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Poster, Friday, 19:00  
Insight into the cell-beam interaction in the Optical Stretcher

Steffen Grosser, Anatol Fritsch, Tobias Kießling, Roland Stange, Josef A. Käs

University of Leipzig, Faculty of Physics and Earth Sciences, Institute of Experimental Physics I, Soft Matter Physics Division, Linnéstraße 5, 04103 Leipzig, Germany
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In the optical cell stretcher, force on cells is exerted by two counter-propagating beams. Usually, these forces are calculated using a ray-optics approach, assuming the same refractive index for all cells. The effect of the cells on the beam has so far been disregarded. 
We use an extension of the optical stretcher setup that allows to partially track the laser beams after having passed the cell in the stretcher chamber. We report that the effect of the cells on the beam can, as a first approach, be described by a cell-lens analogy: The cells reduce the divergence of the laser beam or can even refocus the beams. Smaller cells have a higher radius of curvature and are thus stronger lenses; an expectation which we experimentally confirm.
Parameters such as cell size and ellipticity are used in a ray-transfer-matrix calculation to predict the expected focussing, making it possible to give an estimate of the refractive index of the individual cell by comparing the expected and the actual measured focussing. This in turn allows to readress the problem of determining the force actually exerted on the cell.

Fig. 1: Phase contrast images of two cells in the stretcher: The cell on the left has, considering its size, a rather low focussing effect; the cell on the right induces comparably high focussing. They are assigned a low (left) respectively high (right) refractive index by our model. This coincides with the low respectively high phase contrast of the cells.
University of Leipzig  |  Faculty of Physics and Earth Sciences  |  Institute of Experimental Physics I  |  Soft Matter Physics Division
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