Dye Attenuation Without Dye: Quantifying Concentration Fields with Short-wave Infrared Imaging*

An integral part of an experimental fluid dynamicist's tool box is the use of tracer particles or dyes in image analysis to perform quantitative measurements of fluid flow and concentration fields. However, there are many applications where the addition of a tracer is not practical. In biological contexts, tracers can affect the biological phenomena being investigated. When investigating flows in porous media with strong chemical interactions between the solid and fluid phases (e.g. highly swelling polymeric systems), interactions between the tracer and the immobile polymer phase can alter the behaviour of the system.

Here, we present Short-wave Infrared (SWIR) based absorbance imaging as a tool to track concentration fields without the use of tracers. Instead, we choose a particular wavelength outside the visible spectrum at which the fluid in question absorbs strongly. Experiments utilising this technique with the fluid of interest being water are presented. They provide in-situ space and time resolved measurements of the depth-integrated water concentration field resulting from when a water drop is placed on both an impermeable glass slide and on an absorbent hydrogel sheet. The applicability of this technique to more general fluid problems is then discussed.