A half-zone experimental set-up was used to study the effects of various liquid bridge and helium flow parameters on the onset of thermocapillary convection in silicone oil liquid bridges. Experiments confirmed that helium flow has a stabilizing effect, with the effect increasing with helium velocity. Furthermore, helium flow in the same direction as surface flow due to Marangoni convection had a more stabilizing effect than countercurrent flow. It was established that increasing helium temperature has a mixed effect, producing a less stable bridge at low helium flow rates, but a more stable flow pattern at higher helium flow rates. Finally, it was confirmed that decreasing the cold disk temperature results in a decrease in critical temperature difference.

No description available.

No description available.

A numerical code for solving the time-dependent incompressible 3D Navier-Stokes equations with finite volumes on overlapping staggered grids in cylindrical and rectangular geometry is developed. In the code, written in FORTRAN, the momentum equation for the velocity is solved by projection method and Poisson equation for the pressure is solved by ADI implicit method in two directions combined with discrete fast Fourier transform in the third direction. A special technique for overcoming the singularity on the cylinder's axis is developed. This code, taking into account dependence upon temperature of the viscosity, density and surface tension of the liquid, is used to study the fluid motion in a cylinder with free cylindrical surface (under normal and zero-gravity conditions); and in a rectangular closed cell with a source of thermocapillary convection (bubble inside attached to one of the cell's faces). They are significant problems in crystal growth and in general experiments in fluid dynamics respectively. Nevertheless, the main study is dedicated to the liquid bridge problem. The development of thermocapillary convection inside a cylindrical liquid bridge is investigated by using a direct numerical simulation of the 3D, time-dependent problem for a wide range of Prandtl numbers, Pr = 0.01 - 108. For Pr > 0.08 (e.g. silicon oils), above the critical value of temperature difference between the supporting disks, two counter propagating hydrothermal waves bifurcate from the 2D steady state. The existence of standing and traveling waves is discussed. The dependence of viscosity upon temperature is taken into account. For Pr = 4, 0-g conditions, and for Pr = 18.8, 1-g case with unit aspect ratio an investigation of the onset of chaos was numerically carried out. For a Pr = 108 liquid bridge under terrestrial conditions , the appearance and the development of thermoconvective oscillatory flows were investigated for different ambient conditions around the free surface. Transition from 2D thermoconvective steady flow to a 3D flow is considered for low-Prandtl fluids (Pr = 0.01) in a liquid bridge with a non-cylindrical free surface. For Pr < 0.08 (e.g. liquid metals), in supercritical region of parameters 3D but non-oscillatory convective flow is observed. The computer program developed for this simulation transforms the original non-rectangular physical domain into a rectangular computational domain. A study of how presence of a bubble in experimental rectangular cell influences the convective flow when carrying out microgravity experiments. As a model, a real experiment called TRAMP is numerically simulated. The obtained results were very different from what was expected. First, because of residual gravity taking place on board any spacecraft; second, due to presence of a bubble having appeared on the experimental cell's wall. Real data obtained from experimental observations were taken for the calculations.

This study uses the Phase-field method to simulate the transient thermal current of the metal surface heated and molten by a massing energy. The flow field uses a two-dimension module, considered with the mass conservation equation, momentum equation, energy equation and level-set equation, to solve for the distribution in whole domain, including the interface, of temperature, velocity, pressure and level-set number. We ignore the effect of concentration diffusion, but consider about the effect of heat translation on the flow field. Finally the results will display the flows of air around molten area forced by buoyancy which is caused by high temperature, and the flows in molten area forced by thermocapillary which is caused by temperature gradient.

No description available.

No description available.

