Precursor films in wetting phenomena

The spontaneous spreading of non-volatile liquid droplets on solid substrates poses a classic problem in the context of wetting phenomena. It is well known that the spreading of a macroscopic droplet is in many cases accompanied by a thin film of macroscopic lateral extent, the so-called precursor film, which emanates from the three-phase contact line region and spreads ahead of the latter with a much higher speed. Such films have been usually associated with liquid-on-solid systems, but in the last decade similar films have been reported to occur in solid-on-solid systems. While the situations in which the thickness of such films is of mesoscopic size are rather well understood, an intriguing and yet to be fully understood aspect is the spreading of microscopic, i.e., molecularly thin films. Here we review the available experimental observations of such films in various liquid-on-solid and solid-on-solid systems, as well as the corresponding theoretical models and studies aimed at understanding their formation and spreading dynamics. Recent developments and perspectives for future research are discussed.Comment: 51 pages, 10 figures; small typos correcte

Methods and Instrumentation of Sample Injection for XFEL Experiments

abstract: ABSTRACT X-Ray crystallography and NMR are two major ways of achieving atomic resolution of structure determination for macro biomolecules such as proteins. Recently, new developments of hard X-ray pulsed free electron laser XFEL opened up new possibilities to break the dilemma of radiation dose and spatial resolution in diffraction imaging by outrunning radiation damage with ultra high brightness femtosecond X-ray pulses, which is so short in time that the pulse terminates before atomic motion starts. A variety of experimental techniques for structure determination of macro biomolecules is now available including imaging of protein nanocrystals, single particles such as viruses, pump-probe experiments for time-resolved nanocrystallography, and snapshot wide- angle x-ray scattering (WAXS) from molecules in solution. However, due to the nature of the "diffract-then-destroy" process, each protein crystal would be destroyed once probed. Hence a new sample delivery system is required to replenish the target crystal at a high rate. In this dissertation, the sample delivery systems for the application of XFELs to biomolecular imaging will be discussed and the severe challenges related to the delivering of macroscopic protein crystal in a stable controllable way with minimum waste of sample and maximum hit rate will be tackled with several different development of injector designs and approaches. New developments of the sample delivery system such as liquid mixing jet also opens up new experimental methods which gives opportunities to study of the chemical dynamics in biomolecules in a molecular structural level. The design and characterization of the system will be discussed along with future possible developments and applications. Finally, LCP injector will be discussed which is critical for the success in various applications.Dissertation/ThesisDoctoral Dissertation Physics 201

Investigations into Convective Deposition from Fundamental and Application-Driven Perspectives

Crystalline particle coatings can provide critical enhancement to wide-ranging energy and biomedical device applications. One method by which ordered particle arrays can be assembled is convective deposition. In convective deposition, particles flow to a surface via evaporation-driven convection, then order through capillary interactions. This thesis will serve to investigate convective deposition from fundamental and application-driven perspectives. Motivations for this work include the development of point-of-care diagnostic devices, macroporous membranes, and various energy applications. Immunoaffinity cell capture devices display enhanced diagnostic capabilities with intelligently varied surface roughness in the form of particle coatings. Relatedly, highly crystalline particle coatings can be used to template the fabrication of macroporous polymer membranes. These membranes display highly monodisperse pores at particle contact points. In addition, ordered areas of particles, acting as microlenses, can enhance LED performance by 2.66-fold and DSSC efficiency by 30%. Previous research has targeted the formation of crystalline monolayers of particles. However, much insight can be gleaned from imperfect coatings. The analysis of submonolayer coatings, exhibiting significant void spaces, provides insight as to the specific mechanisms and timescales for flow and crystallization. A pair of competing deposition modes, termed ballistic and locally-ordered, enables the intelligent design of experiments and enables significant enhancement in control of resultant thin film morphology. Surface tension-driven particle assembly is subject to a number of native instabilities and macroscale defects that can irreversibly compromise coating uniformity. These include the formation of three-dimensional streaks, where surface tension-driven flow spurs on the nucleation of large imperfections. These imperfections, once nucleated, exhibit a feedback loop of dramatically enhanced evaporation and resultant flow. In addition, thick nanoparticle coatings, subject to enormous drying stresses, exhibit highly uniform crack formation and spacing in an attempt to minimize system energy. Both these imperfections yield insight on convective deposition as a fundamental phenomenon, and intelligent design of experiments moving forward. Cracking can be suppressed through layer-by-layer particle assembly, whereas streaking can be controlled via several significant process enhancements. Process enhancements include the addition of smaller constituent, as packing aids, to suspension, the application of lateral vibration, and the reversal of relevant surface tension gradients. The transition from unary to binary suspensions represents a significant improvement to convective deposition as a process. Nanoparticles act as packing, and flow, aids, wholly suppress macroscale defects under ideal conditions. A relative deficiency or excess of nanoparticles can generate complex coating morphologies including multilayers and transverse stripes. The application of lateral vibration to convective deposition allows the assembly of monolayer particle coatings under a larger range of operating conditions and at a faster rate. Macroscale defect formation can increased through an enhancement of the natural condition, where evaporative cooling generates a thermal gradient in drying droplets. Conversely, these defects can be suppressed with a reversal of this gradient, which will reverse the direction of surface tension-driven recirculation. These fundamental developments in understanding, and associated process enhancements, are critical in current efforts to scale up convective deposition. As convective deposition evolves from laboratory-scale batch experiments to continuous, large scale, coatings, repeatability and robustness, as well as an ability to controllably change thin film morphology, will be essential

