189 research outputs found
Two-dimensional flow of foam around a circular obstacle: local measurements of elasticity, plasticity and flow
We investigate the two-dimensional flow of a liquid foam around circular
obstacles by measuring all the local fields necessary to describe this flow:
velocity, pressure, bubble deformations and rearrangements. We show how our
experimental setup, a quasi-2D "liquid pool" system, is adapted to the
determination of these fields: the velocity and bubble deformations are easy to
measure from 2D movies, and the pressure can be measured by exploiting a
specific feature of this system, a 2D effective compressibility. To describe
accurately bubble rearrangements, we propose a new, tensorial descriptor. All
these quantities are evaluated via an averaging procedure that we justify
showing that the fluctuations of the fields are essentially random. The flow is
extensively studied in a reference experimental case; the velocity presents an
overshoot in the wake of the obstacle, the pressure is maximum at the leading
side and minimal at the trailing side. The study of the elastic deformations
and of the velocity gradients shows that the transition between plug flow and
yielded regions is smooth. Our tensorial description of T1s highlight their
correlation both with the bubble deformations and the velocity gradients. A
salient feature of the flow, notably on the velocity and T1 repartition, is a
marked asymmetry upstream/downstream, signature of the elastic behaviour of the
foam. We show that the results do not change qualitatively when various control
parameters vary, identifying a robust quasistatic regime. These results are
discussed in the frame of the actual foam rheology literature, and we argue
that they constitute a severe test for existing rheological models, since they
capture both the elastic, plastic and fluid behaviour of the foam.Comment: 41 pages, 25 figures, submitted to Journal of Fluid Mechanics (but
not in JFM style), short version of the abstrac
Deformation of soap films pushed through tubes at high velocity
International audienceThe behaviour of soap films pushed through tubes at large velocities, up to several m/s, is investigated. The film shape deviates from its equilibrium configuration perpendicular to the walls and gets curved downstream. A simple model relates the radius of curvature of the film to the friction in the lubrication films touching the wall, and the scaling of Bretherton (1961) holds up to surprisingly high velocities, at which the capillary and Weber numbers are no longer small parameters. The tube geometry is varied and accounted for through the notion of hydraulic diametre. A limit of stability of the film, beyond which the films burst or evolve unsteadily, is predicted, and captures quantitatively the observations. The new questions raised by our results on the dissipation in soap films are discussed, especially the role of Plateau borders and inertial effects
Microbubble formation and pinch-off scaling exponent in flow-focusing devices
We investigate the gas jet breakup and the resulting microbubble formation in
a microfluidic flow-focusing device using ultra high-speed imaging at 1 million
frames/s. In recent experiments [Dollet et al., Phys. Rev. Lett. 100, 034504
(2008)] it was found that in the final stage of the collapse the radius of the
neck scales with time with a 1/3 power-law exponent, which suggested that gas
inertia and the Bernoulli suction effect become important. Here, ultra
high-speed imaging was used to capture the complete bubble contour and quantify
the gas flow through the neck. It revealed that the resulting decrease in
pressure, due to Bernoulli suction, is too low to account for an accelerated
pinch-off. The high temporal resolution images enable us to approach the final
moment of pinch-off to within 1 {\mu}s. We observe that the final moment of
bubble pinch-off is characterized by a scaling exponent of 0.41 +/- 0.01. This
exponent is approximately 2/5, which can be derived, based on the observation
that during the collapse the neck becomes less slender, due to the exclusive
driving through liquid inertia
Liquid films with high surface modulus moving in tubes: dynamic wetting film and jumpy motion
We investigate the motion through a wet tube of transverse soap films, or
lamellae, of high surface dilatationnal modulus. Combining local thickness and
velocity measurements in the wetting film, we reveal a zone of several
centimeters in length, the dynamic wetting film, which is significantly
influenced by a moving lamella. The dependence of this influence length on
lamella velocity and wetting film thickness provides a discrimination among
several possible surfactant minimal models. A spectacular jumpy mode of
unsteady motion of a lamella is also evidenced
Coupled vibrations of a meniscus and liquid films
International audienceWe investigate the vibration properties of a circular horizontal film, that is bounded by a meniscus (or Plateau border) and suspended to two catenary films. The suspending films act as capillary springs, and the system is thus free to oscillate around its equilibrium position. We study successively its free and forced oscillations. In our experiments, we track simultaneously the positions of the Plateau border and of the film. The model that we present predicts the eigenfrequency of the system, and its resonance characteristics (in forced oscillations). In particular, we show that the dynamics of both the Plateau border and the film have to be taken into account, in order to provide an accurate prediction of the oscillation frequency
Non-locality and viscous drag effects on the shear localisation in soft-glassy materials
We study the Couette flow of a quasi-2d soft-glassy material in a Hele-Shaw
geometry. The material is chosen to be above the jamming point, where a yield
stress emerges, below which the material deforms elastically and
above which it flows like a complex fluid according to a Herschel-Bulkley (HB)
rheology. Simultaneously, the effect of the confining plates is modelled as an
effective linear friction law, while the walls aside the Hele-Shaw cell are
sufficiently close to each other to allow visible cooperativity effects in the
velocity profiles (Goyon et al., Nature 454, 84-87 (2008)). The effects of
cooperativity are parametrized with a steady-state diffusion-relaxation
equation for the fluidity field , defined as the ratio
between shear rate and shear stress . For particular
rheological flow-curves (Bingham fluids), the problem is tackled analytically:
we explore the two regimes and
and quantify the effect of the extra localisation induced by the wall friction.
