SURFACE ACOUSTIC WAVE (SAW) DESIGN AND APPLICATIONS IN MICROFLUIDICS RAHUL KISHOR INTERDISCIPLINARY GRADUATE SCHOOL NANYANG ENVIRONMENT & WATER RESEARCH INSTITUTE (NEWRI) 2017 SURFACE ACOUSTIC WAVE (SAW) DESIGN AND APPLICATIONS IN MICROFLUIDICS RAHUL KISHOR INTERDISCIPLINARY GRADUATE SCHOOL NANYANG ENVIRONMENT & WATER RESEARCH INSTITUTE (NEWRI) A Thesis Submitted to the Nanyang Technological University in partial fulfilment of the requirement for the degree of Doctor of Philosophy 2017 Acknowledgements With great pleasure I would like to thank my thesis advisor Prof Zheng Yuanjin for his support and guidance provided to me during my stint at NTU as a PhD student. I am deeply indebted to you for the faith and confidence bestowed on me over these years. My heartfelt thanks to Prof Lim Teik Thye and Dr Wang Zhenfeng, my co- supervisor and Prof Richard Webster, my mentor for their invaluable suggestions and recommendations. The completion of this highly interdisciplinary undertaking could not have been possible without the assistance of my teammates. I would like to especially thank Dr Feng Xiaohua, Dr Ding Ran, Dr Gao Fei, Dr Zhu Yao and Liu Siyu for their help and support. A special gratitude to Seah Yen Peng Daphne for her guidance and unwavering support. Dr Sivaramapanicker Sreejith , thanks for your extreme patience in taking my thoughts and making them look highly professional with your amazing skills. The days in NTU was enjoyable from the friends I made over these years.Thank you all for being with me throughout these years. I thank NTU and NEWRI for awarding me the postgraduate scholarship to support my studies. This journey would have been difficult without the backing of my in-laws and relatives. To my sister Heera, you deserve my whole-hearted thanks for being there i always to encourage me. To my father, who is always in my thoughts- you are missed. To my mother, Padmaja Kishor, who was my first teacher and taught me the value of hard work and education. Thank you for your unwavering love and faith in me. Words are not enough to express my love and gratitude to my beloved wife, Sreedevi.You have been with me throughout this journey, and I couldn’t complete this without your selfless support, sacrifices and patience. This thesis is dedicated to you. ii Abstract Lab-on-a-chip (LOC) is the current trend towards developing point-of-care devices. LOC finds its application in fields of medicine, environmental monitoring and towards a multitude of industrial applications. It involves integrating all the functions that were done in a laboratory, from input samples to results delivery in an easy-to-handle de- vice. This was realised due to the advent of microfluidics and the microelectronics technology. Physically, an LOC consists of four main parts: microfluidics, actuators, sensors and readout circuits. Many research groups along with startup companies have developed technologies to realise fluid actuation/control and signal detection with impressive capabilities. However, there is a dearth of LOC’s available in the market. They are still confined within the spaces of the laboratory and regarded as a ”chip-on- lab” functionality, justifiably due to the lack of an integrated platform that performs the different assay procedures in a seamless and automated fashion. This work is in pur- suit of developing an integrated platform to realise an LOC utilising surface acoustic wave (SAW) devices. SAW are nanometer amplitude vibrations that are generated on a piezoelec- tric substrate. During the last decade, SAW has been intensively used and researched for microfluidic applications majorly as an actuator. SAW capabilities as a sensor for immunoassay was also explored. This unique feature of SAW to act both as an actu- ator and sensor makes it easier for integration. In this thesis, we first study the use of iii SAW as an actuator of the fluids by establishing a novel mechanism for characterising the SAW energy transmission in fluidic channels, which is essential for all the SAW microfluidics design, using a mixing structure. We developed analytical models in this work that could be used to optimise the power transmission coefficient and hence increase actuation efficiency. Besides focusing on the application of SAW as an actuator, its usage as a sensor was primarily relying on the immunoassay technique which required compli- cated surface preparation steps. In the subsequent work, we proposed and demon- strated a new sensing methodology utilising photoacoustic induced surface acoustic wave (SAW-PA) for simultaneous optical and mechanical property characterization of analytes (including cells, nanoparticle and dyes). A nanosecond pulsed laser exci- tation on a sample triggers a longitudinal acoustic wave in the fluid which is mode- converted into a Rayleigh SAW on the piezoelectric substrate and