8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P9.8Tue 611:23-14:00Particle image velocimetry with “phantom” particles:tracking below the resolution limitStefano Buzzaccaro, 1 Eleonora Secchi, 1 and Roberto Piazza 11 Department of Chemistry (CMIC), Via Ponzio 34/3, 20133, Milano, ItalyThe experimental challenges associated with the investigation of systems exhibiting complex heterogeneousdynamics have recently spurred to devise new optical techniques, exploiting intensitycorrelation methods to obtain structural and dynamic information from the scattered radiation collectedwithin a suitably defined “near-field” region close to the sample. These techniques providevery useful insights about the links between scattering and imaging approaches, and yield a deeperunderstanding of the relation between spatial coherence and image formation [1]. Inspired by thesedevelopments, we propose a new velocimetry technique (Differential Velocimetry Microscopy,DVM), requiring only a standard white-light optical microscope equipped with a digital cameraand allowing to track the flow in microfluidic channel with temporal and spatial micrometric 3Dresolution. This methods offers the key opportunity of using “phantom” tracers, namely very smallscatterers with a size far below the diffraction limit, still retaining the same 3D resolution of a standardmicroscope. As an example of the great potential of DVM, we present measurements of the3D velocity field in complex microfluidic geometries. The usefulness of the principles underlyingDVM is not however restricted to microfluidics applications, for the same idea can be exploitedto monitor the hydrodynamic flow on macroscopic scales, by substituting the microscope witha low cost light source and controlling spatial coherence by optical stop-down methods, whichenable to generate a “speckled” image. This simple optical setup allows us to describe the collapsedynamics of colloidal gels with a spatial resolution of 25µm on an overall scale of 4cm [2, 3].[1] R. Cerbino, A. Vailati, Curr. Opin. Colloid Interface Sci, 14: 416 (2009).[2] G. Brambilla, S. Buzzaccaro, R. Piazza, L. Berthier, L. Cipelletti, Phys. Rev. Lett. 106:118302(2011).[3] E. Secchi, S. Buzzaccaro, R. Piazza, in preparation.8