790,069 research outputs found
Colloidal particles at a nematic-isotropic interface: effects of confinement
When captured by a flat nematic-isotropic interface, colloidal particles can
be dragged by it. As a result spatially periodic structures may appear, with
the period depending on a particle mass, size, and interface
velocity~\cite{west.jl:2002}. If liquid crystal is sandwiched between two
substrates, the interface takes a wedge-like shape, accommodating the
interface-substrate contact angle and minimizing the director distortions on
its nematic side. Correspondingly, particles move along complex trajectories:
they are first captured by the interface and then `glide' towards its vertex
point. Our experiments quantify this scenario, and numerical minimization of
the Landau-de Gennes free energy allow for a qualitative description of the
interfacial structure and the drag force.Comment: 7 pages, 9 figure
Interaction of colloids with a nematic-isotropic interface
The Landau-de Gennes free energy is used to calculate the interaction between
long cylindrical colloids and the nematic-isotropic (NI) interface. This
interaction has two contributions: one is specific of liquid crystals and
results from the deformation of the director field close to the particles or to
the interface, while the other is generic and results from wetting and surface
tension effects.
Deep in the nematic phase the director field of long cylindrical colloids,
with strong homeotropic anchoring, exhibits two half-integer defect lines. As
the colloid moves towards the interface, the director configuration changes
through a series of discontinuous transitions, where one or two of the defects
are annihilated. In addition, the NI interface bends towards the colloid in
order to minimize the elastic free energy in the nematic. In the isotropic
phase, the colloid is surrounded by a thin nematic layer that reduces the
surface free energy under favorable wetting conditions.
The interaction has a well-defined minimum near the interface. In this region
the director and interfacial structures are complex and cannot be described
analytically. Using the numerical results for the Landau-de Gennes free energy
in the harmonic region, we obtained simple scaling laws for the (linear) force
on the colloid
On stoichiometry and intermixing at the spinel/perovskite interface in CoFe2O4/BaTiO3 thin films
The performance of complex oxide heterostructures depends primarily on the interfacial coupling of the two component structures. This interface character inherently varies with the synthesis method and conditions used since even small composition variations can alter the electronic, ferroelectric, or magnetic functional properties of the system. The focus of this article is placed on the interface character of a pulsed laser deposited CoFe2O4/BaTiO3 thin film. Using a range of state-of-the-art transmission electron microscopy methodologies, the roles of substrate morphology, interface stoichiometry, and cation intermixing are determined on the atomic level. The results reveal a surprisingly uneven BaTiO3 substrate surface formed after the film deposition and Fe atom incorporation in the top few monolayers inside the unit cell of the BaTiO3 crystal. Towards the CoFe2O4 side, a disordered region extending several nanometers from the interface was revealed and both Ba and Ti from the substrate were found to diffuse into the spinel layer. The analysis also shows that within this somehow incompatible composite interface, a different phase is formed corresponding to the compound Ba2Fe3Ti5O15, which belongs to the ilmenite crystal structure of FeTiO3 type. The results suggest a chemical activity between these two oxides, which could lead to the synthesis of complex engineered interfaces
Dynamic model and stationary shapes of fluid vesicles
A phase-field model that takes into account the bending energy of fluid
vesicles is presented. The Canham-Helfrich model is derived in the
sharp-interface limit. A dynamic equation for the phase-field has been solved
numerically to find stationary shapes of vesicles with different topologies and
the dynamic evolution towards them. The results are in agreement with those
found by minimization of the Canham-Helfrich free energy. This fact shows that
our phase-field model could be applied to more complex problems of
instabilities.Comment: Accepted for publication in EPJE. 9 pages, 7 figure
Quantization of Hall Resistance at the Metallic Interface between an Oxide Insulator and SrTiO
The two-dimensional metal forming at the interface between an oxide insulator
and SrTiO3 provides new opportunities for oxide electronics. However, the
quantum Hall effect, one of the most fascinating effects of electrons confined
in two dimensions, remains underexplored at these complex oxide
heterointerfaces. Here, we report the experimental observation of quantized
Hall resistance in a SrTiO3 heterointerface based on the modulation-doped
amorphous-LaAlO/SrTiO heterostructure, which exhibits both high
electron mobility exceeding 10000 cm/Vs and low carrier density on the
order of ~10 cm. Along with unambiguous Shubnikov-de Haas
oscillations, the spacing of the quantized Hall resistance suggests that the
interface is comprised of a single quantum well with ten parallel conducting
two-dimensional subbands. This provides new insight into the electronic
structure of conducting oxide interfaces and represents an important step
towards designing and understanding advanced oxide devices
Ion size effects at ionic exclusion from dielectric interfaces and slit nanopores
A previously developed field-theoretic model [R.D. Coalson et al., J. Chem.
Phys. 102, 4584 (1995)] that treats core collisions and Coulomb interactions on
the same footing is investigated in order to understand ion size effects on the
partition of neutral and charged particles at planar interfaces and the ionic
selectivity of slit nanopores. We introduce a variational scheme that can go
beyond the mean-field (MF) regime and couple in a consistent way pore modified
core interactions, steric effects, electrostatic solvation and image-charge
forces, and surface charge induced electrostatic potential. We show that in the
dilute limit, the MF and the variational theories agree well with MC simulation
results, in contrast to a recent RPA method. The partition of charged Yukawa
particles at a neutral dielectric interface (e.g air-water or protein-water
interface) is investigated. It is shown that as a result of the competition
between core collisions that push the ions towards the surface, and repulsive
solvation and image forces that exclude them from the interface, a
concentration peak of finite size ions sets in close to the dielectric
interface. We also characterize the role played by the ion size on the ionic
selectivity of neutral slit nanopores. We show that the complex interplay
between electrostatic forces, excluded volume effects induced by core
collisions and steric effects leads to an unexpected reversal in the ionic
selectivity of the pore with varying pore size: while large pores exhibits a
higher conductivity for large ions, narrow pores exclude large ions more
efficiently than small ones
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