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Some swimming microorganisms are sensitive to light, and this can affect the way in which they negotiate their environment. In particular, photophobic cells are repelled from unfavourable light conditions, and in a quiescent fluid environment this can be observed as elevated cell levels in regions away from these light conditions. This photoresponsive effect is of interest due to its potential technological applications. For example, the use of light to focus and direct cells could be used as a convenient means to separate out the algae used in biofuel production (for example, hydrogen), or exploited within devices for biodetection of environmental contaminants. However, in these types of situations the swimming cells will usually be suspended in a flow with shear. In this environment, it has previously been shown that cells can become hydrodynamically trapped in regions of high fluid shear, and so the extent to which photofocusing can occur under these conditions is not immediately clear. Moreover, in applications where the light must pass through appreciable volumes of the suspension, cells will typically absorb light and so shade each other from the illumination. As such, the intensity at any point in the flow is dependent upon the global cell concentration. Hence, in this study we model the coupled influence of fluid shear and cell photosensitivity on a suspension of swimming microorganisms, and ask under what circumstances a suspension of photophobic cells might be focused into high concentration regions.
