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AN INTRODUCTION TO
MICROFLUIDICS :
Lecture n°1
Patrick TABELING, [email protected]
ESPCI, MMN, 75231 Paris
0140795153
Outline of Lecture 1
1 - Past and present of microfluidics
2 - Microfluidics, nanofluidics and macroscopic approach.
3 - The changes in the balances of forces that result from
miniaturization.
SOME REFERENCES
Oxford Univ Press
Translation by Suelin CHEN
Oxford University Press
To appear, 20 Oct 2005
MEMS =
MICRO ELECTROMECHANICHAL SYTEMS
Systems whose sizes lie in the range 1 -300 microns
A new situation arose in the seventies, further to the tremendous
development of microelectronics : it became possible to fabricate
all sorts of miniaturized objects : microcondensators,
microvalves, micropumps, microresonators, microdispenser...
by exploiting an important accumulation of technological
knowledge, and taking advantage of the availability of
sophisticated equipment.
This generated a substantial economical
activity
Airbag Sensor
- Analog Device
300 mm
3 mm
Commercial Inkjet using MEMS technology
2 mm
Perhaps, everything started with a talk given by R. Feynman….
There's Plenty of Room at the Bottom
An Invitation to Enter a New Field of Physics
R Feynman, CALTECH, Dec 1959
I would like to describe a field, in which little has been done, but in
which an enormous amount can be done in principle. This field is not quite
the same as the others in that it will not tell us much of fundamental
physics (in the sense of, ``What are the strange particles?'') but it is
more like solid-state physics in the sense that it might tell us much of
great interest about the strange phenomena that occur in complex
situations.
Furthermore, a point that is most important is that it would have an
enormous number of technical applications.
Micro-Electro-Mechanical -System
MEMS
First Silicon Beams
1988
Spring
Howe & Muller 1982
1982
Fan, Tai & Muller, 1988
First micromotor
(1989)
Fan,Tai and Muller 1989
Quic kT ime™ et un
déc ompres seur Cinepak Codec by Radius
s ont requis pour visi onner c ette image.
Quic kT ime™ et un
déc ompres seur Cinepak Codec by Radius
s ont requis pour visi onner c ette image.
Insect spinning on a micromotor
100 mm
Craighead (Cornell)
HOW DO WE FABRICATE
A MEMS ?
Si
Microfabrication of a
membrane
Si
Oxydation
Si
Développement
Si
Si
Ouverture et strippage
Depot de resine
masque
Si
Insolation
Si
Attaque par KOH
Microfluidics = Realization and study of flows
and transfers in (artificial) microsystems
A few milestones
1970 - 1990 : Essentially nothing (apart from the Stanford gas
chromatographer)
1990 : First liquid chromatograph (Manz et al)
mTAS concept (Manz, Graber, Widmer, Sens.Actuator, 1991)
1990 -1998 : First elementary microfluidic systems (micromixers,
microréactors, separation systems,..)
1998-2004 : Appearance of soft lithography technology, which
fostered the domain. All sorts of microfluidic systems with various
levels of complexity are made, using different technologies
First microfluidic system : Terry (1975) (Stanford)
Injection valve
Canal de 1.5 m long
Thermal
sensor
Reyes et al, Anal Chem, 74, 2623 (2002)
A microfluidic system for DNA separation
From AgilentCaliper
Allow to characterize DNA
Fragments with excellent
resolution, and in a small
time
A system which will probably have an impact in biology
Les opérations élémentaires
Chargement, compartimentage
Mélange, purge.
(Quake et al, Science 2002)
An elementary
Lab-on-a-chip
LAB-ON A CHIP BIOSITE
DIAGNOSES
HEART ATTACK
WITHIN 10 MN
PERSPECTIVES OF MICROFLUIDICS
Microfluidics is increasingly used in an impressive number
of domains
- Food industry
- Chemistry
- Biotechnology
- Oil industry
- Drug discovery
In these domains, microfluidic systems of various complexities are
needed, and the challenge is to be able to respond to these needs.