Other rheo-thinning fluids are explored with the help of numerical simulations
based on lattice Boltzmann models, revealing a robustness of the analytical
findings. Synergies and comparisons with other existing works in the literature
(Barry et al., Phil. Mag. Lett. 91, 432-440 (2011)) are also discussed
Preferred sizes and ordering in surface nanobubble populations
Two types of homogeneous surface nanobubble populations, created by different
means, are analyzed statistically on both their sizes and spatial positions. In
the first type (created by droplet-deposition, case A) the bubble size R is
found to be distributed according to a generalized gamma law with a preferred
radius R*=20 nm. The radial distribution function shows a preferred spacing at
~5.5 R*. These characteristics do not show up in comparable Monte-Carlo
simulations of random packings of hard disks with the same size distribution
and the same density, suggesting a structuring effect in the nanobubble
formation process. The nanobubble size distribution of the second population
type (created by ethanol-water exchange, case B) is a mixture of two clearly
separated distributions, hence, with two preferred radii. The local ordering is
less significant, due to the looser packing of the nanobubbles.Comment: 5 pages, 5 figure
Model for the growth and the oscillation of a cavitation bubble in a spherical liquid-filled cavity enclosed in an elastic medium
International audienceEquations are derived that describe the growth and subsequent damped oscillation of a cavitation bubble in a liquid-filled cavity surrounded by an elastic solid. It is assumed that the nucleation and the growth of the bubble are caused by an initial negative pressure in the cavity. The liquid is treated as viscous and compressible. The obtained equations allow one to model, by numerical computation, the growth and the oscillation of the bubble in the cavity and the oscillation of the cavity surface. It is shown that the equilibrium radius reached by the growing bubble decreases when the absolute magnitude of the initial negative pressure decreases. It is also found that the natural frequency of the bubble oscillation increases with increasing bubble radius. This result is of special interest because in an unbounded liquid, the natural frequency of a bubble is known to behave oppositely, namely it decreases with increasing bubble radius
Two-dimensional flows of foam: drag exerted on circular obstacles and dissipation
A Stokes experiment for foams is proposed. It consists in a two-dimensional
flow of a foam, confined between a water subphase and a top plate, around a
fixed circular obstacle. We present systematic measurements of the drag exerted
by the flowing foam on the obstacle, \emph{versus} various separately
controlled parameters: flow rate, bubble volume, solution viscosity, obstacle
size and boundary conditions. We separate the drag into two contributions, an
elastic one (yield drag) at vanishing flow rate, and a fluid one (viscous
coefficient) increasing with flow rate. We quantify the influence of each
control parameter on the drag. The results exhibit in particular a power-law
dependence of the drag as a function of the solution viscosity and the flow
rate with two different exponents. Moreover, we show that the drag decreases
with bubble size, increases with obstacle size, and that the effect of boundary
conditions is small. Measurements of the streamwise pressure gradient,
associated to the dissipation along the flow of foam, are also presented: they
show no dependence on the presence of an obstacle, and pressure gradient
depends on flow rate, bubble volume and solution viscosity with three
independent power laws.Comment: 23 pages, 13 figures, proceeding of Eufoam 2004 conferenc
- …