48 research outputs found
A microfluidic technique for generating monodisperse submicron-sized drops
International audienceWe present a route for producing monodisperse micro and nanodrops that is based on a liquid-gas phase transition occurring within a microfluidic device. A gas which is soluble in water is mixed with an insoluble one and injected into an aqueous surfactant solution, using a microfluidic device that produces monodisperse bubbles. As the soluble gas diffuses out of the bubbles, they shrink and the remaining insoluble gas condenses into drops. Their radius can be tuned over a wide range by changing the initial gas mixing ratio
A high rate flow-focusing foam generator
We use a rigid axisymetric microfluidic flow focusing device to produce
monodisperse bubbles, dispersed in a surfactant solution. The gas volume
fraction of the dispersion collected out of this device can be as large as 90%,
demonstrating that foam with solid-like viscoelastic properties can be produced
in this way. The polydispersity of the bubbles is so low that we observe
crystallization of our foam. We measure the diameter of the bubbles and compare
these data to recent theoretical predictions. The good control over bubble size
and foam gas volume fraction shows that our device is a flexible and promising
tool to produce calibrated foam at a high flow rate
Coarsening transitions of wet liquid foams under microgravity conditions
We report foam coarsening studies which were performed in the International
Space Station (ISS) to suppress drainage due to gravity. Foams and bubbly
liquids with controlled liquid fractions between 15 and 50\% were
investigated to study the transition between bubble growth laws previously
reported near the dry limit and the dilute limit (Ostwald ripening). We determined the coarsening rates; for the
driest foams and the bubbly liquids, they are in close agreement with
theoretical predictions. We observe a sharp cross-over between the respective
laws at a critical value . At liquid fractions beyond this transition,
neighboring bubbles are no longer all in contact, like at a jamming transition.
Remarkably is significantly larger than the random close packing
volume fraction of the bubbles which was determined
independently. We attribute the differences between and
to a weakly adhesive bubble interaction that we have
studied in complementary ground-based experiments
Aqueous foams in microgravity, measuring bubble sizes
The paper describes a study of wet foams in microgravity whose bubble size
distribution evolves due to diffusive gas exchange. We focus on the comparison
between the size of bubbles determined from images of the foam surface and the
size of bubbles in the bulk foam, determined from Diffuse Transmission
Spectroscopy (DTS). Extracting the bubble size distribution from images of a
foam surface is difficult so we have used three different procedures : manual
analysis, automatic analysis with a customized Python script and machine
learning analysis. Once various pitfalls were identified and taken into
account, all the three procedures yield identical results within error bars.
DTS only allows the determination of an average bubble radius which is
proportional to the photon transport mean free path . The relation
between the measured diffuse transmitted light intensity and {}
previously derived for slab-shaped samples of infinite lateral extent does not
apply to the cuboid geometry of the cells used in the microgravity experiment.
A new more general expression of the diffuse intensity transmitted with
specific optical boundary conditions has been derived and applied to determine
the average bubble radius. The temporal evolution of the average bubble radii
deduced from DTS and of the same average radii of the bubbles measured at the
sample surface are in very good agreement throughout the coarsening. Finally,
ground experiments were performed to compare bubble size distributions in a
bulk wet foam and at its surface at times so short that diffusive gas exchange
is insignificant. They were found to be similar, confirming that bubbles seen
at the surface are representative of the bulk foam bubbles
Bubble dynamics relaxation in aqueous foam probed by multispeckle diffusing-wave spectroscopy
International audienceWe study the bubble rearrangement dynamics in aqueous foam during the passage from liquidlike to solidlike behavior which follows a transient shear deformation that perturbs the bubble packing. The local dynamics is probed using multispeckle diffusing-wave spectroscopy. We show that following the perturbation the average time between rearrangements relaxes exponentially, with time elapsed since the end of the perturbation. The observed scaling of the characteristic relaxation time with perturbation amplitude and foam age is explained by a schematic coarse-grained model based on the scaling state hypothesis
Capture of particles in soft porous media
International audienceWe investigate the capture of particles in soft porous media. Liquid foam constitutes a model system for such a study, allowing the radii of passage in the pore space to be tuned over several orders of magnitude by adjusting the liquid volume fraction. We show how particle capture is determined by the coupling of interstitial liquid flow and network deformation, and present a simple model of the capture process that shows good agreement with our experimental data
Flow in Foams and Flowing Foams
International audienceAqueous foams are complex fluids composed of gas bubbles tightly packed in a surfactant solution. Even though they generally consist only of Newtonian fluids, foam flow obeys nonlinear laws. This can result from nonaffine deformations of the disordered bubble packing as well as from a coupling between the surface flow in the surfactant monolayers and the bulk liquid flow in the films, channels, and nodes. A similar coupling governs the permeation of liquid through the bubble packing that is observed when foams drain due to gravity. We review the experimental state of the art as well as recent models that describe the interplay of the processes at multiple length scales involved in foam drainage and rheology
High-resolution diffusing-wave spectroscopy using optimized heterodyne detection
International audienceWe show experimentally and theoretically that the use of optimized heterodyne detection in a diffusing-wave spectroscopy experiment leads to the detection of much smaller intensity autocorrelations than with conventional (either homodyne or heterodyne) setups. This enhanced resolution may be useful for the study of longtime dynamics of multiple-scattering disordered systems. (C) 2003 Optical Society of America