MicroPIV and micromixing study of corona wind induced microcentrifugation flows in a cylindrical cavity

Abstract

Recently, a novel way of driving rapid microcentrifugation was discovered using ionic wind via ionization of the atmosphere around a singular electrode tip, driving liquid recirculation in a small cylindrical cavity due to interfacial shear. In the original work, the primary azimuthal surface recirculation was speculated to drive a secondary flow in the bulk of the liquid which resembles a helical swirling flow that tapers toward a pseudo-stagnation point at the cavity floor, analogous to Batchelor flows between co-axially placed stationary and rotating disks. Here, we employ microParticle Image Velocimetry (microPIV) together with numerical simulations to verify this speculation. Good qualitative and reasonable quantitative agreements were obtained between the experiments and numerical simulations. In both, we were able to capture salient features of the three-dimensional flow; for the experiments, this was achieved by the reconstruction of the three-dimensional flow field from the planar two-dimensional velocity fields obtained in a confocal-like manner. In addition, we formally quantify the micromixing enhancement first demonstrated, but not quantified, in the original experiments. Our results show a mixing enhancement close to two orders of magnitude approaching vigorous mixing intensities as the surface vortices suffer from various instabilities leading towards their breakdown into subvortices at large applied voltages and AC frequencies, reminiscent of that in the original work.

On‐Chip Generation of Vortical Flows for Microfluidic Centrifugation

“…Finally, interfacial vortices can also be driven electrokinetically through a discharge‐driven mechanism in which a singular electrode, held above the free surface and raised above a critical potential beyond which atmospheric breakdown occurs, leads to the generation of an ionic wind that can be exploited to shear the interface of a liquid in the form of a sessile droplet or in a microwell ( Figure a). This was then demonstrated for the trapping of particles within a vortex cluster due to shear induced migration along the surface or a converging–stagnating flow in the bulk generated by the interfacial azimuthal rotation for applications such as the separation of red blood cells from plasma or for sample preconcentration to enable enhanced spectroscopic detection . A theory that describes how the particles are trapped within the vortex center to a point or limit cycle, and strategies to break the vortex trap so as to release the particles, is given in Wang et al Additionally, it has also been shown that vortices can be generated within sessile droplets using electrowetting, wherein the application of the electric field leads to a change in the interfacial tension γ of the liquid and hence the contact angle θ of the droplet in which θ 0 is the native contact angle of the droplet prior to the application of the electric field, V is the applied potential and d the thickness of the dielectric layer separating the droplet from the plate electrode beneath it.…”

“…• Discharge-driven [83][84][85][86][87] • Electrowetting [88] Ease of electrode fabrication and integration into microfluidic device if large applied potentials requiring bulky amplifiers are not required Electric field intensity can be tuned to control vortex characteristics and strength Chemical surface modification is an added complexity in channel fabrication; functionalization can wear off and needs to be reap- [89][90][91][92][93][94][95][96][97][98][99] -Posts/pillars/protrusions/ chamber [100][101][102][103][104][105] -Membranes [106] -Resonant cavities [107,108] -Microarray titer plate [109] -Plasmonic nanoparticles (photoacoustic effect) [110,111] • Direct contact with liquid [112][113][114][115][116][117] Surface vibration…”

“…Mechanical actuation using micro‐electromechanical systems (MEMS), for example, the rotating microimpellers fabricated in SU‐8 and housed within a pump chamber, and driven either electrically, through rotational vibration displacements induced by an asymmetric SAW with which the impeller is directly in contact with, or even by the aforementionmed SAW‐generated azimuthal microfluidic centrifugation flow itself, for example, are effective ways for generating vortical flows, but the use of rotors can be subject to degradation in performance and reliability due to issues associated with clogging and wear as with all mechanically moving components at these scales. Pneumatic means, for example, the generation of an airflow to shear the surface of a liquid contained in a microchamber to induce it to rotate in the same way as that driven by ionic wind (Section ), have also been proposed, although such systems are often hampered by the incompatibilities in integrating large compressed air sources or pneumatic pumps with small scale microfluidic devices.…”

Simulation and Experimental Characterization of Microscopically Accessible Hydrodynamic Microvortices

“…Micro particle-image velocimetry then became a popular method for understanding detailed fluid motion inside of microfluidic channels [23]. One relevant study applied µ-PIV to investigate the microvortex produced by active microcentrifugation with electrodes [24]. Later studies showed further progress toward effective µ-PIV investigation of microvortices [20].…”