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

Shear-horizontal surface acoustic wave microfluidics for lab-on-chip applications

Surface acoustic wave (SAW) devices based on the piezoelectric principle have been used extensively in telecommunication applications over the last 20 years, but have recently shown promise in the area of biomedical applications due to their efficient micro-fluidic functions and highly sensitive and label-free detection of pathogens, bacteria, cells, DNA and proteins. There are two types of surface acoustic wave modes: i.e., Rayleigh SAW (R-SAW) and shear horizontal SAW (SH-SAW). R-SAW is widely used for microfluidics and sensing in dry conditions, whereas SH-SAW is mainly used for sensing in liquid conditions. This thesis firstly reviewed the current theoretical and research progress related to these devices and application within the biomedical fields to date, and then the SH-SAW was applied into a novel lab-on-chip combining both bio-sensing and micro-fluidic functions. Simulations of the SH-SAW propagation on 36o Y-cut LiTaO3 were undertaken. Results showed a weak vertical wave component, and at a 90° rotation cut, the crystal was able to support a vertical Rayleigh component showing mixed sensing and streaming possibilities on a single crystal. Experimental investigation of the SH-SAW identified the ability for the shear wave to support mixing, pumping, heating, nebulisation and ejection of sessile droplets on the surface of the crystal with a theoretical explanation for the behaviour presented. A comparison with a standard R-SAW devices made of 128o Y-cut LiNbO3 and sputtered ZnO films was performed. This novel behaviour of digital microfludics, i.e., using sessile droplet with the SH-SAW, demonstrated by this work offers the possibility to manufacture a fully integrated micro-fluidic bio-sensing platform using a single crystal to realise a range of micro-fluidic functions

Evaporation of sessile droplets on pinning-free surfaces

Contact-line pinning is a fundamental limitation of diffusion-limited evaporation of sessile droplets. Sessile droplet evaporation is pervasive in a wide range of situations from ink-jet printing, to pesticide sprays and spotted microarrays. Contact-line pinning drives, for example, stick-slip motion and non-uniform deposition of solute within the droplet. Moreover, contact-line pinning is problematic in a wide range of situations, such as droplet microfluidics where capillary forces dominate the motion of liquid fronts. Recently, Slippery Liquid-Infused Porous Surfaces (SLIPS) have shown excellent droplet shedding abilities by use of a lubricating liquid, imbibed into a porous structure, immiscible to droplets on the surface. However, the lubricating liquid removes the droplet-solid-interaction, can cloak the droplet, can be several microns in thickness and the porous structure can be fragile to external mechanical forces. Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) is a new type of liquid-like surface, which is an attached coating rather than a retained liquid. SOCAL promises pinning-free properties while being nanometres thick and demonstrating mechanical robustness. Few research groups have reported successful creation of SOCAL surfaces. This thesis shows an optimised methodology to make reliably, pinning-free low-hysteresis SOCAL surfaces. This is done by modifying the parameters to create SOCAL and measuring the contact-angle hysteresis. A low contact-angle hysteresis of < 1° is achieved. These surfaces then show, for the first time, constant contact angle mode evaporation of sessile water droplets from a solid surface. This allows for the accurate measurement of the diffusion coefficient of water. An unexpected feature of the evaporation sequences is a step change increase in contact angle reminiscent of a type V adsorption isotherm. Attempts are made to characterise this using Dynamic Vapour Sorption (DVS) and Quartz Crystal Microbalance (QCM) techniques. This thesis also shows voltage-programmable control of water droplets on SOCAL using electrowetting. The unexpected behaviour of droplets on SOCAL is investigated and the electrowetting device is optimised. This allows control of the constant contact angle evaporation on both SLIPS and SOCAL. This is used to study the effect on the contact angle during the evaporation of sessile water droplets. The results of this thesis will benefit the aforementioned applications overcoming contact-line pinning and introducing new methods of controlling sessile droplet evaporation

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

Integration of virus-like particle macromolecular bioreceptors in electrochemical biosensors

Rapid, sensitive and selective detection of chemical hazards and biological pathogens has shown growing importance in the fields of homeland security, public safety and personal health. In the past two decades, efforts have been focusing on performing point-of-care chemical and biological detections using miniaturized biosensors. These sensors convert target molecule binding events into measurable electrical signals for quantifying target molecule concentration. However, the low receptor density and the use of complex surface chemistry in receptors immobilization on transducers are common bottlenecks in the current biosensor development, adding to the cost, complexity and time. This dissertation presents the development of selective macromolecular Tobacco mosaic virus-like particle (TMV VLP) biosensing receptor, and the microsystem integration of VLPs in microfabricated electrochemical biosensors for rapid and performance-enhanced chemical and biological sensing. Two constructs of VLPs carrying different receptor peptides targeting at 2,4,6-trinitrotoluene (TNT) explosive or anti-FLAG antibody are successfully bioengineered. The VLP-based TNT electrochemical sensor utilizes unique diffusion modulation method enabled by biological binding between target TNT and receptor VLP. The method avoids the influence from any interfering species and environmental background signals, making it extremely suitable for directly quantifying the TNT level in a sample. It is also a rapid method that does not need any sensor surface functionalization process. For antibody sensing, the VLPs carrying both antibody binding peptides and cysteine residues are assembled onto the gold electrodes of an impedance microsensor. With two-phase immunoassays, the VLP-based impedance sensor is able to quantify antibody concentrations down to 9.1 ng/mL. A capillary microfluidics and impedance sensor integrated microsystem is developed to further accelerate the process of VLP assembly on sensors and improve the sensitivity. Open channel capillary micropumps and stop-valves facilitate localized and evaporation-assisted VLP assembly on sensor electrodes within 6 minutes. The VLP-functionalized impedance sensor is capable of label-free sensing of antibodies with the detection limit of 8.8 ng/mL within 5 minutes after sensor functionalization, demonstrating great potential of VLP-based sensors for rapid and on-demand chemical and biological sensing