The influence of injection velocity and relaxation time on the spreading of tracers in viscoelastic liquids agitated by submerged, recirculating jets with low Reynolds numbers

We provide experimental demonstration that the spread of tracer elements in a tank containing a viscoelastic liquid and agitated by a submerged jet pointing to the base of the tank can be influenced by the relaxation time of the liquid. We analyzed the temporal spreading of the boundary of a tracer-front in two dimensions using flow visualization at early stages and found that for a given fluid, the evolution of the tracer-front at various injection velocities follows a universal trajectory when considered on a normalized time scale of math formula, where t is observation time, math formula is injection velocity and math formula is the effective diffusivity of the tracer elements in the medium. For a different fluid, at a given math formula, the trajectory scales with the relaxation time of the fluid. The importance of relaxation time to the evolution of a tracer-front is something previously unreported.