Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L20: Biological Fluid Dynamics: Single Cells and Bacteria |
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Chair: Eva Kanso, University of Southern California Room: Georgia World Congress Center B308 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L20.00001: A unified theory of the mitotic spindle dynamics during cell division Ehssan Nazockdast, Hai-Yin Wu, Daniel J Needleman, Michael John Shelley The proper positioning of the mitotic spindle in eukaryotic cells is crucial for accurate chromosome segregation and the progression of life. It involves the interaction of microtubule assemblies with motor-proteins and subcellular organelles. In an earlier study we used a combination of cytoplasmic flow measurements and computational fluid dynamics simulations to show that the positioning is achieved by pulling forces of motor-proteins bound to the cell boundary on microtubules. Here we propose a kinetic theory based on such active forces and fluid drag that can give analytical prediction of the experimentally measured dynamical response of the mitotic spindle structure in different stages of cell division. The predictions the pronuclear migration, and metaphase spindle dynamics as well as transition to spindle oscillation in anaphase. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L20.00002: Fluid mechanics at the cellular membrane Guillermo Javier Amador, Dennis van Dijk, Marie-Eve Aubin-Tam, Daniel Seewai Tam The mechanical and chemical interactions between cells and their surroundings dictate their behavior, such as motion, growth, and proliferation. Such interactions are mediated by the cell’s membrane, a thin layer composed of a lipid bilayer and embedded proteins. The membrane’s functions include the transport of molecules and ions, and sensing (or signaling); therefore, it is crucial for homeostasis, or the maintenance of a stable internal state. Controlled studies into membrane mechanics have been limited by complexities in geometry and detection of forces at the scale of pico-Newtons. To address these challenges, we use novel force measurement techniques with optical tweezers to probe the hydrodynamic flow around free-standing lipid bilayers within microfluidic channels. The aims of these measurements are to quantify the interfacial tension of lipid bilayers and fluid slip close to the layer surface, building towards a fundamental understanding of the physical principle governing the hydrodynamics around membranes. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L20.00003: Self-assembly and Hydrodynamics of Coarse-Grained Amphiphilic Lipids Szu-Pei P. Fu, Rolf J. Ryham, Yuan-Nan Young In macroscopic models, the well-known Helfrich Hamiltonian membrane model has been extensively used to capture macroscopic physical properties of a lipid bilayer membrane. However, some phenomena such as membrane fusion and micelle formation are challenging to be described using a macroscopic framework. Therefore, in order to include the salient molecular details in a coarse-grained manner, we study the dynamics of immersed lipid bilayer membrane using Janus-type particle configurations to represent collections of amphiphilic lipids. The main idea is to solve an action field due to the hydrophobic tail-tail interactions and to obtain the domain variation of the system. From this approach, we also examine the amphiphilic lipid dynamics by forming force and torque in terms of integral formulations. We adopt an integral formulation for the numerical simulations to efficiently investigate the particle self-assembly and hydrodynamics. We will present the collecting behavior for large system and simulations of lipid bilayer membranes in a viscous fluid in this talk. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L20.00004: Numerical simulation of cell adhesion in microfluidic devices Jifu Tan, Wei Li Understanding cell adhesion dynamics under fluid flow is important for many biotransport problems. We investigated the influence of cell size, ligand coating density, and micropost layout on cell adhesion in a microfluidic device. The cells were modeled as coarse grained cell membranes and the adhesion was modeled as interacting potentials, while the fluid was solved using the lattice Boltzmann method. The coupling between the cell and the fluid was achieved though the immersed boundary method. Simulation results showed that the competition between hydrodynamics and cell membrane adhesion determines the cell rolling speed and adhesion status. Higher ligand coating density and lower flow rate increase the ligand receptor interaction time and enhance the cell adhesion. The cell showed higher stress on the trailing edge of the membrane when rolling on a micropost surface. Intercellular collision can enhance or deteriorate cell adhesion, depending on the orientation of the incoming collision. Cells showed preferred adhesion in the vicinity of stagnant points, which is consistent with microfluidics based experimental results. