Statistical mechanics and hydrodynamics of bacterial suspensions

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 106; no. 37; pp. 15567 - 15572
• Main Authors: Baskaran, Aparna, Marchetti, M. Cristina
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 15.09.2009; National Acad Sciences
• Subjects: Algae; Bacteria; Bacterial Physiological Phenomena; Bacteriology; Biological Sciences; Biomechanical Phenomena; Chemical suspensions; Chlamydomonas; Cilia; Classification; Density; Flagella; Flow velocity; Fluid mechanics; Geometric centers; Hydrodynamic equations; Hydrodynamics; Low reynolds number; Melanocytes; Models, Biological; Movement - physiology; Particle interactions; Physical Sciences; Physics; Rheology; Statistical mechanics; Statistics; Stroke
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0906586106

Unicellular living organisms, such as bacteria and algae, propel themselves through a medium via cyclic strokes involving the motion of cilia and flagella. Dense populations of such "active particles" or "swimmers" exhibit a rich collective behavior at large scales. Starting with a minimal physical model of a stroke-averaged swimmer in a fluid, we derive a continuum description of a suspension of active organisms that incorporates fluid-mediated, long-range hydrodynamic interactions among the swimmers. Our work demonstrates that hydrodynamic interactions provide a simple, generic origin for several nonequilibrium phenomena predicted or observed in the literature. The continuum model derived here does not depend on the microscopic physical model of the individual swimmer. The details of the large-scale physics do, however, differ for "shakers" (particles that are active but not self-propelled, such as melanocytes) and "movers" (self-propelled particles), "pushers" (most bacteria) and "pullers" (algae like Chlamydomonas). Our work provides a classification of the large-scale behavior of all these systems.