19 research outputs found
Tunable Three-Dimensional Architecture of Nematic Disclination Lines
Disclinations lines play a key role in many physical processes, from the
fracture of materials to the formation of the early universe. Achieving
versatile control over disclinations is key to developing novel electro-optical
devices, programmable origami, directed colloidal assembly, and controlling
active matter. Here, we introduce a theoretical framework to tailor
three-dimensional disclination architecture in nematic liquid crystals
experimentally. We produce quantitative predictions for the connectivity and
shape of disclination lines found in nematics confined between two thinly
spaced glass substrates with strong planar anchoring. By drawing an analogy
between nematic liquid crystals and magnetostatics, we find that: i)
disclination lines connect defects with the same topological charge on opposite
surfaces, and ii) disclination lines are attracted to regions of the highest
twist. Using polarized light to pattern the in-plane alignment of liquid
crystal molecules, we test these predictions experimentally and identify
critical parameters that tune the disclination lines' curvature. We verify our
predictions with computer simulations and find non-dimensional parameters
enabling us to match experiments and simulations at different length scales.
Our work provides a powerful method to understand and practically control
defect lines in nematic liquid crystals
Enhanced gel formation in binary mixtures of nanocolloids with short-range attraction
© 2018 Author(s). Colloidal suspensions transform between fluid and disordered solid states as parameters such as the colloid volume fraction and the strength and nature of the colloidal interactions are varied. Seemingly subtle changes in the characteristics of the colloids can markedly alter the mechanical rigidity and flow behavior of these soft composite materials. This sensitivity creates both a scientific challenge and an opportunity for designing suspensions for specific applications. In this paper, we report a novel mechanism of gel formation in mixtures of weakly attractive nanocolloids with modest size ratio. Employing a combination of x-ray photon correlation spectroscopy, rheometry, and molecular dynamics simulations, we find that gels are stable at remarkably weaker attraction in mixtures with size ratio near two than in the corresponding monodisperse suspensions. In contrast with depletion-driven gelation at larger size ratio, gel formation in the mixtures is triggered by microphase demixing of the species into dense regions of immobile smaller colloids surrounded by clusters of mobile larger colloids that is not predicted by mean-field thermodynamic considerations. These results point to a new route for tailoring nanostructured colloidal solids through judicious combination of interparticle interaction and size distribution
Smectic ordering in liquid crystal - aerosil dispersions II. Scaling analysis
Liquid crystals offer many unique opportunities to study various phase
transitions with continuous symmetry in the presence of quenched random
disorder (QRD). The QRD arises from the presence of porous solids in the form
of a random gel network. Experimental and theoretical work support the view
that for fixed (static) inclusions, quasi-long-range smectic order is destroyed
for arbitrarily small volume fractions of the solid. However, the presence of
porous solids indicates that finite-size effects could play some role in
limiting long-range order. In an earlier work, the nematic - smectic-A
transition region of octylcyanobiphenyl (8CB) and silica aerosils was
investigated calorimetrically. A detailed x-ray study of this system is
presented in the preceding Paper I, which indicates that pseudo-critical
scaling behavior is observed. In the present paper, the role of finite-size
scaling and two-scale universality aspects of the 8CB+aerosil system are
presented and the dependence of the QRD strength on the aerosil density is
discussed.Comment: 14 pages, 10 figures, 1 table. Companion paper to "Smectic ordering
in liquid crystal - aerosil dispersions I. X-ray scattering" by R.L. Leheny,
S. Park, R.J. Birgeneau, J.-L. Gallani, C.W. Garland, and G.S. Iannacchion
Slow dynamics, aging, and glassy rheology in soft and living matter
We explore the origins of slow dynamics, aging and glassy rheology in soft
and living matter. Non-diffusive slow dynamics and aging in materials
characterised by crowding of the constituents can be explained in terms of
structural rearrangement or remodelling events that occur within the jammed
state. In this context, we introduce the jamming phase diagram proposed by Liu
and Nagel to understand the ergodic-nonergodic transition in these systems, and
discuss recent theoretical attempts to explain the unusual,
faster-than-exponential dynamical structure factors observed in jammed soft
materials. We next focus on the anomalous rheology (flow and deformation
behaviour) ubiquitous in soft matter characterised by metastability and
structural disorder, and refer to the Soft Glassy Rheology (SGR) model that
quantifies the mechanical response of these systems and predicts aging under
suitable conditions. As part of a survey of experimental work related to these
issues, we present x-ray photon correlation spectroscopy (XPCS) results of the
aging of laponite clay suspensions following rejuvenation. We conclude by
exploring the scientific literature for recent theoretical advances in the
understanding of these models and for experimental investigations aimed at
testing their predictions.Comment: 22 pages, 5 postscript figures; invited review aricle, to appear in
special issue on soft matter in Solid State Communication
Driven Topological Transitions in Active Nematic Films
The topological properties of many materials are central to their behavior,
with the dynamics of topological defects being particularly important to
intrinsically out-of-equilibrium, active materials. In this paper, local
manipulation of the ordering, dynamics, and topological properties of
microtubule-based extensile active nematic films is demonstrated in a joint
experimental and simulation study. Hydrodynamic stresses created by
magnetically actuated rotation of disk-shaped colloids in proximity to the
films compete with internal stresses in the active nematic, enabling local
control of the motion of the +1/2 charge topological defects that are intrinsic
to spontaneously turbulent active films. Sufficiently large applied stresses
drive the formation of +1 charge topological vortices in the director field
through the merger of two +1/2 defects. The directed motion of the defects is
accompanied by ordering of the vorticity and velocity of the active flows
within the film that is qualitatively unlike the response of passive viscous
films. Many features of the film's response to the disk are captured by Lattice
Boltzmann simulations, leading to insight into the anomalous viscoelastic
nature of the active nematic. The topological vortex formation is accompanied
by a rheological instability in the film that leads to significant increase in
the flow velocities. Comparison of the velocity profile in vicinity of the
vortex with fluid-dynamics calculations provides an estimate of film viscosity
Nanorod Mobility within Entangled Wormlike Micelle Solutions
In the semidilute regime, wormlike
micelles form an isotropic entangled microstructure that is similar
to that of an entangled polymer solution with a characteristic, nanometer-scale
entanglement mesh size. We report a combined X-ray photon correlation
spectroscopy (XPCS) and rheology study to investigate the translational
dynamics of gold nanorods in semidilute solutions of entangled wormlike
micelles formed by the surfactant cetylpyridinium chloride (CPyCl)
and the counterion sodium salicylate (NaSal). The CPyCl concentration
is varied to tune the entanglement mesh size over a range that spans
from approximately equal to the nanorod diameter to larger than the
nanorod length. The NaSal concentration is varied along with the CPyCl
concentration so that the solutions have the maximum viscosity for
given CPyCl concentration. On short time scales the nanorods are localized
on a length scale matching that expected from the high-frequency elastic
modulus of the solutions as long as the mesh size is smaller than
the rod length. On longer time scales, the nanorods undergo free diffusion.
At the highest CPyCl concentrations, the nanorod diffusivity approaches
the value expected based on the macroscopic viscosity of the solutions,
but it increases with decreasing CPyCl concentration more rapidly
than expected from the macroscopic viscosity. A recent model by Cai
et al. [Cai, L.-H.; Panyukov, S.; Rubinstein, M. Macromolecules 2015, 48, 847−862] for nanoparticle
“hopping” diffusion in entangled polymer solutions accounts
quantitatively for this enhanced diffusivity