Fluid flow around swimming microorganisms

As tiny creatures swim, the flows they create in their liquid environment can affect the motion of their neighbors and the viscosity of the ambient suspension. Those flows have been mathematically modeled, but, until recently, they had not been imaged near a swimming microorganism. The feat has now been accomplished by two research teams working independently, one at the University of Cambridge in the UK and one at Haverford College in the US. Both obtained flow fields by taking sequential photographs of microscopic tracer particles displaced by the alga Chlamydomonas reinhardtii, which swims by executing a breast-stroke-like motion with its two approximately 10-µm-long flagella. The image, from the Cambridge group, shows the streamlines created by the swimming alga, averaged over a stroke cycle (left). That fluid flow, the researchers note, is well described by a simple model (right) in which C. reinhardtii exerts two point forces to balance the drag force on its body. (The Cambridge team also studied the larger alga Volvox carteri.) The Haverford collaboration focused on the changes in the alga’s flow field during the course of a swimming cycle (see the video that accompanies this item online). The group discovered, for example, that as it carries out its forth-and-back motion, C. reinhardtii expends about four times the energy it would if it could swim steadily along its trajectory. (K. Drescher et al., Phys. Rev. Lett. 105 , 168101, 2010 http://dx.doi.org/10.1103/PhysRevLett.105.168101 ; J. S. Guasto, K. A. Johnson, J. P. Gollub, Phys. Rev. Lett. 105 , 168102, 2010 http://dx.doi.org/10.1103/PhysRevLett.105.168102 .)