  Fluorescent latex beads have been widely used as tracers in microfluidics
over for the last decades. They have the advantages to be density matched with
water and to be easily localizable using fluorescence microscopy. We have
recently synthesized silver-coated oil droplets that are both luminescent and
absorptive, by first coating the oil interface with a polydopamine layer and
then depositing a silver layer by a redox process. They have a mean diameter of
6 \mu m and a their density has been matched to the density of water by
adjusting the thickness of the metallic layer. In this work we used these
particles as tracers to measure the velocity profile of a water flow in a PDMS
microchannel with a rectangular cross-section, which allowed us to confirm the
predictions of the Stokes equation with results comparable to those of common
submicronic polystyrene particles.
