Large scale flow visualization and anemometry applied to lab on chip   models of porous media
  The following is a report on an experimental technique allowing to quantify and map the velocity field with a very high resolution and a simple equipment in large 2D devices. A simple Shlieren technique is proposed to reinforce the contrast in the images and allow you to detect seeded particles that are pixel-sized or even inferior to it. The velocimetry technique that we have reported on is based on auto-correlation functions of the pixel intensity, which we have shown are directly related to the magnitude of the local average velocity. The characteristic time involved in the decorrelation of the signal is proportional to the tracer size and inversely proportional to the average velocity. We have reported on a detailed discussion about the optimization of relevant involved parameters, the spatial resolution and the accuracy of the method. The technique is then applied to a model porous media made of a random channel network. We show that it is highly efficient to determine the magnitude of the flow in each of the channels of the network, opening the road to the fundamental study of the flows of complex fluids. The latter is illustrated with yield stress fluid, in which the flow becomes highly heterogeneous at small flow rates. 
Johan Paiola, Harold Auradou, Hugues Bodiguel
physics.flu-dyn