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L20.00005: Instability-driven Oscillations of Elastic Microfilaments Feng Ling, Hanliang Guo, Eva Kanso Cilia and flagella are slender organelles that exhibit a variety of rhythmic beating patterns from conic-like motions to planar wave deformations. Although their internal structure, composed of a microtubule-based axoneme driven by dynein motors, is known, the mechanism responsible for these beating patterns remains elusive. Existing theories suggest that the dynein activity is dynamically regulated, via a geometric feedback from the cilium's mechanical deformation to the dynein forces. Recently, an open-loop mechanism was proposed based on a `flutter’-like instability. Here, we show that an elastic filament in viscous fluid, pinned at one end, and acted on by a distribution of axial forces exhibits a Hopf bifurcation, but this bifurcation generally leads to non-planar spinning of the filament, at a buckled configuration with locked curvature. We also show the existence of a second bifurcation, at larger forces, that causes a transition from spinning to planar wave-like deformations. To analyze these instabilities, we use a combination of numerical analysis, linear stability theory, and multi-link models. Our results support the theory that an instability-driven mechanism could explain the wide variety of beating patterns observed in cilia and flagella. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L20.00006: ElectroDeformation-Relaxation of Cells in Suspension to Characterize Mechanical Properties Yasir Demiryurek, Seyedsajad Moazzeni, Miao Yu, David I. Shreiber, Jeffrey D. Zahn, Ramsey A Foty, Jerry W Shan, Liping Liu, Hao Lin Mechanical properties of biological cells are closely linked with their physiological and pathological states. To characterize such properties, we have developed a technique based on on-chip electrodeformation-relaxation of cells in suspension. Using an indium-tin-oxide (ITO) coated glass slide, the experimental platform set up parallel electrodes along the edge which were lithographically defined by etching ITO. Cells were exposed to high frequency and amplitude electric pulses. Images of electrodeformation and the subsequent relaxation upon pulse cessation were analyzed to quantify shape evolution during the process. Two distinctive regimes were identified by data analysis. If cells were deformed for shorter than a threshold of approximately tens of milliseconds, the relaxation timescales are independent of the pulse duration, indicating invariant mechanical properties. When deformed for longer than the threshold, the relaxation time scales linearly with the pulse duration, which is typically seen in soft glassy materials. This behavior is found to be coherent across the various cell types examined, providing insights into understanding cellular response to mechanical cues. |
Monday, November 19, 2018 5:23PM - 5:36PM |
L20.00007: Cancer cells in flow: particle-based modeling Igor V. Pivkin, Kirill Lykov, Yasaman Nematbakhsh, Menglin Shang, Chwee Teck Lim Cells have complex mechanical properties which have proven to be important in many biological processes, requiring detailed models to give an insight into the cell rheology. We developed particle-based mesoscale computational model that is suitable for modeling of cells with wide range of viscoelastic properties and, at the same time, computationally efficient to be employed to large and complex flow domains. The cell model comprises three main components: cell membrane, cytoskeleton and nucleus. The model was calibrated using micropipette aspiration experiment data. Its performance was tested using microfluidics experiment simulations. Applications in simulations of MCF-7, MDA-MB-231 and isolated circulating tumor cells will be presented. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L20.00008: Rotating biological cells in a non-rotating AC electric field Viktor Shkolnikov, Daisy Xin, Yang Lei Electrically induced particle rotation (ROT) has been widely used to obtain particle effective conductivity and permittivity as a function of frequency and to distinguish particle types. Traditionally, a rotating AC electric field applied by microelectrodes rotates a single particle on an axis perpendicular to the electric field (Born-Lertes effect). However, producing a rotating electric field on microscale is often cumbersome, and does not permit the rotation (and analysis) of a large number of particles simultaneously. Here we discuss and demonstrate rotation of cells in a non-rotating, non-uniform AC electric field using simple planar electrodes, with the axis of rotation perpendicular to the applied field. The rotation of the particle is asynchronous and is non-linearly dependent on the field strength and frequency. We present an electrohydrodynamic model for particle rotation and discuss our experimental observations of rotation. We also demonstrate the use of this technique for reconstruction of the 3D geometry of the cell, as a low cost alternative to stepping confocal microcopy, especially for a large number of single, non-adherent cells. