Direct Measurement of the Flow Field around Swimming Microorganisms
Entry by Leon Furchtgott, APP 225 Fall 2010.
Knut Drescher, Raymond E. Goldstein, Nicolas Michel, Marco Polin, and Idan Tuval. "Direct Measurement of the Flow Field Around Swimming Microorganisms". Physical Review Letters 105, 168101 (2010)
Swimming microorganisms create flows that influence their mutual interactions and modify the rheology of their suspensions. These flows have been studied theoretically but have not been measured. In this paper, the authors take measurements of the flow field around swimming Volvox carteri and Chlamydomonas reinhardtii. They find that flows around V. carteri have strong Stokeslet contribution whereas C. reinhardtii is best modeled as a stresslet but in the near field as 3 off-centered Stokeslets.
Microorganisms are present in fluids in every part of the biosphere. Much of the behavior of microorganims such as bacteria and protozoa takes place at low Reynolds number and in large collective groups. Advances in imaging techniques have allowed for careful study of the motion of flagella that have been pinned down but there is much less known about the behavior of freely swimming microorganisms. This paper presents the first of such measurements.
Shown in Figure 1 is the flow field of a freely swimming V. cateri in the laboratory frame. The authors made such measurements using a technique called particle image velocimetry, which consists of tracking the motion of small passive tracer beads suspended in a fluid. The authors recorded the motions of small tracer particles and using this information determined the instantaneous velocities and the flow field at each time point.