Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session NC: Drops and Bubbles X |
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Chair: N. Aubrey, Carnegie Mellon University Room: Salt Palace Convention Center 150 G |
Tuesday, November 20, 2007 11:35AM - 11:48AM |
NC.00001: Deformation and transport of particle laden droplets in micro devices Sai Nudurupati, Pushpendra Singh, Nadine Aubry Transporting droplets in microfluidic devices can be achieved efficiently by applying a non-uniform electric field. While this approach has been successfully used in the past to transport rigid particles, we show here that it can be applied to droplets, particularly those which carry particles on their surface. A droplet not only experiences a net force which transports it, but also an electric stress on its surface which deforms it and can even break it into one or more droplets if the applied electric is sufficiently strong. In this work, we use both experiments and direct numerical simulations (DNS) to study the problem of deformation and transport of a dielectric drop in a non-uniform electric field. The electric field is generated by placing electrodes at the bottom of the microfluidic device. Experiments show that the distribution of particles on the surface of a drop becomes non-uniform when an external electric field is applied. The DNS technique is based on a finite element scheme in which the droplet and its surrounding fluid are moved using the fundamental equations of motion. The interface is tracked by the level set method and the electric forces are computed using the Maxwell stress tensor. [Preview Abstract] |
Tuesday, November 20, 2007 11:48AM - 12:01PM |
NC.00002: Self-Assembly of Droplets in a Nematic Liquid Crystal Chunfeng Zhou, Pengtao Yue, James J. Feng In recent experiments, micro-particles and droplets were observed to self-assemble into specific patterns when they were dispersed in a nematic liquid crystal. We use a diffuse-interface method to simulate one such system. Five types of pairwise interactions are found in this study: dipolar attracion, dipolar repulsion, anti-dipolar repulsion, parallel dipolar repulsion and anti-parallel dipolar attraction. These results are consistent with prior results in the literature. In particular, the dipolar attracion force obeys the $-4^{th}$ power law reported earlier. Furthermore, multiple droplets self-assemble into a long chain similar to experimental observations. [Preview Abstract] |
Tuesday, November 20, 2007 12:01PM - 12:14PM |
NC.00003: Combined optical and acoustical characterization of coated microbubbles Michel Versluis, Jeroen Sijl, Detlef Lohse, Rik Vos, Nico de Jong Optical ultra high-speed imaging of ultrasound contrast agents has revealed new detailed information on the dynamics of coated microbubbles, e.g. surface modes and ``compression-only'' behavior. How these non-spherical and non-symmetrical bubble oscillations translate into an acoustic response is unknown. Acoustic studies of individual microbubbles have been hindered by the ability to isolate a single contrast bubble and by the transducer calibration and its corresponding sensitivity. Here we present a combined optical and acoustical setup to characterize individual ultrasound contrast agents. Bubbles were isolated in a capillary fibre by an active flow control. The receiving transducer was accurately calibrated, therefore both the optical and acoustical recordings provide quantitative information on the microbubble response, allowing for a direct comparison between the two methods. For larger bubbles oscillating in the linear regime, the measured acoustic bubble response was in good agreement with the response predicted from the optically recorded radial bubble dynamics. [Preview Abstract] |
Tuesday, November 20, 2007 12:14PM - 12:27PM |
NC.00004: Conical singularities inside cone-jet electrosprays Alvaro G. Marin, Ignacio G. Loscertales, Antonio Barrero In coaxial jet electrosprays inside liquid baths, a conductive liquid forms a cone-jet electrospray in a bath containing a dielectric liquid. An additional dielectric liquid is injected inside the Taylor cone forming a liquid meniscus. In certain circumstances, however, we have observed that the dielectric menisci present extremely sharp tips, without mass emission, that can be stabilized and made completely steady. In this presentation we will first explore the parametrical range of liquid properties, mainly viscosities and surface tensions, under which these sharp tips take place. Secondly, we have developed a simple analytical model for the very complex electro-hydrodynamical flow, which predicts the angle of the tip as a function of the liquid properties. Therefore, we are able to compare it with the results of the experiments. When the liquid meniscus is slowly fed, the cusped interface turns into a spout which flows coated by the conducting liquid forming the electrified coaxial jet which has been successfully employed for the production of double emulsions (Marin et al., Phys. Rev. Lett. 98, 014502, 2007). [Preview Abstract] |
Tuesday, November 20, 2007 12:27PM - 12:40PM |
NC.00005: Nanometer-resolved collective micromenisci oscillations through optical diffraction Helmut Rathgen, Kazuyasu Sugiyama, Detlef Lohse, Frieder Mugele We study the dynamics of periodic arrays of micrometer-sized liquid-gas menisci formed at superhydrophobic surfaces immersed into water. By measuring the intensity of optical diffraction peaks in real time we are able to resolve nanometer scale oscillations of the menisci with sub-microsecond time resolution. Upon driving the system with an ultrasound field at variable frequency we observe a pronounced resonance at a few hundred kHz, depending on the exact geometry. Modeling the system using the time-dependent Stokes equation, we find that this unexpectedly low resonance frequency is caused by a collective mode of the acoustically coupled oscillating menisci. [Preview Abstract] |
