Magnetic polymer actuators for microfluidics

161 Pages·2017·6.11 MB·English

Magnetic polymer actuators for microfluidics
Citation for published version (APA):
Fahrni, F. (2009). Magnetic polymer actuators for microfluidics. [Phd Thesis 1 (Research TU/e / Graduation
TU/e), Applied Physics and Science Education]. Technische Universiteit Eindhoven.
https://doi.org/10.6100/IR654208
DOI:
10.6100/IR654208
Document status and date:
Published: 01/01/2009
Document Version:
Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the version of the article upon submission and before peer-review. There can be
important differences between the submitted version and the official published version of record. People
interested in the research are advised to contact the author for the final version of the publication, or visit the
DOI to the publisher's website.
• The final author version and the galley proof are versions of the publication after peer review.
• The final published version features the final layout of the paper including the volume, issue and page
numbers.
Link to publication
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners
and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
• You may not further distribute the material or use it for any profit-making activity or commercial gain
• You may freely distribute the URL identifying the publication in the public portal.
If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please
follow below link for the End User Agreement:
www.tue.nl/taverne
Take down policy
If you believe that this document breaches copyright please contact us at:
[email protected]
providing details and we will investigate your claim.
Download date: 07. Feb. 2023
Magnetic polymer actuators
for micro(cid:29)uidics
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van de
rector magni(cid:28)cus, prof.dr.ir. C.J. van Duijn, voor een
commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen
op donderdag 17 december 2009 om 16.00 uur
door
Francis Fahrni
geboren te Lausanne, Zwitserland
Dit proefschrift is goedgekeurd door de promotoren:
prof.dr.ir. M.W.J. Prins
en
prof.dr.ir. J.M.J. den Toonder
Copromotor:
dr. L.J. van IJzendoorn
A catalogue record is available from the Eindhoven University of Technology
Library
ISBN: 978-90-386-2103-6
Copyright ' 2009 by F. Fahrni
Printed by the Eindhoven University Press, the Netherlands.
The research described in this thesis forms part of the research programme of the
Dutch Polymer Institute (DPI), Technology Area Functional Polymer Systems,
DPI project #532.
Contents
Summary v
1 Introduction 1
1.1 Micro(cid:29)uidics and its applications . . . . . . . . . . . . . . . . . . . 1
1.2 Integrated (cid:29)uid actuation . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Mixing in micro(cid:29)uidics . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Magnetic actuation . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Polymer micro-actuators . . . . . . . . . . . . . . . . . . . . . . . . 6
1.6 Device concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.7 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 Magnetic micro-actuation 13
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Miniaturising magnetic actuation . . . . . . . . . . . . . . . . . . . 13
2.3 Magnetism and magnetic particles . . . . . . . . . . . . . . . . . . 19
2.3.1 Ferromagnetism and superparamagnetism . . . . . . . . . . 21
2.3.2 Selection of magnetic particles . . . . . . . . . . . . . . . . 23
2.3.3 Limitations of magnetic particles . . . . . . . . . . . . . . . 24
2.4 Elastomers with magnetic particles . . . . . . . . . . . . . . . . . . 25
2.4.1 PDMS as elastomer for micro-actuators . . . . . . . . . . . 25
2.4.2 Dispersion of magnetic particles in PDMS . . . . . . . . . . 28
2.4.3 Elastic modulus of PDMS with magnetic particles . . . . . 31
2.4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.5 Scaling behaviours of magnetic actuation . . . . . . . . . . . . . . 34
2.5.1 Expressions for magnetic force and torque . . . . . . . . . . 34
2.5.2 Magnetic forces for dispersed particles . . . . . . . . . . . . 37
2.5.3 Scaling behaviour of the magnetic gradient force . . . . . . 38
2.5.4 Scaling behaviour of the magnetic torque . . . . . . . . . . 43
2.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
ii Magnetic polymer actuators for micro(cid:29)uidics
3 Magnetic particle interactions in ferro(cid:29)uids and polymers 51
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2 The Langevin model . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.3 Inter-particle interactions in ferro(cid:29)uids . . . . . . . . . . . . . . . . 54
