:: Microfluidics ::

Microfluidic Immunomagnetic Multi-Target Sorting

Sorting of cells and biological material is typically done using a fluorescence detection system that has a large footprint. Cells are also sorted by their attachment to antibody-coated magnetic beads in magnetic separation systems. Here, a microfluidic system is developed that uses a magnetic field to sort magnetic beads by deflecting them in the direction normal to the flow. A mathematical model is derived that predicts the magnitude of deflection based on the bead size and susceptibility, magnet strength, fluid speed and viscosity, and device geometry. The system can be applied to sort cells and biological materials by binding them to beads of different sizes and susceptibilities.

(Collaborating with Ian Griffiths and Howard Stone)

Microfluidic Particle Conformal Coating Device

Coating of cells with a layer of polymeric membrane can help protects the cells from the host's immune system during drug delivery. We demonstrate conformal coating of protein-bound, micron-size paramagnetic beads with a very thin O(1 micron) layer of coating fluid, by pulling the beads from an aqueous phase to an oil phase in a microfluidic co-flowing system and control the coating thickness with the aqueous phase viscosity.

(Collaborating with Jason Wexler, Jiandi Wan and Howard Stone)

:: Interfacial Deformation ::

Particle Collection and Hump Formation At A Liquid-Air Interface

When a suspension of paramagnetic beads is in a sufficiently strong magnetic field gradient, a jet forms. Based on this approach, we report a technique for depositing an aggregate of paramagnetic beads on a substrate. In response to a weak magnetic field, all of the beads collect at the almost planar interface, which then deforms modestly as the field strength is increased to form a hump. Above a critical field strength, the hump where the beads have collected goes unstable to form a jet. We use high-speed videos to study the system’s hump-jet transition. We also propose an analytical scaling model that predicts the critical conditions for the transition by the balance of magnetic and capillary forces acting on the aggregate of beads.

(Collaborating with Ian Griffiths, Zhenzhen Li, Pilnam Kim and Howard Stone)

Magnetic Capillary Rafts

By balancing capillary attraction and magnetic repulsion, magnetized spheres form equilibrium patterns on an oil-water interface. The distance between particles is controlled by the magnitude of the magnetic field, which reaches a critical point when the field is sufficiently weak, the formation collapses, sinks, and drags some of the oil with it.

(Collaborating with Krzysztof Sadlej and Howard Stone)

:: Capillarity Dynamics ::

Droplet Splash Control With Tangential Velocity

Drops falling onto a moving or inclined surface experience normal and tangential impact velocities. Asymmetrically triggering and inhibiting splashes is accomplished by varying the relative tangential velocity between the falling drop and the impacted surface. A model is developed that predicts the wetting to splashing transition, taking into account the tangential velocity.

(Collaborating with Jacy Bird and Howard Stone)

Spreading and Jetting of Electrically Driven Films

We study the electrically driven spreading of dielectric liquid films in wedge-shaped gaps across which a potential difference is applied. Our experiments are in a little-studied regime where the electrical relaxation time is long compared to the time for charge to be convected by the fluid motion. We observe that at a critical gap height, or critical normal electric field, the hump-shaped leading edge undergoes an instability in the form of a Taylor cone and periodic jetting ensues, after which traveling waves occur along the trailing thin film. We propose a convection-dominated mechanism for charge transport to describe the observed dynamics and rationalize the viscosity dependence of the dynamics.

(Collaborating with Pilnam Kim, Camille Duprat and Howard Stone)

:: Publications and Presentations ::

Peer-Reviewed Journal Publications

1. S. S. H. Tsai, J. S. Wexler, J. Wan, and H. A. Stone. "Microfluidic Ultralow Interfacial Tensiometry By Magnetic Forcing." Submitted. (2012) [Preprint available]
2. S. S. H. Tsai, I. M. Griffiths, Z. Li, P. Kim, and H. A. Stone. "Microfluidic Immunomagnetic Multi-Target Sorting – A Model For Controlling Deflection Of Paramagnetic Beads."Lab on a Chip, 11 (15), 2577 - 2582. (2011) [Link]
3. S. S. H. Tsai, J. S. Wexler, J. Wan, and H. A. Stone. "Conformal Coating of Particles In Microchannels By Magnetic Forcing." Applied Physics Letters, 99, 153509. (2011) [Link]
4. P. Kim, C. Duprat, S. S. H. Tsai, and H. A. Stone. "Selective Spreading And Jetting Of Electrically Driven Eielectric Films." Physical Review Letters, 107, 034502 (2011) [Link]
5. J. C. Bird, S. S. H. Tsai, and H. A. Stone. "Inclined To Splash: Triggering And Inhibiting A Splash With Tangential Velocity." New Journal of Physics, 11, 063017. (2009) [Link]

Peer-Reviewed Proceedings

1. S. S. H. Tsai, J. S. Wexler, I. M. Griffiths, and H. A. Stone. "Separation of Magnetic Beads In A Microfluidic Device - Modeling and Experimentation." Proceedings of the ASME 2011 International Mechanical Engineering Congress and Exposition. Denver, Colorado. (2011)
2. S. S. H. Tsai and H. A. Stone. "Microfluidic Magnetic Multi-Cell Sorting." Proceedings of the 8th International Conference on Nanochannels, Microchannels, and Minichannels. Montreal, Canada. (2010)
3. S. S. H. Tsai, R. Ben Mrad, and P. E. Sullivan. "Numerical Modeling of EWOD In Digital Microfluidics." Proceedings of the Canadian Congress of Applied Mechanics. Toronto, Canada. (2007)

Invited Talks and Seminars

1. "Magnetic particles In Viscous Flows And At Interfaces: Separation, Coating, and Jetting." Department of Mechanical and Industrial Engineering, University of Toronto, June 7, 2011
2. "Microfluidic Magnetic Sorting and Coating." Ontario On A Chip 2011. Toronto, Canada. June 9, 2011
3. "Magnetic Particles In Viscous Flows And At Interfaces: Separation, Coating, and Jetting." Mathematical Institute, University of Oxford. May 18, 2011

Contributed Oral Presentations

1. "Microfluidic Controlled Conformal Coating Of Particles."
   64th APS Meeting of the Division of Fluid Dynamics, Baltimore, Maryland. (2011) (Oral presenter)
2. "Magnetic Spheres And Microfluidics: Problems In Sorting And Coating."
   ASME International Mechanical Engineering Congress And Exposition, Denver, Colorado. (2011) (Oral presenter)
3. "Magnetic Fields Applied to Paramagnetic Suspensions: The Hump-Jet Transition." 
   63rd APS Meeting of the Division of Fluid Dynamics, Long Beach, California. (2010) (Oral presenter)
4. "Electrically Driven Motion of Thin Films of Dielectric Liquids."
   63rd APS Meeting of the Division of Fluid Dynamics, Long Beach, California. (2010)
5. "Microfluidic Magnetic Multi-Cell Sorting."
   Proceedings of the 8th International Conference on Nanochannels, Microchannels, and Minichannels, Montreal, Canada. (2010) (Oral presenter)
6. "Microfluidic-Micromagnetic Separation of Blood Borne Pathogens in Whole Blood."
   16th US National Congress of Theoretical and Applied Mechanics, State College, Pennsylvania. (2010)
7. "Droplet Impact Onto A Flat Plate: Inclined Versus Moving Surfaces."
   61st APS Meeting of the Division of Fluid Dynamics, San Antonio, Texas. (2008) (Oral presenter)
   
8. "Numerical Modeling of EWOD in Digital Microfluidics."
   Proceedings of the Congress of Applied Mechanics, Toronto, Canada. (2007) (Oral presenter)