Electrohydrodynamically-Induced Interfacial Vortex & Spiral Flows

                                                                                                         Ionic or corona wind, generated when a sharp electrode tip is raised beyond a threshold ionisation potential (just like lightning), can be directed towards a liquid surface slightly below the electrode to produce surface and bulk liquid recirculation in a small cylindrical microfluidic chamber as a consequence of interfacial shear. Recently, we demonstrated how the surface vortices can be used to drive efficient micro-mixing, as shown in the movie. This is extremely useful, for example, in high throughput screening microarrays in order to overcome the long times associated with microscale diffusional transport. The same micropipettes that deliver the reagents can be used as the sharp electrode tip to generate the ionic wind.

In addition, it is also possible to use the surface vortices generated to trap microparticles, as depicted in the movie. The particles are drawn into the vortex streamlines due to positive dielectrophoresis and subsequently migrate towards the centre of the vortex due to shear gradients. This particle concentration mechanism is useful in biosensor technology due to the limitations of current biosensors to detect small amounts of pathogens.

An alternative mechanism for particle trapping is to employ the bulk liquid recirculation. Curiously, a similar phenomenon occurs in stirred tea cups - the tea leaves are observed to accumulate in the centre at the bottom of the tea cup instead of being expelled outwards. This paradoxical behaviour was explained by Einstein in 1926. He showed that the friction at the bottom of the tea cup generated an inward radial force in a thin layer just above the cup's base. This together with the azimuthal rotation at the top surface gives rise to a spiral-like flow trajectory, which is known as Batchelor flows, named after GI Batchelor, who showed that a similar
flow phenomenon occurred in a liquid column trapped between stationary and rotating disks. Particles in the flow follow the liquid trajectory and are spun down in a similar fashion towards the base of the chamber in the centre. If the sedimentation forces are sufficiently large to overcome the convective forces, the particles remain trapped at the base instead of being recirculated with the liquid back up a central spinal column. This bulk recirculation phenomenon is used to effectively separate red blood cells from plasma in a tiny device. This microcentrifugation technology is hence useful for the development of miniature chip-scale devices for point-of-care diagnostics.
  1. 1.DR Arifin, LY Yeo, JR Friend. Microfluidic Blood Plasma Separation Via Bulk Electrohydrodynamic Flows. Biomicrofluidics 1, 014103 (2007) (PDF).

  2. 2.LY Yeo, JR Friend, DR Arifin. Electric Tempest in a Teacup - The Tea Leaf Analogy to Microfluidic Blood Plasma Separation. Appl Phys Lett 89, 103516 (2006) (PDF).

  3. 3.LY Yeo, D Hou, S Maheshswari, H-C Chang. Electrohydrodynamic Surface Micro-Vortices for Mixing and Particle Trapping. Appl Phys Lett 88, 233512 (2006) (PDF).

  4. 4.JJ Qin, LY Yeo, JR Friend. MicroPIV and Micromixing Study of Corona Wind Induced Microcentrifugation Flows in a Cylindrical Cavity. Microfluid Nanofluid 8, 231-241 (2010) (PDF).

Press Releases:

  1. 1.If Research is Your Cup of Tea, this Eureka Moment will Stir the Blood, The Age, 16 August 2008.

  2. 2.Einstein's Tea Leaves Cause a Stir in Medical Circles, ABC Catalyst, Television Broadcast: 19 July 2007.

  3. 3.Stirring Things Apart, The Economist, 20 January 2007 (Print Version).

  4. 4.Einstein's Tea Leaves Inspire Gadget, Discovery Channel News, 19 January 2007.

  5. 5.Taking Blood Cells for a Spin, ScienceNOW, Science Magazine Daily News, 18 January 2007.

  6. 6.Einstein's Tea Leaves Inspire New Gadget, ABC Science Online, 17 January 2007.

  7. 7.Unique Blood Testing Method Developed, The Washington Times, 16 January 2007.

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