3.4 Inter-particle interactions in polymers . . . . . . . . . . . . . . . . 58
3.5 Induced magnetic anisotropy in polymers . . . . . . . . . . . . . . 61
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4 Local actuation of superparamagnetic actuators 67
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.2 Modelling of the device concept . . . . . . . . . . . . . . . . . . . . 68
4.2.1 Geometry of the device . . . . . . . . . . . . . . . . . . . . 68
4.2.2 Heat dissipation in a current wire and related scaling. . . . 71
4.2.3 De(cid:29)ection induced by a current wire . . . . . . . . . . . . . 75
4.3 Fabrication and experimental methods . . . . . . . . . . . . . . . . 79
4.3.1 Lithographic structuring of a high aspect ratio mould . . . 80
4.3.2 High aspect ratio actuators by mould replication . . . . . . 84
4.3.3 Integrated current wires . . . . . . . . . . . . . . . . . . . . 86
4.3.4 Device assembly . . . . . . . . . . . . . . . . . . . . . . . . 88
4.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.4.1 Maximum current pulse in micro-fabricated wires . . . . . . 89
4.4.2 Temperature dependent swelling of PDMS in solvents . . . 92
4.4.3 De(cid:29)ection of micro-actuators close to current wires . . . . . 93
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5 External actuation of superparamagnetic actuators 101
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.2 Superparamagnetic and ferromagnetic torque . . . . . . . . . . . . 102
5.3 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . 105
5.3.1 Quadrupole setup for rotating magnetic (cid:28)elds . . . . . . . . 105
5.3.2 Setup for actuation in (cid:29)uid and image analysis . . . . . . . 109
5.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . 110
5.4.1 Micro-actuators in a rotating magnetic (cid:28)eld . . . . . . . . . 110
5.4.2 Amplitude of de(cid:29)ection of micro-actuators . . . . . . . . . . 114
5.4.3 De(cid:29)ection of micro-actuators at higher frequencies . . . . . 115
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Magnetic polymer actuators for micro(cid:29)uidics iii
6 Ferromagnetic actuators and induced (cid:29)uid manipulations 119
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6.2 Fabrication and experimental methods . . . . . . . . . . . . . . . . 120
6.2.1 Fabrication of high aspect ratio lying arti(cid:28)cial cilia . . . . . 120
6.2.2 Actuation experiments in (cid:29)uid . . . . . . . . . . . . . . . . 123
6.2.3 Closedmicro(cid:29)uidicchannelandparticletrackingexperiments123
6.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . 124
6.3.1 Magnetisation of ferromagnetic PDMS composite . . . . . . 124
6.3.2 Actuation of arti(cid:28)cial cilia . . . . . . . . . . . . . . . . . . . 125
6.3.3 Fluid manipulations by arti(cid:28)cial cilia . . . . . . . . . . . . . 135
6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
References 139
Acknowledgements 147
Curriculum Vitae 149

Magnetic polymer actuators for micro(cid:29)uidics
Summary
The manipulation of (cid:29)uids on the sub-millimetre scale (cid:21) micro(cid:29)uidics (cid:21) (cid:28)nds ap-
plication in the miniaturisation and integration of biological analysis, chemical
synthesis, optics and information technology. In a micro(cid:29)uidic device, (cid:29)uids need
tobetransported,mixed,separatedanddirectedinandthroughamicro-scalesys-
tem. Thee(cid:30)cientmixingof(cid:29)uids(cid:21)particularlyneededforanalysisorsynthesis(cid:21)
presentsalargechallengeinmicro(cid:29)uidics. Mixingcannotoccurbyturbulencebe-
cause of the low Reynolds number that prevails in micro-channels, and molecular
di(cid:27)usion is rather slow in achieving mixing on the scale of a micro(cid:29)uidic channel.
A solution for obtaining mixing on e(cid:30)cient time scales has been to passively or
activelymanipulatethe(cid:29)uidstoinducechaoticadvectionandincreasetheinterfa-
cialareaoftwo(cid:29)uidsprogressively,therebydecreasingthelengthscaleoverwhich
di(cid:27)usion has to take place to mix the (cid:29)uids.
In this thesis we investigate magnetic polymer micro-actuators that can be
incorporated on the walls of micro(cid:29)uidic channels and can be actuated with mag-
netic (cid:28)elds. A magnetic stimulus that addresses micro-actuators is very robust,
because of the low interaction between magnetic (cid:28)elds and (bio)chemical (cid:29)uids.
The use of polymeric materials for producing micro-actuators potentially allows
for cost-e(cid:27)ective micro-devices with integrated (cid:29)uidic actuation. The aim of the
thesis is to provide generic and advanced (cid:29)uid control inside micro(cid:29)uidic devices,
e.g. for the purpose of integrated pumping or for the purpose of mixing.
