Numerical Modeling of Deformation, Oscillation, Spreading and Collision Characteristics of Droplets in an Electric Field

Electric field induced flows, or electrohydrodynamics (EHD), have been promising in many fast-growing technologies, where droplet movement and deformation can be controlled to enhance heat transfer and mass transport. Several complex EHD problems existing in many applications were investigated in this thesis. Firstly, this thesis presents the results of numerical simulations of the deformation, oscillation and breakup of a weakly conducting droplet suspended in an ambient medium with higher conductivity. It is the first time that the deformation of such a droplet was investigated numerically in a 3D configuration. We have determined three types of behavior for the droplets, which are less conducting than ambient fluid: 1) oblate deformation (which can be predicted from the small perturbation theory), 2) oscillatory oblate-prolate deformation and 3) breakup of the droplet. Secondly, a numerical study of droplet oscillation placed on different hydrophobic surfaces under the effect of applied AC voltage including the effect of ambient gas was investigated. The presented algorithm could reproduce droplet oscillations on a surface considering different contact angles. It has been found that the resonance frequency of the water droplet depends on the surface property of the hydrophobic materials and the electrostatic force. Thirdly, a new design of an electrowetting mixer using the rotating electric field was proposed which offers a new method to effectively mix two droplets over a different range of AC frequencies. Two regimes were observed for droplet coalescence: 1) coalescence due to the high droplet deformation, 2) coalescence due to the interaction of electrically induced dipoles. Fourthly, the spreading and retraction control of millimetric water droplets impacting on dry surfaces have been investigated to examine the effect of the surface charge density and electric field intensity. The effect of the surface charge on the spreading of droplets placed gently on surfaces was investigated in the first part. It was found that the maximum spreading diameter increases with an increasing charge. In the second part, the impact of a droplet on a ground electrode was considered. It was also found that in order to keep the maximum diameter after the impact, less charge is needed for surfaces with lower contact angle. Finally, the interaction between two identical charged droplets was investigated numerically. The effects of the impact velocity, drop size ratio and electric charge on the behavior of the combined droplet were investigated. It was shown that two conducting droplets carrying charges of the same polarity under some conditions may be electrically attracted. The formation of charged daughter droplets has been investigated and it was found that the number of the satellite droplets after collision appears to increase with an increase in the droplet charge

Experimental studies of liquid marbles and superhydrophobic surfaces

The interaction of water droplets with hydrophobic or rough, superhydrophobic solid surfaces has been studied. Such surfaces may be found in the natural world and their potential applications range from waterproof and self-cleaning surfaces to droplet microfluidics. A measure of hydrophobicity is obtained from the angle between the liquid and solid surface measured from the solid through the liquid, known as the contact angle. Variations in this angle can indicate not only a level of ‘wetting’ of the surface but also small amounts of droplet movement and may be achieved by electrowetting, the application of a voltage between a liquid droplet and a substrate, and/or by varying the local topography of the surface. Photolithography and thin-film deposition fabrication techniques have been used to create hydrophobic and superhydrophobic surfaces for use in electrowetting experiments. Both AC and DC electrowetting behaviour has been investigated and the results have been shown to be in agreement with past work and well established theory. Liquid marbles have been investigated as water drops displaying extreme non-wetting behaviour, with conformal coatings forming textures similar to those formed by the topography of a super-hydrophobic surface

Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

Recently, piezoelectric thin films including zinc oxide (ZnO) and aluminium nitride (AlN) have found a broad range of lab-on-chip applications such as biosensing, particle/cell concentrating, sorting/patterning, pumping, mixing, nebulisation and jetting. Integrated acoustic wave sensing/microfluidic devices have been fabricated by depositing these piezoelectric films onto a number of substrates such as silicon, ceramics, diamond, quartz, glass, and more recently also polymer, metallic foils and bendable glass/silicon for making flexible devices. Such thin film acoustic wave devices have great potential for implementing integrated, disposable, or bendable/flexible lab-on-a-chip devices into various sensing and actuating applications. This paper discusses the recent development in engineering high performance piezoelectric thin films, and highlights the critical issues such as film deposition, MEMS processing techniques, control of deposition/processing parametres, film texture, doping, dispersion effects, film stress, multilayer design, electrode materials/ designs and substrate selections. Finally, advances in using thin film devices for lab-on-chip applications are summarised and future development trends are identified.The authors acknowledge support from the Innovative electronic Manufacturing Research Centre (IeMRC) through the EPSRC funded flagship project SMART MICROSYSTEMS (FS/01/02/10), Knowledge Transfer Partnership No KTP010548, EPSRC project EP/L026899/1, EP/F063865/1; EP/F06294X/1, EP/P018998/1, the Royal Society-Research Grant (RG090609) and Newton Mobility Grant (IE161019) through Royal Society and NFSC, the Scottish Sensing Systems Centre (S3C), Royal Society of Edinburgh, Carnegie Trust Funding, Royal Academy of Engineering-Research Exchange with China and India, UK Fluidic Network and Special Interest Group-Acoustofluidics, the EPSRC Engineering Instrument Pool. We also acknowledge the National Natural Science Foundation of China (Nos. 61274037, 51302173), the Zhejiang Province Natural Science Fund (No. Z11101168), the Fundamental Research Funds for the Central Universities (No. 2014QNA5002), EP/D03826X/1, EP/ C536630/1, GR/T24524/01, GR/S30573/01, GR/R36718/01, GR/L82090/01, BBSRC/E11140. ZXT acknowledges the supports from the National Natural Science Foundation of China (61178018) and the NSAF Joint Foundation of China (U1630126 and U1230124) and Ph.D. Funding Support Program of Education Ministry of China (20110185110007) and the NSAF Joint Foundation of China (Grant No. U1330103) and the National Natural Science Foundation of China (No. 11304209). NTN acknowledges support from Australian Research Council project LP150100153. This work was partially supported by the European Commission through the 6th FP MOBILIS and 7th FP RaptaDiag project HEALTH-304814 and by the COST Action IC1208 and by the Ministerio de Economía y Competitividad del Gobierno de España through projects MAT2010-18933 and MAT2013-45957R

Microfluidics and Nanofluidics Handbook

The Microfluidics and Nanofluidics Handbook: Two-Volume Set comprehensively captures the cross-disciplinary breadth of the fields of micro- and nanofluidics, which encompass the biological sciences, chemistry, physics and engineering applications. To fill the knowledge gap between engineering and the basic sciences, the editors pulled together key individuals, well known in their respective areas, to author chapters that help graduate students, scientists, and practicing engineers understand the overall area of microfluidics and nanofluidics. Topics covered include Finite Volume Method for Numerical Simulation Lattice Boltzmann Method and Its Applications in Microfluidics Microparticle and Nanoparticle Manipulation Methane Solubility Enhancement in Water Confined to Nanoscale Pores Volume Two: Fabrication, Implementation, and Applications focuses on topics related to experimental and numerical methods. It also covers fabrication and applications in a variety of areas, from aerospace to biological systems. Reflecting the inherent nature of microfluidics and nanofluidics, the book includes as much interdisciplinary knowledge as possible. It provides the fundamental science background for newcomers and advanced techniques and concepts for experienced researchers and professionals

PVDF as a Biocompatible Substrate for Microfluidic Fuel Cells

A reliable, flexible, and biocompatible power source for implantable and wearable devices has always been one of the biggest challenges for medical device design engineers. Microfluidic fuel cells (MFCs) are one of the candidates to generate a constant and reliable energy. However, the aspects of this approach, such as use of expensive materials, limitation of achievable power density and biocompatibility, have not yet been fully addressed. These challenges have restricted the application of MFCs to lab-on-chip systems that are deemed to be promising for implantable medical devices. Recently, porous materials such as natural papers and synthetic polymers (in the form of either nanofibers or porous membranes), when used as the MFC substrate, have shown that they can address the above-mentioned challenges. More importantly, these porous materials induce an inherent capillary flow in the fuel, eliminating the need of a pump. This may lead to an increased fuel efficiency and miniaturization of MFCs. However, the search for a porous biomaterial that displays high mechanical strength but remains flexible without degrading in a biological environment is not straightforward. In this research, Polyvinylidene Fluoride (PVDF), a non-biodegradable, biocompatible, flexible, and inexpensive material, was investigated for the first time as a channel substrate in a dynamic state MFC. To achieve the desired porosity, flexibility, and material strength of the substrate, PVDF nanofibers were fabricated using the electrospinning technique. Furthermore, hydrophilic PVDF nanofibers were successfully achieved by oxygen plasma surface treatment. The desired PVDF-based MFC was conceptualized using Axiomatic Design Theory (ADT) and FCBPSS (F: function, C: context, B: behavior, P: principle, SS: structure-state) methods. To investigate the electrochemical output of the designed PVDF-based MFC, a hydrophilic porous PVDF membrane was used as the substrate to induce a capillary action in the fuel (hydrogen peroxide). The PVDF-based MFC studied here successfully produced a power density of 0.158 mW/cm^2 at 0.08 V that is ~60% higher compared to the previous dynamic state paper-based biofuel cell reported in the literature. Moreover, the power density of MFC studied here is comparable to previous studies of static state single compartment MFCs using the same fuel type and concentration. Therefore, the results from this work demonstrate, for the first time, that the porous PVDF is a suitable material for the channel substrate in a dynamic state MFC. The potential application of this cell, in medicine, is utilizing the hydrophilic porous PVDF in electrochemical, implantable, and wearable medical devices. This approach can also be applied to any self-powered point-of-care diagnostic system

