Tailored micromixing in chemically patterned microchannels undergoing electromagnetohydrodynamic flow

The study unveils a simple, non-invasive method to perform micromixing with the help of spatiotemporal variation in the Lorentz force inside a microchannel decorated with chemically heterogeneous walls. Computational fluid dynamics simulations have been utilized to investigate micromixing under the coupled influence of electric and magnetic fields, namely, electromagnetohydrodynamics, to alter the direction of the Lorentz force at the specific locations by creating the reverse flow zones where the pressure gradient, ∇ p = 0 . The study explores the impact of periodicity, distribution, and size of electrodes alongside the magnitude of applied field intensity, the flow rate of the fluid, and the nature of the electric field on the generation of the mixing vortices and their strength inside the microchannels. The results illustrate that the wall heterogeneities can indeed enforce the formation of localized on-demand vortices when the strength of the localized reverse flow overcomes the inertia of the mainstream flow. In such a scenario, while the vortex size and strength are found to increase with the size of the heterogeneous electrodes and field intensities, the number of vortices increases with the number of heterogeneous electrodes decorated on the channel wall. The presence of a non-zero pressure-driven inflow velocity is found to subdue the strength of the vortices to restrict the mixing facilitated by the localized variation of the Lorentz force. Interestingly, the usage of an alternating current (AC) electric field is found to provide an additional non-invasive control on the mixing vortices by enabling periodic changes in their direction of rotation. A case study in this regard discloses the possibility of rapid mixing with the usage of an AC electric field for a pair of miscible fluids inside a microchannel.

Bubble-Enhanced Mixing Induced by Standing Surface Acoustic Waves (SSAWs) in Microchannel

BAW-based micromixers usually achieve mixing enhancement with acoustic-induced bubbles, while SAW-based micromixers usually enhance mixing efficiency by varying the configuration of IDTs and microchannels. In this paper, bubble-enhanced acoustic mixing induced by standing surface acoustic waves (SSAWs) in a microchannel is proposed and experimentally demonstrated. Significant enhancement in the mixing efficiency was achieved after the bubbles were stimulated in our acoustofluidic microdevice. With an applied voltage of 5 V, 50 times amplified, the proposed mixing microdevice could achieve 90.8% mixing efficiency within 60 s at a flow rate of 240 μL/h. The bubbles were generated from acoustic cavitation assisted by the temperature increase resulting from the viscous absorption of acoustic energy. Our results also suggest that a temperature increase is harmful to microfluidic devices and temperature monitoring. Regulation is essential, especially in chemical and biological applications.

Efficient electroosmotic mixing in a narrow-fluidic channel: the role of a patterned soft layer

We propose a novel and efficient mixing technique in a soft narrow-fluidic channel under the influence of electrical forcing. We show that a grafted polyelectrolyte layer (PEL) added as a patch to the channel wall modulates the electrical double layer (EDL) so that an applied electric field initiates a local electroosmotic flow (EOF) at the patched section. This EOF develops in the opposite direction to the primary pressure-driven flow. This localized EOF leads to the formation of Lamb vortices at the patched sections through the phenomenon of momentum exchange with the primary stream and promotes the mixing therein. Our study, consistent with the stream-function/vorticity approach, primarily focuses on the numerical analysis of the mixing phenomena. Through a quantitative description, we reveal the effect of different patterns on the underlying mixing phenomena in the convective mixing regime. We also discuss the impact of key parameters on the mixing efficiency, the onset of the recirculation zone, variation in the mixing length, and the shear-driven aggregation kinetics in soft matter systems. Finally, considering the practicability of the present problem, we unveil the values of several design parameters for which the mixing efficiency in the channel reaches the maximum.

A hybrid electrically-and-piezoelectrically driven micromixer built on paper for microfluids mixing

This study aims to explore the channel patterns and the characteristic parameters of the zigzag microchannel based on microfluidic paper-based analytical devices (μPADs), in which the mixing efficiency and speed can be greatly enhanced. Better mixing of the solutions was obtained by adding a simple directing electric field to the optimized structure of the zigzag microchannel on paper-based chips instead of the traditional complex devices. A higher mixing efficiency was reached when the direct-current (DC) power supply reached 20 V. Meanwhile, a piezoelectric transducer (PZT) driver was used in the mixing experiment with the paper-based zigzag microchannel. The results show that the mixing efficiency reached a maximum value when the input voltage and frequency were 30 V and 150 Hz, respectively. These paper-based devices meet the requirements of the biochemical analysis field because they are low cost, easy to operate, and have high efficiencies, giving them good prospects for future applications. Graphical Abstract.

Electromagnetophoretic Micro-convection around a Droplet in a Capillary

The electromagnetophoretic behavior of organic droplets in an electrolyte solution was investigated in a silica capillary cell using a superconducting bulk magnet (3.5 T) and a magnetic circuit (2.7 T). The initially dispersed emulsion droplets of dodecane migrated to the wall of the capillary, responding to the direction of an electric current, and coalesced to form smaller and larger droplets after some repeated migrations. When the electric current was applied continuously, the larger droplets became arranged with regular intervals on the wall, and smaller droplets rotated around the larger droplets. These interesting behaviors were analyzed while taking into account the local electric current density determined by the flow velocity of the ionic current around a droplet, which was lowest on the electrode sides of the droplet. The difference in the local electric current density generated the Lorentz-force difference in the medium, which lead to local micro-convection around the droplet, and also the alignment of larger droplets by a repelling effect between the adjacent micro-convections.