Chapter 1 Introduction
1.2 Optical manipulation for small objects
Starting in 1980s, to overcome the force limitation of optical tweezers, an optothermal mechanism was found to provide a much larger force to allow faster droplet manipulation (cm/ s) [18]
.
One typical phenomenon is thermocapillary, which is based on the Marangoni effect (Figure 1-1). The local illuminated area has higher temperature and smaller surface stress. It means the droplet can be pumped toward the dark and cold side due to its larger surface stress. In this decade, thermocapillary can be used for fast transportation, trapping, and sorting the droplets in microfluidic studies (Figure 1-2) [19, 20, 21].
Figure 1-1. Thermocapillary mechanism of to push the droplet.
(a)
(b)
Figure 1-2. (a) The 1.53 mm radius bubble migration with temperature gradient G=2.77 K/
mm [19]. (b) The bubbles aggregation experiment by Hiroki Kasumi et al. [21].
However, the high power pulse laser still generates undesirable heat on the specimens.
Another optothermal mechanism, optothermal cavitation bubbles, was proposed around 1990s. The high power pulse laser is also used as a heating source. This focused laser produces a rapidly expanding vapor bubble in the water through nonlinear optical absorption. And then the explosive evaporation occurs when a thin layer of liquid reaches the critical temperatures in a very short period of time (Figure 1-3a) [22]. Nowadays, this pressure impulse made from the thermal-bubble is widely applied in biological chips (Figure 1-3b, 1-3c) [23, 24, 25, 26]
.
(a)
(b) (c)
Figure 1-3. (a) Schematic processes of laser excited bubble cavitation [22]. (b) Schematic of the cell sorter within few microseconds response [23]. (c) The localized
cell concentrator by acoustically activating the optothermally bubbles [24].
Although the optothermal cavitation bubbles mechanism, can indirectly generate larger force on the specimens without photo damage, the researchers continue to pursue a lower-cost and lower-power light source to access the opto-mechanical control. Since 2000s, opto-electrowetting (OEW) and optical dielectrophoresis (ODEP) techniques have been proposed to move liquid droplets and microparticleswith high performance, reliability, simplicity, and fast response. Instead of the expensive and high power (hundred milliwatts to watt) laser, the OEW and ODEP are set up with the liquid crystal displays or projectors [27].
The opto-electrowetting (OEW), which was proposed by Chiou et al. in 2003 [28], now is an effective method for manipulating micro droplets. (Figure 1-4a) depicts a typical OEW device; the droplet is sandwiched between a top hydrophobic surface and a bottom OEW surface [29, 30, 31, 32]. The bottom OEW structure is made by an
electrodes array on a photoconductive material. In the light illuminating area, the electric conductivity of the photoconductive layer increases, causing more voltage across the droplet. Base on the relationship between the contact angle θ and the applied voltage across the droplet in equation 1-1 [33], the contact angle of the droplet decrease and it moves toward the dark side.
(1-1) In equation 1-1, θ0 is the original contact angle, γ is the surface tension between the droplet and surrounding medium, ε0 is vacuum permittivity, εr is the dielectric constant, t is the thickness of the dielectric layer, and V is the voltage. Recently, many modified OEW techniques are proposed for integrating with other microfluidic components. For example, Chuang et al. presented an open optoelectrowetting (o-OEW) device with a translational speed up to 3.6 mm/ s (Figure 1-4b) [34]. Park et al. proposed the single-sided continuous optoelectrowetting (SCOEW) to overcome the droplet size limitation in the sandwich OEW device (Figure 1-4c)[35].
(a)
(b) (c)
Figure 1-4. The scheme of (a) a typical OEW device [29], (b) open optoelectrowetting (o-OEW) device [34], and (c) Single-sided continuous optoelectrowetting (SCOEW)
working principle with its equivalent circuit model [35].