Current estimates indicate microfluidic demands will grow at a
fast rate over the next 5 years, generating visible economical activity
One day, we’ll perhaps receive this strange watch
as a birthday gift
It is not sure however we will be capable soon to
mimick a number of natural systems
The tree
The spider
FLUIDS FLOWING IN NANOMETRIC DEVICES
- NANOFLUIDICS
Nanofluidics
1nm
10nm
Single
molecule
100nm
1mm
1mm
10mm
Microfluidics
100mm 1mm
Two admissible definitions of nanofluidics
Definition 1 (engineer definition) :
Nanofluidics deals with fluids flowing in systems whose
Characteristic sizes range between 10 and 300 nm
Definition 2 (physicist definition) :
Nanofluidics deals with fluids flowing in conditions where
interactions between micro and macroscopic scales play a
crucial role.
Some notions on the ranges of influence of
Intermolecular microscopic forces
MOSY OF WHAT
WE KNOW ON
THE BEHAVIOUR
OF SIMPLE
LIQUIDS AT THE
NANOSCALE
COMES FROM
THIS MACHINE
(Tabor,
Israelachvilii
~1980)
This is not the case for the Van der Waals forces between
surfaces in the vacuum, whose extent lies in the nm range
FORCES LINKED TO THE PRESENCE OF ADSORBED LAYERS
Debye layers may have sizes comparable to
Submicrometric channels.
In the presence of an electrolyte, Debye layers develop
DEBYE-HUCKEL layers - typically 100 nm
up to 1mm thick in pure water
Mean free path in gases
Thermal capillarity length
Nanofluidics is a host of
Many novel phenomena,
Involving interactions between
Microscopic and macroscopic scales
Bubble nucleation barrier
Debye layer thickness
Fluctuation forces range
VdW force range
Nanofluidics
1nm
10nm
Single
Molecule
studies
100nm
1mm
10mm
100mm 1kmm
Microfluidics
BREAKUP OF A NANOJET ( NUMERICAL EXPERIMENTS)
M. Moseler, U. LandmanScience, 289, 5482, 1165 - 1169 (2000)
Nanojets do not behave like ordinary jets
Microjet
Nanojet
The reason is that capillary thermal scale matters : l=(kT/g)1/2
Working with negative pressures becomes feasible
Macroscopic approach generally assumes that the interfaces
are infinitely thin
Boundary conditions
Laplace law
Speculating about possible effects in nanochannels
Laminar flow are not parabolic; they probe the nature
of the surfaces exposed to the fluid
Free interfaces behave in a strange way in nanochannels
Hydrodynamic instabilities behave differently
Fabricate superfluid hydrogen.
500 nm
Nanofluidics is not just an exotic subject : we already
use nanofabricated nanochannels in a number of
applications
Separation of long strands of DNS by usine nanopillars
(Baba et al, Univ. Tokyo)
A broad prospective on nanofluidics (from A. Van den Berg)
Physical aspects of microfluidics
Mean free path in gases
Thermal capillarity length
There exists interactions between
microscopic and macroscopic scales
in microfluidic systems
Bubble nucleation barrier
Debye layer thickness
Fluctuation forces range
VdW force range
Nanofluidics
1nm
10nm
Single
Molecule
studies
100nm
1mm
10mm
100mm 1kmm
Microfluidics
Experiment by S. Chu et al (1994)
The cell and a number of its components
have sizes comparable to microsystems
Cells can be
manipulated individually in microfluidic
systems.
PLAYING WITH
CELLS AND
CONCENTRATION
GRADIENTS
Cell sorting (Quake et al, 2000)
There exist microscopic scales which are comparable to microsystem sizes
The mean free path in gases may reach micrometers
The notion of fluid particle in hydrodynamics
(According to Batchelor)
l should be much smaller than the system size for ordinary
Hydrodynamics to apply :
Kn 
l
L
1
Gas flow regimes
MICROFLUIDICS
0.1
« Ordinary »
hydrodynamic
regime
Slip flow
regime
0.6
20
Kn
Transitionnal
regime
Rarefied gas
regime
Mercury
column
Pressure
sensor I
Pressurized
gas tank
Pressure
sensor O
PI
(2)
(3)
(4)
(1)
Variable
resistance
PO
vacuum
pump
Théory with
s ~ 0.9
9
8
7
S
6
5
4
3
2
Channel
1.14±0.02 mm
in heigth
200 mm wide
1+6Kn
1
0
0,2
0,4
0,6
0,8
1
Kn
12 mRTLQm
S
3
PPm wb
S=1
“Ordinary” hydrod.
