(350c) Measuring Concentration Fields in Microfluidic Channels in Situ with a Fabry–Perot Interferometer

Spatio-temporal concentration
profiles arise in a variety of physical systems; for example, those in which
materials precipitate, dissolve, react, or equilibrate. While many of these
systems can be studied in a microfluidic environment, there are few general
techniques which can measure spatio-temporal
concentration profiles without fluorescent labeling. Here we describe a Fabry-Perot interferometric technique, capable of measuring
concentration profiles in situ,
without any chemical label, by tracking Fringes of Equal Chromatic Order
(FECO). The technique has a sensitivity of 10^-5 RIU, which can be
used to track local solute changes of ~0.05% (w/w). The technique is spatially
resolved (1 mm) and easily measures evolving concentration fields with ~20 Hz
rate. Here, we demonstrate by measuring the transport of various solutes and
solvents through polyethylene glycol diacrylate
(PEG-DA) hydrogels with and without embedded oil droplets. We anticipate that
our technique will prove useful for directly measuring evolving concentration
profiles, opening new possibilities for the direct interrogation of dynamic,
material systems.