Optical dielectrophoresis (ODEP) is another optoelectronic manipulation method, which was first proposed by Chiou et al. in 2005 for the parallel manipulation of single particles in (Figure 1-5a) [28]. Recently this technique is widely used in microfluidic and biological chips for cell patterning [36], sorting [37, 38], transfection [39], and separation [40]. An ODEP device consists of top and bottom transparent electrodes, and the bottom electrode is a photoconductive layer (Figure 1-5b). The photoconductive layer is used as localized virtual electrodes with incident light, making an intensive electric field in the bright area. Based on the conventional time average DEP force exerted on a sphere in a fluidic medium (equation 1-2), the particles in the aqueous can be collected in the bright areas. In this equation, R is the particle radius, εm denotes the permittivity of the liquid medium, Erms is the root-mean square magnitude of the electric field. K(ω) is the
Clausius-Mossotti (CM) factor, which is related to the particle and liquid medium’s permittivity (εp and εm), conductivity (σp and σm), and the applied frequency across the liquid medium (ω).
(1-2)
(a) (b)
Figure 1-5. (a) The ODEP device for massive and parallel particles manipulation [27]. (b) The scheme of ODEP mechanism [38].
Comparing the OEW and ODEP, due to their different working frequencies (10~20 kHz for OEW; 100~200 kHz for ODEP), the OEW is usually used for droplet manipulation,
but the ODEP can further control particles in microfluidic devices. In (Figure 1-6), below fmin, the impedance of the dielectric layer is larger than the dark impedance of the photoconductive layer (such as a-Si:H or TiOPc), so most of the voltage across the dielectric layer with or without light illumination. Between fmin and fc, under illumination, the cross voltage switch from the photoconductive layer to the dielectric layer, causing OEW to occur. Between fc and fmax, under illumination, the field now drops primarily across the liquid layer, resulting in OET. Above fmax, the impedance of the liquid becomes so low that it drops below Zl inhibiting effective voltage switching [41].
Figure 1-6. The frequency response figure (not to scale) of OEW and ODEP [41].
However, these conventional optoelectronic technologies are still only available for manipulation at nano-to sub-micro Newton scales. To obtain a better manipulation efficiency in a micro-environment, a much larger force should be discovered, developed and integrated with optical control.
1.3 Piezoelectric manipulations and control system
Over the last few decades, piezoelectric materials are widely applied in mechanical system as actuators or sensors. Piezoelectric actuators can generate small strain (usually
<0.1%) and high stress (MPa) in a fast response time, so they can provide large force but small displacement, which can do suitable pumping [42, 43, 44, 45, 46], sorting [47, 48], or mixing in microfluidic devices [49, 50]. Furthermore, based on the direct piezoelectric effect [51], some flexible and transparent piezoelectric polymers are popularly used as pressure sensors in biological researches (Figure 1-7) [52, 53, 54, 55]. These polymer films have inertness to chemical agents, high sensitivity and electrical response over a wide frequency range.
(a) (b)
(c) (d)
Figure 1-7. Piezoelectric effect in microfluidic device for (a) pumping [46],(b) sorting [47], (c) mixing [49], and (d) pressure monitoring[54].
Beside these vibration and deformation, the distributed piezoelectric elements or the so-called “smart-structures” provide the controlling ability in dynamic systems [56, 57].
Over the past 25 years, many researchers demonstrated the active control and adaptive structures with piezoelectric materials. Some early efforts focused on the integration of distributed sensors and actuators to target specific structural modes for application in sensing, actuation, or both. In 1989, Tzou et al. first derived the theories of distributed sensing and active vibration controlling for flexible shell structures [58]. In 1995, Qiu et al. controlled the bending moment of a circular cylindrical shell by the distributed inner and
outer PVDF actuators with same values but opposite phases (Figure 1-8a) [59]. Some others studies discussed the interaction between the piezoelectric material and the structure
so that they could use the piezoelectric actuators to reduce the sound pressure or vibration.
In 1991, Dimitriadis et al. demonstrated that bonding the piezoelectric elements on a plate could reduce the harmonic sound transmitted or radiated from the plate (Figure 1-8b) [60].