Modelling of dynamical effects related to the wettability and capillarity of simple and complex liquids

This Thesis explores physical phenomena characteristic for thin liquid films and small droplets of simple and complex liquids on solid substrates for which wettability and capillarity control their statical and dynamical properties. We start by discussing the general concepts of wettability and capillarity and introduce the common mathematical framework of the lubrication approximation for studies of thin liquid films and small contact angle drops. We demonstrate the derivation of the generic equation describing the evolution of a film of simple liquid from the Navier-Stokes equations. We show how this model can be further extended to incorporate various effects relevant to the case of complex liquids. The results described in the Thesis comprise three projects with the common main theme of the influence of wettability and capillarity on the statics and dynamics of the studied systems, namely (i) Evaporating sessile droplets fed through the solid substrate - a geometry that allows us to discuss steady states of the system and their role in the time evolution of freely evaporating droplets without influx in an isothermal case; (ii) The influence of a solute--dependent wettability on the stability, static and dynamical properties of thin films and drops of non-volatile mixtures, suspensions and solutions; (iii) A parameter-passing scheme between particle-based Molecular Dynamics simulations and the continuum lubrication model which allows us to discuss equilibrium properties of small polymeric droplets. We present the physical questions arising in the three systems and discuss approaches and results as well as possible extensions

Enhanced Temperature Control Method Using ANFIS with FPGA

Temperature control in etching process is important for semiconductor manufacturing technology. However, pressure variations in vacuum chamber results in a change in temperature, worsening the accuracy of the temperature of the wafer and the speed and quality of the etching process. This work develops an adaptive network-based fuzzy inference system (ANFIS) using a field-programmable gate array (FPGA) to improve the effectiveness. The proposed method adjusts every membership function to keep the temperature in the chamber stable. The improvement of the proposed algorithm is confirmed using a medium vacuum (MV) inductively-coupled plasma- (ICP-) type etcher

ULTRAFAST OPTICAL RESPONSE AND TRANSPORT PROPERTIES OF STRONTIUM TITANATE-BASED COMPLEX OXIDE NANOSTRUCTURES

As the silicon-based semiconductor integrated circuits led by Moore's Law approaching their physical limits, the search for a new generation of nanoelectronic and nanophotonic devices is becoming a hot topic in this post-Moore era. The strontium titanate-based complex oxide heterostructure appears to be a promising alternative due to its diverse emergent properties. Being able to control the metal-insulator transition at the polar/nonpolar LaAlO3/SrTiO3 interface using conductive atomic force microscopy (c-AFM) lithography has made LaAlO3/SrTiO3, in particular, an attractive platform. Expanding the class of heterostructures which can be controlled at nanoscale dimensions is important for alternative oxide-based nanodevices. In this dissertation, the writing and erasing of nanostructures at the nonpolar/nonpolar oxide interface of CaZrO3/SrTiO3 using c-AFM lithography is investigated. Conducting nanostructures as narrow as 1.2 nm at room temperature is achieved. Low-temperature transport measurements based on these nanostructures provide insight into the electronic structure of the CaZrO3/SrTiO3 interface. Such extreme nanoscale control, with dimensions comparable to most single-walled carbon nanotubes, holds great promise for oxide-based nanoelectronic devices. Nanophotonic devices operating at terahertz frequencies, on the other hand, offer unique information for many applications. In this dissertation, broadband nanoscale terahertz generators based on c-AFM lithography defined LaAlO3/SrTiO3 nanojunctions are proved to be able to detect the plasmonic response of a single gold nanorod. By femtosecond pulse shaping using a home-built pulse shaper, over 100 THz bandwidth selective difference frequency generation at LaAlO3/SrTiO3 nanojunctions is also demonstrated, which has great potential in both studying fundamental light-matter interaction and realizing selective control of rotational or vibrational resonances in nanoparticles. With this unprecedented control of THz field, the two-dimensional (2D) material graphene and its coupling with the quasi-2D LaAlO3/SrTiO3 interface are also under investigation. The preliminary data shows evidence for graphene response up to 60 THz. These results help to fill the terahertz gap as well as offer new opportunities for oxide-based nanophotonic devices or even hybrid optoelectronic integrated circuits

