Hydrogen sulfide (H 2 S) is emerging as an important gasotransmitter in both physiological and pathological states. Rapid measurement of H 2 S remains a challenge. We report a microfluidic method for rapid measurement of sulphide in blood plasma using Dansyl-Azide, a fluorescence (FL) based probe. We have measured known quantities of externally added (exogenous) H 2 S to both buffer and human blood plasma. Surprisingly, a decrease in FL intensity with increase in exogenous sulphide concentration in plasma was observed which is attributed to the interaction between the proteins and sulphide present in plasma underpinning our observation. The effects of mixing and incubation time, pH, and dilution of plasma on the FL intensity is studied which revealed that the FL assay required a mixing time of 2 min, incubation time of 5 min, a pH of 7.1 and performing the test within 10 min of sampling; these together constitute the optimal parameters at room temperature. A linear correlation (with R 2 ≥ 0.95) and an excellent match was obtained when a comparison was done between the proposed microfluidic and conventional spectrofluorometric methods for known concentrations of H 2 S (range 0–100 µM). We have measured the baseline level of endogenous H 2 S in healthy volunteers which was found to lie in the range of 70 μM – 125 μM. The proposed microfluidic device with DNS-Az probe enables rapid and accurate estimation of a key gasotransmitter H 2 S in plasma in conditions closely mimicking real time clinical setting. The availability of this device as at the point of care, will help in understanding the role of H 2 S in health and disease.

We report the dynamics of aqueous droplets of different size and viscosity at the interface of a coflowing stream of immiscible oils (i.e., primary and secondary continuous phases) in a microchannel, at low Re. The lateral migration of droplets introduced into the primary continuous phase toward the interface and subsequent selective migration of droplets across the interface into the secondary continuous phase is investigated. The interplay between the competing noninertial lift and interfacial tension forces, which govern the interfacial migration of the droplets, is presented and discussed. The velocity and strain rate profiles, and interface location, which are critical for calculating the lift force and migration behavior of droplets, are presented. The trajectories of droplets of different size and viscosity in the primary continuous phase are obtained for different interface locations. During interfacial migration, the deformation behavior of droplets of different viscosities is studied. Finally, sorting of droplets based on size contrast is demonstrated and sorting efficiency is found. A new paradigm of migration and sorting of droplets is reported, which could find importance in chemical and biological applications.

Demulsification of droplets stabilized with surfactant is very challenging due to their low surface energy. We report ultralow voltage-based electrocoalescence phenomenon for the demulsification of aqueous droplets with an aqueous stream. In the absence of electric field, due to the disjoining pressure resulting from the tail-tail interaction between the surfactant molecules present on the aqueous droplets and interface, coalescence of aqueous droplets with the aqueous stream is prevented. However, above a critical electric field, the electrical stress overcomes the disjoining pressure, thus leading to the droplet coalescence. The influence of surfactant concentration, droplet diameter, and velocity on the electrocoalescence phenomena is studied. The macroscopic contact between the aqueous droplet with the aqueous stream enables droplet coalescence at much lower voltage (10 to 90 V), which is at least two orders of magnitude smaller than voltages used in prior works (1.0 to 3.0 kV). The electrocoalescence phenomena is used for the extraction of microparticles encapsulated in aqueous droplets into the aqueous stream and size-based selective demulsification. A new paradigm of droplet electrocoalescence and content extraction is presented that would find significant applications in chemistry and biology.

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