412 research outputs found
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
On the sound of snapping shrimp
Fluid dynamics video: Snapping shrimp produce a snapping sound by an
extremely rapid closure of their snapper claw. Our high speed imaging of the
claw closure has revealed that the sound is generated by the collapse of a
cavitation bubble formed in a fast flowing water jet forced out from the claws
during claw closure. The produced sound originates from the cavitation collapse
of the bubble. At collapse a short flash of light is emitted, just as in single
bubble sonoluminescence. A model based on the Rayleigh-Plesset equation can
quantitatively account for the visual and acoustical observations.Comment: Fluid dynamics vide
iLIF: illumination by Laser-Induced Fluorescence for single flash imaging on a nanoseconds timescale \ud
The challenge in visualizing fast microscale fluid motion phenomena is to record high-quality images free of motion-blur. Here, we present an illumination technique based on laser-induced fluorescence which delivers high-intensity light pulses of 7 ns. The light source consists of a Q-switched Nd:YAG laser and a laser dye solution incorporated into a total internal reflection lens, resulting in a uni-directional light beam with a millimeter-sized circular aperture and 3° divergence. The laser coherence, considered undesirable for imaging purposes, is reduced while maintaining a nanoseconds pulse duration. The properties of the illumination by laser-induced fluorescence (iLIF) are quantified, and a comparison is made with other high-intensity pulsed and continuous light source
Ultrasound-induced Gas Release from Contrast Agent Microbubbles
We investigated gas release from two hard-shelled ultrasound contrast agents by subjecting them to high-mechanical index (MI) ultrasound and simultaneously capturing high-speed photographs. At an insonifying frequency of 1.7 MHz, a larger percentage of contrast bubbles is seen to crack than at 0.5 MHz. Most of the released gas bubbles have equilibrium diameters between 1.25 and 1.75 /spl mu/m. Their disappearance was observed optically. Free gas bubbles have equilibrium diameters smaller than the bubbles from which they have been released. Coalescence may account for the long dissolution times acoustically observed and published in previous studies. After sonic cracking, the cracked bubbles stay acoustically active
Optical observations of acoustical radiation force effects on individual air bubbles
Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US parameters (pulse length, acoustic pressure) on different bubble\ud
patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 um to 79 um was generated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, reaching a maximum at the resonance size of 48 um. The predicted displacement values differ by 15% from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radiation forces that are strong enough to affect the bubble position
Role of the Channel Geometry on the Bubble Pinch-Off in Flow-Focusing Devices
The formation of bubbles by flow focusing of a gas and a liquid in a rectangular channel is shown to depend strongly on the channel aspect ratio. Bubble breakup consists in a slow linear 2D collapse of the gas thread, ending in a fast 3D pinch-off. The 2D collapse is predicted to be stable against perturbations of the gas-liquid interface, whereas the 3D pinch-off is unstable, causing bubble polydispersity. During 3D pinch-off, a scaling wm~tau1/3 between the neck width wm and the time tau before breakup indicates that breakup is driven by the inertia of both gas and liquid, not by capillarity
Self-wrapping of an ouzo drop induced by evaporation on a superamphiphobic surface
Evaporation of multi-component drops is crucial to various technologies and
has numerous potential applications because of its ubiquity in nature.
Superamphiphobic surfaces, which are both superhydrophobic and superoleophobic,
can give a low wettability not only for water drops but also for oil drops. In
this paper, we experimentally, numerically and theoretically investigate the
evaporation process of millimetric sessile ouzo drops (a transparent mixture of
water, ethanol, and trans-anethole) with low wettability on a superamphiphobic
surface. The evaporation-triggered ouzo effect, i.e. the spontaneous
emulsification of oil microdroplets below a specific ethanol concentration,
preferentially occurs at the apex of the drop due to the evaporation flux
distribution and volatility difference between water and ethanol. This
observation is also reproduced by numerical simulations. The volume decrease of
the ouzo drop is characterized by two distinct slopes. The initial steep slope
is dominantly caused by the evaporation of ethanol, followed by the slower
evaporation of water. At later stages, thanks to Marangoni forces the oil wraps
around the drop and an oil shell forms. We propose an approximate diffusion
model for the drying characteristics, which predicts the evaporation of the
drops in agreement with experiment and numerical simulation results. This work
provides an advanced understanding of the evaporation process of ouzo
(multi-component) drops.Comment: 41 pages, 8 figure
Cell permeabilisation and transport focused around oscillating microbubbles
The ultrasound-driven oscillation of a microbubble drives a steady streaming focused around the bubble. The study of individual bubbles attached to a wall shows vivid recirculations. When cells are in the vicinity of these bubbles, also used in medecine as contrast agent for ultrasound echography, they experience considerable shear rates. We introduce in the flow giant unilamelar lipid vesicles, acting as artificial cells. Rupture of the lipidic membrane with the opening of pores is revealed by high-speed camera recordings. A reversible permeation of the membrane wall can also be obtained, demonstrating at the micron scale the efficiency of microbubbles to deliver drugs in cells. The streaming flow of bubble on a surface can be further controlled, with the adjunction of a solid obstacle nearby: the flow turns to be directed. We will present a microfluidic device using the principle of bubble/obstacle doublets to locally transport small objets such as cells
Evaporation-triggered segregation of sessile binary droplets
Droplet evaporation of multicomponent droplets is essential for various
physiochemical applications, e.g. in inkjet printing, spray cooling and
microfabrication. In this work, we observe and study phase segregation of an
evaporating sessile binary droplet, consisting of a mixture of water and a
surfactant-like liquid (1,2-hexanediol). The phase segregation (i.e., demixing)
leads to a reduced water evaporation rate of the droplet and eventually the
evaporation process ceases due to shielding of the water by the non-volatile
1,2-hexanediol. Visualizations of the flow field by particle image velocimetry
and numerical simulations reveal that the timescale of water evaporation at the
droplet rim is faster than that of the Marangoni flow, which originates from
the surface tension difference between water and 1,2-hexanediol, eventually
leading to segregation
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