematic medium are used as an
experimental model system to explore the effects of anisotropic screening,
demonstrating competing anisotropic elastic and electrostatic effective pair
interactions for colloidal surface charges tunable from neutral to high,
yielding particle-separated metastable states. Generally, our work contributes
to the understanding of electrostatic screening in nematic anisotropic media.Comment: 15 pages, 5 figures, SM under ancillary file
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