Micromixer design, fabrication, and performance characterization and validation

1. ME EN-6730: Microfluidic Chip Design
Micromixer Design Project
Udayan Ghosh

2. Objective and rationale
• To design a passive micromixer in order to mix two fluids of equal density and viscosity.
• Conditions:
• Mixer should perform for three different flow rate: 0.1 mL/hr, 1 mL/hr and 10 mL/hr
• Channel height 50 µm
• Aspect ratio of any channel or structure should be 3(height):1(width)
Considered Physics:
• Diffusion: The goal is to increase the diffusion surface
area and to increase diffusion time
• Dean flow: Rotation of the flow due to circular curve

3. Considered variables
Three considered variables:
• Inlet type
• Number of serpentine turns
• Number of turns for dean flow
Considered parameter/feature for simulation:
• Diffusion coefficient: 2E-6 𝑐𝑚2
/𝑠𝑒𝑐
• Crosswind diffusion type: Codina

4. Simulation – Inlet selection
• Two different inlet setup has been tried to observe mixing
• The goal was to increase the surface of diffusion
Figure: Inlet channels without division Figure: Inlet channels with division of one of
the inlets

5. Simulation – Inlet selection
Figure: Mixing for inlet channels without division
Figure: Mixing for inlet channels with division of
one of the inlets
Figure: Considered channel geometry for inlet selection
Figure: Mixing for inlet channels without division
Figure: Mixing for inlet channels with division of
one of the inlets

6. Simulation – Inlet selection
Figure: Mixing for inlet channels without division
Figure: Mixing for inlet channels with division of one of the inlets
• For all three flow rate,
better mixing is s
observed inlet channel
with divisions.
Flow rate
(mL/hr)
Mixing Index for
inlet channel
without division
(%)
Mixing Index for
inlet channel
with division
(%)
0.1 84.18 99.77
1 51.67 94.92
10 37.35 63.46

7. Simulation – Length of the serpentine channel
Figure: One turn of serpentine channel Figure: Two turn of serpentine channel
• Length of the serpentine channel has been optimized
• Expected increased diffusion time will induce better mixing at low flow rate
• Expected vortices due to geometry in the channel

8. Simulation – Length of the serpentine channel
Figure: Two serpentine turns (Flow rate 0.1mL/hr) Figure: Two serpentine turns (Flow rate 1 mL/hr) Figure: Two serpentine turns (Flow rate 10 mL/hr)
Figure: One serpentine turns (Flow rate 10 mL/hr)Figure: One serpentine turns (Flow rate 1 mL/hr)Figure: One serpentine turns (Flow rate 0.1mL/hr)

9. Simulation – Length of the serpentine channel
Flow rate
(mL/hr)
Mixing Index for one
turn
(%)
Mixing Index for two
turn
(%)
0.1 98.93 99.77
1 88.83 94.92
10 85.07 63.46
• Observed better mixing index by
increasing the channel length
• For high flow rate, it seems there is
error in mixing index. The source of
the error can be meshing.

10. Simulation – Interlocking spiral channel length
• As at high flow rate, mixing was
unsatisfactory, next “Dean Flow”
concept is integrated to achieve
higher mixing
• Interlocking serpentine channel is
integrated.
• Each cell contains 16 interlocking
spirals.
• Optimized the cell for interlocking
spiral channel.
Figure: Interlocking microchannel after the serpentine channel

11. Simulation – Spiral channel length
Flow rate
(mL/hr)
Mixing Index for one
cell
(%)
Mixing Index for two
cell
(%)
0.1 99.96 99.95
1 96.36 97.01
10 93.93 94.44

12. Metrics and success
• Inlet with division is considered for the final design
• Two turns of the serpentine channel is considered
• Two cells of interlocking spiral is considered in final
design
Figure: Considered geometry for 2D and 3D simulation
comparison
Overall footprint: 10.25 * 10.42 mm

13. 2D simulation
Challenges:
• Dean flow can not be fully captured in 2D simulation
• Maximum mesg size
was used as 0.01 mm
and minimum was
used as 0.0000154
mm
• Mixing index was
calculated as 94.45%
Figure: Concentration distribution for flow rate 10 mL/hrFigure: 2D contour plot of surface concertation

14. 3D simulation
• Fine mesh has been
used for the 3D
simulation
• Mixing index was found
as 99.92%
Challenges:
• 3D simulation can capture the effect of “Dean Flow”. However, using fine mesh,
mixing is observed at the serpentine channel
• By refining mesh, result doesn’t converge.
Figure: 3D contour plot of surface concertation Figure: Concentration distribution for flow rate 10 mL/hr

15. Comparison between 2D and 3D simulations
For comparison, flow rate 10 mL/hr has been considered.
Mixing Index
(%)
Pressure drop
(psi)
Remarks/Challenges
2D 94.45 4.53
Difficult to capture dean
flow
3D 99.92 60.34 Limitation in meshing

16. Fabrication and design recommendation
• The smallest feature in the design has dimension of 50 micron.
• One layer design – so one mask will be sufficient for fabrication
Recommendation:
• To run at least one complete 3D simulation with small mesh element size to
see how dean flow affects. My plan is to run the simulation only for spiral
channel without considering the serpentine channel.
• Based on the observation, I may switch the position of the serpentine channel
and spiral channel.

17. Thank you