Development of a Digital Microfluidic Toolkit: Alternative Fabrication Technologies for Chemical and Biological Assay Platforms

This thesis proposes the development of a digital microfluidics (DMF) device using alternative fabrication methods and materials for application in chemical and biological assays. DMF technology which relies on electrowetting-on-dielectric (EWOD) mechanism, offers several advantages such as reduced sample volume, faster analysis, device flexibility, and portability. It is however not without shortcomings as the fabrication of DMF devices is expensive while the reliability of such devices is reduced due to surface contamination when highly concentrated biomolecular samples (e.g. protein and cells) are used. The first experimental work in this thesis aims to reduce the cost of electrode patterning of DMF devices by investigating the use of inkjet printing method in conjunction with several combinations of conductive ink and substrate. It has been found that EWOD device made of PEDOT:PSS, a type of conductive polymer ink printed on Melinex®, a polyethylene terephthalate substrate presents the most reliable droplet actuation performance with velocity comparable to the standard chrome-on-glass device. Two types of inkjet-printed PEDOT:PSS-on-Melinex® device have been fabricated; one is a 3D 4 × 4 electrode array device and the other is a magnetic micro-immunoassay device establishing the feasibility of the proposed method. The 3D 4 × 4 electrode array device which utilises both sides of the substrate (i.e. top and bottom surfaces) for electrode patterning allows for future construction of multi-level DMF devices with large functional area. Implementation of such electrode design increases throughput as it made multiple parallel assays possible. The second inkjet-printed device demonstrates the possibility of employing the PEDOT:PSS-on-Melinex® device in heterogeneous immunoassay by successfully performing mixing and merging of two droplets and more importantly the magnetic beads separation operation. The second experimental investigation concerns the search for substitute materials for the dielectric and hydrophobic components of EWOD device using off-the-shelf products. For the dielectric component, the best performing material in terms of electrowetting reversibility is Rust-Oleum® Polyurethane Finish while for the hydrophobic surface is Top Coating of NeverWet® superhydrophobic material. Both are low-cost materials which employ a very simple spraying technique as their fabrication method. The NeverWet® superhydrophobic material has been selected for detailed investigation due to its other potential function as an anti-biofouling surface to either eliminate or minimise the biomolecules adsorption problem. The superhydrophobic material has shown great potential by demonstrating droplet contact angle reversibility and low roll-off angle for highly concentrated protein solution indicating low adsorption of protein on its surface. A superhydrophobic EWOD device has been fabricated using the Top Coating of NeverWet® as the actuating surface and the device has reliably transported concentrated protein droplets across its surface. It is hoped that the findings in the thesis will assist towards the future realisation of low-cost and robust DMF devices for a wide range of biological and chemical assays applications outside of conventional laboratory environment

Dynamic spatially resolved unilateral NMR measurements of liquid ingress and vapour adsorption and desorption in heterogeneous layered fabrics

This thesis presents investigations of liquid ingress and vapour uptake in different porous media including textile fabrics and activated carbons, monitored by means of a unilateral NMR instrument. The aim of this work is to assess protective materials which prevent toxic liquid ingress and toxic vapour uptake from contaminating materials and personnel. A high performance fabric made of a combination of coated and not coated fibres can provide extremely high protection against toxic liquids. By incorporating an adsorbent layer between two highly repellent layers, an “intelligent” fabric that can prevent complete penetration through the composite system by toxic vapours can be constructed. This project was undertaken with a low-field unilateral profile NMR Mouse® (MObile Universal Surface Explorer) which can collect signal from a thin and flat sensitive volume (ca. 1.5 cm x 1.5 cm x 0.6 mm) up to 10 mm above it, and in a non-invasive manner. The instrument uses a strong inherent magnetic field gradient (11.38 T.m-1) in conjunction with pulsed radio frequency waves. The method makes use of Fourier Transformed NMR in order to spatially resolve 1D vertical profiles for each measurement over a field of view exceeding 500 µm and with a spatial resolution of 15 µm. One system investigated was a laminate heterogeneous layered fabric, made of a horizontal stack of three individual layers, each 70 μm thick, constructed from entangled fibres of 10 µm in diameter. The top and bottom layers are strongly repellent to the oil used as a model that represents a simulant for a toxic liquid, whilst the middle layer is non-repellent and allows oil to absorb inside