Bulletin of the American Physical Society
2017 Annual Meeting of the Far West Section
Friday–Saturday, November 3–4, 2017; Merced, California
Session B4: Bio and Soft Matter Physics |
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Chair: Ajay Gopinathan, University of California Merced Room: COB1 120 |
Saturday, November 4, 2017 2:00PM - 2:12PM |
B4.00001: Computational study of a nanoparticle-rich domain formation in a nematic liquid crystal Charles Melton, Sheida Riahinasab, Robin Selinger, Linda Hirst Past work has shown that when functionalized quantum dots are introduced into a nematic liquid crystal host and then cooled through the isotropic-nematic phase transition, structures of numerous sizes and morphologies are formed via self-assembly. The sizes of these structures are controlled by tuning the cooling rate of the liquid crystal. During the transition, the segregation of isotropic and nematic domains behaves as a classic phase separation phenomenon. We study this phenomenon by using the Cahn-Hilliard equation in conjunction with the nematic liquid crystal order parameter. By combining these two systems, we successfully model phase domain separation that follows the lower order parameter of the liquid crystal, as seen in experiments. We calculate isotropic domain size as a function of cooling rate, and find a power law relation that is differs from experimental observations, yet shows similar behavior. [Preview Abstract] |
Saturday, November 4, 2017 2:12PM - 2:24PM |
B4.00002: Investigating the mixing quality of a biological active nematic Amanda Tan, Eric Roberts, Kevin Mitchell, Linda Hirst Active matter consists of individual entities that consume energy and collectively move. Active matter systems can form emergent patterns. We study an active matter system composed of biomaterials that forms a self-mixing network with nematic liquid crystal characteristics. This active network system is composed of biopolymers (microtubules), and molecular motors (kinesin) confined in 2D at an oil-water interface. When confined in 2D, the network resembles an active nematic system that self-mixes. We are interested in quantifying the mixing quality by measuring the rate of separation of the filaments. We bound beads to the network, and measure the separation distance as a function of time for bead pairs. We found that the network exhibits exponential stretching, which may imply that it is a good mixer. We further investigate how changing the velocity in the system affects the mixing quality. We change the velocity of the filaments moving by altering the ATP concentration. [Preview Abstract] |
Saturday, November 4, 2017 2:24PM - 2:36PM |
B4.00003: Designing Liquid Crystalline Ligands for Increased QD Photovoltaic Efficiency Sheida.T Riahinasab, Amir Keshavarz, Benjamin J. Stokes, Linda S. Hirst High efficiency quantum dot (QD) photovoltaics are potential candidates for use in space mission's due to their long lifetimes and stable photonic properties under high photon flux, however one limitation of this technology is in the loss of efficiency due to inter-dot energy transfer. In a recent publication we found that mesogenic ligand surface attachment can promote long-term QD photo-stability. The mesogenic ligands reduced inter-dot energy transfer, produced stable recombination rates and steady emission color over more than an hour of continuous photo-excitation. In this project we tune the spacing between QDs by using mesogenic ligands (rod-like molecules attached to the particle by a flexible alkyl chain) to decrease the lost of energy. This strategy will provide an effective route towards improving the functional and structural characteristics of QD hybrid devices. [Preview Abstract] |
Saturday, November 4, 2017 2:36PM - 2:48PM |
B4.00004: Substrate mobility produces velocity time dependence in microtubule gliding Joseph Lopes, David Quint, Dail Chapman, Ajay Gopinathan, Linda Hirst, Jing Xu Molecular motor based transport is critical for all eukaryotic cell function. Motors often work in small teams to transport a cargo in-vivo, however understanding the factors that control and regulate the group function of multiple motors bound to a lipid membrane remains a challenge. Here we couple kinesin motors to a lipid bilayer, utilizing the microtubule gliding assay, recording and analyzing gliding velocity as a function of time. We observe a constant gliding velocity on glass that is characteristic of solid substrates, while gliding on membrane resulted in a larger than two-fold increase in velocity. When microtubules are immobilized in the absence of ATP, microtubule-bound motors are observed to build up over time. We propose an analytical model relating time dependent motor protein density to microtubule velocity, giving us a motor disassociation rate and a mechanism for the observed velocity increase. Our results provide evidence that motors coupled to a fluid-like membrane exhibit significantly different gliding behavior than observed on rigid substrates such as glass and hypothesize that motor diffusion in the membrane may play a role in biological transport processes. [Preview Abstract] |
Saturday, November 4, 2017 2:48PM - 3:00PM |
B4.00005: Run and Tumble of \textit{Escherichia coli} in Micropillar Arrays Pooja Chopra, Bin Liu Responses of microorganisms to emergent environmental conditions compose the dynamic nature of these biological systems. In an aqueous environment, an \textit{Escherichia coli} bacterium responds to a gradient of chemical attractant or repellant by frequently switching between its `run' and `tumble' modes. Despite these extensively studied chemotactic behaviors, it remains unclear how and whether an individual bacterium responds to mechanical signals, such as physical contacts with boundary walls. Such a potential mechanosensing to solid boundaries is associated with bacterial adhesions and thus crucial for formation of their aggregates, known as biofilms. Here, we applied a patterned array of micropillars as well-controlled mechanical stimuli to aqueous bacteria. We examined the run-and-tumble swimming of \textit{E.coli} subjected to these pillars. The long-term behaviors of individual bacteria were captured by a 3D tracking microscope for obtaining cell-specific statistics. By correlating the cellular behaviors to the pillar geometry and the detailed interactions, we explored the mechanisms of bacterial sensing and responding to solid structures by run-tumble statistics. [Preview Abstract] |
