Micro Mixers

1. Micro Mixers Presentation for: Dr. Liou’s ME 595/695 class ME 595 Special Topics in Mechanical Engineering – Micro Fluidics Western Michigan University Presented by: John Tomasi November 30th, 2004

2. Micro Mixers J. Tomasi I. Introduction to Mixing • Macroscopically, mixing is usually accomplished through turbulence • transverse velocity components • spontaneous flow fluctuations • folding and stretching of fluid particles • Chaotic behavior that randomly redposits particles until homogeneity is achieved • An everyday occurrence that is usually taken for granted

3. Micro Mixers J. Tomasi I. Introduction to Mixing • Microscopically, turbulence is difficult to achieve • reduced length scale results in low Re and highly laminar flows • fluids tend to move in layers • mixing is achieved solely through diffusion along the fluid-fluid boundary • slow and/or lengthy process • Péclet Number is ~ 100,

4. Micro Mixers J. Tomasi II. Importance of Mixing • Sample Analyzing • proteins • DNA • Drug/Sample Delivery • Chemical Reaction Kinematics • Additional laboratory procedures necessary for a laboratory to fit on a “chip”

5. Micro Mixers J. Tomasi III. Types of Mixers • Active All micro mixing devices and techniques that utilize devices separate from the flow channel to induce mixing. • Passive All micro mixing devices and techniques that operate independent of external intervention.

6. Micro Mixers J. Tomasi IV. Active Mixers • Electro-osmosis with AC current applied • mixing occurs at particular frequencies • operation credited to instability that develops at particular frequencies • experimentally shown to reduce mixing time from 500 sec. to as low as 3 sec.

7. Micro Mixers J. Tomasi IV. Active Mixers • Electro-osmosis with AC current applied • Advantages: • utilizes common microfluidics device • Disadvantages: • requires high voltage • sensitive to contaminates in the system • only low flow rates created

8. Micro Mixers J. Tomasi IV. Active Mixers • Orthogonal Perturbations • transverse velocity components artificially created by periodic injections • cross-flow establishes folding of fluid layers • operates best with particular injection frequencies

9. Micro Mixers J. Tomasi IV. Active Mixers • Orthogonal Perturbations

10. Micro Mixers J. Tomasi IV. Active Mixers • Electrowetting-On-Dielectric (EWOD) • droplet based • sandwiched bi-fluid droplet with only two sides of contact • BC’s allow fluid layers to fold as droplet moves • matrix of electrically charged plates direct droplet to roll in a box pattern • each complete roll increases the number of fluid interfaces exponentially (2n)

11. Micro Mixers J. Tomasi IV. Active Mixers • Electrowetting-On-Dielectric (EWOD)

12. Micro Mixers J. Tomasi IV. Active Mixers • Electrowetting-On-Dielectric (EWOD) • Advantages: • requires only 30 Vrms • experimentally shown to reduce mixing times by as much as 50x • compact • Disadvantages: • complex construction • small throughput

13. Micro Mixers J. Tomasi V. Passive Mixers • Serpentine and Helical Channel • fluids mix while the follow a winding path • operating principle: chaotic advection

14. Micro Mixers J. Tomasi V. Passive Mixers • Serpentine and Helical Channel • Advantages: • compactly places long channel required for diffusive mixing on a microdevice • very simple to operate • Disadvantages: • effective in flows with Re > 1 • helical channel extremely difficult to construct • no great reductions in mixing times

15. Micro Mixers J. Tomasi V. Passive Mixers • Staggered Herringbone Mixer (SHM) • raised ridges on the channel floor cause the fluids to rotate within the channel • increases contact area and induces layer folding • Requires ridge heights of only 20 -30 % of mean channel height • various configurations of ridges

16. Micro Mixers J. Tomasi V. Passive Mixers • Staggered Herringbone Mixer (SHM) • Advantages: • requires only one additional step during the construction phase over regular channels • functions independent of Re for Re<100 • superiority to regular channels increases with flow velocity • 1cm/s flow in 100μm square channel requires 100cm for diffusion alone, but only 1 cm with SHM • same fluid in the same channels flowing at 10cm/s would require 1,000cm for diffusion, but only 1.5 cm for SHM

17. Micro Mixers J. Tomasi V. Passive Mixers • Hydrodynamic Focusing • achieves mixing by reducing the layer thickness over which diffusion must operate • utilizes a microjet and two side channels to create three thin fluid layers • the thickness of each layer is controlled by its injection rate

18. Micro Mixers J. Tomasi V. Passive Mixers • Hydrodynamic Focusing

19. Micro Mixers J. Tomasi V. Passive Mixers • Hydrodynamic Focusing • Advantages: • extremely quick mixing times – as low as 10 and 20 μs • small volume consumption ~ 5nl/s • properties of system well understood • Disadvantages: • best suited for reaction kinematics investigations

20. Micro Mixers J. Tomasi V. Passive Mixers • Droplet Based Recirculating Flow • fluids to be mixed and water are injected into a stream of fluid moving through a microchannel • recirculating flow within droplet folds fluid layers into one another • requires that “channel fluid” and droplet fluid be immiscible • surface tension between water and “channel fluid” must be greater than between water and droplet fluids • “twirling” of droplet tip initiates mixing • droplet length controlled by relative injection rate

21. Micro Mixers J. Tomasi V. Passive Mixers • Droplet Based Recirculating Flow

22. Micro Mixers J. Tomasi V. Passive Mixers • Droplet Based Recirculating Flow • Advantages: • quick mixing times ~ as low as 25 μs • Disadvantages: • restrictive on fluid combinations • droplet length needs to be within a specific range for efficient mixing

23. Micro Mixers J. Tomasi Thank You Thank You Questions? for listening