Origin of Polar Order in Dense Suspensions of Phototactic Micro-Swimmers

Year: 2012

Authors: Furlan S., Comparini D., Ciszak M., Beccai L., Mancuso S., Mazzolai B.

Autors Affiliation: Center for Micro-BioRobotics, Istituto Italiano di Tecnologia, Pontedera, Italy; The BioRobotics Institute, Scuola Superiore Sant’Anna, Pontedera, Italy; Laboratorio Internazionale di Neurobiologia Vegetale – Department of Plant Soil & Environmental Science, University of Florence, Florence, Italy; Consiglio Nazionale delle Ricerche – Istituto Nazionale di Ottica, Florence, Italy

Abstract: A main question for the study of collective motion in living organisms is the origin of orientational polar order, i.e., how organisms align and what are the benefits of such collective behaviour. In the case of micro-organisms swimming at a low Reynolds number, steric repulsion and long-range hydrodynamic interactions are not sufficient to explain a homogeneous polar order state in which the direction of motion is aligned. An external symmetry-breaking guiding field such as a mechanism of taxis appears necessary to understand this phonemonon. We have investigated the onset of polar order in the velocity field induced by phototaxis in a suspension of a motile micro-organism, the algae Chlamydomonas reinhardtii, for density values above the limit provided by the hydrodynamic approximation of a force dipole model. We show that polar order originates from a combination of both the external guiding field intensity and the population density. In particular, we show evidence for a linear dependence of a phototactic guiding field on cell density to determine the polar order for dense suspensions and demonstrate the existence of a density threshold for the origin of polar order. This threshold represents the density value below which cells undergoing phototaxis are not able to maintain a homogeneous polar order state and marks the transition to ordered collective motion. Such a transition is driven by a noise dominated phototactic reorientation where the noise is modelled as a normal distribution with a variance that is inversely proportional to the guiding field strength. Finally, we discuss the role of density in dense suspensions of phototactic micro-swimmers.

Journal/Review: PLOS ONE

Volume: 7 (6)      Pages from: e38895  to: e38895

KeyWords: article; biological model; cell density; cell motility; Chlamydomonas reinhardtii; dipole; force; fungal cell culture; hydrodynamics; microbiological phenomena and functions; microorganism; noise; nonhuman; particle image velocimetry; phototactic microswimmer; phototaxis; polar order state; population density; velocity, Cyanobacteria; Microfluidics; Models, Theoretical; Motion; Rheology; Suspensions
DOI: 10.1371/journal.pone.0038895

Citations: 6
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