Explosive Micro-Bubble Actuator Dennis van den Broek

175 Pages·2008·10.72 MB·English

Explosive Micro-Bubble Actuator
Dennis van den Broek
The research described in this thesis was carried out at the Transducers Science and
Technology Group of the MESA+ Research Institute at the University of Twente, En-
schede, The Netherlands. The project was financially supported by the Dutch Technology
Foundation (STW).
Promotiecommissie:
Voorzitter
prof. dr. ir. J. van Amerongen Universiteit Twente
Secretaris
prof. dr. ir. A.J. Mouthaan Universiteit Twente
Promotor
prof. dr. M.C. Elwenspoek Universiteit Twente
Referent
prof. dr. A. van Keulen Technische Universiteit Delft
Leden
prof. dr. A. Prosperetti Universiteit Twente
prof. dr. ir. G.J.M. Krijnen Universiteit Twente
prof. dr. C.J. Kim University of California, Los Angeles
prof. dr. D. Lohse Universiteit Twente
prof. dr. T.H. van der Meer Universiteit Twente
van den Broek, Dennis
Explosive Micro-Bubble Actuator
Ph.D. Thesis, University of Twente, Enschede, The Netherlands
ISBN: 978-90-365-2743-9
Copyright (cid:2)c 2008 by D.M. van den Broek, Enschede, The Netherlands
Explosive Micro-Bubble Actuator
proefschrift
ter verkrijging van
de graad van doctor aan de Universiteit Twente
op gezag van de rector magnificus
prof. dr. W.H.M. Zijm,
volgens besluit van het College voor Promoties
in het openbaar te verdedigen
op vrijdag 31 oktober 2008 om 15:00 uur
door
Dennis Micha van den Broek
geboren op 11 augustus 1979
te Vlaardingen
Dit proefschrift is goedgekeurd door de promotor:
prof. dr. M.C. Elwenspoek
aan mijn vader
Contents
1 Actuators and Bubbles 1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Actuation principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Thermo-pneumatic actuation . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3.1 Thermal expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3.2 Phase-change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3.3 Actuator design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Bubble actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.1 Inkjet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.2 Bubble pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.3 Mechanical displacement . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4.4 Electrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Bubble nucleation and bubble dynamics . . . . . . . . . . . . . . . . . . . 6
1.6 Motivation of the work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.7 Aim of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.8 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Theory 11
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 The 1-D conduction model . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2 The 2-D conduction model . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.3 Repetition frequency . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2.4 Summary and conclusions on the heat transfer . . . . . . . . . . . . 28
2.3 Bubble Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.1 The phase transition . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3.2 Kinetic limit of superheat . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.3 Nucleation probability . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.3.4 Spontaneous nucleation . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.4 Bubble dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.4.1 Explosive evaporation . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.4.2 Bubble collapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.3 The intermediate stage . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.4 Summary and conclusions on the bubble dynamics . . . . . . . . . 39
2.5 Membrane deflection theory . . . . . . . . . . . . . . . . . . . . . . . . . . 40
i
ii Contents
2.5.1 Static deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.5.2 Membrane vibrations . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.5.3 Summary and conclusions on the membrane response . . . . . . . . 46
3 Design and Fabrication 49
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.2 Design and fabrication of the heaters . . . . . . . . . . . . . . . . . . . . . 50
3.2.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.2.2 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3 Fabrication by KOH etching . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3.1 Mask design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.4 Fabrication with Reactive ion etching . . . . . . . . . . . . . . . . . . . . 60
3.5 The completed device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.6 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4 Bubble dynamics 65
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.2 Stroboscopic imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.3 Resistance Thermometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.3.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.4 Bubble nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.4.1 Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.4.2 Average heater temperature . . . . . . . . . . . . . . . . . . . . . . 69
4.4.3 Influence of applied power on the bubble nucleation . . . . . . . . . 70
4.4.4 Reproducibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.5 Different heater geometries . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.5.1 Early nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.5.2 Current density simulation . . . . . . . . . . . . . . . . . . . . . . . 79
