Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D17: Biofluids: Microswimmers and Hydrodynamic Interactions |
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Chair: Raymond Goldstein, University of Cambridge Room: 28C |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D17.00001: \textit{E. coli} in a wall bounded shear flow Mehdi Molaei, Jian Sheng Understanding bacteria motility over a wall in a shear flow is critical to determine those crucial biophysical processes involved in the biofilm formation and the shear erosion. Using digital holographic microscopy combined with microfluidics we capture three-dimensional swimming patterns of wild-type \textit{E. coli} bacteria in a straight micro-channel subjecting to a carefully controlled flow shear. Three-dimensional locations and orientations of bacterial are extracted with a resolution of 0.185 $\mu $m in lateral directions and 0.5 $\mu $m in the wall normal direction. Robust statistics based on thousands of trajectories allow us to characterize bacteria swimming over a surface under flow shear. These characteristics, including swimming velocity, tumbling frequencies, cellular attachment, and suspension dispersion, will be used to elucidate the cell wall interactions in shear flows. Current analysis will focus on the hydrodynamic mechanisms other than near field interfacial forces on cell migration and orientation near a sheared surface. Preliminary data on bacteria over a chemically modified surface will also be presented. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D17.00002: Bacteria motility at oil-water interfaces Gabriel Juarez, Steven Smirga, Vicente Fernandez, Roman Stocker The swimming dynamics of bacteria are strongly influenced by interfaces: Motile bacteria often accumulate at rigid boundaries, such as liquid-solid interfaces, and at soft boundaries, such as liquid-air or liquid-liquid interfaces. Attachment of bacteria to these interfaces is crucial for the formation of biofilms (liquid-solid), pellicles (liquid-air), and oil-degrading communities (liquid-liquid). We investigated the motility of the oil-degrading bacteria \textit{Marinobacter aquaeolei} in the presence of oil droplets. We created individual oil droplets using dedicated microfluidic devices and captured the swimming behavior of individual bacteria near the interface and their attachment dynamics to the droplets with high-speed and epifluorescent microscopy. We find that \textit{Marinobacter aquaeolei} has a high affinity towards interfaces and their swimming dynamics at soft interfaces differ from both those in the bulk and at rigid boundaries. Characterizing the interaction and attachment of motile bacteria to liquid-liquid interfaces will promote a fundamental understanding to oil-microbe interactions in aquatic environments and potentially lead to improved oil bioremediation strategies. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D17.00003: Accumulation of swimming bacteria near an interface Jay Tang, Guanglai Li Microbes inhabit planet earth over billions of years and have adapted to diverse physical environment of water, soil, and particularly at or near interfaces. We focused our attention on the locomotion of Caulobacter crescentus, a singly flagellated bacterium, at the interface of water/solid or water/air. We measured the distribution of a forward swimming strain of C. crescentus near a surface using a three-dimensional tracking technique based on dark field microscopy and found that the swimming bacteria accumulate heavily within a micrometer from the surface. We attribute this accumulation to frequent collisions of the swimming cells with the surface, causing them to align parallel to the surface as they continually move forward. The extent of accumulation at the steady state is accounted for by balancing alignment caused by these collisions with rotational Brownian motion of the micrometer-sized bacteria. We performed a simulation based on this model, which reproduced the measured results. Additional simulations demonstrate the dependence of accumulation on swimming speed and cell size, showing that longer and faster cells accumulate more near a surface than shorter and slower ones do. The overarching goal of our study is to describe interfacial microbial behavior through detailed analysis of their motion. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D17.00004: Bacterial trapping in shear Roberto Rusconi, Jeffrey S. Guasto, Roman Stocker Bacteria are ubiquitously exposed to flow, both in natural environments and artificial devices (e.g., catheters), where confining surfaces create non-uniform shear. While the effects of shear on passive particles are well understood, little is known about the consequences of shear on motile bacteria. We exposed bacteria having different motility strategies (e.g., run-and-tumble, run-and-reverse) to microfluidic Poiseuille flows and quantified the swimming kinematics and cell distribution in the channel using video-microscopy. We discovered that the coupling of motility and a spatially varying shear results in a dramatic trapping of motile cells in high-shear regions, and conversely a strong depletion in the low-shear portion of the channel. We demonstrate experimentally that this trapping process is robust across species such as \textit{Bacillus subtilis} and \textit{Pseudomonas aeruginosa}, and can have far-reaching consequences on bacterial transport, by (i) counteracting bacterial chemotactic responses; and (ii) enhancing surface attachment and thus biofilm formation by trapping cells near walls. More generally, this work shows that--despite the low Reynolds number--the coupling of flow and self-propulsion can be nonlinear and not simply a superposition of the two effects. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D17.00005: Swimming of \textit{E. coli} near micro-structured surfaces Vasily Kantsler, Jorn Dunkel, Raymond E. Goldstein Understanding the mechanisms that govern surface accumulation of swimming bacteria is a key challenge for controlling biofilm formation. Here, we report detailed measurements of density and orientation distributions for \textit{Escherichia coli} bacteria as a function of the distance from a solid surface. Experiments were performed for wild-type and non-tumbling strains in both quasi-2D and 3D microfluidic chambers. We find that, for both geometries, the density profile in dilute suspensions decays sharply within a few microns from flat surfaces approaching a constant value in the bulk. Our measurements of the orientation distributions show that bacteria preserve memory of aligning collisions with surfaces for surprisingly long periods of time even after escaping into the bulk fluid. These experimental results agree well with numerical simulations of a minimal mechanistic model that accounts for steric interactions between bacteria and surfaces. We further demonstrate that optimal micro-scale surface patterning can substantially decrease accumulation of swimming bacteria, thereby providing a novel mechanism for preventing biofilm formation. