micro fluid i c design principles

7/30/2019 Micro Fluid i c Design Principles

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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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Next: Hands on learning

Day 1

Multiplexed Mixer


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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


6/6622 November 2010 Intro to Microfluidics 6

Microfluidics

at a low level

Primary Tasks

Input Selection

Flow distribution

Mixing

Storage

Primary elements

Channel

Valve

Chamber



Course chip designs decomposed

Lines

Curves

Stars


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Part 1

Distribution: design of on-chip flow



Lines and Curves

Flow

Control

Color convention

The most important elements of a microfluidic

deviceChannels for stuff



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



Challenging geometry

Large Flat Chambers

hw

w >> 10 h

side

top

lCollapse zone



Challenging geometry

Large Flat Chambers

hw

w >> 10 h

side

top

l

Solution: posts



Typical channel geometry

width

Height/depth

Length100um

10um

whatever you need

it to be

These dimensions arespecial



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



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



Laminar flow

Fluid velocity profile in an infinitelywide channel

z

x

h

L

vx

P 1 P 2

1Re



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?



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


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


Control channel

w (100um, typical)

w

Overhang ~ 20-30um


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



Cross-over: does not produce valve


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Crossing over



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


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



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Cleaner MUX



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Cleaner MUX



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Valves controlling valves -

latching



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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



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Valve enhanced mixing



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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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micro fluid i c design principles

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