Tuesday, November 20, 2007 12:40PM - 12:53PM |
NC.00006: Model of Therapeutic Ultrasound Contrast Agent Dynamics Chao-Tsung Hsiao, Xiaozhen Lu, Georges Chahine Targeted drug and gene delivery are rapidly emerging applications for ultrasound contrast agents since this could reduce potential deleterious side effects to healthy tissue and minimize the overall dose needed. Therapeutic ultrasound contrast agents are encapsulated microbubbles usually composed of a high molecular weight gas core and a highly viscous thick liquid shell. Development of new contrast agents requires a good understanding of the stability and breakup mechanisms of the liquid shell when subjected to ultrasonic acoustic waves. A novel numerical code, which enables one to investigate the dynamics of thick-shelled contrast agents and the interaction between multiple agents and with nearby boundaries has been developed by coupling a Boundary Element Method solver and a finite-volume Navier-Stokes solver. We have applied the coupled code to examine shell breakup mechanisms for contrast agents near a solid wall. We found that the shell thickness varies significantly from location to location due to non-spherical deformations and that the contrast agent may break up due to local shell thinning and stretching as the non-spherical deformation is significant. [Preview Abstract] |
Tuesday, November 20, 2007 12:53PM - 1:06PM |
NC.00007: Cavitation in microscale confinement: new concept of mild brain injury. Iskander Akhatov, Cheng Wang, Mariusz Ziejewski The present effort is to understand the possible damages in brain caused by the cavitation bubbles generated when the impacting shock waves passing through human head. In order to build an adequate mathematical model of this phenomenon, one should be able to model inception and dynamics of cavitation in biological liquid confined in macroscale or microscale space between solids, elastic surfaces, or membranes -- biological tissues, in general. A more in-depth understanding of the outcomes from the dynamic response of brain tissue, including the location, size, and geometry of the damage site, will be of assistance to physicians in the properly interpreting the neurodiagnostic results. In the present study it is stated that in micro scale confinement bubble collapse can not cause any damage. This is due to the fact that collapse is damped by viscous dissipation in micro channels. Otherwise, the bubble inception itself may cause damage. It is shown that cavitation inception in micro scale may happen for much higher tensions than in infinite liquid. At such a strong tension substantial amount of elastic energy is stored in liquid. This energy being released during cavitation inception generates `recoil pressure' that may be high enough to damage biological tissue. [Preview Abstract] |
Tuesday, November 20, 2007 1:06PM - 1:19PM |
NC.00008: Explosive Microsphere Particle Standards for Trace Explosive Detection Instruments Matthew Staymates, Robert Fletcher, Greg Gillen Increases in Homeland Security measures have led to a substantial deployment of trace explosive detection systems within the United States and US embassies around the world. One such system is a walk-through portal which aerodynamically screens people for trace explosive particles. Another system is a benchtop instrument that can detect explosives from swipes used to collect explosive particles from surfaces of luggage and clothing. The National Institute of Standards and Technology is involved in a chemical metrology program to support the operational deployment and effective utilization of trace explosive and narcotic detection devices and is working to develop a measurement infrastructure to optimize, calibrate and standardize these instruments. Well characterized test materials are essential for validating the performance of these systems. Particle size, chemical composition, and detector response are particularly important. Here, we describe one method for producing monodisperse polymer microspheres encapsulating trace explosives, simulants, and narcotics using a sonicated co-flow Berkland nozzle. The nozzle creates uniform droplets that undergo an oil/water emulsion process and cure to form hardened microspheres containing the desired analyte. Issues such as particle size, particle uniformity and levels of analyte composition will be discussed. [Preview Abstract] |
Tuesday, November 20, 2007 1:19PM - 1:32PM |
NC.00009: The liquid micro-jet from laser induced cavitation bubbles. Jack Abboud, Ghanem Oweis A vaporous cavitation bubble grows spherically in an infinite medium to a maximum radius, collapses in a spherical manner to a minimum volume, and then may rebound one or more times or disintegrate. When the bubble collapses above a solid boundary, the asymmetry of the surrounding flow field will cause the upper bubble surface to cave in, resulting in a fast liquid jet that penetrates its lower surface and continues towards the solid boundary. This fast jet formation is one perceived mechanism for cavitation damage in hydro-machinery. If a hole is intentionally drilled in the solid boundary underneath the collapsing bubble, the fast micro-jet can continue its path and be cultivated for a variety of applications such as micro surgery of soft tissue. In this study, cavitation bubbles are generated by focusing the pulsed IR beam from an Nd-YAG laser above a solid surface. The forming liquid micro-jet is investigated in the cases of blank and drilled solid boundaries. [Preview Abstract] |
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