detected using the metal electrodes (interdigital transducer, IDT). We further developed a numerical model to study the wave conversion process (longitudinal acoustic waves to SAW) and demonstrate that the PA generated in the microfluidic channel acts as a mechanical resonator dependent on the dimensions of the microfluidic channel. We experimen- tally verified that a SAW device matched to the channel resonant frequency could improve the sensitivity. Finally, we propose a platform combining the SAW actuators and SAW-PA sensor, which is closer to realising the initial objective of an integrated platform for LOC. In this work, we have developed an integrated microfluidic system that com- bined high-efficiency (> 95%) tilted-angle standing surface acoustic wave (taSSAW) based particle separation, particle concentration inside an open microfluidic cham- ber and sensing, on a single LiNbO3 substrate. The platform demonstrated real-time quantitative detection of 10 µm polystyrene beads down to 7 particles in 10 µl of the sample volume in 15 minutes. iv Contents Acknowledgements ...............................i Abstract ...................................... iii List of Figures ..................................x List of Tables ................................... xiv List of Abbreviations ............................... xv 1 Introduction 1 1.1 Motivation for this thesis . .2 1.2 Objectives . .3 1.3 Thesis outline . .5 2 Literature Review 7 2.1 Defining Lab-on-a-chip . .7 2.2 Processes involved in an LOC . .8 v 2.2.1 Sample handling . .8 2.2.2 Separation . 12 2.2.3 Detection . 13 2.3 Surface acoustic wave microfluidics . 18 2.3.1 SAW fluid actuation and manipulation . 22 3 Acoustofluidic mixing: The study of an acoustically coupled multi-layered microfluidic platform on SAW substrate 25 3.1 Introduction . 26 3.2 Materials and methods . 28 3.2.1 SAW device design and simulation . 28 3.2.2 Micromixer design . 30 3.3 Experimental setup . 31 3.3.1 Electrical setup . 31 3.3.2 Coupling layer . 32 3.3.3 Placement of the microfluidic channel . 33 3.3.4 Microfluidic setup . 33 3.3.5 Mixing efficiency calculation . 34 3.4 Results and discussion . 36 vi 3.4.1 Transient behaviour of mixing . 36 3.4.2 Effect of voltage on mixing . 37 3.4.3 Effect of coupling layer thickness on mixing efficiency . 37 3.4.4 Effect of frequency on mixing . 40 3.4.5 Acoustic heating . 43 3.5 Concluding remarks . 45 4 Photoacoustic induced surface acoustic wave (SAW PA) sensor for sens- ing analytes in a microfluidic channel 47 4.1 Introduction . 48 4.2 Materials and Methods . 50 4.2.1 SAW device design and fabrication . 50 4.2.2 Design of microfluidic channel . 51 4.2.3 Reagent and solutions . 51 4.2.4 Experimental setup . 52 4.2.5 Sensor theory and data processing . 54 4.3 Experimental Results and discussion . 60 4.3.1 SAW-PA signal from standard dye solutions . 60 4.3.2 SAW-PA signals from gold nanoparticles . 64 vii 4.4 FEM modelling of the SAW-PA sensor . 70 4.4.1 Theoretical analysis . 70 4.4.2 Finite element mode . 71 4.4.3 Experimental measurement . 77 4.4.4 Sensitivity calculation . 80 4.5 Conclusion . 80 5 Integrated microfluidic system using SAW for real-time separation and detection of particles on a single substrate 82 5.1 Introduction . 83 5.2 Materials and methods . 85 5.2.1 Device design and operation . 85 5.2.2 SAW device fabrication . 88 5.2.3 SAW device characterization . 88 5.2.4 Functionality of the integrated device . 88 5.2.5 Materials . 96 5.3 Results and discussion . 96 5.3.1 Separation of particles . 96 5.3.2 Concentration of particles . 98 viii 5.3.3 Sensing of particles . 100 5.4 Conclusion . 106 6 Conclusions and Future Work 108 6.1 Conclusions . 108 6.2 Future work . 110 Appendix A Lamb waves in a plate 112 A.1 Derivation of stress in an isotropic solid . 114 A.2 Derivation of mechanical displacement components of Lamb wave . 115 Appendix B Transmission coefficient of a sound wave from an arbitrary number of layers 119 Bibliography 129 ix List of Figures 2.1 Photoacoustic principle . 18 2.2 Schematic of a surface acoustic wave device . 20 2.3 Schematic of the SAW interacting with a droplet . 21 3.1 Fabricated SAW devices at a centre frequency of 50 MHz . 29 3.2 Measured S11 using the network analyser . 30 3.3 Reusable SAW mixing schematic . 31 3.4 PDMS as a reservoir to control coupling layer thickness . 32 3.5 Microfluidic Y channel placed on top of the substrate via coupling layer . 33 3.6 Y shaped microchannel on PDMS . 34 3.7 Images of the SAW mixing experiment . 35 3.8 Transient mixing phenomena . 36 3.9 Mixing efficiency variation with voltage . 38 3.10 Acoustic wave transmission through the various layers . 39 x 3.11 Mixing efficiency variation with coupling layer thickness . 40 3.12 Mixing efficiency variation with frequency . 41 3.13 Acoustic wave transmission with the wave refracting angles .