8,624 research outputs found
Acoustophoresis in Variously Shaped Liquid Droplets
The ability to precisely trap, transport and manipulate micrometer-sized
objects, including biological cells, DNA-coated microspheres and
microorganisms, is very important in life science studies and biomedical
applications. In this study, acoustic radiation force in an ultrasonic standing
wave field is used for micro-objects manipulation, a technique termed as
acoustophoresis. Free surfaces of liquid droplets are used as sound reflectors
to confine sound waves inside the droplets. Two techniques were developed for
precise control of droplet shapes: edge pinning and hydrophilic/hydrophobic
interface pinning. For all tested droplet shapes, including circular, annular
and rectangular, our experiments show that polymer micro particles can be
manipulated by ultrasound and form into a variety of patterns, for example,
concentric rings and radial lines in an annular droplet. The complexity of the
pattern increases with increasing frequency, and the observations are in line
with simulation results. The acoustic manipulation technique developed here has
the potential to be integrated into a more complex on-chip microfluidic
circuit. Especially because our method is well compatible with electrowetting
technology, which is a powerful tool for manipulating droplets with free
surfaces, the combination of the two methods can provide more versatile
manipulation abilities and may bring a wealth of novel applications. In the
end, we demonstrate for the first time that acoustophoresis can be used for
manipulating Caenorhabditis elegans
Cavitation Induction by Projectile Impacting on a Water Jet
The present paper focuses on the simulation of the high-velocity impact of a projectile impacting on a water-jet, causing the onset, development and collapse of cavitation. The simulation of the fluid motion is carried out using an explicit, compressible, density-based solver developed by the authors using the OpenFOAM library. It employs a barotropic two-phase flow model that simulates the phase-change due to cavitation and considers the co-existence of non-condensable and immiscible air. The projectile is considered to be rigid while its motion through the computational domain is modelled through a direct-forcing Immersed Boundary Method. Model validation is performed against the experiments of Field et al. [Field, J., Camus, J. J., Tinguely, M., Obreschkow, D., Farhat, M., 2012. Cavitation in impacted drops and jets and the effect on erosion damage thresholds. Wear 290–291, 154–160. doi:10.1016/j.wear.2012.03.006. URL http://www.sciencedirect.com/science/article/pii/S0043164812000968 ], who visualised cavity formation and shock propagation in liquid impacts at high velocities. Simulations unveil the shock structures and capture the high-speed jetting forming at the impact location, in addition to the subsequent cavitation induction and vapour formation due to refraction waves. Moreover, model predictions provide quantitative information and a better insight on the flow physics that has not been identified from the reported experimental data, such as shock-wave propagation, vapour formation quantity and induced pressures. Furthermore, evidence of the Richtmyer-Meshkov instability developing on the liquid-air interface are predicted when sufficient dense grid resolution is utilised
Super-Droplet Approach to Simulate Precipitating Trade-Wind Cumuli - Comparison of Model Results with RICO Aircraft Observations
In this study we present a series of LES simulations employing the
Super-Droplet Method (SDM) for representing aerosol, cloud and rain
microphysics. SDM is a particle-based and probabilistic approach in which a
Monte-Carlo type algorithm is used for solving the particle collisions and
coalescence process. The model does not differentiate between aerosol
particles, cloud droplets, drizzle or rain drops. Consequently, it covers
representation of such cloud-microphysical processes as: CCN activation,
drizzle formation by autoconversion, accretion of cloud droplets,
self-collection of raindrops and precipitation including aerosol wet
deposition. Among the salient features of the SDM, there are: (i) the
robustness of the model formulation (i.e. employment of basic principles rather
than parametrisations) and (ii) the ease of comparison of the model results
with experimental data obtained with particle-counting instruments.
The model set-up used in the study is based on observations from the Rain In
Cumulus over Ocean (RICO) field project (the GEWEX Cloud System Study Boundary
Layer Cloud Working Group RICO case). Cloud and rain droplet size spectrum
features obtained in the simulations are compared with previously published
aircraft observations carried out during the RICO field project. The analysis
covers height-resolved statistics of simulated cloud microphysical parameters
such as droplet number concentration, effective radius, and the width of the
cloud droplet size spectrum. The sensitivity of the results to the grid
resolution of the LES, as well as to the sampling density of the probabilistic
(Monte-Carlo type) model is discussed.Comment: Paper presented at the 16-th International Conference on Clouds and
Precipitation ICCP-2012, Leipzig, Germany. Revised version with corrected LWC
plots in Figures 2,3 and 4, and an updated discussion of the LWC plots in
section 4.
