Time: 4:30 p.m.
Place: UHW 11/1102
Christian Wagner, Department of Experimental Physics, Universität des Saarlandes, Saarbrücken, Germany
We use optical tweezers both to study ﬂuctuations in hydrodynamic systems and cell cell adhesion in biological systems. For the ﬁrst case, shear-induced cross-correlations of particle ﬂuctuations perpendicular and along stream-lines are investigated experimentally and theoretically. Direct measurements of the Brownian motion of micron-sized beads, held by optical tweezers in a shear-flow cell, show a strong time-asymmetry in the cross-correlation, which is caused by the non-normal amplification of ﬂuctuations. Complementary measurements on the single particle probability distribution substantiate this behavior and both results are consistent with a Langevin model. In addition, a shear-induced anti-correlation between orthogonal random-displacements of two trapped and hydrodynamically interacting particles is detected, having one or two extrema in time, depending on the positions of the particles.
For the second system we use holographical optical tweezers to investigate the role of red blood cells in thrombus formation. Red blood cells were suspended in a buﬀer solution and an irreversible cell-cell adhesion could be induced by the addition of the lysophosphatidic acid (LPA). LPA is released as a second messenger by activated platelets. Both substances lead to an inﬂux of Ca2+ and eﬂux of K-followed by the activation of lipid scramblase Ca2+ leading to an exposure of phosphatidylserine on the outer leaflet. The adhesion forces could be determined to be larger than 25 pN and thus larger than any hydrodynamic forces in physiological blood ﬂow that could eventually break up the formed aggregates.