In 2008, Kozie’n et al. analyzed the possibility to active noise and vibration cancellation for the realistic machine structure with the distributed piezoelectric elements [61]. Recently, dell’Isola et al. proposed a damage detector which electric signals come from the state variables of the main structure and a distributed set of piezoelectric patches [62]. Wang et al. integrated piezoelectric actuating and sensing elements to be a cantilever-based
oscillating type MEMS dc current sensor [63].
Particularly in 1989, Lee and Moon introduced the patterned electrode on a continuous piezoelectric film to measure somespecific vibration modals [64, 65, 66]; these sensors with the spatialdistribution electrodes are denoted as modal sensors (Figure 1-8c) [67]. In 2001, Hsu and Lee further designed the miniature Autonomous Phase-gain ROtation/ linear Piezoelectric Optimal Sensing (APROPOS) for a fundamental a free-fall motion [68].
They also proposed a no-phase delay low-pass filter to tailor the sensor transfer function with a symmetric weighting electrode (Figure 1-8d) [69].
(a) (b)
(c)
(d)
Figure 1-8. Distributed piezoelectric actuators or sensors for vibration controlling systems.
(a) Configuration of a bending control model by Qiu et. al. [59]. (b) The sound pressure pattern of a plate vibration with and without piezoelectric actuators controlling [60]. (c) The uniform and mode 1 electrode patterns and their respective transfer functions [66]. (d) The schematic of a symmetric piezoelectric distributed sensor and its transfer function (dark
line) versus a uniform sensor (gray line) [69].
1.4 Optopiezoelectric control system
Based on the promising efficiency of piezoelectric actuators and sensors, it might be possible to develop an optical piezoelectric coupled mechanism for larger force control with novel opto-piezoelectric materials [49, 70]. Taking the convertible advantage of photoconductive materials, some photoconductive molecules are integrated with piezoelectric devices as optical virtual electrodes. In 2011, Chen et al. proposed an optically induced piezoelectric vibration control mechanism, which was achieved with a coupling of spiropyran-doped liquid crystal on PZT plates [71, 72] (Figure 1-9a). They
In 2013, Huang designed a deformable mirror using a spatially modulated TiOPc/ PZT actuator [73]. A white light triggered photoconductive material, TiOPc, was coated on the
PZT buzzer as virtual electrode. By projecting different light patterns, the spatially distributed actuating forces made this TiOPc/PZT actuator deforms in respective shapes (Figure 1-9b). One year later, Chang et al. proposed an optical spatially modulated TiOPc/ piezo buzzer actuator driven by optical patterns (Figure 1-9c) [74]. By varying light illumination pattern, the spatial force enhancement was found to change the acoustic beam pattern and directivity.
(a)
(b) (c)
Figure 1-9. Light modulated optopiezoelectric controlling system. (a) Schematic of the TiOPc thin film-based optically modulated first modal sensor system [71, 72]. (b) The
spatial deformations from a spatial illuminated PZT plate by the electronic speckle pattern interferometry (ESPI) measurement [73]. (c) The acoustic beam pattern directivity of “light/ dark (LD)” and “dark/ light (DL)” of a piezoelectric speaker [74].
1.5 Motivation and purpose
These previous studies demonstrated the performance of piezoelectric sensors and actuators. They also showed the possibility of light controlled piezoelectric devices. In this study, we further discuss the capability of optical piezoelectric manipulation for distributed sensors and piezoelectric actuators. In Chapter 3, a NMR compatible C.
elegant trapper will be proposed for demonstrating the performance of PZT in microfluidic
device. In Chapter 4, a novel photoconductive piezoelectric composite (P(VDF-TrFE)/
TiOPc) will be developed and analyzed. This lead-free composite is better than PZT to be a biocompatible microfluidic chip substrate. Furthermore in Chapter 5, we will compare the mechanical sensing ability between this composite (P(VDF-TrFE)/ TiOPc) and a double layers structure (TiOPc-PZT) through a bending sensor.