Saturday, November 4, 2017 3:00PM - 3:12PM |
B4.00006: Active motility in bimodular bacterial aggregates Yu Zeng, Bin Liu Dispersal capability is essential for microorganisms to achieve long-distance translocation, thus crucial for their abundance in various environments. In general, active dispersals are attributed to the movements of self-powered planktonic cells, while sessile cells that live a colonial life often disperse passively through flow entrainments. Here, we report another means of active dispersal employed by aggregates of sessile cells. The spherical rosette colonies of the bacterium \textit{Caulobacter crescentus} are aggregates of sessile stalked cells, of which a small proportion undergo cell division, grow active flagella and effect whole-rosette motility. We show that these rosettes actively disperse both in bulk water and near the solid-liquid interface. In particular, the proximity of a self-powered rosette to the solid surface promotes a rolling movement, leading to its persistent transportation along the solid boundary. The active dispersal of these rosettes demonstrated a novel mode of colonial transportation that is based on the division of labor between sessile and motile cells. [Preview Abstract] |
Saturday, November 4, 2017 3:12PM - 3:24PM |
B4.00007: Non-perturbative manipulation through a 3D microfluidic treadmill Jeremias Gonzalez, Bin Liu Our capabilities of micromanipulation have evolved with advances in contact-free trapping techniques under various disciplines, such as optical, magnetic, and microfluidic traps. In these techniques, a microscale particle is held in place under compression due to electromagnetic or hydrodynamic forces. In this work, we present a trap-free design of a microfluidic ``treadmill'' (MFC), realized by a uniform flow along arbitrary directions in a 3D microfluidic device, which is composed of a central chamber and pairs of $x-$ and $y-$channels at different elevations. Through boundary element simulations, we demonstrate that 3D background flows along any direction can be generated in the middle of the chamber, controlled by a set of syringe pumps. By tuning the detailed geometry of the MFC, we show the optimized shape of the device that leads to minimized strain rate, allowing for manipulation of the suspended particles with negligible perturbations. We also show an experimental realization of the MFC device, using laser stereolithography. The $x-$, $y-$, and $z-$ manipulation modes are obtained independently by a syringe pump with push/pull mechanisms, and are compared with the above simulation results. [Preview Abstract] |
Saturday, November 4, 2017 3:24PM - 3:36PM |
B4.00008: Fingering instabilities in bacterial community phototaxis Ritwika VPS, Rosanna Man Wah Chau, Kerwyn Casey Huang, Ajay Gopinathan Synechocystis sp PCC 6803 is a phototactic cyanobacterium that moves directionally in response to a light source. During phototaxis, these bacterial communities show emergent spatial organisation resulting in the formation of finger-like projections at the propagating front. In this study, we propose an analytical model that elucidates the underlying physical mechanisms which give rise to these spatial patterns. We describe the migrating front during phototaxis as a one-dimensional curve by considering the effects of phototactic bias, diffusion and surface tension. By considering the propagating front as composed of perturbations to a flat solution and using linear stability analysis, we predict a critical bias above which the finger-like projections appear as instabilities. We also predict the wavelengths of the fastest growing mode and the critical mode above which the instabilities disappear. We validate our predictions through comparisons to experimental data obtained by analysing images of phototaxis in Synechocystis communities. Our model also predicts the observed loss of instabilities in taxd1 mutants (cells with inactive TaxD1, an important photoreceptor in finger formation), by considering diffusion in mutually perpendicular directions and a lower, negative bias. [Preview Abstract] |
Saturday, November 4, 2017 3:36PM - 3:48PM |
B4.00009: Optimizing success of random searches in various search environments Farnaz Golnaraghi, Ajay Gopinathan Many animals such as the albatross, gray seal, and deer are known to exhibit foraging patterns where the distances they travel in a given direction are drawn from a particular heavy tailed distribution (a power law) known as a Levy distribution. Previous studies have shown that, under conditions of sparse resources, this search process is optimized with respect to the efficiency, defined as the ratio of total number of targets found to the total distance travelled, when the power-law has an exponent equal to 2. Although single agent Levy search processes have been studied well in the literature, little is known about multi-agent search processes. In many natural settings, there are typically multiple foragers who can interact with each other in different ways including either cooperating or competing with each other. We develop a stochastic agent-based simulation to study the effect of the number of agents, and various types of interactions between them on the search efficiency, and present our results for the optimum search strategy for cases in which foragers try to avoid encountering each other in different ways. [Preview Abstract] |
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