The impact of liquid drops on solid surfaces is a ubiquitous phenomenon in our everyday experience; nevertheless, a general understanding of the dynamics governing droplet impact remains elusive. The impact event is understood within a commonly accepted hydrodynamic picture: impact initiates with a rapid shock and a subsequent ejection of a sheet leading to beautiful splashing patterns. However, this picture ignores the essential role of the air that is trapped between the impacting drop and the surface. We describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate on the air film laterally outward at surprisingly high velocities, consistent with theoretical predictions. We directly visualize the rapid spreading dynamics succeeding the impact of a droplet of fluid on a solid, dry surface. We show that the approach of the spreading liquid toward the surface is unstable, and lift-off of the spreading front away from the surface occurs. Lift-off ensues well before the liquid contacts the surface, in contrast with prevailing paradigm where lift-off of the liquid is contingent on solid-liquid contact and the formation of a viscous boundary layer. We show that when a drop impacts an atomically smooth mica surface, a strikingly stable nanometer thin layer of air remains trapped between the liquid and the solid. This layer occludes the formation of contact, and ultimately causes the complete rebound of the drop. / Engineering and Applied Sciences

The ζ-adsorption isotherm was used along with Gibbsian thermodynamics to determine an expression for the surface tension of solid-vapour interface. This expression was examined at low pressures to predict the surface tension of solids in the absence of adsorption, γS0. The method indicated the same value of γS0 for a solid using different vapour adsorption isotherms. A method based on the system stability was developed to predict the contact angle. The findings indicated that the contact angle is a thermodynamic property which depends on the state of the system. Furthermore, the dependence of contact angle on the curvature of three-phase contact line was described by the adsorption at the solid-liquid interface without the introduction of line tension. The energy transport mechanisms during steady-state evaporation of water-sessile droplets were studied. By suppressing the buoyancy-driven convection, the active modes of energy transport were thermal conduction and thermocapillary convection. The experiments on Cu, Au (111) and PDMS showed that the dominant mode of energy transport varies along the liquid-vapor interface. Near the droplet apex, thermal conduction provides enough energy for the evaporation. However, close to three-phase contact line where most of the evaporation occurs, thermocapillary convection is by far the dominant mode of energy transport. In the evaporation experiments on PDMS, the measured directions of thermocapillary convection were opposite of the predicted ones by other studies, since the energy carried by thermocapillary convection was neglected in the previous studies. The study was followed by examination of temperature boundary condition and energy transport at the solid-liquid interface. It was concluded that there is an adsorbed layer at the solid-liquid interface with different thermal properties compared to those of bulk liquid phase. This layer causes a resistance (Kapitsa resistance) and consequently a temperature discontinuity at the adsorbed layer-bulk liquid interface. Due to the high resistance at this interface, only a small portion of energy conducted by solid substrate enters directly to the bulk liquid phase. The remainder was transported through the adsorbed layer to the three-phase contact line. This energy was then distributed along the liquid-vapour interface by thermocapillary convection to be consumed by the evaporation process.

The ζ-adsorption isotherm was used along with Gibbsian thermodynamics to determine an expression for the surface tension of solid-vapour interface. This expression was examined at low pressures to predict the surface tension of solids in the absence of adsorption, γS0. The method indicated the same value of γS0 for a solid using different vapour adsorption isotherms. A method based on the system stability was developed to predict the contact angle. The findings indicated that the contact angle is a thermodynamic property which depends on the state of the system. Furthermore, the dependence of contact angle on the curvature of three-phase contact line was described by the adsorption at the solid-liquid interface without the introduction of line tension. The energy transport mechanisms during steady-state evaporation of water-sessile droplets were studied. By suppressing the buoyancy-driven convection, the active modes of energy transport were thermal conduction and thermocapillary convection. The experiments on Cu, Au (111) and PDMS showed that the dominant mode of energy transport varies along the liquid-vapor interface. Near the droplet apex, thermal conduction provides enough energy for the evaporation. However, close to three-phase contact line where most of the evaporation occurs, thermocapillary convection is by far the dominant mode of energy transport. In the evaporation experiments on PDMS, the measured directions of thermocapillary convection were opposite of the predicted ones by other studies, since the energy carried by thermocapillary convection was neglected in the previous studies. The study was followed by examination of temperature boundary condition and energy transport at the solid-liquid interface. It was concluded that there is an adsorbed layer at the solid-liquid interface with different thermal properties compared to those of bulk liquid phase. This layer causes a resistance (Kapitsa resistance) and consequently a temperature discontinuity at the adsorbed layer-bulk liquid interface. Due to the high resistance at this interface, only a small portion of energy conducted by solid substrate enters directly to the bulk liquid phase. The remainder was transported through the adsorbed layer to the three-phase contact line. This energy was then distributed along the liquid-vapour interface by thermocapillary convection to be consumed by the evaporation process.