Overall, we try to set out a path to achieve the optical piezoelectric manipulation and sensing (Figure 1-10). In this thesis, we demonstrate the ability of piezoelectric devices in microfluidic devices, and then developing the optopiezoelectric material for further optopiezoelectric microfluidic application.
Figure 1-10. Overview of optical actuators and sensors for mechanical system.
Chapter 2. Material
In this chapter, piezoelectric material and photoconductive material used in this study are discussed. In Chapter 3, we will integrate piezoelectric ceramic (PZT) into microfluidic device. In Chapter 4, optopiezoelectric actuator made of a spiropyran/ liquid crystal (SP/ LC) material layer on a PZT laminate will be examined. The optopiezoelectric sensor composed of piezoelectric polymer (P(VDF-TrFE) and photoconductive particles, titanyl phthalocyanine(TiOPc), are to be explored as well. In this chapter, the physical and chemical properties and applications of these materials are detailed.
2.1 Piezoelectric material
The piezoelectric effect was discovered by Pierre and Jacques Curie in 1880. But practical applications started after 1940s when sonars development started. Piezoelectric materials can be used as an actuator to transfer electrical signal into mechanical deformation (direct effect), or it can be used as a sensor to transform vibrations into electrical energy (converse effect) (Figure 2-1). The basic but detailed piezoelectric theory will be discussed in Chapter 3.
2.1.1 Lead Zirconium Titanate (PZT)
The most commercialized and widely applied piezoelectric material is lead zirconate titanate (PZT) (Pb[Zr(x)Ti(1-x)]O3). PZT has excellent piezoelectric properties, high Curies temperature (>150。C), high spontaneous polarization (remnant charge > 50 μC/cm2)
and high electromechanical coupling coefficient (d31 , d33 equal to few hundreds pC/N). It has become the basis of many important industrial products. Over the years, PZT has been applied in underwater sensors [75, 76], biosensors [77, 78], and energy harvesters [79, 80], etc.
Figure 2-1. The working function of positive and converse piezoelectric effect.
2.1.2 Piezoelectric polymer
Piezoelectric polymers have been discovered for more than 40 years, but in recent years they have progressively developed due to its flexible, lightweight and transparent advantages. The properties of polymers are very different from PZT in (Table 2-1). The piezoelectric polymers have much higher piezoelectric stress constant (g31), which indicates that they are much better sensors than ceramics [81]. Polymers also typically possess high dielectric breakdown and high operating field strength, which means they can withstand much higher driving fields than ceramics. Furthermore, with spin coating or electro spinning process, different mechanical properties and applications of the piezoelectric
polymer can be fabricated by using the sol-gel processes [82, 83]. Thus piezoelectric polymers surely establish its technical applications and useful device configurations.
Table 2-1. The piezoelectric properties comparison of PZT and PVDF film [81].
Nowadays, Poly(vinylidene fluoride) (PVDF) is one of the most applied piezoelectric polymers. It was found in 1969 to have a very large piezoelectric coefficient 6- 7 pC/N [84, 85]. Its chemical structure of (-CH2-CF2-) repeat units provides large chain flexibility. The highly repulsive forces of the fluorine atoms making the polymer polarized naturally (Figure 2-2). The most common and thermodynamically stable phase is the α-phase, which has a trans-gauche (TGTG') conformation and does not show a net lattice polarization, i.e., this phase presents no piezoelectric effect. After stretching and poling processes, the α-phase transfers to β-phase in an all-trans (TTTT) zig-zag conformation (Figure 2-3). The C-F dipoles all align in the same direction providing the β-phase PVDF a spontaneous lattice polarization, which is necessary to ferroelectric effect [86].
Features Brittle, heavy, toxic Flexible, lightweight, low mechanical impedance
Figure 2-2. Molecular conformations and unit cells of the two common polymorphs of PVDF [86].
