Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session LW: Biofluids VIII: Cellular II |
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Chair: Hao Lin, Rutgers University Room: 208A-D |
Monday, November 23, 2009 3:35PM - 3:48PM |
LW.00001: Mechanics of bacterial biofilm expansion: differentiation and nutrient deprivation Agnese Seminara, Tommy Angelini, Roberto Kolter, David Weitz, Michael Brenner Biofilms are sessile colonies of micro-organisms, usually encased in a protective extracellular matrix and associated to a surface. Cells inside a biofilm differentiate in precise patterns in space and time, so that biofilms are often compared to multicellular organisms. Nutrient deprivation is thought to be the main trigger for the onset of differentiation. Here we explore this idea quantitatively by modeling the mechanics of biofilm growth coupled to reaction and diffusion of nutrient concentration. The results give insight on the time evolution of biofilm morphology and provides a quantitative description of the stresses developed within the biofilm during expansion. We develop a theoretical framework applicable to a variety of microorganisms and compare the predictions to time-lapse microscopy data on Bacillus Subtilis biofilms. [Preview Abstract] |
Monday, November 23, 2009 3:48PM - 4:01PM |
LW.00002: Shear-induced adhesion of bacterial cells Sigolene Lecuyer, Roberto Rusconi, Yi Shen, Alison Forsyth, Howard Stone Bacterial adhesion is the first step in the development of surface-associated communities known as biofilms. The formation of these microbial structures is the cause of many different problems in medical devices and industrial water systems. Despite an extensive literature, the underlying mechanisms of the initial reversible attachment are not fully understood. We have investigated the effects of hydrodynamics on the probability of adsorption and detachment of bacteria on model surfaces by using phase-contrast microscopy in straight microchannels. In this way we have been able to measure the time that each bacterium spends on the surface and to analyze the mobility as a function of the flow rate. The main finding of our experiments and analyses is a counter-intuitive enhanced adhesion as the shear stress is increased over a wide range of shear rates. [Preview Abstract] |
Monday, November 23, 2009 4:01PM - 4:14PM |
LW.00003: Leukocyte transport by red blood cells in a microvessel Jonathan Freund A simulation model is used to study the transport of relatively large, spherical, and stiff white blood cells (leukocytes) by the relatively smaller and highly flexible red cell as they flow in the microcirculation. Their interaction dynamics are thought to be an important component of the inflammation response, in which leukocytes bind to the walls of blood vessels. The red cells are modeled in the simulations as highly deformable three-dimensional shells encasing a Newtonian fluid, and the viscous-flow equation is solved via a boundary integral formulation in which the cell shapes discretized by global spectral basis functions. For slow flow rates, it is found that the leukocyte is predominantly adjacent the vessel walls, whereas for faster flow rates this configuration appears to become unstable and the leukocyte traverses the whole vessel in a seemingly random fashion. For the straight round tubes simulated thus far, the stable leukocyte stand-off distance is always beyond the range of the binding molecules that capture it, which suggests that vessel inhomogeneities or interactions with other white cells are needed to create contact and thereby binding with the vessel walls. [Preview Abstract] |
Monday, November 23, 2009 4:14PM - 4:27PM |
LW.00004: Role of thermal fluctuations in vesicle dynamics Konstantin Turitsyn, Sergey Vergeles, Vladimir Lebedev Lipid bilayer vesicles can exhibit several regimes of motion when subjected to external flow: tumbling, tank-treading, trembling. Theoretical predictions based on deterministic models proved to be very successful in describing the corresponding phase diagram on qualitative level. However, recent experimental studies [Deschamps et.al., Phys.Rev.Lett. 178102 (2007)] identified significant quantitative discrepancies between theory and experiment, related mainly to the transition between tumbling and trembling regimes. Here we show that some of these discrepancies can be attributed to the role of thermal fluctuations. We extend the theoretical model to account for their effect. Fluctuations of the membrane give rise to effective compressibility, and lead to renormalization of its surface area. Resulting dynamical equations are still simple enough to be studied analytically and the resulting phase diagram is consistent with new experimental observations. [Preview Abstract] |
Monday, November 23, 2009 4:27PM - 4:40PM |
LW.00005: Modeling of Fluid-Membrane Interaction in Cellular Microinjection Process Mehdi Karzar-Jeddi, Jhon Diaz, Nejat Olgac, Tai-Hsi Fan Cellular microinjection is a well-accepted method to deliver matters such as sperm, nucleus, or macromolecules into biological cells. To improve the success rate of \textit{in vitro} fertilization and to establish the ideal operating conditions for a novel computer controlled rotationally oscillating intracytoplasmic sperm injection (ICSI) technology, we investigate the fluid-membrane interactions in the ICSI procedure. The procedure consists of anchoring the oocyte (a developing egg) using a holding pipette, penetrating oocyte's zona pellucida (the outer membrane) and the oolemma (the plasma or inner membrane) using an injection micropipette, and finally to deliver sperm into the oocyte for fertilization. To predict the large deformation of the oocyte membranes up to the piercing of the oolemma and the motion of fluids across both membranes, the dynamic fluid-pipette-membrane interactions are formulated by the coupled Stokes' equations and the continuum membrane model based on Helfrich's energy theory. A boundary integral model is developed to simulate the transient membrane deformation and the local membrane stress induced by the longitudinal motion of the injection pipette. The model captures the essential features of the membranes shown on optical images of ICSI experiments, and is capable of suggesting the optimal deformation level of the oolemma to start the rotational oscillations for piercing into the oolemma. [Preview Abstract] |
Monday, November 23, 2009 4:40PM - 4:53PM |
