121 research outputs found
Viscoelastic effects and anomalous transient levelling exponents in thin films
We study theoretically the profile evolution of a thin viscoelastic film
supported onto a no-slip flat substrate. Due to the nonconstant initial
curvature at the free surface, there is a flow driven by Laplace pressure and
mediated by viscoelasticity. In the framework of lubrication theory, we derive
a thin film equation that contains local viscoelastic stress through the
Maxwell model. Then, considering a sufficiently regular small perturbation of
the free surface, we linearise the equation and derive its general solution. We
analyse and discuss in details the behaviour of this function. We then use it
to study the viscoelastic evolution of a Gaussian initial perturbation through
its transient levelling exponent. For initial widths of the profile that are
smaller than a characteristic length scale involving both the film thickness
and the elastocapillary length, this exponent is shown to reach anomalously
high values at the elastic-to-viscous transition. This prediction should in
particular be observed at sufficiently short times in experiments on thin
polymer films.Comment: 4 figure
Wake and wave resistance on viscous thin films
The effect of an external pressure disturbance, being displaced with a
constant speed along the free surface of a viscous thin film, is studied
theoretically in the lubrication approximation in one- and two-dimensional
geometries. In the comoving frame, the imposed pressure field creates a
stationary deformation of the interface - a wake - that spatially vanishes in
the far region. The shape of the wake and the way it vanishes depend on both
the speed and size of the external source and the properties of the film. The
wave resistance, namely the force that has to be externally furnished in order
to maintain the wake, is analysed in details. For finite-size pressure
disturbances, it increases with the speed, up to a certain transition value
above which a monotonic decrease occurs. The role of the horizontal extent of
the pressure field is studied as well, revealing that for a smaller disturbance
the latter transition occurs at higher speed. Eventually, for a Dirac pressure
source, the wave resistance either saturates in a 1D geometry, or diverges in a
2D geometry
Flow-injection of branched polymers inside nanopores
Flexible chains (linear or branched) can be forced to enter into a narrow
capillary by using a hydrodynamic flow. Here, we correct our earlier
description of this problem by considering the progressive nature of the
suction process. We find that the critical current for penetration, , is
controlled by the entry of a single blob of the capillary size, and that its
scaling structure is the same for branched and linear chains.Comment: Submitted to Europhysics Letter
Elastowetting of Soft Hydrogel Spheres
When a soft hydrogel sphere is placed on a rigid hydrophilic substrate, it
undergoes arrested spreading by forming an axisymmetric foot near the contact
line, while conserving its global spherical shape. In contrast, liquid water
(that constitutes greater than 90% of the hydrogel's volume) spreads into a
thin film on the same surface. We study systematically this elastowetting of
gel spheres on substrates of different surface energies, and find that their
contact angle increases as the work of adhesion between the gel and the
substrate decreases, as one would observe for drops of pure water - albeit
being larger than in the latter case. This difference in the contact angles of
gel and water appears to be due to the elastic shear stresses that develop in
the gel and oppose its spreading. Indeed, by increasing the elastic modulus of
the gel spheres, we find that their contact angle also increases. In addition,
the length of the contact foot increases with the work of adhesion and sphere
size, while it decreases when the elastic modulus of the gel is increased. We
discuss those experimental results in light of a minimal analysis based on
energy minimization, volume conservation, and scaling arguments
Self-Similarity and Energy Dissipation in Stepped Polymer Films
The surface of a thin liquid film with nonconstant curvature is unstable, as
the Laplace pressure drives a flow mediated by viscosity. We present the
results of experiments on one of the simplest variable curvature surfaces: a
stepped polymer film. Height profiles are measured as a function of time for a
variety of molecular weights. The evolution of the profiles is shown to be
self-similar. This self-similarity offers a precise measurement of the
capillary velocity by comparison with numerical solutions of the thin film
equation. We also derive a master expression for the time dependence of the
excess free energy as a function of the material properties and film geometry.
The experiment and theory are in excellent agreement and indicate the
effectiveness of stepped polymer films to elucidate nanoscale rheological
properties.Comment: 5 pages, 4 figures, article accepted for publication in Physical
Review Letter
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