70 research outputs found
Nucleated dewetting in supported ultra-thin liquid films with hydrodynamic slip
This study reveals the influence of the surface energy and solid/liquid
boundary condition on the breakup mechanism of dewetting ultra-thin polymer
films. Using silane self-assembled monolayers, SiO substrates are rendered
hydrophobic and provide a strong slip rather than a no-slip solid/liquid
boundary condition. On undergoing these changes, the thin-film breakup
morphology changes dramatically -- from a spinodal mechanism to a breakup which
is governed by nucleation and growth. The experiments reveal a dependence of
the hole density on film thickness and temperature. The combination of lowered
surface energy and hydrodynamic slip brings the studied system closer to the
conditions encountered in bursting unsupported films. As for unsupported
polymer films, a critical nucleus size is inferred from a free energy model.
This critical nucleus size is supported by the film breakup observed in the
experiments using high speed \emph{in situ} atomic force microscopy.Comment: 8 pages, 9 figures, including supplementary materia
Capillary leveling of stepped films with inhomogeneous molecular mobility
A homogeneous thin polymer film with a stepped height profile levels due to
the presence of Laplace pressure gradients. Here we report on studies of
polymeric samples with precisely controlled, spatially inhomogeneous molecular
weight distributions. The viscosity of a polymer melt strongly depends on the
chain length distribution; thus, we learn about thin-film hydrodynamics with
viscosity gradients. These gradients are achieved by stacking two films with
different molecular weights atop one another. After a sufficient time these
samples can be well described as having one dimensional viscosity gradients in
the plane of the film, with a uniform viscosity normal to the film. We develop
a hydrodynamic model that accurately predicts the shape of the experimentally
observed self-similar profiles. The model allows for the extraction of a
capillary velocity, the ratio of the surface tension and the viscosity, in the
system. The results are in excellent agreement with capillary velocity
measurements of uniform mono- and bi-disperse stepped films and are consistent
with bulk polymer rheology.Comment: Accepted for publication in Soft Matter, Themed Issue on "The
Geometry and Topology of Soft Materials
Controlling Marangoni induced instabilities in spin-cast polymer films: how to prepare uniform films
In both research and industrial settings spin coating is extensively used to
prepare highly uniform thin polymer films. However, under certain conditions,
spin coating results in films with non-uniform surface morphologies. Although
the spin coating process has been extensively studied, the origin of these
morphologies is not fully understood and the formation of non-uniform spincast
films remains a practical problem. Here we report on experiments demonstrating
that the formation of surface instabilities during spin coating is dependent on
temperature. Our results suggest that non-uniform spincast films form as a
result of the Marangoni effect, which describes flow due to surface tension
gradients. We find that both the wavelength and amplitude of the pattern
increase with temperature. Finally, and most important from a practical
viewpoint, the non-uniformities in the film thickness can be entirely avoided
simply by lowering the spin coating temperature.Comment: 8 pages, 6 figures. electronic supplementary material: 3 pages, 4
figure
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
Capillary-driven flow induced by a stepped perturbation atop a viscous film
Thin viscous liquid films driven by capillarity are well described in the
lubrication theory through the thin film equation. In this article, we present
an analytical solution of this equation for a particular initial profile: a
stepped perturbation. This initial condition allows a linearization of the
problem making it amenable to Fourier analysis. The solution is obtained and
characterized. As for a temperature step in the heat equation, self-similarity
of the first kind of the full evolution is demonstrated and a long-term
expression for the excess free energy is derived. In addition, hydrodynamical
fields are described. The solution is then compared to experimental profiles
from a model system: a polystyrene nanostep above the glass transition
temperature which flows due to capillarity. The excellent agreement enables a
precise measurement of the capillary velocity for this polymeric liquid,
without involving any numerical simulation. More generally, as these results
hold for any viscous system driven by capillarity, the present solution may
provide a useful tool in hydrodynamics of thin viscous films.Comment: Accepted for publication in Physics of Fluid
Elastohydrodynamic relaxation of soft and deformable microchannels
Hydrodynamic flows in compliant channels are of great interest in physiology
and microfluidics. In these situations, elastohydrodynamic coupling leads to:
(i) a nonlinear pressure-vs.-flow-rate relation, strongly affecting the
hydraulic resistance; and (ii), because of the compliance-enabled volume
storage, a finite relaxation time under a step-wise change in pressure. This
latter effect remains relatively unexplored, even while the time scale can vary
over a decade in typical situations. In this study we provide time-resolved
measurements of the relaxation dynamics for thin and soft, rectangular
microfluidic channels. We describe our data using a perturbative lubrication
approximation of the Stokes equation coupled to linear elasticity, while taking
into account the effect compliance and resistance of the entrance. The
modelling allows to completely describe all of the experimental results. Our
work is relevant for any microfluidic scenario wherein a time-dependent driving
is applied and provides a first step in the dynamical description of compliant
channel networks
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