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D17.00006: Helical swimming in confined geometries Kenneth S. Breuer, Bin Liu, Thomas R. Powers We discuss how bacterial swimming is affected by spatial confinement at the micron scale, as in a porous medium. We model a bacterial swimmer in a porous medium by a rotating rigid helix in a cylindrical cavity with smooth walls. A novel boundary element method is introduced to make full use of the helical symmetry. This method allows us to investigate situations of tight confinement in which the helix comes very close to the walls. We show that the confinement enhances the swimming efficiency, especially when the circumference of the tube matches the contour length of one helical pitch. To our surprise, at fixed power consumption, a highly-coiled swimmer swims faster in a narrower confinement, while a more open coil swims faster in a cavity with a wider opening. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D17.00007: Hydrodynamic synchronization of flagella on the surface of the colonial alga \textit{Volvox carteri} Douglas Brumley, Marco Polin, Raymond Goldstein, Timothy Pedley Whether on the surface of unicellular ciliates or in the respiratory epithelium, groups of eukaryotic cilia and flagella are capable of coordinating their beating over large scales. The mechanism responsible for the emergence of these metachronal waves is still unclear, mostly because finding an experimental system in which the beating filaments can be followed individually is challenging. We propose the multicellular green alga \textit{Volvox carteri} as an ideal model system to study metachronal coordination, and report the existence of robust metachronal waves on its surface. Inspired by flagellar tip trajectories of \textit{Volvox} somatic cells, we model a flagellum using a sphere of radius $a$ elastically bound to a circular orbit of radius $r_0$, perpendicular to a no-slip plane. This elastohydrodynamic model of weakly-coupled self-sustained oscillators can be recast in terms of interacting phase oscillators, offering an intuitive understanding of the mechanism driving the emergence of coordination. Our results confirm that elasticity is fundamental to guarantee fast and robust synchronization, and that sufficiently compliant trajectories lead to the emergence of metachronal waves in a manner essentially independent of boundary conditions. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D17.00008: Do proximate, C. elegans swimmers synchronize their gait? Jinzhou Yuan, David Raizen, Haim Bau We imaged two C. elegans swimming, one after the other, in a tapered conduit. The conduit was subjected to a DC electric field, with the negative pole at the narrow end and applied flow directed from the narrow end. As a result of their attraction to the negative pole (electrotaxis), both animals swam upstream. As the conduit narrowed, the average adverse flow velocity increased and the swimming speed of the leading animal decreased faster than that of the trailing animal, allowing the latter to catch up with the former. We quantified synchronization by measuring the phase lag between the gait of one animal and the extended wave pattern of the other as a function of the distance between the two animals. Only when the distance between the two animals' body centers was nearly equal to or smaller than one body length were the animals' motions synchronized. When the nematodes were parallel to one another, synchronization was essential to prevent the animals from colliding. Direct numerical simulations indicate that when the trailing animal's head is immediately downstream of the leading animal's tail, the animals derive just a slight hydrodynamic advantage from their proximity compared to a single swimmer. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D17.00009: Simulations of artificial swimmers in confined flows Luca Brandt, Lailai Zhu, Eerik Gj{\O}lberg Miniature swimmming robots are potentially powerful for microobject manipulation, such as flow control in lab-on-a-chip, localized drug delivery and screening for diseases. Magnetically driven artificial bacterial flagella (ABF) performing helical motion is advantegous due to high swimming speed and accurate control. Using boundary element method, we numerically investigate the propulsion of ABF in free space and near solid boundaries. Step-out at high actuation frequencies, wobbling and near-wall drifting are documented, in qualitative agreement with recent experiments. We aim to explore the effect of swimmer shape on the performance, thus benefiting design of efficient microswimmers. Propulsion of ABF confined by a solid wall with and without background shear flow is also studied, with a focus on wall-induced hydrodynamic interaction and its influence on the stability of the motion. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D17.00010: Hydrodynamical entrapment of ciliates at the air-liquid interface Jonathan Ferracci, Hironori Ueno, Keiko Numayama-Tsuruta, Yohsuke Imai, Takami Yamaguchi, Takuji Ishikawa We found the new phenomenon of the entrapment of ciliates at the air-water interface, though they are not trapped by a solid interface. We first characterize the behaviours of cells at the interface by comparing it to those away from interfaces. The results showed that the cell's swimming velocity is considerably reduced at the air-water interface. In order to experimentally verify the possible physiological causes of the entrapment, we observed their behaviours in absence of positive chemotaxis for oxygen and the negative geotaxis. The results illustrated that the entrapment phenomenon was not dependent on these physiological conditions. The experiments using surfactant revealed that the entrapment phenomenon was strongly affected by the velocity-stress conditions at the interface. This fact was confirmed numerically by a boundary element method, i.e. the stress-free condition at the air-liquid interface is one of the main mechanisms of the entrapment phenomenon found in the experiments. Since the entrapment phenomenon found in this study affects the cell-cell interactions and the mass transport at the interface, the knowledge obtained in this study is useful for better understanding the complex behaviours of swimming microorganisms in nature. [Preview Abstract] |
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