83 research outputs found
Coffee-stain growth dynamics on dry and wet surfaces
The drying of a drop containing particles often results in the accumulation
of the particles at the contact line. In this work, we investigate the drying
of an aqueous colloidal drop surrounded by a hydrogel that is also evaporating.
We combine theoretical and experimental studies to understand how the
surrounding vapor concentration affects the particle deposit during the
constant radius evaporation mode. In addition to the common case of evaporation
on an otherwise dry surface, we show that in a configuration where liquid is
evaporating from a flat surface around the drop, the singularity of the
evaporative flux at the contact line is suppressed and the drop evaporation is
homogeneous. For both conditions, we derive the velocity field and we establish
the temporal evolution of the number of particles accumulated at the contact
line. We predict the growth dynamics of the stain and the drying timescales.
Thus, dry and wet conditions are compared with experimental results and we
highlight that only the dynamics is modified by the evaporation conditions, not
the final accumulation at the contact line
Self-crumpling elastomers: bending induced by the drying stimulus of a nanoparticle suspension
We report an experimental study of the drying-induced peeling of a bilayer,
consisting of an elastomeric disk coated with a suspension of nanoparticles. We
show that although capillary forces associated with the scale of the droplet
can not compete with the adhesion of the elastomer on a surface, nevertheless
large tensile stresses develop in the coating, which results in a moment
bending the bilayer. We attribute this stress to the nano-menisci in the pores
of the colloidal material and we propose a model that describes successfully
the early stage curvature of the bilayer. Thus, we show that the peeling can be
conveniently controlled by the particle size and the coating thickness.Comment: 6 pages, 5 figures, 1 table, accepted in EP
Instability and morphology of polymer solutions coating a fiber
We report an experimental study on the dynamics of a thin film of polymer
solution coating a vertical fiber. The liquid film has first a constant
thickness and then undergoes the Rayleigh-Plateau instability which leads to
the formation of sequences of drops, separated by a thin film, moving down at a
constant velocity. Different polymer solutions are used, i.e. xanthan solutions
and polyacrylamide (PAAm) solutions. These solutions both exhibit shear-rate
dependence of the viscosity, but for PAAm solutions, there are strong normal
stresses in addition of the shear-thinning effect. We characterize
experimentally and separately the effects of these two non-Newtonian properties
on the flow on the fiber. Thus, in the flat film observed before the emergence
of the drops, only shear-thinning effect plays a role and tends to thin the
film compared to the Newtonian case. The effect of the non-Newtonian rheology
on the Rayleigh-Plateau instability is then investigated through the
measurements of the growth rate and the wavelength of the instability. Results
are in good agreement with linear stability analysis for a shear-thinning
fluid. The effect of normal stress can be taken into account by considering an
effective surface tension which tends to decrease the growth rate of the
instability. Finally, the dependence of the morphology of the drops with the
normal stress is investigated and a simplified model including the normal
stress within the lubrication approximation provides good quantitative results
on the shape of the drops.Comment: Accepted in Journal of Fluid Mechanic
Annular cracks in thin films of nanoparticle suspensions drying on a fiber
We report an experimental study of the crack pattern formed during the drying
of a colloidal suspension. A horizontal fiber, which provides a one
dimensional, boundary-free substrate, is coated by a film of micronic
thickness. The geometry imposes a remarkable annular crack pattern and allowing
precise measurements of the crack spacing over a short range of film thickness
(between 2 and 10 m) which varies linearly with the film height. We
compare our experimental data with a model proposed by Kitsunezaki which
suggests that the variation of the crack spacing with the film thickness
depends on the ratio between a critical stress at cracking and a critical
stress for slipping on the substrate. By measuring the friction force of the
colloidal gels on a hydrophobic surface through a cantilever technique, we can
deduce the critical crack stress for these colloidal gels simply by measuring
the crack spacing of the pattern.Comment: Accepted in EP
Homogeneous deposition of particles by absorption on hydrogels
When a drop containing colloidal particles evaporates on a surface, a
circular stain made of these particles is often observed due to an internal
flow toward the contact line. To hinder this effect, several approaches have
been proposed such as flow modification by addition of surfactants or control
of the interactions between the particles. All of these strategies involve the
liquid phase while maintaining the drying process. However, substitution of
evaporation by absorption into the substrate of the solvent has been
investigated less. Here, we show that a droplet containing colloidal particles
deposited on swelling hydrogels can lead to a nearly uniform coating. We report
experiments and theory to explore the relation between the gel swelling,
uniformity of deposition and the adsorption dynamics of the particles at the
substrate. Our findings suggest that draining the solvent by absorption
provides a robust route to homogeneous coatings
Drop impact on a flexible fiber
When droplets impact fibrous media, the liquid can be captured by the fibers
or contact then break away. Previous studies have shown that the efficiency of
drop capture by a rigid fiber depends on the impact velocity and defined a
threshold velocity below which the drop is captured. However, it is necessary
to consider the coupling of elastic and capillary effects to achieve a greater
understanding of the capture process for soft substrates. Here, we study
experimentally the dynamics of a single drop impacting on a thin flexible
fiber. Our results demonstrate that the threshold capture velocity depends on
the flexibility of fibers in a non-monotonic way. We conclude that tuning the
mechanical properties of fibers can optimize the efficiency of droplet capture.Comment: Soft Matter (2015
Protocol to perform pressurized blister tests on thin elastic films
This work aims to identify common challenges in the preparation of the
blister test devices designed for measurement of energy release rate for
brittle thin films and to propose easy-to-implement solutions accordingly. To
this end, we provide a step-by-step guide for fabricating a blister test device
comprised of thin polystyrene films adhered to glass substrates. Thin films are
first transferred from donor substrates to an air-water interface, which is
then used as a platform to locate them on a receiver substrate. We embed a
microchannel at the back of the device to evacuate the air trapped in the
opening, through which the pressure is applied. We quantify the height and the
radius of the blister to estimate the adhesion energy using the available
expressions correlating the normal force and the moment with the shape of the
blister. The present blister test provided adhesion energy per unit area of \mbox{mJ}/{\mbox m}^2 for polystyrene on glass, which is in good
agreement with the measurement of \mbox{mJ}/{\mbox m}^2 found
in our independent cleavage test
Damping of liquid sloshing by foams: from everyday observations to liquid transport
We perform experiments on the sloshing dynamics of liquids in a rectangular
container submitted to an impulse. We show that when foam is placed on top of
the liquid the oscillations of the free interface are significantly damped. The
ability to reduce sloshing and associated splashing could find applications in
numerous industrial processes involving liquid transport.Comment: Accepted for publication in Journal of Visualizatio
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