Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R27: Biofluids: General II: Microbubbles and Droplets |
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Chair: Kaushik Sarkar, University of Delaware Room: Ballroom I |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R27.00001: The Acoustic Atomization of Droplets within a Bubble David Li, Robinson Seda-Padilla, J. Brian Fowlkes, Joseph Bull The process of vaporizing liquid microdroplets using ultrasound is known as acoustic droplet vaporization (ADV). Gas embolotherapy is a proposed cancer therapy that uses the ADV process to selectively generate microbubbles, which can then lodge in the microvasculature to occlude blood flow and starve the tumor. We have observed that continued ultrasound exposure to microbubbles adhering to a wall induces in a droplet atomization process. The atomization process originates at the gas-liquid interface and produces a spray of liquid droplet within the microbubble along the axis of the acoustic beam. Single pulses with 30 cycles from 3.5 and 7.5 MHz single element focused transducers operating at peak negative pressures ranging from 4 to 8 MPa were used to generate atomization. The atomization process was observed in microbubbles ranged from 30 $\mu $m to 1 mm in diameter. The extent of the atomization had a direct relationship with acoustic pressure. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R27.00002: Bubble Transport and Splitting in a Symmetric Bifurcation Adnan Qamar, Joseph Bull Transport and splitting of gas bubbles through a geometrically symmetric bifurcation is investigated numerically as a model of cardiovascular gas bubble transport in air embolism and Gas Embolotherapy. An interface capturing Volume of Fluid Method on an Eulerian fixed grid is used to compute the bubble splitting at the symmetric bifurcation. Bubble transport and splitting is investigated for a range of roll angles, capillary numbers, Reynolds numbers and Bond numbers. Results indicate that splitting is observed to be more homogenous at higher capillary numbers and lower roll angles. It is observed that at nonzero roll angles and small bubble lengths, there is a critical value of the capillary number below which the bubbles do not split and are transported entirely into the upper branch. The value of the critical capillary number increases with roll angle and the bubble length. Shear stress distribution at the bifurcation carina increases several folds as the bubble tip reaches the carina. These findings suggest that, in large vessels, gas emboli tend to be transported upward unless flow is unusually strong. In smaller vessels more even splitting of bubbles is predicted. The endothelial cells at a vessel bifurcation would be potentially exposed to higher stress levels, which might induce bioeffects. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R27.00003: Frequency dependent subharmonic threshold for contrast microbubbles Amit Katiyar, Kausik Sarkar We numerically investigate the predictions from several contrast microbubble models to determine the excitation threshold for subharmonic generation. All models are transformed into a common interfacial rheological form, where encapsulation is represented by two radius dependent surface properties---effective surface tension and surface dilatational viscosity. In contrast to the classical perturbative result, the minimum threshold for subharmonic generation is not always obtained near twice the resonance frequency; instead it can occur over a range of frequency from resonance to twice the resonance frequency. The quantitative variation of the threshold with frequency depends on the model, bubble radius and encapsulation properties. Some models incorporate an upper limit on effective surface tension (resulting from strain softening or rupture of the encapsulation during expansion). Without this upper limit, the threshold is extremely large especially near the resonance frequency and there is a global minimum near twice the resonance frequency. On the other hand, having zero surface tension in the buckled state (the lower limit) increases the threshold especially near twice the resonance frequency which in presence of the upper limit results in a possible shift of the minimum threshold towards resonance. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R27.00004: The fluid mechanics of nutrient transport within biofilms Michael Brenner, Agnese Seminara, Naveen Sinha, James Wilking, Tommy Angelini, Roberto Kolter, David Weitz Bacterial biofilms are interface-associated colonies of bacteria embedded in an extracellular matrix that is composed primarily of polymers and proteins. During the growth of a biofilm, nutrient is taken up by the surface of the biofilm, and contained by cells in the bulk. A critical problem is that above a critical size there is necessarily a growth bottleneck, in which the biofilm cannot take up enough nutrients to feed all of the cells within it. We discuss, through theory and experiments, several strategies that are employed by biofilms of Bacillus subtilus to avoid this growth bottleneck. These include clever use and control of osmotic pressure (through the expression of polymeric extracellular matrix); the excretion of surfactants and the use of associated marangoni stresses; and the distribution of flagella (used as mixers) within the bulk of the biofilm. Some speculations about other potential mechanisms (for which there is no current experimental support) will also be presented. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R27.00005: Modeling and 3-D Simulation of Biofilm Dynamics in Aqueous Environment Qi Wang We present a complex fluid model for biofilms growing in an aqueous environment. The modeling approach represents a new paradigm to develop models for biofilm-environment interaction that can be used to systematically incorporate refined chemical and physiological mechanisms. Special solutions of the model are presented and analyzed. 3-D numerical simulations in aqueous environment with emphasis on biofilm- ambient fluid interaction will be discussed in detail. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R27.00006: Variation of flow-induced stresses within scaffolds used in bone tissue engineering Dimitrios Papavassiliou, Ngoc Pham, Roman Voronov, Vassilios Sikavitsas Bone tissue engineering is often based on seeding adult stem cells on porous scaffolds and subsequently placing these scaffolds in flow perfusion bioreactors to stimulate cell differentiation and cell growth. In the present study, the distribution of stresses in structured porous scaffolds under flow is investigated by calculating the probability density function of flow-induced stresses in different scaffold geometries with simulations. The physical reason for the development of particular stress distributions is further explored, and it is found that the direction of flow relative to the internal architecture of the porous scaffold is important for stress distributions. When the flow direction is random relative to the configuration of the geometric elements making up the scaffold, it is found that a common distribution, such as the one suggested by Voronov et al. (Appl. Phys. Let., 2010, 97:024101), can be used to describe the stress distribution. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R27.00007: From thermodynamics to fluid mechanics: Enhancing the 2-D protein crystallization process James Young, David Posada, Amir Hirsa, Juan Lopez The leading method to determine protein structure is to perform X-ray diffraction on protein crystals. However, crystallizing protein is a challenging task which is usually met with limited success. 2-D protein crystals at the air/water interface are commonly obtained under quiescent conditions. Recently, the formation of such crystals was shown to be enhanced by the presence of flow. Here, we examine both the kinetics of the process, including the detrimental effects of protein aggregation, as well as the fluid dynamics associated with successful crystallization events. The deep-channel surface viscometer geometry is utilized which consists of the annular flow between two stationary cylinders and a rotating floor. For a particular protein surface concentration, a Reynolds number threshold has been identified above which crystals grow and below which they do not. This flow system also allows for the determination of the surface shear viscosity, which provides an indication of the mesoscale interactions associated with protein crystals. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R27.00008: Constrained droplets for high resolution microscopy of protein fibrillization David Posada, Peter Tessier, Amir Hirsa The use of constrained droplets (droplets with pinned contact lines on solid surfaces) is proposed here as a method for sample support in optical microscopy studies. Capillarity acts to contain the liquid sample, allowing access for observations in the bulk and at the gas/liquid interface. At the capillary length scale, surface tension forms stable interfaces, virtually immune to gravity and with curvatures that can be adjusted. This is particularly useful when studying the gas/liquid interface and its vicinity under high resolution optical microscopy. Such observations are normally performed using oil immersion objectives which must be positioned within distances only tens of microns from the region of interest. Constrained droplets can also be used at small scales, requiring minute volumes of analyte. The use of the constrained droplet method is demonstrated by studying the aggregation of insulin into amyloid fibrils in the solution and at the gas/liquid interface, where proteins are prone to denaturation and subsequent fibrillization. Such an aggregation process is associated with many neurodegenerative diseases, including Alzhemier's. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R27.00009: Material characterization of poly-lactic acid shelled ultrasound contrast agent and their dynamics Shirshendu Paul, Daniel Russakow, Tyler Rodgers, Kausik Sarkar, Michael Cochran, Margaret Wheatley Micron-size gas bubbles encapsulated with lipids and proteins are used as contrast enhancing agents for ultrasound imaging. Biodegradable polymer poly-lactic acid (PLA) has recently been suggested as a possible means of encapsulation. Here, we report \textit{in vitro} measurement of attenuation and scattering of ultrasound through an emulsion of PLA agent as well as theoretical modeling of the encapsulated bubble dynamics. The attenuation measured with three different transducers of central frequencies 2.25, 3.5 and 5 MHz, shows a peak around 2-3 MHz. These bubbles also show themselves to possess excellent scattering characteristics including strong non-linear response that can be used for harmonic and sub-harmonic contrast imaging. Our recently developed interfacial rheological models are applied to describe the dynamics of these bubbles; rheological model properties are estimated using measured attenuation data. The model is then applied to predict nonlinear scattered response, and the prediction is compared against experimental observation. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R27.00010: Blood Flow through an Open-Celled Foam Jason Ortega, Duncan Maitland The Hazen-Dupuit-Darcy (HDD) equation is commonly used in engineering applications to model the pressure gradient of flow through a porous media. One major advantage of this equation is that it simplifies the complex geometric details of the porous media into two coefficients: the permeability, K, and form factor, C. However through this simplification, the flow details within the porous media are no longer accessible, making it difficult to study the phenomena that contribute to changes in K and C due to clotting of blood flow. To obtain a more detailed understanding of blood flow through a porous media, a direct assessment of the complex interstitial geometry and flow is required. In this study, we solve the Navier-Stokes equations for Newtonian and non-Newtonian blood flow through an open-celled foam geometry obtained from a micro-CT scan. The nominal strut size of the foam sample is of $O$(10e-5) m and the corresponding Reynolds number based upon this length ranges up to $O$(10). Fitting the pressure gradient vs. Darcy velocity data with the HDD equation demonstrates that both viscous and inertial forces play an important role in the flow through the foam at these Reynolds numbers. Recirculation zones are observed to form in the wake of the pore struts, producing regions of flow characterized by both low shear rates and long fluid residence times, factors of which have been shown in previous studies to promote blood clotting. [Preview Abstract] |
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