25 research outputs found
Density functional theory study of the nematic-isotropic transition in an hybrid cell
We have employed the Density Functional Theory formalism to investigate the
nematic-isotropic capillary transitions of a nematogen confined by walls that
favor antagonist orientations to the liquid crystal molecules (hybrid cell). We
analyse the behavior of the capillary transition as a function of the
fluid-substrate interactions and the pore width. In addition to the usual
capillary transition between isotropic-like to nematic-like states, we find
that this transition can be suppressed when one substrate is wet by the
isotropic phase and the other by the nematic phase. Under this condition the
system presents interface-like states which allow to continuously transform the
nematic-like phase to the isotropic-like phase without undergoing a phase
transition. Two different mechanisms for the disappearance of the capillary
transition are identified. When the director of the nematic-like state is
homogeneously planar-anchored with respect to the substrates, the capillary
transition ends up in a critical point. This scenario is analogous to the
observed in Ising models when confined in slit pores with opposing surface
fields which have critical wetting transitions. When the nematic-like state has
a linearly distorted director field, the capillary transition continuously
transforms in a transition between two nematic-like states.Comment: 31 pages, 10 figures, submitted to J. Chem. Phy
Vortex waves in trapped Bose-Einstein condensates
We have theoretically studied vortex waves of Bose-Einstein condensates in
elongated harmonic traps. Our focus is on the axisymmetric varicose waves and
helical Kelvin waves of singly quantized vortex lines. Growth and decay
dynamics of both types of vortex waves are discussed. We propose a method to
experimentally create these vortex waves on demand.Comment: minor changes, 7 pages, 7 figure
Quantized vortices and superflow in arbitrary dimensions: Structure, energetics and dynamics
The structure and energetics of superflow around quantized vortices, and the
motion inherited by these vortices from this superflow, are explored in the
general setting of the superfluidity of helium-four in arbitrary dimensions.
The vortices may be idealized as objects of co-dimension two, such as
one-dimensional loops and two-dimensional closed surfaces, respectively, in the
cases of three- and four-dimensional superfluidity. By using the analogy
between vorticial superflow and Ampere-Maxwell magnetostatics, the equilibrium
superflow containing any specified collection of vortices is constructed. The
energy of the superflow is found to take on a simple form for vortices that are
smooth and asymptotically large, compared with the vortex core size. The motion
of vortices is analyzed in general, as well as for the special cases of
hyper-spherical and weakly distorted hyper-planar vortices. In all dimensions,
vortex motion reflects vortex geometry. In dimension four and higher, this
includes not only extrinsic but also intrinsic aspects of the vortex shape,
which enter via the first and second fundamental forms of classical geometry.
For hyper-spherical vortices, which generalize the vortex rings of three
dimensional superfluidity, the energy-momentum relation is determined. Simple
scaling arguments recover the essential features of these results, up to
numerical and logarithmic factors.Comment: 35 pages, 7 figure
Kelvin Modes of a fast rotating Bose-Einstein Condensate
Using the concept of diffused vorticity and the formalism of rotational
hydrodynamics we calculate the eigenmodes of a harmonically trapped
Bose-Einstein condensate containing an array of quantized vortices. We predict
the occurrence of a new branch of anomalous excitations, analogous to the
Kelvin modes of the single vortex dynamics. Special attention is devoted to the
excitation of the anomalous scissors mode.Comment: 7 pages, 3 figures, submitted to Phys. Rev.
From one cell to the whole froth: a dynamical map
We investigate two and three-dimensional shell-structured-inflatable froths,
which can be constructed by a recursion procedure adding successive layers of
cells around a germ cell. We prove that any froth can be reduced into a system
of concentric shells. There is only a restricted set of local configurations
for which the recursive inflation transformation is not applicable. These
configurations are inclusions between successive layers and can be treated as
vertices and edges decorations of a shell-structure-inflatable skeleton. The
recursion procedure is described by a logistic map, which provides a natural
classification into Euclidean, hyperbolic and elliptic froths. Froths tiling
manifolds with different curvature can be classified simply by distinguishing
between those with a bounded or unbounded number of elements per shell, without
any a-priori knowledge on their curvature. A new result, associated with
maximal orientational entropy, is obtained on topological properties of natural
cellular systems. The topological characteristics of all experimentally known
tetrahedrally close-packed structures are retrieved.Comment: 20 Pages Tex, 11 Postscript figures, 1 Postscript tabl
Physics of solid–liquid interfaces: From the Young equation to the superhydrophobicity (Review Article)
The state-of-art in the field of physics of phenomena occurring at solid/liquid interfaces is presented. The notions of modern physics of wetting are introduced and discussed including: the contact angle hysteresis, disjoining pressure and wetting transitions. The physics of low temperature wetting phenomena is treated. The general variational approach to interfacial problems, based on the application of the transversality conditions to variational problems with free endpoints is presented. It is demonstrated that main equations, predicting contact angles, namely the Young, Wenzel, and Cassie–Baxter equations arise from imposing the transversality conditions on the appropriate variational problem of wetting. Recently discovered effects such as superhydrophobicity, the rose petal effect and the molecular dynamic of capillarity are reviewed