J.Maurer et al (2002)
RECENT NUMERICAL SIMULATIONS
INDICATE THAT ORDINARY HYDRODYNAMICS
IS RECOVERED IN THE SUBMICRON RANGE
THE PHYSICS OF MINIATURIZATION
The spectacular changes of the balances of forces as
we go to small scales.
Scaling laws
Remarks
- Animal maximum speeds do not depend on the scale
- But the fluid velocity, at low Reynolds numbers, varies as
the scale.
All animals run at the same speed
Lower members oscillate with a period
T~l
Velocity is V~l/T ~l0, size independent
A mechanical example of a scaling law
Vibration frequency of a Cantilever beam
hc
f 
2
2L
h
f ~l
1
L
At what speed does the Thyrannosaurus run ?
20 m/s ?
11 m/s
An apparently controversial issue
J.R. Hutchintson, M. Garcia, Nature, 415, 1018 (2002)
Reasonings on the physics of miniaturization
Méthod 1 : Compare the exponents of the scaling laws. The
smallest “wins”.
Example : Insects are easily caught by water drops
Fmusc~ l2, Fcap ~ l
Fmusc << Fcap
Méthod 2 : More accurate : using P theorem
Consider a physical quantity function of n other quantities
a = f(a1,a2,…..an)
In a system with k dimensions.
We are thus dealing with n+1 quantities
The physical law reduces to a simpler expression :
P=g(P1, P2,…. Pn+k-1)
Involving n-k+1 variables instead of n+1
Example :
Hydrodynamic flows, characterized by a single
scale, have a velocity field which satisfies :
u = U g(x/l,Re)
Reynolds number = Ul/n
As we miniaturize, the Reynolds number goes to zero, and
thus one may conclude that in microfluidic systems, flows
are laminar and stable.
Argument :
u(x) = f(x,U,r,m,l)
n+1=6
k=3
On peut donc définir 6-3 nombres sans dimensions
x

u
 g ,Re
l

U
Avec Re=Ul/n, le nombre de Reynolds
Analysis of a microjet
100 m m
2
mU
r
a
g
2
Re 
 10 ; Ca 
 10
; Bo 
 103
n
g
g
Ua
Conclusion : le jet est laminaire (donc facilement controlable),
les gouttes sont sphériques et la gravité est négligeable
Scaling laws in nature
Reasonings on scaling laws are often used to explain a
number of apparently strange phenomena in nature
The smallest size of the mammifers
Thermal power lossed by conduction with the environnement,
for a fixed T ~ T l
Power extracted from the digestion of the food ~ N l3
To reach a steady temperature, loss and gain must balance :
l ~ (T/N)1/2
Since one cannot take an infinite number of meals per day, one
cannot miniaturize mammifers at will
Smallest mammifer is ~2 cm
2cm
Smallest mammifer :
The pygmee shred
The shred
Advantages being miniaturized :
jump high (H~ l0),
walk on water
Disadvantage : being easily caught by a water drop
Scaling laws for the
electrostatic micromotor
Scheme of the electrostatic micromotor
E
Torque C ~ Fl ~ l
P  C ~ l
3
Small torque, small power
(unless we rotate fast)
3
Réalisation d’un micro moteur
Sacrificial Etching
Sacrificial Layer
Structure Layer
MIT micro-turbine project
- Diameter-heigth : 12 mm/3mm
- Air flow-rate : 0.15 g/s
- Outlet temperature : 1600 K
-Rotation speed: 2.4 106 tr/mn
- Power : 16W
- Weight : 1g
- Fuel consumption : 7g/h
Some words….
Limits of the scaling arguments
1)- The detailed factors coming with the scaling laws
Their analysis allows to determine the range of validity of the
reasoning.
2) The spatial structure of the forces at hand.
CONCLUSIONS OF LECTURE 1
1 - Microfluidics is an interdisciplinary domain, driven by
applications (existing or potential), in which interesting physics
can be done
2 - Most of the phenomena taking place in microsystems can be
described in a macroscopic framework; however, for a number
of systems (gases, macromolecules,..) the microscopic scales
interfere directly with the microsystem size.
3 - Balances of forces are deeply modified as we go from the
ordinary to the micro world. Reasoning on scaling laws is a
powerful approach to anticipate the changes one may expect
from miniaturizing a given system.