162 research outputs found
Phase-separation of miscible liquids in a centrifuge
We show that a liquid mixture in the thermodynamically stable homogeneous
phase can undergo a phase-separation transition when rotated at sufficiently
high frequency . This phase-transition is different from the usual case
where two liquids are immiscible or where the slow sedimentation process of one
component (e.g. a polymer) is accelerated due to centrifugation. For a binary
mixture, the main coupling is due to a term ,
where is the difference between the two liquid densities and
the distance from the rotation axis. Below the critical temperature there is a
critical rotation frequency , below which smooth density gradients
occur. When , we find a sharp interface between the low
density liquid close to the center of the centrifuge and a high density liquid
far from the center. These findings may be relevant to various separation
processes and to the control of chemical reactions, in particular their
kinetics.Comment: 6 pages, 3 figure
Diblock copolymer ordering induced by patterned surfaces
We use a Ginzburg-Landau free energy functional to investigate diblock
copolymer morphologies when the copolymer melt interacts with one surface or is
confined between two chemically patterned surfaces. For temperatures above the
order-disorder transition a complete linear response description of the
copolymer melt is given, in terms of an arbitrary two-dimensional surface
pattern. The appearance of order in the direction parallel to the surface is
found as a result of the order in the perpendicular direction. Below the
order-disorder point and in a thin-film geometry, our procedure enables the
analytic calculation of distorted perpendicular and tilted lamellar phases in
the presence of uniform or modulated surface fields.Comment: 8 pages, 3 figures, to be published in Europhys. Let
Diblock Copolymer Ordering Induced by Patterned Surfaces Above the Order-Disorder Transition
We investigate the morphology of diblock copolymers in the vicinity of flat,
chemically patterned surfaces. Using a Ginzburg-Landau free energy, spatial
variations of the order parameter are given in terms of a general
two-dimensional surface pattern above the order-disorder transition. The
propagation of several surface patterns into the bulk is investigated. The
oscillation period and decay length of the surface -modes are calculated in
terms of system parameters. We observe lateral order parallel to the surface as
a result of order perpendicular to the surface. Surfaces which has a finite
size chemical pattern (e.g., a stripe of finite width) induces lamellar
ordering extending into the bulk. Close to the surface pattern the lamellae are
strongly perturbed adjusting to the pattern.Comment: 9 pages, 14 figures, to be published in Macromolecule
Attraction Between Like-Charge Surfaces in Polar Mixtures
We examine the force between two charged surfaces immersed in aqueous
mixtures having a coexistence curve. For a homogeneous water-poor phase, as the
distance between the surfaces is decreased, a water-rich phase condenses at a
distance in the range 1-100nm. At this distance the osmotic pressure can
become negative leading to a long-range attraction between the surfaces. The
osmotic pressure vanishes at a distance , representing a very deep
metastable or globally stable energetic state. We give analytical and numerical
results for and on the Poisson-Boltzmann level.Comment: 6 pages, 6 figure
Diblock copolymer thin films: Parallel and perpendicular lamellar phases in the weak segregation limit
We study morphologies of thin-film diblock copolymers between two flat and
parallel walls. The study is restricted to the weak segregation regime below
the order-disorder transition temperature. The deviation from perfect lamellar
shape is calculated for phases which are perpendicular and parallel to the
walls. We examine the undulations of the inter material dividing surface and
its angle with the walls, and find that the deviation from its unperturbed
position can be much larger than in the strong segregation case. Evaluating the
weak segregation stability of the lamellar phases, it is shown that a surface
interaction, which is quadratic in the monomer concentration, favors the
perpendicular lamellar phase. In particular, the degeneracy between
perpendicular and unfrustrated parallel lamellar phases for walls without a
preferential adsorption is removed.Comment: 10 pages, 9 figures, submitted to European Physical Journal
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