118 research outputs found
Stretching Folding Instability and Nanoemulsions
Here we show a folding-stretching instability in a microfluidic flow focusing
device using silicon oil (100cSt) and water. The fluid dynamics video
demonstrates an oscillating thread of oil focused by two co-flowing streams of
water. We show several high-speed sequences of these oscillations with 30,000
frames/s. Once the thread is decelerated in a slower moving pool downstream an
instability sets in and water-in-oil droplets are formed. We reveal the details
of the pinch-off with 500,000 frames/s. The pinch-off is so repeatable that
complex droplet patterns emerge. Some of droplets are below the resolution
limit, thus smaller than 1 micrometer in diameter
Characterization and modification of cavitation pattern in shock wave lithotripsy
The temporal and spatial dynamics of cavitation bubble cloud growth and collapse in extracorporeal shock wave lithotripsy (ESWL) is studied experimentally. The first objective is obtaining reproducible cloud patterns experimentally and comparing them with FDTD-calculations. Second, we describe a method to modify the cavitation pattern by timing two consecutive pressure waves at variable delays. It is found that the spatial and temporal dynamics of the cavitation bubble can be varied in large ranges. The ability to control cavitation dynamics allows discussing strategies for improvement of medical and biological applications of shock waves such as cell membrane poration and stone fragmentation
Controlled Cavitation in Microfluidic Systems
We report on cavitation in confined microscopic environments which are commonly called microfluidic or lab-on-a-chip systems. The cavitation bubble is created by focusing a pulsed laser into these structures filled with a light-absorbing liquid. At the center of a 20 µm thick and 1 mm wide channel, pancake-shaped bubbles expand and collapse radially. The bubble dynamics compares with a two-dimensional Rayleigh model and a planar flow field during the bubble collapse is measured. When the bubble is created close to a wall a liquid jet is focused towards the wall, resembling the jetting phenomenon in axisymmetry. The jet flow creates two counter-rotating vortices which stir the liquid at high velocities. For more complex geometries, e.g., triangle- and square-shaped structures, the number of liquid jets recorded correlates with the number of boundaries close t
Oscillate Boiling from Electrical Microheaters
Oscillate boiling offers excellent heat transfer at temperatures above the
Leidenfrost temperature. Here we realize an electrical microheater with an
integrated thermal probe and resolve the thermal cycle during the
high-frequency bubble oscillations. Thermal rates of K/s were found
indicating its applicability for compact and rapid heat transfer from micro
electrical devices
Finite amplitude wave propagation through bubbly fluids
The existence of only a few bubbles could drastically reduce the acoustic
wave speed in a liquid. Wood's equation models the linear sound speed, while
the speed of an ideal shock waves is derived as a function of the pressure
ratio across the shock. The common finite amplitude waves lie, however, in
between these limits. We show that in a bubbly medium, the high frequency
components of finite amplitude waves are attenuated and dissipate quickly, but
a low frequency part remains. This wave is then transmitted by the collapse of
the bubbles and its speed decreases with increasing void fraction. We
demonstrate that the linear and the shock wave regimes can be smoothly
connected through a Mach number based on the collapse velocity of the bubbles
Merging of Soap Bubbles and Why Surfactant Matters
The merging of two soap bubbles is a fundamental fluid mechanical process in
foam formation. In the present experimental study the liquid films from two
soap bubbles are brought together. Once the liquid layers initially separated
by a gas sheet are bridged on a single spot the rapid merging of the two liquid
films proceed. Thereby the connecting rim is rapidly accelerated into the
separating gas layer. We show that due to the dimple formation the velocity is
not uniform and the high acceleration causes initially a Rayleigh-Taylor
instability of the liquid rim. At later times, the rim takes heals into a
circular shape. However for sufficient high concentrations of the surfactant
the unstable rim pinches off microbubbles resulting in a fractal dendritic
structure after coalescence
Bubble nucleation and jetting inside a millimetric droplet
In this work, we present experiments and simulations on the nucleation and
successive dynamics of laser-induced bubbles inside liquid droplets in
free-fall motion, i.e. a case where the bubbles are subjected to the influence
of a free boundary in all directions. The droplets of a millimetric size are
released from a height of around 20\,cm and acquire a velocity of around 2\,m/s
at the moment the bubble is nucleated. Within this droplet, we have
investigated the nucleation of secondary bubbles induced by the rarefaction
wave that is produced when the shock wave emitted by the laser-induced plasma
reflects at the drop surface. Interestingly, three-dimensional clusters of
cavitation bubbles are observed. Their shape is compared with the negative
pressure distribution computed with a CFD model and allows us to estimate a
cavitation threshold value. High-speed recordings of the drop/bubble dynamics
are complemented by the velocity and pressure fields simulated for the same
initial conditions. The effect of the proximity of a curved free surface on the
jetting dynamics of the bubbles was qualitatively assessed by classifying the
cavitation events using a non-dimensional stand-off parameter which depends on
the drop size, the bubble maximum radius and the relative position of the
bubble inside the drop. Here, we found that the curvature of the free surface
does not play a determinant role on the jet dynamics, being the distance to the
surface the dominant parameter. The oscillation of the laser-induced bubbles
promote the onset of Rayleigh-Taylor and Rayleigh-Plateau instabilities,
observed on the drop's surface. The specific mechanisms leading to the
destabilisation of the droplet surface were identified through a careful
inspection of the high speed images
Birth and growth of cavitation bubbles within water under tension confined in a simple synthetic tree
Water under tension, as can be found in several systems including tree
vessels, is metastable. Cavitation can spontaneously occur, nucleating a
bubble. We investigate the dynamics of spon- taneous or triggered cavitation
inside water filled microcavities of a hydrogel. Results show that a stable
bubble is created in only a microsecond timescale, after transient
oscillations. Then, a diffusion driven expansion leads to filling of the
cavity. Analysis reveals that the nucleation of a bubble releases a tension of
several tens of MPa, and a simple model captures the different time scales of
the expansion process
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Acoustic resonance effects and cavitation in SAW aerosol generation
The interaction of surface acoustic waves (SAWs) with liquids enables the production of aerosols with adjustable droplet sizes in the micrometer range expelled from a very compact source. Understanding the nonlinear acousto-hydrodynamics of SAWs with a regulated micro-scale liquid film is essential for acousto-microfluidics platforms, particularly aerosol generators. In this study, we demonstrate the presence of micro-cavitation in a MHz-frequency SAW aerosol generation platform, which is touted as a leap in aerosol technology with versatile application fields including biomolecule inhalation therapy, micro-chromatography and spectroscopy, olfactory displays, and material deposition. Using analysis methods with high temporal and spatial resolution, we demonstrate that SAWs stabilize spatially arranged liquid micro-domes atop the generator's surface. Our experiments show that these liquid domes become acoustic resonators with highly fluctuating pressure amplitudes that can even nucleate cavitation bubbles, as supported by analytical modeling. The observed fragmentation of liquid domes indicates the participation of three droplet generation mechanisms, including cavitation and capillary-wave instabilities. During aerosol generation, the cavitation bubbles contribute to the ejection of droplets from the liquid domes and also explain observed microstructural damage patterns on the chip surface eventually caused by cavitation-based erosion
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