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This study concerns the use of counter based laser Doppler anemometry in a wet steam flow of variable wetness fraction. Velocity measurements across the flow were made under different steam conditions. Comparison was made with a theoretical profile based upon a simple flow analysis. A small radial turbocharger was used as a means of extracting enthalpy homogeneously from a dry superheated flow of steam using the compressor as a brake. The wetness fraction of the exhaust was estimated using measured values of the thermodynamic properties. A laser extinction method was used to determine the number concentration and mean radius of the water droplets acting as natural scatterers in the wet steam. A laser anemometer was designed which made use of the properties of a propagating gaussian beam to produce a small probe volume. This was required to reduce the number of water droplets likely to be present simultaneously in the measuring volume. Good Doppler signals were obtained and these have been presented for a range of wet steam conditions. A computer model was developed to predict the scattering of laser light through wet steam. Results have shown that this can be accurately modelled using a particle size distribution function. The program written to perform the simulation takes into account both single and multiple scattering events. The parameters used for the distribution function required a knowledge of the wetness fraction and so provided a useful means of checking the estimate based upon the thermodynamic measurements. It has been demonstrated that the ability to obtain Doppler signals from the wet steam can be predicted by computation of the signal-to-noise ratio for the medium. Good results were obtained for the wet steam conditions under investigation.

The extraction of bitumen (heavy oil) from the oil sands is predominantly achieved through a water-based technology. This involves a slurrying process, typically called conditioning, which is categorized into three equally important steps: bitumen-sand liberation, bitumen coalescence, and air-bitumen attachment. Previous studies found that bitumen recovery was dependent upon process variables such as energy dissipation rate, temperature and caustic addition. Correlations between bitumen droplet size and recovery have also been established; however no investigations linking the aforementioned process variables to the resultant bitumen droplet size had been performed. This work investigates the development of a Batch Extraction Unit built specifically for this investigation as well as a study of the bitumen droplet size as a function of the rate of mechanical energy input. / Chemical Engineering

Droplet growth processes in a cumulus cloud, beginning from its inception at sub-micron scale up to drizzle drop size of few hundred microns, in an average duration of about half hour, has been a topic of intense research. In particular role of turbulence in aiding droplet growth in clouds has been of immense interest. Motivated by this question, we have performed experiments in which turbulent natural convection coupled with phase change is set up inside a tall vertical insulated tube, by heating water located at tube bottom and circulating cold air at tube top. The resulting moist turbulent natural convection flow in the tube is expected to be axially homogeneous. Mixing of air masses of differing temperature and moisture content leads to condensation of water vapor into droplets, on aerosols available inside the tube. We there-fore have droplets in a turbulent flow, in which phase change is coupled to turbulence dynamics, just as in clouds. We obtain a linear mean-temperature pro le in the tube away from its ends. Because there is net flux of water vapor through the tube, there is a weak mean axial flow, but which is small compared to turbulent velocity fluctuations. We have experimented with two setups, the major difference between them being that in one setup, called AC setup, tube is open to atmosphere at its top and hence has higher aerosol concentration inside the tube, while the other setup, called RINAC setup, is closed to atmosphere and due to presence of aerosol filters has lower aerosol concentration inside the tube. Also in the latter setup, cold air temperature at tube top can be reduced to sub-zero levels. In both setups, turbulence attains a stationary state and is characterized by Rayleigh number based on temperature gradient inside the tube away from its ends, which is 107. A significant result from our experiments is that in RINAC setup, we obtain a broadened droplet size distribution at mid-height of tube which includes a few droplets of size 36 m, which in real clouds marks the beginning of rapid growth of droplets due to collisions among them by virtue of their interaction with turbulence. This shows that for broadening of droplet size distribution, high turbulence levels prevalent in clouds is not strictly necessary. Second part of our study comprises two pieces of theoretical work. First, we deal with the problem of a large collector drop settling amidst a population of smaller droplets whose spatial distribution is homogeneous in the direction of fall. This problem is relevant to the last stage of droplet growth in clouds, when the droplets have grown large enough that they interact weakly with turbulence and begin to settle under gravity. We propose a new method to solve this problem in which collision process is treated as a discrete stochastic process, and reproduce Telford's solution in which collision is treated as a homogeneous Poisson process. We then show how our method may be easily generalized to non-Poisson collision process. Second, we propose a new method to detect droplet clusters in images. This method is based on nearest neighbor relationship between droplets and does not employ arbitrary numerical criteria. Also this method has desirable invariance properties, in particular under the operation of uniform scaling of all distances and addition/deletion of empty space in an image, which therefore renders the proposed method robust. This method has advantage in dealing with highly clustered distributions, where cluster properties vary over the image and therefore average of properties computed over the entire image could be misleading.