A NUMERICAL STUDY ON THE DYNAMICS OF DROPLET-WALL INTERACTIONS

Ph.DDOCTOR OF PHILOSOPH

Second Microgravity Fluid Physics Conference

The conference's purpose was to inform the fluid physics community of research opportunities in reduced-gravity fluid physics, present the status of the existing and planned reduced gravity fluid physics research programs, and inform participants of the upcoming NASA Research Announcement in this area. The plenary sessions provided an overview of the Microgravity Fluid Physics Program information on NASA's ground-based and space-based flight research facilities. An international forum offered participants an opportunity to hear from French, German, and Russian speakers about the microgravity research programs in their respective countries. Two keynote speakers provided broad technical overviews on multiphase flow and complex fluids research. Presenters briefed their peers on the scientific results of their ground-based and flight research. Fifty-eight of the sixty-two technical papers are included here

Advances in Optofluidics

Optofluidics a niche research field that integrates optics with microfluidics. It started with elegant demonstrations of the passive interaction of light and liquid media such as liquid waveguides and liquid tunable lenses. Recently, the optofluidics continues the advance in liquid-based optical devices/systems. In addition, it has expanded rapidly into many other fields that involve lightwave (or photon) and liquid media. This Special Issue invites review articles (only review articles) that update the latest progress of the optofluidics in various aspects, such as new functional devices, new integrated systems, new fabrication techniques, new applications, etc. It covers, but is not limited to, topics such as micro-optics in liquid media, optofluidic sensors, integrated micro-optical systems, displays, optofluidics-on-fibers, optofluidic manipulation, energy and environmental applciations, and so on

Electrical control and enhancement of dropwise condensation

Condensation of vapor typically occurs via the formation of condensate films on condensing surfaces; however, the liquid film imposes a substantial thermal resistance to heat transfer. Filmwise condensation heat transfer can be enhanced by 5-7X by condensing vapor as droplets, which roll-off the surface, thereby preventing buildup of a liquid film. Dropwise condensation heat transfer can be enhanced by the use of electrowetting (EW) to enhance coalescence, growth and shedding of condensed droplets. This dissertation includes several fundamental studies on EW-enhanced dropwise condensation. Experiments, analytical modeling and statistical modeling are used to gain a deeper understanding of droplet growth, coalescence and shedding under EW. Chapter 1 details the motivation for this study and the objectives of this dissertation. Chapter 2 includes a literature review of condensation, electrowetting and data science- based statistical methods. Chapter 3 presents a detailed experimental study of dropwise condensation of humid air under the influence of electrowetting fields. An analytical heat transfer model, which accounts for the presence of non-condensable gases, is used to predict the heat transfer benefits associated with electrowetting-assisted condensation. Chapter 4 presents a detailed analysis of electrowetting-induced coalescence dynamics of a distribution of water droplets. Statistical modeling-based algorithms are used to identify key electrowetting-related parameters that influence droplet coalescence; the influence of these parameters on coalescence is quantified. Chapter 5 studies droplet shedding dynamics under electrowetting and shows that an intermittent electric field can significantly increase condensation rates (as compared to a continuous electric field). A key finding is the almost complete removal of water from surfaces in very short durations (< 1 sec) is observed. It is also found that the extent and rate of water removal depends on the applied voltage and frequency of the AC EW waveform, respectively. Chapter 6 presents a novel approach and an experimentally validated model to analyze the oscillations of water droplets under the influence of AC electrowetting. Chapter 7 summarizes key conclusions and outlines suggestions for future work. Overall, the research reported in this dissertation has led to fundamental contributions in the areas of condensation and microfluidics. This multidisciplinary work has involved experiments, analytical modeling and statistical modeling. Results show that electrowetting fields influence all the phenomena important in dropwise condensation (growth, coalescence, shedding of droplets). Electrowetting is therefore a powerful tool to control and enhance condensation heat transfer. This research impacts applications in energy (steam condensation, refrigeration), water (atmospheric water harvesting, desalination) and infrastructure (self-cleaning).Mechanical Engineerin