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AN INTRODUCTION TO MICROFLUIDICS Lecture n1
Patrick TABELING, patrick.tabeling_at_espci.fr ESPCI,
MMN, 75231 Paris 0140795153
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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.
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SOME REFERENCES
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Translation by Suelin CHEN Oxford University
Press To appear, 20 Oct 2005
Oxford Univ Press
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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.
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This generated a substantial economical activity

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Airbag Sensor - Analog Device
300 mm
3 mm
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Commercial Inkjet using MEMS technology
2 mm
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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


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Micro-Electro-Mechanical -System
MEMS
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First micromotor (1989)
Insect spinning on a micromotor
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100 mm
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Craighead (Cornell)
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HOW DO WE FABRICATE A MEMS ?
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Microfabrication of a membrane
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Microfluidics came later, in the nineties
Microfluidics Realization and study of flows
and transfers in (artificial) microsystems
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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
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First microfluidic system Terry (1975)
(Stanford)
Injection valve
Canal de 1.5 m long
Thermal sensor
Reyes et al, Anal Chem, 74, 2623 (2002)
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A microfluidic system for DNA separation
From Agilent- Caliper Allow to characterize DNA
Fragments with excellent resolution, and in a
small time
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A system which will probably have an impact in
biology
Les opérations élémentaires
Chargement, compartimentage Mélange, purge.
(Quake et al, Science 2002)
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An elementary Lab-on-a-chip
LAB-ON A CHIP
BIOSITE
DIAGNOSES HEART ATTACK WITHIN 10 MN
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PERSPECTIVES OF MICROFLUIDICS
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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
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One day, well perhaps receive this strange
watch as a birthday gift
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It is not sure however we will be capable soon
to mimick a number of natural systems
The tree
The spider
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FLUIDS FLOWING IN NANOMETRIC DEVICES-
NANOFLUIDICS
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Nanofluidics
1nm
100nm
100mm
10nm
1mm
1mm
1mm
10mm
Microfluidics
Single molecule
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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.
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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)
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This is not the case for the Van der Waals forces
between surfaces in the vacuum, whose extent
lies in the nm range
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FORCES LINKED TO THE PRESENCE OF ADSORBED LAYERS
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Debye layers may have sizes comparable
to Submicrometric channels.
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In the presence of an electrolyte, Debye layers
develop
DEBYE-HUCKEL layers - typically 100 nm up to 1mm
thick in pure water
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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
100nm
1kmm
10nm
10mm
1mm
100mm
Microfluidics
Single Molecule studies
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BREAKUP OF A NANOJET ( NUMERICAL EXPERIMENTS)
M. Moseler, U. LandmanScience, 289, 5482, 1165 -
1169 (2000)
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Nanojets do not behave like ordinary jets
Microjet
Nanojet
The reason is that capillary thermal scale
matters l(kT/g)1/2
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Working with negative pressures becomes feasible
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Macroscopic approach generally assumes that the
interfaces are infinitely thin
Boundary conditions
Laplace law
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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.
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500 nm
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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)
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A broad prospective on nanofluidics (from A. Van
den Berg)
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Physical aspects of microfluidics
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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
100nm
1kmm
10nm
10mm
1mm
100mm
Microfluidics
Single Molecule studies
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Experiment by S. Chu et al (1994)
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The cell and a number of its components have
sizes comparable to microsystems
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Cells can be manipulated individually in
microfluidic systems.
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PLAYING WITH CELLS ANDCONCENTRATIONGRADIENTS
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Cell sorting (Quake et al, 2000)
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There exist microscopic scales which are
comparable to microsystem sizes
The mean free path in gases may reach micrometers
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The notion of fluid particle in hydrodynamics
(According to Batchelor)
l should be much smaller than the system size for
ordinary Hydrodynamics to apply
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Gas flow regimes
MICROFLUIDICS
Kn
0.1
0.6
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 Ordinary  hydrodynamic regime
Transitionnal regime
Slip flow regime
Rarefied gas regime
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Rarefied gas effects can be substantial in small
microchannels
Mercury column
Pressure sensor I
Pressure sensor O
Pressurized gas tank
PI
PO
(2)
(3)
(4)
vacuum pump
(1)
Variable resistance
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Rarefied gas effects can be substantial in small
microchannels
Théory with s 0.9
16Kn
Channel 1.140.02 mm in heigth 200 mm wide
S1 Ordinary hydrod.
J.Maurer et al (2002)
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RECENT NUMERICAL SIMULATIONS INDICATE THAT
ORDINARY HYDRODYNAMICS IS RECOVERED IN THE
SUBMICRON RANGE
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THE PHYSICS OF MINIATURIZATION
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The spectacular changes of the balances of forces
as we go to small scales.
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Scaling laws
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Remarks - Animal maximum speeds do not depend on
the scale - But the fluid velocity, at low
Reynolds numbers, varies as the scale.
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All animals run at the same speed
Lower members oscillate with a period T
l Velocity is Vl/T l0, size independent
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A mechanical example of a scaling law
Vibration frequency of a Cantilever beam
h
L
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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)
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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 ltlt Fcap
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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 n1 quantities The
physical law reduces to a simpler expression
Pg(P1, P2,.
Pnk-1) Involving n-k1 variables instead of n1
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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.
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Argument
u(x) f(x,U,r,m,l)
n16 k3
On peut donc définir 6-3 nombres sans dimensions
Avec ReUl/n, le nombre de Reynolds
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Analysis of a microjet
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Conclusion le jet est laminaire (donc
facilement controlable), les gouttes sont
sphériques et la gravité est négligeable
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Scaling laws in nature
Reasonings on scaling laws are often used to
explain a number of apparently strange phenomena
in nature
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The smallest size of the mammifers
Thermal power lossed by conduction with the
environnement, for a fixed DT DT l Power
extracted from the digestion of the food N
l3 To reach a steady temperature, loss and gain
must balance l (DT/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
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Advantages being miniaturized jump
high (H l0), walk on water
Disadvantage being easily caught by a water
drop
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Scaling laws for the electrostatic micromotor
Scheme of the electrostatic micromotor
E
Small torque, small power (unless we rotate fast)
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Réalisation dun micro moteur
Sacrificial Etching
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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
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Some words.
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Limits of the scaling arguments
We forgot
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.
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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.