Exposure of a volatile organic vapour could set in powerful rotational motion a microdroplet composed of an aqueous salt solution loaded with metal nanoparticles. The solutal Marangoni motion on the surface originating from the sharp difference in the surface tension of water and organic vapour stimulated the strong vortices inside the droplet. The vapour sources of methanol, ethanol, diethyl ether, toluene, and chloroform stimulated motions of different magnitudes could easily be correlated to the surface tension gradient on the drop surface. Interestingly, when the nanoparticle laden droplet of aqueous salt solution was connected to an external electric circuit through a pair of electrodes, an ∼85–95% reduction in the electrical resistance was observed across the spinning droplet. The extent of reduction in the resistance was found to have a correlation with the difference in the surface tension of the vapour source and the water droplet, which could be employed to distinguish the vapour sources. Remarkably, the power density of the same prototype was estimated to be around 7 μW cm⁻², which indicated the potential of the phenomenon in converting surface energy into electrical in a non-destructive manner and under ambient conditions. Theoretical analysis uncovered that the difference in the ζ-potential near the electrodes was the major reason for the voltage generation. The prototype could also detect the repeated exposure and withdrawal of vapour sources, which helped in the development of a proof-of-concept detector to sense alcohol issuing out of the human breathing system.

Electric field mediated instabilities in a tri-layer oil-water flow inside a microchannel have been explored with the help of the analytical models and computational fluid dynamic simulations. The twin oil-water interfaces undergo either in-phase bending or antiphase squeezing mode of deformation when a direct current (DC) electric field is applied locally inside the channel. The selection of modes largely depends on the magnitudes of the electric field intensity and oil-water interfacial tension. The instability modes grow to form an array of miniaturized oil-droplets with a significantly higher surface to volume ratio. While squeezing mode leads to a time-periodic dripping of droplets at relatively lower field intensities, the bending mode develops into a whiplash ejection of miniaturized droplets at higher field intensities. Subsequently, a transition from purely laminar to chaotic flow is observed, resembling the von Kármán vortex street from a flow past immersed body, suitable for augmented heat, mass, and momentum transport inside a microfluidic channel. Under these conditions, the simulations also reveal the formation of multiple microvortices inside and outside the droplets, which helps in increase in the local Reynolds number for a better mixing efficiency in such microflows. Use of alternating current electric field instead of DC is also found to create on-demand flow features in a time-periodic manner following the mode selection. The amplitude, frequency, and waveform of such electric field is found to generate miniaturized oil-droplets along with the formation of an array of flow features, namely, thread, slugs, plugs, among others.

We explore pumping and digitization of two-phase flow patterns inside a microchannel with the help of a unidirectional and oscillating Lorentz force. For this purpose, an electric field has been coupled with an oscillating (or unidirectional) magnetic field to generate a sinusoidal (or unidirectional) Lorentz force in the channel filled with a pair of Newtonian, immiscible, and electrically conducting fluids. Application of the steady or oscillating Lorentz force is found to enhance the throughput of a pressure-driven flow in conjunction with the mixing of the phases by creating discrete and miniaturized flow structures. Numerical simulations show that the application of Lorentz force in an oil-water stratified flow leads to the digitization of the flow patterns together with enhanced transport due to the magnetohydrodynamic pumping of the fluids. The size and frequency of the flow patterns and the throughput of the flow can be noninvasively altered by tuning the intensity of the electric or magnetic field, frequency of the magnetic field, and fluid properties. An oscillatory Lorentz force with periodic change in direction can lead to time-periodic forward and backward motions of the fluids to prompt a unique reciprocating motion of the flow features while they translate along the channel. The oscillation frequency of some of the flow features is found to follow a linear correlation with the frequency of the magnetic field suitable for pumping applications. The proposed pumping and digitization strategies can be of significance in the design and development of next-generation microscale reactors, mixers, pumps, and microelectromechanical systems (MEMS) devices.

An externally applied alternating current (AC) electrostatic field can deform the interface of a pair of weakly conducting liquids to engender droplet flow patterns inside the ‘T’ shaped microchannels.