LW.00006: The role of ion electrophoresis in electroporation-mediated molecular delivery Jianbo Li, Hao Lin Electroporation is a widely applied technique to deliver active molecules into the cellular compartment, to perform a variety of tasks such as gene therapy and directed stem cell differentiation. In this technique, an electric field transiently permeabilizes the cellular membrane to facilitate molecular exchange. While the permeabilization process is relatively well-understood, the transport mechanisms for molecular delivery are still under debate. In this work, the role of ion electrophoresis in electroporation-mediated molecular delivery is investigated using numerical simulations. The result indicates that ion electrophoresis is the dominant mode of transport in the delivery of small charged molecules. Furthermore, the achievable intracellular concentration is strongly influenced by the conductivity difference between the cytoplasm and the buffer, a phenomenon known as ``field-amplified sample stacking''. The result agrees well with the fluorescence measurement by Gabriel and Teissi\'{e} (1999), and suggests a new possibility to simultaneously improve cell viability and efficiency in electroporation-mediated molecular delivery. [Preview Abstract] |
Monday, November 23, 2009 4:53PM - 5:06PM |
LW.00007: Stability of a Lipid Bilayer Membrane Subjected to a DC Electric Pulse Jonathan Schwalbe, Petia Vlahovska, Michael Miksis An analytical theory is developed to study the dynamics of a lipid bilayer membrane subjected to a DC electric pulse. The thin lipid membrane is impermeable to ions and thus acts as a capacitor. The model consists of conservation of current, which obeys Ohm's law, and the Stokes equations to describe fluid motion. The effects of membrane fluidity and incompressibility, variations in lipid density along the monolayers, and resistance to bending are taken into account. Small amplitude perturbations of a planar membrane are considered. The result is a time dependent system of equations for the growth rate as a functions of wave number. Variation of the applied voltage and a difference in the conductivities of the bulk fluids yield a long-wave instability in the system. The theory highlights that the membrane charging time is critical for the instability. Our theoretical findings are relevant to understanding the physical mechanisms of electroporation of biomembranes. [Preview Abstract] |
Monday, November 23, 2009 5:06PM - 5:19PM |
LW.00008: Soluble surfactants favorably modify fluid structure and wall shear stress profiles during near-occluding bubble motion in a computational model of intravascular gas embolism T.N. Swaminathan, P.S. Ayyaswamy, D.M. Eckmann Finite sized gas bubble motion in a blood vessel causes temporal and spatial gradients of shear stress at the endothelial cell surface lining the vessel wall as the bubble approaches the cell, moves over it and passes it by. Rapid reversals occur in the sign of the shear stress imparted to the cell surface during this motion. The sign-reversing shear is a potently coupled source of cell surface mechanical stretch, potentiating cell injury. The presence of a suitable soluble surfactant in the bulk medium considerably reduces the level of the shear stress gradients imparted to the cell surface as compared to an equivalent surfactant-free system. The bubble shape and the film thickness between the bubble and the vessel wall are also different. Furthermore, the bubble residence time near the proximity of a cell surface changes in comparison. These results based on our modeling may help explain several phenomena observed in experimental studies related to gas embolism, a significant problem in cardiac surgery and decompression sickness. [Preview Abstract] |
Monday, November 23, 2009 5:19PM - 5:32PM |
LW.00009: Three-dimensional simulation of $10^3$ deformable capsules in Poiseuille flow R. Murthy Kalluri, Sai Doddi, Prosenjit Bagchi Three-dimensional simulation using front-tracking methods are presented on the motion of $10^3$ deformable elastic capsules at semi-dense suspension in Poiseuille flow in microvessels, typical of microcirculation and microfluidic devices. The computational framework considered here can resolve the dynamics of individual deformable cell with high fidelity, yet can consider a large number of hydrodynamically interacting cells. In the simulations, the flow field is resolved using up to $300^3$ Eulerian grid points, and each capsule surface is resolved by up to 1280 triangular elements. Flow visualization, and analysis of cell trajectory and velocity of the multi-file motion are presented as functions of the cell deformability, and volume fraction. The simulations are computation- and data- intensive, and the first of their kind in the context of deformable capsule suspension. They provide a wealth of information on the dynamics of semi-dense suspension of liquid capsules, in particular, and of deformable particles, in general. The numerical results allow us to analyze various microrheological phenomena, such as the particle migration and formation of near-wall depletion layer, plug-flow velocity, and Fahraeus and Fahraeus-Lindqvist effects. [Preview Abstract] |
Monday, November 23, 2009 5:32PM - 5:45PM |
LW.00010: Manipulation of red blood cells with electric field Hossain Saboonchi, Asghar Esmaeeli Manipulation of bioparticles and macromolecules is the central task in many biological and biotechnological processes. The current methods for physical manipulation takes advantage of different forces such as acoustic, centrifugal, magnetic, electromagnetic, and electric forces, as well as using optical tweezers or filtration. Among all these methods, however, the electrical forces are particularly attractive because of their favorable scale up with the system size which makes them well-suited for miniaturization. Currently the electric field is used for transportation, poration, fusion, rotation, and separation of biological cells. The aim of the current research is to gain fundamental understanding of the effect of electric field on the human red blood cells (RBCs) using direct numerical simulation. A front tracking/finite difference technique is used to solve the fluid flow and electric field equations, where the fluid in the cell and the blood (plasma) is modeled as Newtonian and incompressible, and the interface separating the two is treated as an elastic membrane. The behavior of RBCs is investigated as a function of the controlling parameters of the problem such as the strength of the electric field. [Preview Abstract] |
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