2,357 research outputs found
Decomposition driven interface evolution for layers of binary mixtures: {II}. Influence of convective transport on linear stability
We study the linear stability with respect to lateral perturbations of free
surface films of polymer mixtures on solid substrates. The study focuses on the
stability properties of the stratified and homogeneous steady film states
studied in Part I [U. Thiele, S. Madruga and L. Frastia, Phys. Fluids 19,
122106 (2007)]. To this aim, the linearized bulk equations and boundary
equations are solved using continuation techniques for several different cases
of energetic bias at the surfaces, corresponding to linear and quadratic
solutal Marangoni effects.
For purely diffusive transport, an increase in film thickness either
exponentially decreases the lateral instability or entirely stabilizes the
film. Including convective transport leads to a further destabilization as
compared to the purely diffusive case. In some cases the inclusion of
convective transport and the related widening of the range of available film
configurations (it is then able to change its surface profile) change the
stability behavior qualitatively.
We furthermore present results regarding the dependence of the instability on
several other parameters, namely, the Reynolds number, the Surface tension
number and the ratio of the typical velocities of convective and diffusive
transport.Comment: Published in Physics of Fluic
Polymer drift in a solvent by force acting on one polymer end
We investigate the effect of hydrodynamic interactions on the non-equilibrium
drift dynamics of an ideal flexible polymer pulled by a constant force applied
at one end of the polymer using the perturbation theory and the renormalization
group method. For moderate force, if the polymer elongation is small, the
hydrodynamic interactions are not screened and the velocity and the
longitudinal elongation of the polymer are computed using the renormalization
group method. Both the velocity and elongation are nonlinear functions of the
driving force in this regime. For large elongation we found two regimes. For
large force but finite chain length the hydrodynamic interactions are
screened. For large chain lengths and a finite force the hydrodynamic
interactions are only partially screened, which in three dimensions results in
unusual logarithmic corrections to the velocity and the longitudinal
elongation.Comment: 6 page
Dynamical Model for Chemically Driven Running Droplets
We propose coupled evolution equations for the thickness of a liquid film and
the density of an adsorbate layer on a partially wetting solid substrate.
Therein, running droplets are studied assuming a chemical reaction underneath
the droplets that induces a wettability gradient on the substrate and provides
the driving force for droplet motion. Two different regimes for moving droplets
-- reaction-limited and saturated regime -- are described. They correspond to
increasing and decreasing velocities with increasing reaction rates and droplet
sizes, respectively. The existence of the two regimes offers a natural
explanation of prior experimental observations.Comment: 4 pages, 5 figure
Interfacial layering in a three-component polymer system
We study theoretically the temporal evolution and the spatial structure of
the interface between two polymer melts involving three different species (A,
A* and B). The first melt is composed of two different polymer species A and A*
which are fairly indifferent to one another (Flory parameter chi_AA* ~ 0). The
second melt is made of a pure polymer B which is strongly attracted to species
A (chi_AB 0). We then show
that, due to these contradictory tendencies, interesting properties arise
during the evolution of the interface after the melts are put into contact: as
diffusion proceeds, the interface structures into several adjacent
"compartments", or layers, of differing chemical compositions, and in addition,
the central mixing layer grows in a very asymmetric fashion. Such unusual
behaviour might lead to interesting mechanical properties, and demonstrates on
a specific case the potential richness of multi-component polymer interfaces
(as compared to conventional two-component interfaces) for various
applications.Comment: Revised version, to appear in Macromolecule
Dynamics of Strongly Deformed Polymers in Solution
Bead spring models for polymers in solution are nonlinear if either the
finite extensibility of the polymer, excluded volume effects or hydrodynamic
interactions between polymer segments are taken into account. For such models
we use a powerful method for the determination of the complete relaxation
spectrum of fluctuations at {\it steady state}. In general, the spectrum and
modes differ significantly from those of the linear Rouse model. For a tethered
polymer in uniform flow the differences are mainly caused by an inhomogeneous
distribution of tension along the chain and are most pronounced due to the
finite chain extensibility. Beyond the dynamics of steady state fluctuations we
also investigate the nonlinear response of the polymer to a {\em large sudden
change} in the flow. This response exhibits several distinct regimes with
characteristic decay laws and shows features which are beyond the scope of
single mode theories such as the dumbbell model.Comment: 7 pages, 3 figure
Stripes of Partially Fluorinated Alkyl Chains: Dipolar Langmuir Monolayers
Stripe-like domains of Langmuir monolayers formed by surfactants with
partially fluorinated lipid anchors (F-alkyl lipids) are observed at the
gas-liquid phase coexistence. The average periodicity of the stripes, measured
by fluorescence microscopy, is in the micrometer range, varying between 2 and 8
microns. The observed stripe-like patterns are stabilized due to dipole-dipole
interactions between terminal -CF3 groups. These interactions are particularly
strong as compared with non-fluorinated lipids due to the low dielectric
constant of the surrounding media (air). These long-range dipolar interactions
tend to elongate the domains, in contrast to the line tension that tends to
minimize the length of the domain boundary. This behavior should be compared
with that of the lipid monolayer having alkyl chains, and which form spherical
micro-domains (bubbles) at the gas-liquid coexistence. The measured stripe
periodicity agrees quantitatively with a theoretical model. Moreover, the
reduction in line tension by adding traces (0.1 mol fraction) of cholesterol
results, as expected, in a decrease in the domain periodicity.Comment: 20 pages, 4 fig
Straightening of Thermal Fluctuations in Semi-Flexible Polymers by Applied Tension
We investigate the propagation of a suddenly applied tension along a
thermally excited semi-flexible polymer using analytical approximations,
scaling arguments and numerical simulation. This problem is inherently
non-linear. We find sub-diffusive propagation with a dynamical exponent of 1/4.
By generalizing the internal elasticity, we show that tense strings exhibit
qualitatively different tension profiles and propagation with an exponent of
1/2.Comment: Latex file; with three postscript figures; .ps available at
http://dept.physics.upenn.edu/~nelson/pull.p
Viscoelastic Effect on Hydrodynamic Relaxation in Polymer Solutions
The viscoelastic effect on the hydrodynamic relaxation in semidilute polymer
solutions is investigated. From the linearized two-fluid model equations, we
predict that the dynamical asymmetry coupling between the velocity fluctuations
and the viscoelastic stress influences on the hydrodynamic relaxation process,
resulting in a wave-number-dependent shear viscosity.Comment: 7pages; To be published in Journal of the Physical Society of
Japan,Vol 72,No2,(2003
Hydrodynamic Self-Consistent Field Theory for Inhomogeneous Polymer Melts
We introduce a mesoscale technique for simulating the structure and rheology
of block copolymer melts and blends in hydrodynamic flows. The technique
couples dynamic self consistent field theory (DSCFT) with continuum
hydrodynamics and flow penalization to simulate polymeric fluid flows in
channels of arbitrary geometry. We demonstrate the method by studying phase
separation of an ABC triblock copolymer melt in a sub-micron channel with
neutral wall wetting conditions. We find that surface wetting effects and shear
effects compete, producing wall-perpendicular lamellae in the absence of flow,
and wall-parallel lamellae in cases where the shear rate exceeds some critical
Weissenberg number.Comment: Revised as per peer revie
- …