On-chip characterization of the mass diffusivity of binary mixtures using Brownian microscopy

We report a technique based on Brownian microscopy for on-chip characterization of the mass diffusivity of binary mixtures. Brownian microscopy tracks the motions of microbeads of precisely known diameter to simultaneously determine local concentration and velocity fields in microchannel flows. We characterize the convective and diffusive mass transports in an H-type microfluidic mixer and extract the diffusivity by analyzing experimentally obtained local concentration and velocity fields. The technique is advantageous for samples that are ill-suited for fluorescence microscopy and for flow conditions that involve significant coupling between convective and diffusive mass transport. The technique is validated for glycerin/water mixtures, which are a model system studied extensively in the past. We also report a detailed uncertainty analysis to help determine the applicability and accuracy of the technique under various sample and flow conditions.