78 research outputs found
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
Orienting Ion-Containing Block Copolymers Using AC Electric Field
We consider orientation mechanisms for block copolymers in an electric field.
Theoretical and experimental studies have shown that nonuniformity of the
dielectric constant gives rise to a preferred orientation of the melt with
respect to the applied field. We show that the presence of ions, as found in
anionically prepared copolymers, may increase the alignment effect markedly.
Time-varying (ac) and static (dc) fields are considered within a unified
framework. We find that orientation of block copolymers can in principle be
achieved without a dielectric contrast if there is a mobility contrast. The
presence of ions is especially important at small field frequencies, as is in
most experiments. Unlike the no-ions case, it is found that orienting forces
depend on the polymer chain lengths. The mobile-ions mechanism suggested here
can be used to reduce the magnitude of orienting fields as well as to
discriminate between block copolymers of different lengths.Comment: 8 pages, 2 figure
Surface induced ordering in thin film diblock copolymers: tilted lamellar phases
We investigate the effect of chemically patterned surfaces on the morphology
of diblock copolymers below the order-disorder transition. Profiles for
lamellar phases in contact with one surface, or confined between two surfaces
are obtained in the weak segregation limit using a Ginzburg-Landau expansion of
the free energy, and treating it with mean-field theory. The periodically
patterned surface induces a tilt of the lamellae in order to match the surface
periodicity. The lamellae relax from the constrained periodicity close to the
surface to the bulk periodicity far from it. The phases we investigate are a
generalization to the mixed (perpendicular and parallel to the surface)
lamellar phases occurring when the two surfaces are homogeneous. A special case
when the surface pattern has a period equal to the bulk lamellar period showing
``T-junction'' morphology is examined. Our analytic calculation agrees with
previous computer simulations and self consistent field theories.Comment: 7 figures, replaced with minor modification
Frustrated Rotations in Nematic Monolayers
Tabe and Yokoyama found recently that the optical axis in a chiral monolayer
of a ferronematic rotates when water evaporates from the bath: the chiral
molecules act as propellers. When the axis is blocked at the lateral walls of
the trough, the accumulated rotation inside creates huge splays and bends. We
discuss the relaxation of these tensions, assuming that a single dust particle
nucleates disclination pairs. For the simplest geometry, we then predict a long
delay time followed by a non-periodic sequence of ``bursts''. These ideas are
checked by numerical simulations.Comment: 5 pages, 4 figures, submitted to Eur. Phys. J.
Block Copolymer at Nano-Patterned Surfaces
We present numerical calculations of lamellar phases of block copolymers at
patterned surfaces. We model symmetric di-block copolymer films forming
lamellar phases and the effect of geometrical and chemical surface patterning
on the alignment and orientation of lamellar phases. The calculations are done
within self-consistent field theory (SCFT), where the semi-implicit relaxation
scheme is used to solve the diffusion equation. Two specific set-ups, motivated
by recent experiments, are investigated. In the first, the film is placed on
top of a surface imprinted with long chemical stripes. The stripes interact
more favorably with one of the two blocks and induce a perpendicular
orientation in a large range of system parameters. However, the system is found
to be sensitive to its initial conditions, and sometimes gets trapped into a
metastable mixed state composed of domains in parallel and perpendicular
orientations. In a second set-up, we study the film structure and orientation
when it is pressed against a hard grooved mold. The mold surface prefers one of
the two components and this set-up is found to be superior for inducing a
perfect perpendicular lamellar orientation for a wide range of system
parameters
Self Trapping of a Single Bacterium in its Own Chemoattractant
Bacteria (e.g. E. Coli) are very sensitive to certain chemoattractants (e.g.
asparate) which they themselves produce. This leads to chemical instabilities
in a uniform population. We discuss here the different case of a single
bacterium, following the general scheme of Brenner, Levitov and Budrene. We
show that in one and two dimensions (in a capillary or in a thin film) the
bacterium can become self-trapped in its cloud of attractant. This should occur
if a certain coupling constant is larger than unity. We then estimate the
reduced diffusion D_eff of the bacterium in the strong coupling limit, and find
D_eff ~ 1/g.Comment: 4 pages, absolutely no figure
Localized Joule heating produced by ion current focusing through micron-size holes
We provide an experimental demonstration that the focusing of ionic currents
in a micron size hole connecting two chambers can produce local temperature
increases of up to C with gradients as large as K. We find a good agreement between the measured temperature profiles and
a finite elements-based numerical calculation. We show how the thermal
gradients can be used to measure the full melting profile of DNA duplexes
within a region of 40 m. The possibility to produce even larger gradients
using sub-micron pores is discussed.Comment: 3 pages, accepted to Appl. Phys. Lett
Phase separation transition in liquids and polymers induced by electric field gradients
Spatially uniform electric fields have been used to induce instabilities in
liquids and polymers, and to orient and deform ordered phases of
block-copolymers. Here we discuss the demixing phase transition occurring in
liquid mixtures when they are subject to spatially nonuniform fields. Above the
critical value of potential, a phase-separation transition occurs, and two
coexisting phases appear separated by a sharp interface. Analytical and
numerical composition profiles are given, and the interface location as a
function of charge or voltage is found. The possible influence of demixing on
the stability of suspensions and on inter-colloid interaction is discussed.Comment: 7 pages, 3 figures. Special issue of the J. Phys. Soc. Ja
The interaction between colloids in polar mixtures above Tc
We calculate the interaction potential between two colloids immersed in an
aqueous mixture containing salt near or above the critical temperature. We find
an attractive interaction far from the coexistence curve due to the combination
of preferential solvent adsorption at the colloids' surface and preferential
ion solvation. We show that the ion-specific interaction strongly depends on
the amount of salt added as well as on the mixture composition. Our results are
in accord with recent experiments. For a highly antagonistic salt of
hydrophilic anions and hydrophobic cations, a repulsive interaction at an
intermediate inter-colloid distance is predicted even though both the
electrostatic and adsorption forces alone are attractive.Comment: 9 pages, 6 figure
Organization of Block Copolymers using NanoImprint Lithography: Comparison of Theory and Experiments
We present NanoImprint lithography experiments and modeling of thin films of
block copolymers (BCP). The NanoImprint lithography is used to align
perpendicularly lamellar phases, over distances much larger than the natural
lamellar periodicity. The modeling relies on self-consistent field calculations
done in two- and three-dimensions. We get a good agreement with the NanoImprint
lithography setups. We find that, at thermodynamical equilibrium, the ordered
BCP lamellae are much better aligned than when the films are deposited on
uniform planar surfaces
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