Search

We explore the chemotaxis of an elliptical double-faced Janus motor (Janusbot) stimulated by a second-order chemical reaction on the surfaces, aA + bB → cC + dD, inside a microfluidic channel. The self-propulsions are modeled considering the full descriptions of hydrodynamic governing equations coupled with reaction–diffusion equations and fluid–structure interaction. The simulations, employing a finite element framework, uncover that the differential rate kinetics of the reactions on the dissimilar faces of the Janusbot help in building up enough osmotic pressure gradient for the motion as a result of non-uniform spatiotemporal variations in the concentrations of the reactants and products around the particle. The simulations uncover that the mass diffusivities of the reactants and products along with the rates of forward and backward reactions play crucial roles in determining the speed and direction of the propulsions. Importantly, we observe that the motor can move even when there is no difference in the total stoichiometry of the reactants and products, (a + b) = (c + d). In such a scenario, while the reaction triggers the motion, the difference in net-diffusivities of the reactants and products develops adequate osmotic thrust for the propulsion. In contrast, for the situations with a + b ≠ c + d, the particle can exhibit propulsion even without any difference in net-diffusivities of the reactants and products. The direction and speed of the motion are dependent on difference in mass diffusivities and reaction rate constants at different surfaces.

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 Karman 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.