(a) (b) (c)
Figure 2-3. Schematic of random crystal lamellae in PVDF polymer: (a) the morphology after the film is melt cast; (b) the film orientation after stretching; and (c) after poling
through the film thickness[87].
In the last decade, copolymers of PVDF with trifluoroethylene (TrFE) (-CHF-CF2-) containing different mole fractions of both components have been studied extensively. It is because they exhibit certain advantages over pure PVDF. P(VDF-TrFE) is synthesized by copolymerization VDF and TrFE monomers (Figure 2-4). Due to the higher repulsive forces of one more fluorine atoms, the ferroelectric phase is inherent in the copolymer at
room temperature. Unlike typical PVDF must be stretched, P(VDF-TrFE) can achieve β-phase conformation by only poling with a high electric field (in the order 50 MV/m) [88~
92]. More piezoelectric properties of PVDF and P(VDF-TrFE) are compared in (Table 2-2).
Figure 2-4. Schematic structure of the molecule chain of P(VDF-TrFE) copolymer in all-trans conformation with a ratio of VDF and TrFE of 75 : 25.
Table 2-2. Comparisons of PVDF and P(VDF-TrFE) [81]
Parameter Symbol and Unit PVDF P(VDF-TrFE)
Young’s modulus (109 N/m2) 2~4 3~5
2.2 Photoconductive material
For developing an optopiezoelectric material, the photoconductive material takes an important role for light-spatial modulation. These materials should be more conductive, i.e. their electrical impedance decreases after light illumination.
2.2.1 Spiropyran
Spiropyran has been a popular photochromic material in photosensitive devices due to its advantageous physical and chemical properties [93, 94, 95]. This molecular can be transformed from a spiropyran (SP) state into a merocyanine (MC) state under UV irradiation, and it is reversible by visible light irradiation or heating (Figure 2-5a). More specifically, the carbon-oxide bond of the spiropyran is opened during transformation, which provides good ionic conductivity. Furthermore, the color also changes from light purple to a darker one under irradiation. Thus, the spiropyran molecules have widely applications include optical memories, sensors and transistors in (Figure 2-5b~ 2-5d) [96~
100].
(a)
(b) (c)
(d)
Figure 2-5. The spiropyran chemical structure and some related applications. (a) The reversible process. (b) The flow rate of photo-controllable electroosmotic in both SP and
MC form, and the poly(styrene-co-divinylbenzene) monolith using 1mM HCl as the electrolyte [98]. (c)A light activated shape memory made of spiropyran doped ethylene-vinyl acetate copolymers [99]. (d) A spiropyran modified micro-fluidic chip
channels for controlling and detecting metal ion accumulation and release [100].
2.2.2 Titanyl Phthalocyanine (TiOPc)
In this study, we also considered titanyl phthalocyanine (TiOPc), which is an organic photovoltaic material. TiOPc is a non-planar phthalocyanines (Pcs) with the Ti=O located slightly above (ca. 0.3Å ) the ring, orthogonal to the molecular plane (Figure 2-6a). Upon photo excitation, an electron can either move from HOMO to LUMO within a molecule (Figure 2-6b) [101]. These surprisingly close π- π molecular contacts indicate a TiOPc has a broadening of the absorbance spectrum toward the near IR region. (Figure 2-6c, 2-6d) [102]. And its high hole mobility (over 1.0 cm2 V-1 s-1) and on/off ratio make it the most efficient organic photoconductors used in more than 90% of the laser printers (Figure 2-6e)
[103]. Recently, because of its characteristics of strong illumination absorption from the visible to the IR region [104~ 106], it is also been taken in optoelectronic chip for the dynamic manipulation [107~ 109]and solar cells [110~ 112] (Figure 2-7).
