Statistical mechanics and hydrodynamics of bacterial suspensions

Aparna Baskaran, M. Cristina Marchetti

Research output: Contribution to journalArticlepeer-review

264 Scopus citations


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, longrange 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.

Original languageEnglish (US)
Pages (from-to)15567-15572
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number37
StatePublished - Sep 15 2009


  • Active suspensions
  • Hydrodynamic interactions
  • Low-Reynolds-number swimming

ASJC Scopus subject areas

  • General


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