52 research outputs found
Mass-Transport Models with Fragmentation and Aggregation
We present a review of nonequilibrium phase transitions in mass-transport
models with kinetic processes like fragmentation, diffusion, aggregation, etc.
These models have been used extensively to study a wide range of physical
problems. We provide a detailed discussion of the analytical and numerical
techniques used to study mass-transport phenomena.Comment: 29 pages, 4 figure
Phase separation of a magnetic fluid: Equilibrium phases and non-equilibrium kinetics
We study self-assembly in a colloidal suspension of magnetic particles by
performing comprehensive molecular dynamics simulations of the Stockmayer (SM)
model which comprises of spherical particles that are decorated by a magnetic
moment. The SM potential incorporates dipole-dipole interactions along with the
usual Lennard-Jones interaction, and exhibits a gas-liquid phase coexistence
observed experimentally in magnetic fluids. When this system is quenched from
the high-temperature homogeneous phase to the coexistence region, the
non-equilibrium evolution to the condensed phase proceeds with the development
of spatial as well as magnetic order. We observe density-dependent coarsening
mechanisms - a diffusive growth law in the nucleation
regime, and hydrodynamics-driven inertial growth law in
the spinodal regimes. [ is the average size of the condensate at time
after the quench.] While the spatial growth is governed by the expected
conserved order parameter dynamics, the growth of magnetic order in the
spinodal regime exhibits unexpected non-conserved dynamics. The equilibrium
morphologies have density-dependent shapes which typically include the
isotropic sphere and spherical bubble morphologies in the nucleation region,
and the anisotropic cylinder, planar slab, cylindrical bubble morphologies in
the spinodal region. The structures are robust and non-volatile and exhibit
characteristic magnetic properties. For example, the oppositely magnetized
hemispheres in the spherical morphology impart the characteristics of a {\it
Janus particle} to it. The observed structures have versatile applications in
catalysis, drug delivery systems, memory devices and magnetic photonic crystals
to name a few.Comment: 12 pages, 8 figure
Hysteresis in a magnetic bead and its applications
We study hysteresis in a micron-sized bead: a non-magnetic matrix embedded
with super- paramagnetic nanoparticles. These hold tremendous promise in
therapeutic applications as heat generating machines. The theoretical
formulation uses a mean-field theory to account for dipolar interactions
between the supermoments. The study enables manipulation of heat dissipation by
a compatible selection of commercially available beads and the frequency f and
amplitude ho of the applied oscillating field in the labortory. We also
introduce the possibility of utilizing return point memory for gradual heating
of a local region.Comment: 8 pages, 4 figure
Emergence of Biaxiality in Nematic Liquid Crystals with Magnetic Inclusions: Some Theoretical Insights
The biaxial phase in nematic liquid crystals has been elusive for several
decades after its prediction in the 1970s. A recent experimental breakthrough
was achieved by Liu et al. [PNAS 113, 10479 (2016)] in a liquid crystalline
medium with magnetic nanoparticles (MNPs). They exploited the different length
scales of dipolar and magneto-nematic interactions to obtain an equilibrium
state where the magnetic moments are at an angle to the nematic director. This
tilt introduces a second distinguished direction for orientational ordering or
biaxiality in the two-component system. Using coarse-grained Ginzburg-Landau
free energy models for the nematic and magnetic fields, we provide a
theoretical framework which allows for the manipulation of morphologies and
quantitative estimates of biaxial order
Surface-directed Dynamics in Living Liquid Crystals
We study living liquid crystals (LLCs), which are an amalgam of nematic
liquid crystals (LCs) and active matter (AM). These LLCs are placed in contact
with surfaces which impose planar/homeotropic boundary conditions on the
director field of the LC and the polarization field of the AM. The interplay of
LC-AM interactions and the surface-directed conditions yield controlled pattern
dynamics in the LLC, which has important technological implications. We discuss
two representative examples of this pattern dynamics
Tailored morphologies in two-dimensional ferronematic wells
We focus on a dilute uniform suspension of magnetic nanoparticles in a nematic-filled micron-sized shallow well with tangent boundary conditions as a paradigm system with two coupled order parameters. This system exhibits spontaneous magnetization without magnetic fields. We numerically obtain the stable nematic and associated magnetization morphologies, induced purely by the geometry, the boundary conditions, and the coupling between the magnetic nanoparticles and the host nematic medium. Our most striking observations pertain to domain walls in the magnetization profile, whose location can be manipulated by the coupling and material properties, and stable interior and boundary nematic defects, whose location and multiplicity can be tailored by the coupling too. These tailored morphologies are not accessible in uncoupled systems and can be used for multistable systems with singularities and stable interfaces
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