(a)
Ti
C
N O
(b) (c)
(d) (e)
Figure 2-6. The physical properties of titanyl phthalocyanine (TiOPc). (a) Schematic molecular model. (b) Molecular stacking of a-TiOPc crystal showing the p-stacking structure with concave pair and convex pair with significant molecular overlaps and very short intermolecular distances [102]. (c) Schematic explaining charges excitons in TiOPc [102]. (d) UV-vis spectra of TiOPc in the colloidal solution and resin film made by Wei Chao et al. [103]. (e) The photo-induced discharging curve of a TiOPc film made by Wei
Chao et al. [103].
Figure 2-7. Operation principle of the light-induced electric field for small particles manipulation by Shih-Mo Yang et al. [107].
Chapter 3. A living worm trapper by PZT actuator
3.1 Introduction
Caenorhabditis elegans (C. elegans) (Figure 3-1) is a 1-mm long transparent
roundworm. It has a two weeks lifespan and is one of the simplest organisms for understanding cell division directly under the microscope. In the early 1960s, Sydney Brenner declared the use of C. elegans as a model to trace how genes made bodies and their behavior. In 2002, Sydney Brenner, H. Robert Horvitz and John E. Sulston received the Nobel Prize in Physiology or Medicine for their discoveries concerning "genetic regulation of organ development and programmed cell death" (an extract from The
Nobel Assembly at Karolinska Institutet, 7, October, 2002). Now the C. elegan has become a widely used model organism in biology.
Figure 3-1. The worm image [113]
Studies of C. elegans often require immobilizing the worm during monitoring or giving stimulation. Traditional techniques were based on manual manipulation on a Petri dish or a multiwall plate, and permanent immobilization for further investigations with cyanoacrylate glue or anesthetics [114, 115]. The first method consumes substantially long time, and the irreversible glues may have some toxicity. The second method gives unavoidably internal biochemical of the worm during injecting the anesthetics [116].
Currently microfluidics and micro-electromechanical system (MEMS) devices provide powerful tools and great advance for worm immobilization.
Besides using these micro chambers and pillars [117, 118], the worm immobilization can be accomplished by temperature [119] or pressure control [120], mechanical [121, 122], electrical [123], and acoustic modulation [124] (Figure 3-2a~ 3-2d).
But all these methods must maintain the pressure, temperature, or electric field continuously in order to keep the worm immobilized. They need to maintain the operation field poses a drawback for instrument such as nuclear magnetic resonance (NMR) that does not have enough spaces in the detection area for the electric wires or fluidic tubes. In this chapter, we develop an on demand piezoelectric worm trapper, which can efficiently trap the C. elegans, and keep it immobilized by hydrostatic equilibrium without using any wires, tubes, and connectors.
(a)
100µm
(b)
(c)
500 µm Before immobiliza on
During immobiliza on
(d)
Figure 3-2. Methods for worm immobilization: (a) by transiently cooling them on-chip to ~ 4 °C [119], (b) by controlling valves states sequence [120], (c) by mechanical clamping
[121], and (d) by acoustic resonance wave [122].
3.2 Theory
In this worm trapper, the worms are immobilized between oil bulges, which are pushed from the piezoelectric generated pressure (Figure 3-3). The related theories include the piezoelectric plate actuator and Laplace pressure.
Figure 3-3. The scheme of the living worm trapper with PZT actuator.
3.2.1 A linear piezoelectric thin plate actuator
The piezoelectric constitutive equations are used to calculate the transfer efficiency between electrical and mechanical signals [125]. In these equations, the piezoelectric
materials are assumed to be a linear material, which is only true when the applying electric field is small. There are four types of the equations as listed below:
Stress-Electric field type:
(3-1)
Strain-Electric field type: (3-2)
Strain-Charge type:
(3-3)
Stress-Charge type:
(3-4) The parameters and variables are detailed in (Table 3-1), and the subscripts refer to IEEE compact matrix notation in (Table 3-2) [125]. The relationships between the variables can also be represented as (Figure 3-4).
Table 3-1. The parameters and variables in (equation 1~ 4).
Symbol Name Unit Relationship
T
p Stress PaSp Strain 1
Ek Electric field V/m
Ek Electric field V/m