Superparamagnetic and ferromagnetic particles have been dispersed in poly-
mers with a low elastic modulus and the composites have been characterised me-
chanically and magnetically. A low elastic modulus polymer enables large de-
(cid:29)ections of micro-actuators with practical magnetic (cid:28)elds. In this thesis, various
typesoftheelastomerpolydimethylsiloxane(PDMS)havebeenusedforconstruct-
ing the polymeric micro-actuators with a low elastic modulus. The e(cid:30)ciency of
magnetic actuation on small scales is discussed for two actuator concepts. It is
shown that actuation by magnetic torque scales neutrally with miniaturisation,
allowing for actuation with externally generated magnetic (cid:28)elds. In contrast, ac-
vi Magnetic polymer actuators for micro(cid:29)uidics
tuation by magnetic gradient force scales inversely proportional to the size of the
actuator. Thereforemagneticgradientactuationcannotbeinducedbyanexternal
electromagnet and requires a localised generation of magnetic (cid:28)elds. Because vis-
cous e(cid:27)ects dominate over inertial e(cid:27)ects at small scales (Reynolds number < 1),
symmetric and in-phase movements of micro-actuators would induce no net (cid:29)uid
(cid:29)ow. Thereforethegenerationofasymmetricorout-of-phasemovementsofmicro-
actuators has been investigated for each actuator concept.
The interaction of magnetic particles embedded in PDMS has been studied
and compared to the interactions in a ferro(cid:29)uid. The enhancement of magnetic
susceptibilityduetotheparticleinteractionswasfoundtobelimitedbecauseofthe
clusteringofmagneticparticlesinthepolymerthatinduceslocaldemagnetisation.
The alignment of clusters of particles in a magnetic (cid:28)eld was investigated and
the resulting magnetic anisotropy was quanti(cid:28)ed. Modelling has established that
such an intrinsic magnetic anisotropy for an actuator can provide an increase in
actuation amplitude up to one order of magnitude, for the same stimulus.
The magnetic PDMS composites developed in this thesis have been used to
fabricate high aspect ratio micro-actuators that are standing or lying on a sub-
strate. Standing superparamagnetic PDMS micro-actuators were produced by
mould replication. The standing micro-actuators have been actuated locally with
thehighmagnetic(cid:28)eldgradientgeneratedbyanintegratedcurrentwire(resulting
in actuation by magnetic gradient force). The local stimulus allows for individual
addressingofthemicro-actuatorsandpotentiallyenablesout-of-phasemovements
of adjacent actuators. Possible geometries for the actuator device have been ex-
plored with models that describe the de(cid:29)ection of the actuators and the heat
dissipation in the current wire. The fabricated micro-actuators were found to re-
spondtothemagneticstimulusofthecurrentwirebutalsotothethermalstimulus
associated to the heat dissipation in the current wire, because of temperature de-
pendent swelling of the micro-actuators in a solvent. The di(cid:27)erent time scales of
magneticandthermalactuationallowedthecreationofanasymmetricmovement.
Thestandingmicro-actuatorshavealsobeenactuatedbyahomogeneousmag-
netic(cid:28)eldgeneratedbyanexternalelectromagnet(resultinginactuationbymag-
netictorque). Anon-constantphaselagwasdemonstratedbetweenactuatorshav-
ingdi(cid:27)erentamplitudesofde(cid:29)ection,whichcanpotentiallyprovidee(cid:30)cientmixing
onsmallscales. Thehighfrequencyactuationofthestandingmicro-actuatorswas
found to be limited to 5 Hz, which we attribute to the viscous behaviour of the
PDMS.
Lying ferromagnetic PDMS micro-actuators were produced with lithographic
and sacri(cid:28)cial layer techniques. The lying micro-actuators have been actuated by
a homogeneous magnetic (cid:28)eld generated by an external electromagnet (resulting
Magnetic polymer actuators for micro(cid:29)uidics vii
in actuation by magnetic torque). The permanent magnetisation of the actuators
allowed for much larger de(cid:29)ections than for the standing superparamagnetic ac-
tuators. For a speci(cid:28)c initial magnetisation of the actuators and using a rotating
magnetic (cid:28)eld, the actuators were shown to exhibit selectively either a symmetric
or an asymmetric movement. The actuation at high frequencies of the micro-
actuators was limited by the viscous drag in (cid:29)uid and, in our experiments, by the
high frequency limitations of the electromagnet. The micro-actuators could oper-
ateuptoafrequencyof50Hz,whichisoneorderofmagnitudehigherthanforthe
standing superparamagnetic actuators. The higher actuation frequency indicated
that the type of PDMS used to fabricate the lying ferromagnetic micro-actuators
exhibits less viscous behaviour.
In a micro(cid:29)uidic cavity, the lying ferromagnetic micro-actuators induced local
vortices or translational net (cid:29)uid (cid:29)ows, depending on their initial magnetisation.
Two micro-actuators pointing in opposite directions were actuated fully indepen-
dently with the same external stimulus, depending on the rotation direction of
the magnetic (cid:28)eld. The di(cid:27)erent re-magnetisation in each case could explain the
possibility for individual actuation. Provided with this independent actuation,
two sets of vortices can be controlled individually in a micro(cid:29)uidic device, which
is particularly promising to mix (cid:29)uids with e.g. a blinking vortex protocol. The
observed translational net (cid:29)uid (cid:29)ows can in principle provide integrated pumping
in micro(cid:29)uidic devices.