Soft Report - Dipartimento di Fisica - Sapienza

Multi-point Holographic Optical micro-VelocimetryMicron-sized particles can be hold stuck by light in a fluidflow. If the lights are turned off for a short time, particleswill be set free to flow for a short distance with the localfluid velocity. This is the core idea of Holographic Opticalmicro-Velocimetry [1], a newly developed technique whichis able to probe flow fields at those micro-scales which areso relevant for biology and micro-fluidics. The idea ofprobing fluid flows following freely flowing tracers has beenlargely applied by Particle Image Velocimetry, formacroscopic flows, and by its more recent miniaturizedversion for micro-fluidics. The main drawback of thisapproach consists in the need of continuously injecting ahigh concentration of tracers in the flowing fluid. This isrequired in order to cover all the interesting points inspace and to be able to average over the thermal noisealways affecting the trajectories of micron sized probes.This condition can be very restrictive in densely packedgeometries and heavily alter the function of theinvestigated micro-device. Holographic Optical micro-Velocimetry offers the possibility of choosing an arbitraryset of interesting points in the flow geometry and only usea small number of tracers, one for each measuring point.The tracers are held in place by the forces exerted by lighttraps. Light traps are submicron regions of high lightintensity whose distribution in space is dynamicallygenerated by a computer controlled hologram [2]. Micronsized dielectric particles are alternatively trapped andreleased from the trapping sites by chopping the trapbeam. Using digital video microscopy we can detect theprobe particles' displacements after a fixed delay timefrom release instant. The velocity field is thus mappeddirectly and simultaneously on the trap sites allowing toreconstruct the two dimensional projection of flow field onthe imaging plane in a three dimensional domain. Mostimportantly, the technique is independent of the trapcharacteristics, viscosity or temperature of the fluid.The validity of the technique has been demonstrated byFig. 2:. Azimuthal components of measured flowvelocities versus the corresponding radial distancefrom the centre of a spinning sphere. Solid linesrepresent the range of the expected velocityvalues for a sphere of radius R=3.7 µm spinning at5.2 Hz.Fig. 3: Measured flow field (arrows) at the outlet ofa 15 µm wide PDMS micro-channel.mapping out the fluid flow around a five microns spinningsphere (Fig.1,2) and at the outlet of a micro-channel(Fig.3). In the future tracers could be permanentlyembedded inside micro-fluidics devices or biologicalsystems and used to probe micro-flows during normaldevice operation.Fig. 1: Measured flow field (arrows) around a 4 µmradius vaterite particle spinning at 5.2 Hz(a) CRS SOFT-INFM-CNR, Roma, Italy.(b) Dipartimento di Fisica, Universita’ di Roma, Italy.References[1] R. Di Leonardo, J. Leach, H. Mushfique, J. Cooper, G.Ruocco, M. Padgett, Phys. Rev. Lett., 96, 134502,(2006).[2]D.G. Grier, Nature, 424, 810, (2003).AuthorsR. Di Leonardo (a), G. Ruocco (b,a)69SOFT Scientific Report 2004-06