364 research outputs found
Effective free energy method for nematic liquid crystals in contact with structured substrates
We study the phase behavior of a nematic liquid crystal confined between a
flat substrate with strong anchoring and a patterned substrate whose structure
and local anchoring strength we vary. By first evaluating an effective surface
free energy function characterizing the patterned substrate we derive an
expression for the effective free energy of the confined nematic liquid
crystal. Then we determine phase diagrams involving a homogeneous state in
which the nematic director is almost uniform and a hybrid aligned nematic state
in which the orientation of the director varies through the cell. Direct
minimization of the free energy functional were performed in order to test the
predictions of the effective free energy method. We find remarkably good
agreement between the phase boundaries calculated from the two approaches. In
addition the effective energy method allows one to determine the energy
barriers between two states in a bistable nematic device.Comment: 10 pages, 7 figures, submitte
Phase behavior of ionic liquid crystals
Bulk properties of ionic liquid crystals are investigated using density
functional theory. The liquid crystal molecules are represented by ellipsoidal
particles with charges located in their center or at their tails. Attractive
interactions are taken into account in terms of the Gay-Berne pair potential.
Rich phase diagrams involving vapor, isotropic and nematic liquid, as well as
smectic phases are found. The dependence of the phase behavior on various
parameters such as the length of the particles and the location of charges on
the particles is studied
Phase behavior of a nematic liquid crystal in contact with a chemically and geometrically structured substrate
A nematic liquid crystal in contact with a grating surface possessing an
alternating stripe pattern of locally homeotropic and planar anchoring is
studied within the Frank--Oseen model. The combination of both chemical and
geometrical surface pattern leads to rich phase diagrams, involving a
homeotropic, a planar, and a tilted nematic texture. The effect of the groove
depth and the anchoring strengths on the location and the order of phase
transitions between different nematic textures is studied. A zenithally
bistable nematic device is investigated by confining a nematic liquid crystal
between the patterned grating surface and a flat substrate with strong
homeotropic anchoring.Comment: 7 pages, 7 figure
Critical Casimir interactions around the consolute point of a binary solvent
Spatial confinement of a near-critical medium changes its fluctuation
spectrum and modifies the corresponding order parameter distribution. These
effects result in effective, so-called critical Casimir forces (CCFs) acting on
the confining surfaces. These forces are attractive for like boundary
conditions of the order parameter at the opposing surfaces of the confinement.
For colloidal particles dissolved in a binary liquid mixture acting as a
solvent close to its critical point of demixing, one thus expects the emergence
of phase segregation into equilibrium colloidal liquid and gas phases. We
analyze how such phenomena occur asymmetrically in the whole thermodynamic
neighborhood of the consolute point of the binary solvent. By applying
field-theoretical methods within mean-field approximation and the
semi-empirical de Gennes-Fisher functional, we study the CCFs acting between
planar parallel walls as well as between two spherical colloids and their
dependence on temperature and on the composition of the near-critical binary
mixture. We find that for compositions slightly poor in the molecules
preferentially adsorbed at the surfaces, the CCFs are significantly stronger
than at the critical composition, thus leading to pronounced colloidal
segregation. The segregation phase diagram of the colloid solution following
from the calculated effective pair potential between the colloids agrees
surprisingly well with experiments and simulations
Accelerating charging dynamics in sub-nanometer pores
Having smaller energy density than batteries, supercapacitors have
exceptional power density and cyclability. Their energy density can be
increased using ionic liquids and electrodes with sub-nanometer pores, but this
tends to reduce their power density and compromise the key advantage of
supercapacitors. To help address this issue through material optimization, here
we unravel the mechanisms of charging sub-nanometer pores with ionic liquids
using molecular simulations, navigated by a phenomenological model. We show
that charging of ionophilic pores is a diffusive process, often accompanied by
overfilling followed by de-filling. In sharp contrast to conventional
expectations, charging is fast because ion diffusion during charging can be an
order of magnitude faster than in bulk, and charging itself is accelerated by
the onset of collective modes. Further acceleration can be achieved using
ionophobic pores by eliminating overfilling/de-filling and thus leading to
charging behavior qualitatively different from that in conventional, ionophilic
pores
Charging Ultra-nanoporous Electrodes with Size-asymmetric Ions Assisted by Apolar Solvent
We develop a statistical theory of charging quasi single-file pores with cations and anions of different sizes as well as solvent molecules or voids. This is done by mapping the charging onto a one-dimensional Blume–Emery–Griffith model with variable coupling constants. The results are supported by three-dimensional Monte Carlo simulations in which many limitations of the theory are lifted. We explore the different ways of enhancing the energy storage which depend on the competitive adsorption of ions and solvent molecules into pores, the degree of ionophilicity and the voltage regimes accessed. We identify new solvent-related charging mechanisms and show that the solvent can play the role of an “ionophobic agent”, effectively controlling the pore ionophobicity. In addition, we demonstrate that the ion-size asymmetry can significantly enhance the energy stored in a nanopore
Critical Casimir effect for colloids close to chemically patterned substrates
Colloids immersed in a critical or near-critical binary liquid mixture and
close to a chemically patterned substrate are subject to normal and lateral
critical Casimir forces of dominating strength. For a single colloid we
calculate these attractive or repulsive forces and the corresponding critical
Casimir potentials within mean-field theory. Within this approach we also
discuss the quality of the Derjaguin approximation and apply it to Monte Carlo
simulation data available for the system under study. We find that the range of
validity of the Derjaguin approximation is rather large and that it fails only
for surface structures which are very small compared to the geometric mean of
the size of the colloid and its distance from the substrate. For certain
chemical structures of the substrate the critical Casimir force acting on the
colloid can change sign as a function of the distance between the particle and
the substrate; this provides a mechanism for stable levitation at a certain
distance which can be strongly tuned by temperature, i.e., with a sensitivity
of more than 200nm/K.Comment: 27 pages, 14 figure
Simple microscopic model of a ternary amphiphilic system---phase diagram and mesoscopic correlations
Critical adsorption on non-spherical colloidal particles
We consider a non-spherical colloidal particle immersed in a fluid close to
its critical point. The temperature dependence of the corresponding order
parameter profile is calculated explicitly. We perform a systematic expansion
of the order parameter profile in powers of the local curvatures of the surface
of the colloidal particle. This curvature expansion reduces to the short
distance expansion of the order parameter profile in the case that the solvent
is at the critical composition.Comment: 9 pages, 7 figure
Probing interface localization-delocalization transitions by colloids
Interface localization-delocalization transitions (ILDT) occur in two-phase
fluids confined in a slit with competing preferences of the walls for the two
fluid phases. At low temperatures the interface between the two phases is
localized at one of the walls. Upon increasing temperature it unbinds. Although
intensively studied theoretically and computationally, such transitions have
not yet been observed experimentally due to severe challenges in resolving fine
details of the fluid structure. Here, using mean field theory and Monte Carlo
simulations of the Ising model, we propose to detect these ILDT by using
colloids. We show that the finite-size and fluctuation induced force acting on
a colloid confined in such a system experiences a vivid change if, upon
lowering the temperature, the interface localizes at one of the walls. This
change can serve as a more easily accessible experimental indicator of the
transition
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