4.5.3 Effects of early nucleation . . . . . . . . . . . . . . . . . . . . . . . 80
4.6 Bubble growth and collapse . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.6.1 The growth stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.6.2 The intermediate stage . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.6.3 The final stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.6.4 The influence of the cavity pressure . . . . . . . . . . . . . . . . . . 90
4.7 Repetition frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.7.1 Effects on the temperature . . . . . . . . . . . . . . . . . . . . . . . 94
4.7.2 Effect on the bubble nucleation . . . . . . . . . . . . . . . . . . . . 96
4.7.3 Effect on the bubble growth and collapse . . . . . . . . . . . . . . . 100
4.8 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5 Response of the membrane 105
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
5.2 Static deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
5.2.1 Different membrane sizes . . . . . . . . . . . . . . . . . . . . . . . . 108
5.3 Dynamic deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Contents iii
5.3.1 Effect of the membrane geometry . . . . . . . . . . . . . . . . . . . 112
5.3.2 Effect of the membrane size . . . . . . . . . . . . . . . . . . . . . . 115
5.3.3 Effect of the applied heating power . . . . . . . . . . . . . . . . . . 116
5.3.4 Effect of the cavity pressure . . . . . . . . . . . . . . . . . . . . . . 117
5.4 Initial dynamic deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
5.4.1 Nucleation and the maximum deflection . . . . . . . . . . . . . . . 120
5.4.2 Effect of the heater size and the membrane size . . . . . . . . . . . 123
5.4.3 Effect of the pressure inside the cavity . . . . . . . . . . . . . . . . 129
5.5 Ideal response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
5.6 Effect of the repetition frequency . . . . . . . . . . . . . . . . . . . . . . . 133
5.6.1 Deflection control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.6.2 Influence of the cavity pressure at high repetition frequencies . . . . 136
5.7 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
6 Conclusion and Outlook 141
6.1 Overall conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6.2 Design improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
6.2.1 Heaters and substrate . . . . . . . . . . . . . . . . . . . . . . . . . 145
6.2.2 Optimal properties of the liquid . . . . . . . . . . . . . . . . . . . . 149
6.2.3 Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
6.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
References 155
Summary 163
Samenvatting 165
Dankwoord 167
Chapter 1
Actuators and Bubbles
A large variety of actuators can be found in literature. This chapter provides a brief
overview of different actuator principles with a focus on thermo-pnuematic actuation and
actuators, which utilize bubbles to provide the actuation force. The second part of this
chapter will give a summary of the research on bubble nucleation and bubble dynamics
under high heat-flux conditions. At the end of this chapter, an outline of this thesis is
given.
1
2 Chapter 1. Actuators and Bubbles
1.1 Introduction
The last decades there has been a trend to miniaturize systems of virtually any kind.
This trend has many reasons: small systems are potentially cheaper to produce, they can
have properties large systems have not, they may facilitate use, and they can be more
energy efficient. This miniaturization has led to systems of a few mm3 and smaller. These
microsystemshavetypicaldetailsinthemicrometerrangeandMicroElectricalMechanical
Systems technology is used to fabricate these systems. In general microsystems comprise
of sensors, components that deliver some information, intelligence to process information
and decide over some particular action, and an actuator that performs the action. Several
types of forces can be used to perform an action. This resulted in a large number of
different microactuator types and an even larger number of actuator designs. The micro-
bubble actuator uses the principle of thermo-pneumatic actuation. The introduction of a
large amount of heat causes explosive evaporation and creates a bubble in a liquid. The
pressureimpulsegeneratedbytheformationofabubblewillcauseamembranedeflection.
Although the force generated by bubbles is used in several successful actuator designs,
bubble generation and bubble dynamics are not completely understood and still topics of
research.
1.2 Actuation principles
There are many ways to generate a force and therefore many different actuator principles.
In general MEMS actuators can placed in four different families and each family can be
divided in several classes. The four families are:
• Electrostatic
• Magnetic
• Thermal
• Piezoelectric
The type of actuator used in a microsystem depends on the required performance of
the actuator. Here one can think of the generated force, the displacement and repetition
frequency. Thisperformanceisdifferentforeachfamilyandeachfamilyhasitslimitations.
Table 1.1 shows the performance and the limitations of the different families.
Force Stroke Frequency Limitation
Electrostatic - +/- + Weak
Piezoelectric +/- +/- + Material
Thermal + + - Low frequency
Magnetic - - +/- Weak
Table 1.1: Preformance and limitations of the different actuator families