Dresden 2014 – scientific programme

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DY: Fachverband Dynamik und Statistische Physik

DY 11: Microswimmers (joint session DY/ BP)

DY 11.5: Talk

Tuesday, April 1, 2014, 10:30–10:45, ZEU 146

Statistical properties of tracer positions, sedimenting in an active fluid — •Thomas John, Matthias Mußler, and Christian Wagner — Experimentalphysik, Universität des Saarlandes

Fluid dynamics on µm scale at velocities in µm/s is characterized by very low Reynolds numbers. Therefore no turbulent behavior and characteristics is expected. Nevertheless, a spatial-temporal random flow field can be present in a media if the fluid contains a lot of active, irregular moving micro-swimmers. We consider trajectories of passive sedimenting beads in such fluids. This trajectories are strongly influenced from the random flow field if the passive particle (tracer) diameter comparable or less then the diameter of the micro-swimmers. We measured such trajectories in suspensions of the green alga chlamydomonas reinhardtii. The alga has two flagella, a diameter of 10 µm and swims as a puller with 50 µm/s. We extract statistical properties of the passive bead positions, e.g. the mean square displacement or the probability density function. Our results are compared with the Brownian motion characteristics of sedimenting particles in very dilute systems and the known non-Brownian characteristics in passive sedimenting particles at higher volume concentrations where the hydrodynamic interactions becomes important.