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...