First Dark Matter Limits from a Large-Mass, Low-Background Superheated Droplet Detector
We report on the fabrication aspects and calibration of the first large
active mass ( g) modules of SIMPLE, a search for particle dark matter
using Superheated Droplet Detectors (SDDs). While still limited by the
statistical uncertainty of the small data sample on hand, the first weeks of
operation in the new underground laboratory of Rustrel-Pays d'Apt already
provide a sensitivity to axially-coupled Weakly Interacting Massive Particles
(WIMPs) competitive with leading experiments, confirming SDDs as a convenient,
low-cost alternative for WIMP detection.Comment: Final version, Phys. Rev. Lett. (in press
Recommended from our members
Controllable direction of liquid jets generated by thermocavitation within a droplet.
A high-velocity fluid stream ejected from an orifice or nozzle is a common mechanism to produce liquid jets in inkjet printers or to produce sprays among other applications. In the present research, we show the generation of liquid jets of controllable direction produced within a sessile water droplet by thermocavitation. The jets are driven by an acoustic shock wave emitted by the collapse of a hemispherical vapor bubble at the liquid-solid/substrate interface. The generated shock wave is reflected at the liquid-air interface due to acoustic impedance mismatch generating multiple reflections inside the droplet. During each reflection, a force is exerted on the interface driving the jets. Depending on the position of the generation of the bubble within the droplet, the mechanical energy of the shock wave is focused on different regions at the liquid-air interface, ejecting cylindrical liquid jets at different angles. The ejected jet angle dependence is explained by a simple ray tracing model of the propagation of the acoustic shock wave inside the droplet
Frequency effect on streaming phenomenon induced by Rayleigh surface acoustic wave in microdroplets
Acoustic streaming of ink particles inside a water microdroplet generated by a surface acoustic wave(SAW) has been studied numerically using a finite volume numerical method and these results have been verified using experimental measurements. Effects of SAW excitation frequency, droplet volume, and radio-frequency (RF) power are investigated, and it has been shown that SAW excitation frequency influences the SAWattenuation length, lSAW , and hence the acoustic energy absorbed by liquid. It has also been observed that an increase of excitation frequency generally enhances the SAW streaming behavior. However, when the frequency exceeds a critical value that depends on the RF power applied to the SAW device, weaker acoustic streaming is observed resulting in less effective acoustic mixing inside the droplet. This critical value is characterised by a dimensionless ratio of droplet radius to SAWattenuation length, i.e., Rd/lSAW . With a mean value of Rd/lSAW  ≈ 1, a fast and efficient mixing can be induced, even at the lowest RF power of 0.05 mW studied in this paper. On the other hand, for the Rd/lSAW ratios much larger than ∼1, significant decreases in streaming velocities were observed, resulting in a transition from regular (strong) to irregular (weak) mixing/flow. This is attributed to an increased absorption rate of acoustic wave energy that leaks into the liquid, resulting in a reduction of the acoustic energy radiated away from the SAW interaction region towards the droplet free surface. It has been demonstrated in this study that a fast and efficient mixing process with a smaller RF power could be achieved if the ratio of Rd/lSAW  ≤ 1 in the SAW-droplet based microfluidics
A robust method for calculating interface curvature and normal vectors using an extracted local level set
The level-set method is a popular interface tracking method in two-phase flow
simulations. An often-cited reason for using it is that the method naturally
handles topological changes in the interface, e.g. merging drops, due to the
implicit formulation. It is also said that the interface curvature and normal
vectors are easily calculated. This last point is not, however, the case in the
moments during a topological change, as several authors have already pointed
out. Various methods have been employed to circumvent the problem. In this
paper, we present a new such method which retains the implicit level-set
representation of the surface and handles general interface configurations. It
is demonstrated that the method extends easily to 3D. The method is validated
on static interface configurations, and then applied to two-phase flow
simulations where the method outperforms the standard method and the results
agree well with experiments.Comment: 31 pages, 18 figure
- …