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22 November 2010Intro to Microfluidics
Day 2
Microfluidic
Design Principles
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22 November 2010 Intro to Microfluidics 2
Microfluidic
device fabrication
concept CAD photolithography molding finishing
Course Instructors Stanford You the students
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22 November 2010 Intro to Microfluidics 3
Next: Hands on learning
Day 1
Multiplexed Mixer
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22 November 2010 Intro to Microfluidics 4
Next: Hands on learning
Day 2
Addressable Array
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Designing your own chips
One size doesnot fit all
Microfludic chips are designed to suit specific needs
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Microfluidics
at a low level
Primary Tasks
Input Selection
Flow distribution
Mixing
Storage
Primary elements
Channel
Valve
Chamber
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Course chip designs decomposed
Lines
Curves
Stars
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Part 1
Distribution: design of on-chip flow
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Lines and Curves
Flow
Control
Color convention
The most important elements of a microfluidic
deviceChannels for stuff
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Typical channel geometry
width
Height/depth
Length100um
10um
whatever you need
it to be
IMPORTANTHeight : Width = 1 : 10
(or larger)
Can create challenges for design
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Challenging geometry
Large Flat Chambers
hw
w >> 10 h
side
top
lCollapse zone
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Challenging geometry
Large Flat Chambers
hw
w >> 10 h
side
top
l
Solution: posts
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Typical channel geometry
width
Height/depth
Length100um
10um
whatever you need
it to be
These dimensions arespecial
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Fluid flow at the microscaleGoverning Equation
vgvvv 2
+=
+
pt
vgvvv 2
Re
1+=+
N p
t
(make dimesionless)
(Navier-Stokes Equation)
For water flow at 1mm/s at 25C through achannel 10m deep:
N Re = 0.01
Inertial (turbulent) flow
Viscous (laminar) flow
vh N =Re
The Reynolds Number
Viscocity
dominated flow
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Deriving further
Assumptions
No local acceleration
Constant density
Incompressible Newtonian fluid
Velocity components only in direction of bulkflow
2
21
dzvd
dxdp x
=
Fluid velocity profile in an infinitelywide channel
z
xh
L
vx
P 1 P 2
Governing Equation
vgvvv 2
+=
+
pt
Forced bypressure
Speed depends only ondepth/height
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Laminar flow
Fluid velocity profile in an infinitelywide channel
z
x
h
L
vx
P 1 P 2
1Re
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Governing Equation
Solving for constant pressure drop:
4
8 LP Q r
=
3
12 LP Q
wd =
Poiseuille (Laminar) flow
Circular Channel
Rectangular Channel
2
21
dzvd
dxdp x
=
z
xh
L
vx
P 1 P 2
V I R= Ohms lawmicrofluidic systems can bedesigned like linear electricalcircuits
Does this look familiar?
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Analog
microfluidic
circuitry
Battery
Resistor
Increase flow/current
Decrease flow/current
Pressure
Channel length/geometry
Channel contraction
Channel expansion
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Test case 2
Which network has equal flow through branches?
A B
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Part 2
Mixing
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Laminar flow and lack of mixing
v = 0
v = 2v = 4
Chemical species in one planeare ignorant
of those in another
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Mixing in purely laminar flow is diffusive
v = 0
v = 2v = 4
Chemical species in one planeare ignorant
of those in another
Injection
point
Molecular
diffusionevident w
Typical diffusiontime:
D
2wt diff
Over short distances
concentrations negligibly mix
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Speeding up diffusion
Solution 1: narrower channels
Diffusion path
Diffusion path
Restrictions?
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Speeding up diffusion
Solution 2: twisted flow
Stroock
et. al. Science (2002)
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Part 3
Input selection
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Simple flow selection
P1
P2
P1
P2
P1 > P2
P1 < P2
Reliable? Robust?
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Input control using laminar flow
To Rest of Chip
To Aux. Waste
To Aux. Waste
To Aux. Waste
Laminar Interface Guidance
Laminar flow interface is coerced across an outputchannel to generate mixing ratios
Excess flow is collected by overflow/bypass channeland sent to waste
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Input control using laminar flow
To Rest of Chip
To Aux. Waste
To Aux. Waste
To Aux. Waste
Laminar Interface Guidance
Laminar flow interface is coerced across an outputchannel to generate mixing ratios
Excess flow is collected by overflow/bypass channeland sent to waste
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Input ratio control A microfluidic
servo: pressure to concentration translation
switching can be binary (off/on) or graded (0, 0.1, 0.2, ... , 0.9, 1.0)
Down stream blending with chaotic mixers
Possible with more than 2 inputs?
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Qin
Q out
P in
Source
Drain
Gate
Microfluidic
Valve MOSFET
Electronic Analogy
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Valves: Up, down, or both
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Valve geometry
Flow channel ValvePad
Control channel
w (100um, typical)
w
Overhang ~ 20-30um
Flow channel ValvePad
Control channel
w
> w
Larger valves better?
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Valve operation
dP
= 10psi
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Crossing over
Cannot routevalve here
Need valve here
Need valve here
Valves under same control
Impossible?
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Valve geometry
Flow channel ValvePad
Control channel
w (100um, typical)
w
Overhang ~ 20-30um
Flow channel ValvePad
Control channel
w
> w
What happens with smaller than w
valves?
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Membrane deflection vs. width
30psi
100 x 100um Pad~2000 uNewtons
30psi100 x 20um Pad
~400 uNewtons
Control
Flow
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Crossing over
Cannot routevalve here
Valves under same control
Cross-over: does not produce valve
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Crossing over
Cannot routevalve here
Valves under same control
More robust Cross-over
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Valve simplified selection
P1
P2
P1
P2
P1 = P2
P1 = P2
V1 = OFF
V2 = ON
V1 = ON
V2 = OFF
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Higher order selection
3 inputs4 inputs
5 inputs6 inputs
16 inputs
3 valves
4 valves
5 valves
6 valves
16 valves?!
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Input Multiplexer (MUX)
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
N inputs selected by only 2*Log2 (N) control valves -
Binary Addressing
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Higher order multiplexing
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
N inputs selected by only N!/(N/2)!^2 control valves(Note: N must be divisible by 2)
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MUX cross contamination
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
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Cleaner MUX
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
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Cleaner MUX
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
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Valves controlling valves -
latching
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
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Valves controlling valves -
latching
Single source for 20control valves (pushdown)
Latches (push-up)control valve state
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Part 4
Pumps, metering, and mixing
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Macrofluidic
pumping
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Microfluidic
peristalsis
See Noels Piano
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Metering
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
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Valve enhanced mixing
Melin & Quake. Ann. Rev. Biophys. Biomol, Struct. (2007)
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Part 5
Storage and filtering
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A simple storage chamber
Issues -
How do you:
controllably capture particles?
access contents without loosing them?
Chamber (when valves are closed)
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A simple storage chamber
Solutions?
Filtering channels
Partially closed valves
Chamber (when valves are closed)
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A chamber with filters
Filters
Chamber
Media
d h
h ~ 0.5*d
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Chamber with partial valve
Not quite a cross over (and/or lower valve pressure)
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The sieve valve
control
flow
glass substrate
control
flow
glass substrate
Sieve pore Allows flow/diffusionBlocks bigger objects
0 psig
30 psig
Rectangular profile
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The sieve valve
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Summary
1:10 h:w
aspect ratio
Only laminar flowFluidic networks designedusing Ohms Law
Mixing by diffusion
Valves are fluidic transistors
Core elements
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Other finer details
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Further Reading
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Further Reading
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