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L20.00009: Orbiting of bacteria around micrometer-sized particles entrapping shallow tents offluids George Araujo, Weijie Chen, Sridhar Mani, Jay X Tang We report discovery of orbital motion of flagellated bacteria when they are confined within a thin layer of water around dispersed micrometer-sized particles sprinkled over a semi-solid agar gel. The liquid layer is shaped like a shallow tent with the height at the center set by the seeding particle and the meniscus profile set by the strong surface tension of water. The thin fluid layer is resilient against evaporation due to agar substrate serving as an enduring reservoir. The tent-shaped constraint and the left handedness of the flagellar filaments result in exclusively clockwise circular trajectories when experiments were done with E. coli and Enterobacter sp. This novel mechanism to entrap bacteria within a minimal volume of fluid is relevant to near surface bacterial accumulation, adhesion, biofilm growth, development of bio-microdevices, and cleansing hygiene. |
Monday, November 19, 2018 6:02PM - 6:15PM |
L20.00010: Equatorial Magnetoaerotaxis of Swimming Bacteria Nicolas Waisbord, Michael Stehnach, Christopher Lefevre, Jeffrey Guasto Magnetotactic bacteria (MTB) synthesize magnetite nano-particles attached to the cell membrane, which mechanically orient the swimming cells parallel to Earth’s magnetic field. The accepted paradigm is that, at a broad range of latitudes, the magnetic field is nearly perpendicular to the surface of the Earth, whereby ‘run-and-reverse’ motility along the magnetic field enables efficient aerotaxis in the water columns of swamps, lakes, and oceans. However, at the equator, in spite of the magnetic field being orthogonal to naturally occurring oxygen gradients, a variety of MTB species are found to thrive there. Using a microfluidic device and Helmholtz coils, we generate and independently control an oxygen gradient and an orthogonal magnetic field, enabling precise measurements of MTB motility. In contrast to the current paradigm, we show that Magnetococcus marinus (MC-1) achieves ‘run-and-tumble’ motility, which facilitates exploration and aerotaxis perpendicular to the magnetic field, establishing a new survival mechanism for equatorial magnetoaerotaxis. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L20.00011: Viscophobic motility of biflagellated microalgae Michael Stehnach, Nicolas Waisbord, Jeffrey S. Guasto Swimming cells often live in environments characterized by spatial gradients of rheological properties, including biofilms and mucus layers. In this work, we demonstrate experimentally that swimming biflagellate cells (Chlamydomonas reinhardtii) exhibit avoidance of high viscosity regions, stemming from a purely hydrodynamic effect. Microfluidic devices are used to generate a spatial gradient of a Newtonian polymer suspension, and video microscopy captures the cell motion. While cells are expected to accumulate in high viscosity due to a mechanical reduction in swimming speed, we observe enhanced cell concentration in low viscosity regions. A statistical analysis of cell motility reveals strongly curved swimming trajectories that are redirected toward low viscosity zones. This viscophobic behavior is explained by an orientation-dependent torque resulting from the differential force generated by the cells’ dual flagella, which sample different fluid viscosities within the gradient, and it is justified by a simple hydrodynamic model. |
Monday, November 19, 2018 6:28PM - 6:41PM |
L20.00012: The effects of external flow on the feeding currents of sessile microorganisms Rachel Pepper, Matthieu Baron, Emily E Riley, Lasse Tor Nielsen, Thomas Kiørboe, Anders Andersen Microscopic sessile suspension feeders (MSSFs) form an important part of aquatic ecosystems. MSSFs live attached to surfaces and consume bacteria and debris. Their environmental impact is mediated by their clearance rate, which depends on the feeding current that they generate to bring in food. The clearance rate has been hypothesized to be limited by recirculating eddies, though eddies are reduced or eliminated if MSSFs push the water at an angle rather than perpendicular to the surface. Those results, however, considered MSSFs in still water, while they live in water with external flow (i.e. the current in a stream or ocean). We investigate the influence of external flow on the MSSF feeding current. Our calculations show that even very slow external flow is sufficient to disrupt the eddies around perpendicular MSSFs, providing a constant supply of fresh nutrients. However, the clearance rate decreases for MSSFs in external flow at a range of non-perpendicular orientations due to the formation of eddies that severely restrict feeding. We observe these eddies experimentally for the common suspension feeder Vorticella, and we also find that some MSSFs may be forced into orientations with reduced clearance rates by flows typical of rivers and streams. |
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