Ultrasound, applied in combination with microbubbles, has potential as a means to enhance the uptake of therapeutic agents, which could include drugs and nucleic acids, into biological cells. This process is commonly referred to as 'sonoporation', and the enhanced uptake can be caused through the incident ultrasonic pressure fi eld causing radial oscillations (cavitation) in the microbubbles, amongst other possibilities. However, the mechanisms responsible for any resultant increase in cell membrane permeability are not yet fully understood. This project focussed on achieving a more fundamental understanding of these salient processes by building on a platform of previous work within the group. One strand of the project involved a complete characterisation of the performance of a rotating mirror high speed camera (Cordin 550-62) that was previously used by our group [and others] to investigate microbubble cavitation phenomena and interactions with proximal cell membranes. Speci cally, I present herein an investigation into the image formation process with this type of camera, the essence of which stymied previous data interpretations. I demonstrate that an inherent asynchrony in the exposure of pixels within individual image frames leads to a temporal anomaly. This was achieved using low cost, flashing LED lights and resulted in the extraction of an algorithm to correct for the temporal anomaly. In a slightly diff erent context, the delivery of suitable ultrasonic fields is necessary to achieve a uniform treatment across a therapeutic target. This thesis also reports on a study on the design of ultrasonic lenses to alter the focal region of a focussed ultrasound transducer with the aim of producing focal regions that can enable sonoporation of tumours of varying sizes. We show that the use of lenses can be an inexpensive alternative to more complex systems such as phased array transducers. Design modelling and experimental testing of lens prototypes are presented along with preliminary results with tissue mimicking polyacrylamide gel phantoms. The target environment in which the process of sonoporation will be clinically useful (i.e. in the physiological circulation) can be simpli ed as a microfluidic system. One strategy for bubble mediated therapy involves the use of a pro-drug approach, that is, when two otherwise benign ingredients are loaded onto separate microbubble populations, but can become mixed at the anatomical target site by the action of focussed ultrasound whereupon a potent drug is produced. The required mixing can be achieved by the violent coalescence of nearby cavitating bubbles, their reaction product then being released and di ffused into the interiour of nearby cells through sonoporation. A study related to this field is presented here where laser induced thermocapillary flows are shown to cause mixing of the content of a drop in a microfluidic channel in a bid to understand the mixing process at a level that may assist future microbubble engineering strategy. To summarise then, the work presented in this thesis has consolidated earlier unpublished data sets achieved by the group, providing new and exacting experimental evidence and an accurate algorithm that will facilitate post-processing of that earlier data (Chapters 2-3). Moreover, group aspirations to translate earlier in-vitro work on sonoporation towards next phase medical-phantom exposures have been boosted through the provision of a new direction involving acoustic lensing, the experimental data from which was used to completely validate existing models for our own design scenarios (Chapter 4). Finally, previous unpublished observations on microbubble coalescence undertaken by the group suggested a means to implement pro-drug delivery with direct in-situ mixing. Such suggestions were explored within microfluidic contexts using lasers to control and visualise the mixing processes that might arise in such situations (Chapter 5). All of these new insights have served to consolidate the group's previous and as yet unpublished data, opening the way for dissemination with confidence in the integrity of that data, and have also extended group capability and expertise in the areas of MHz-rate high speed framing cameras, the fabrication of acoustic lenses, and with microfluidic mixing.

A droplet may be prevented from wetting a solid surface by the existence of a lubricating film of air, driven by theromcapillary convection, between liquid and solid surfaces. The noncontact nature and the load-carrying capability of a nonwetting droplet lead to potential engineering applications, e.g., low-friction bearings. The present research consists of two thrusts. The first is aimed at quantifying nonwetting-system failures (film and pinning) triggered by application of a mechanical load, gaining insights to failure mechanisms. Experimental results show that film failure occurs over a wide range of droplet volumes when the temperature difference between the droplet and the plate, the driving potential of the free-surface motion, is small. Interferometric observations reveal flow instability just prior to film failure, with the growth of a nonaxisymmetric disturbance on a free surface (m = 1). Pinning failure becomes more prevalent as the temperature difference is increased, stabilizing the film flow. As part of the present investigation, a system was devised, allowing an oscillating free-surface to be reconstructed from a series of interferograms. The dynamic responses of the free surface reveal mode coupling, with harmonics of the input frequency excited through nonlinearity. The second thrust of the research succeeded in levitating and translating a droplet using the mechanism of permanent nonwetting. In this scheme, the droplet is heated by a CO2 laser and is placed above a cooled glass surface in order to drive the lubricating film that supports the weight of the drop. Furthermore, the position of the droplet can be controlled by moving the heating location, which leads to an asymmetry of the flow fields, driving air from the cooler-end of the droplet and propelling it towards the heat source. These demonstrations suggest the techniques potential use as a liquid-delivery scheme in a Lab-On-a-Chip system. Modeling is carried out to estimate propulsive forces on the droplet and to explain oscillatory behavior observed when excessive heating is applied on the drop. The concept to sandwich a droplet between two plates, a necessary configuration for levitating smaller droplets (less than mm-scale), is also discussed.

Thin liquid layers have been proposed for heat extraction and protection of the solid surfaces of divertors in magnetic fusion reactors. A number of conceptual designs for plasma-facing components (PFC) use stationary and flowing liquid layers as a renewable first wall for reactor chambers to remove heat and shield solid surfaces from damaging radiation while maintaining acceptable plasma purity levels. Such liquid-protected PFC have the potential to make fusion more commercially attractive by increasing reactor lifetimes and decreasing failure rates. The results of this research will help identify the parameter ranges for successful operation of such protection schemes. This thesis investigates the thermocapillary behavior of axisymmetric horizontal liquid layers with initial heights from 0.27 to 3.0 mm. A negative radial temperature gradient is imposed at the bottom of the liquid layer. Experimental, numerical and asymptotic analyses were carried out for thin layers where buoyancy forces are negligible. A novel asymptotic solution for this axisymmetric geometry was derived from the previous two-dimensional long-wave analysis by Sen et al. (1982). A numerical simulation using the level contour reconstruction method was used to follow the evolution of the liquid-gas interface above an axisymmetric non-isothermal solid surface. Experimental validation of the theoretical and numerical studies was performed using silicone oils of various viscosities (μ = 0.48 × 10-2 9.6 × 10-2 N s/m2). Two measurement techniques, a needle contact method and laser-confocal displacement method, were employed to obtain height profiles for applied temperature differences up to 65°C. Finally, reflectance shadowgraphy was used to visualize free-surface deformation and classify flow regimes in thick layers, where the assumptions of negligible buoyancy and axisymmetric flow are no longer valid. The results of this investigation will allow designers to determine operating windows for successful implementation of liquid-protected PFC.