Droplet-based microfluidic devices have become a preferred versatile platform for various fields in physics, chemistry and biology to manipulate small amounts of liquid samples. In addition to microdroplets, microbubbles are also needed for various pro- cesses in the food, healthcare and cosmetic industries. Polydimethylsiloxane (PDMS) soft lithography, the mainstay for fabricating microfluidic devices, usually requires the usage of expensive apparatus and a complex manufacturing procedure. In ad- dition, current methods have the limited capabilities for fabrication of microfluidic devices within three dimensional (3D) structures. Novel methods for fabrication of droplet-based microfluidic devices for the generation microdroplets and microbubbles are therefore of great interest in current research. In this thesis, we have developed several simple, rapid and low-cost methods for fabrication of microfluidic devices, especially for generation of microdroplets and mi- crobubbles. We first report an inexpensive full-glass microfluidic devices with as- sembly of glass capillaries, for generating monodisperse multiple emulsions. Different types of devices have been designed and tested and the experimental results demon- strated the robust capability of preparing monodisperse single, double, triple and multi-component emulsions. Second, we propose a similar full-glass device for generation of microbubbles, but with assembly of a much smaller nozzle of a glass capillary. Highly monodisperse microbubbles with diameter range from 3.5 to 60 microns have been successfully produced, at rates up to 40 kHz. A simple scaling law based on the capillary number and liquid-to-gas flow rate ratio, successfully predicts the bubble size. Recently, the emergent 3D printing technology provides an attractive fabrication technique, due to its simplicity and low cost. A handful of studies have already demonstrated droplet production through 3D-printed microfluidic devices. However, two-dimensional (2D) flow structures are still used and the advantage of 3D-printing technique has not been fully exploited. Therefore, we apply 3D printing technology to fabricate 3D-miniaturized fluidic device for droplet generation (single emulsion) and droplet-in-droplet (double emulsion) without the need for surface wettability treat- ment of the channel walls, by utilizing 3D geometry design and fabrication. A scaling law is formulated to predict the drop size generated in the device. Furthermore, magnetically responsive microspheres are also produced with our emulsion templates, demonstrating the potential applications of this 3D emulsion generator in chemical and material engineering. Finally, we design and 3D-print a hybrid ?plug-and-play? microfluidic droplet generator, which involves a 3D-printed channel chamber and commercial tubings and fittings. By combination of 3D-printed part and market-available parts, this device can be easily assembled and disassembled, which provides a great flexibility for different demands. A scaling law has been proposed for prediction of drop size generated in the device. Furthermore, a 3D-printed concentration gradient generator and a droplet merging device based on the droplet generator have been developed to demonstrate the great scalability of 3D-printing technology.

Biodiesel has been proposed as a substitute for diesel given that biodiesel has lower net average greenhouse gas emissions than diesel. Additionally, alcohol may be added to biodiesel to improve biodiesel’s performance in a diesel engine as well to reduce engine emissions. This work will study the droplet evaporation process of alcohol-biodiesel blends. Due to alcohol’s polar nature and the fatty acid methyl esters’s (FAME) slightly polar nature, an appropriate method must be chosen to represent the evaporation process of a non-ideal mixture. The vapour-liquid equilibria was modelled in two ways: the first method uses only Raoult’s Law, while the second method uses Raoult’s law modified with activity coefficients calculated using the UNIFAC method. The comparison of the modelled results with experimental vapour-liquid equilibria data has shown that activity coefficients calculated using the UNIFAC method are able to accurately represent alcohol-biodiesel systems. Droplet evaporation experiments have been performed for biodiesel-propanol and biodiesel-pentanol blends at temperatures of 450°C and 700°C with the alcohol concentrations of 5%, 10%, 15%, and 20%. Additionally, the droplet evaporation was numerically modelled using two different models to represent the liquid state: a model with a well-mixed liquid phase and a model which includes component diffusion in the liquid phase. Comparing the experimental droplet temperatures to the numerical models has shown that the diffusion-limited model best represents the droplet evaporation process, suggesting that some of the alcohol components remain in the center of the droplet even when the droplet temperature is greater than the boiling temperature of the alcohol. This was further confirmed by observations of bubbling within the droplet during evaporation of the biodiesel-alcohol blends, in which there were both small bubbles and large bubbles forming. The formation of large bubbles has shown to correspond with the difference between experimental droplet diameter and the diffusion-limited model’s droplet diameter.

This thesis examines droplets striking water repelling surfaces as well as the movement of a soil based bacteria under various light and heat conditions. Droplet impact studies have shown that introducing a macroscopic feature to a water repelling surface can reduce the amount of time that droplet is in contact with the surface. By manipulating water droplets to impact different sized needles at varying speeds, we present how a needle can induce a similar reduction in the residence time of the droplet to more widely studied features. Results show the spreading and lift-off characteristics of the droplet are dependent on the impact speed as well as the size of macroscopic feature. A separate topic examines environmental motivators for mobility in a terrestrial cyanobacteria species called Microcoleus vaginatus. This cyanobacteria is indigenous to cold deserts, such as the Colorado Plateau or Mojave Desert in North America, and is essential to the health and preservation of the biological soil crust. These bacteria are the first organisms to grow in new soil, secreting a carbohydrate that acts as soil glue, thereby increasing soil adhesion. It has been shown that these bacteria will rise to the surface of the soil from their subsurface homes after rainfall, but it is unclear how they are able to make this journey. It is also unclear if other factors, such as nutrient levels or heat and light, affect their movement. Here we present an investigation of M. vaginatus’ response to light and heat in order to determine if these basic stimuli affect movement, thereby informing future restorative models.