67 research outputs found
Wall slip of complex fluids: interfacial friction or slip length?
Using a dynamic Surface Force Apparatus, we demonstrate that the notion of
slip length used to describe the boundary flow of simple liquids, is not
appropriate for viscoelastic liquids. Rather, the appropriate description lies
in the original Navier's partial slip boundary condition, formulated in terms
of an interfacial friction coefficient. We establish an exact analytical
expression to extract the interfacial friction coefficient from oscillatory
drainage forces between a sphere and a plane, suitable for dynamic SFA or
Atomic Force Microscopy non-contact measurements. We use this model to
investigate the boundary friction of viscoelastic polymer solutions over 5
decades of film thicknesses and one decade in frequency. The proper use of the
original Navier's condition describes accurately the complex hydrodynamic force
up to scales of tens of micrometers, with a simple "Newtonian-like" friction
coefficient, not frequency dependent, and reflecting closely the dynamics of an
interfacial depletion layer at the solution/solid interface.Comment: 7 pages, 5 figure
Thermally Activated Dynamics of the Capillary Condensation
This paper is devoted to the thermally activated dynamics of the capillary
condensation. We present a simple model which enables us to identify the
critical nucleus involved in the transition mechanism. This simple model is
then applied to calculate the nucleation barrier from which we can obtain
informations on the nucleation time. We present a simple estimation of the
nucleation barrier in slab geometry both in the two dimensional case and in the
three dimensional case. We extend the model in the case of rough surfaces which
is closer to the experimental case and allows comparison with experimental
datas.Comment: 6 pages, 3 figures, Submitted to J. Phys. : Condens. Matter,
Proceedings of the IV Liquid Matter Conference - Grenada(Spain) july 199
Measurement of the temperature decrease in evaporating soap films
Recent advances have demonstrated that evaporation can play a significant
role on soap film stability, which is a key concern in many industrial areas
but also for children playing with bubbles. Thus, evaporation leads to a film
thinning but also to a film cooling, which has been overlooked for soapy
objects. Here, we study the temperature variation of an evaporating soap film
for different values of relative humidity and glycerol concentrations. We
evidence that the temperature of soap films can decrease after their creation
up to 8C. We propose a model describing the temperature drop of
soap films after their formation that is in quantitative agreement with our
experiments. We emphasize that this cooling effect is significant and must be
carefully considered in future studies on the dynamics of soap films.Comment: 6 page
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
Generation of Giant Soap Films
Artists regularly make soap bubbles several meters long. In this article we
make soap films up to two meters high by pulling a horizontal fishing line
driven by belts out of a soapy solution at velocities ranging from 20 cm/s to
250 cm/s. We characterize the thickness profile of the central part of the film
that behaves like a rubber band under tension. We show that its thickness
profile is well described by a static model in which a homogeneous elastic film
is stretched by its own weight. This leads to an exponential thickness profile
with a characteristic length given by a competition between gravity and surface
elasticity. The prefactor is fixed by the shape and area of the film, governed
by the fishing line motion but also by a continuous extraction of foam film
from the lateral menisci, thicker than the central part, and that progressively
invades the film from its lateral boundaries. The model we propose captures the
subtle interplay between gravity, film elasticity and film extraction and leads
to predictions in good agreement with our experimental data
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