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Evaporation and wetting of droplets are a phenomena present in everyday life and in many industrial, biological or medical applications; thus controlling and understanding the underlying mechanisms governing this phenomena becomes of paramount importance. More recently, breakthroughs in the fabrication of new materials and nanomaterials have led to the synthesis of novel nanoscale particulates that dispersed into a base fluid modify the properties of this latter. Enhancement in heat transfer or the self-assembly of the particles in suspension during evaporation, are some of the areas in which nanofluids excel. Since it is a relatively new area of study, the interplay particle-particle, particle-fluid or particle-substrate at the macro-, micro-, and nanoscale is yet poorly understood. This work is an essay to elucidate the fundamental physics and mechanisms of these fluids during free evaporation, of great importance for the manipulation and precise control of the deposits. The evaporative behaviour of pure fluids on substrates varying in hydrophobicity has been studied and an unbalance Young’s force is proposed to explain the effect of substrate hydrophilicity on the pinning and the depinning forces involved during droplet evaporation. On other hand, the addition of nanoparticles to a base fluid modifies the evaporative behaviour of the latter and: a more marked “stick-slip” behaviour is observed when increasing concentration on hydrophobic substrates, besides the longer pinning of the contact line reported on hydrophilic ones when adding nanoparticles. A deposition theory to explain the final deposits observed, for the outermost ring, after the complete vanishing of a 0.1% TiO2-ethanol nanofluid droplet has also been developed. In addition, the evaporation of pinned nanofluid droplets on rough substrates at reduced pressures has been systematically studied. A revisited Young-Lippmann equation is proposed as one of the main findings to explain the enhancement on electrowetting performance of nanoparticle laden fluid droplets when compared to the pure fluid case. On the other hand, of relevant importance is the absence of “stick-slip” behaviour and the more homogeneous deposits found after the complete evaporation of a nanofluid droplet under an external electric field applied when compared to free evaporation of these fluids.

This study explores an innovative approach to control the droplet size distribution produced by an electrospray with the intention of eventually being able to deliver precisely controlled quantities of precursor materials for nanofabrication. The technique uses a thermionic cathode to charge the droplets in excess of the Rayleigh limit, leading to droplet breakup or fission. The objective of these experiments was to assess whether the proposed technique could be used to produce a new droplet size distribution with a smaller mean droplet diameter without excessively broadening the distribution. An electrospray was produced in a vacuum chamber using a dilute mixture of ionic liquid. During their transit from the capillary source to a diagnostic instrument, the resulting droplets were exposed to an electron stream with controlled flux and kinetic energy. The droplets were sampled in an inductive charge detector to characterize changes in the size distribution. A positively biased anode electrode was used to collect electron current during droplet exposure. This collected current was used as the primary control variable and used as a measure of the electron flux. The anode bias voltage was a secondary control variable and used as a measure of the electron energy. In a series of seven tests, two sets showed evidence of fission having occurred resulting in the formation of two droplet populations after electron bombardment. Three sets of results showed evidence of a single droplet population after electron bombardment, but shifted to a smaller mean diameter, and one set of results was inconclusive. Because of the large standard deviation in the droplet diameter distributions, the two cases in which a second population was evident were the strongest indication that droplet fission had occurred.

The effects of liquid superheat on the disruption of liquid droplets accelerated in a supersonic flow were examined experimentally in a drawdown supersonic wind tunnel. Monodisperse 60 ìm diameter droplets of two test fluids (methanol and ethanol) were generated upstream of the entrance to the tunnel and accelerated with the supersonic flow such that their maximum velocities relative to the air flow were transonic. Droplets were imaged by shadowgraphy and by multiple-exposure direct photography using planar laser sheet illumination. In addition to providing information on droplet lifetime, the latter technique allows measurement of the droplet downstream distance versus time, from which the velocity and acceleration during disruption can be inferred. All droplets were unheated upon injection. Depending on the vapor pressure of the liquid, the droplets achieved varying levels of liquid superheat as they experienced low static pressure in the supersonic flow. Histograms of the droplet population downstream of the supersonic nozzle throat indicate that the lifetime of droplets in supersonic flow decreases with an increasing amount of droplet superheat. The shorter lifetime occurs even as the droplet Weber number (based on initial droplet size) decreases initially due to the lower relative velocity of the methanol droplets to that of ethanol droplets. This is due to a higher acceleration than ethanol droplets of comparable initial size. This is consistent with the more rapid disruption and the faster decrease in mass for the methanol droplets. The droplets, depending on the level of superheating, in some cases underwent disruption modes different than those expected for the corresponding values of Weber number.

We present an experimental study on the behavior of bubbles captured in a Taylor vortex. The gap between a rotating inner cylinder and a stationary outer cylinder is filled with a Newtonian mineral oil. Beyond a critical rotation speed (ω[subscript c]), Taylor vortices appear in this system. Small air bubbles are introduced into the gap through a needle connected to a syringe pump. These are then captured in the cores of the vortices (core bubble) and in the outflow regions along the inner cylinder (wall bubble). The flow field is measured with a two-dimensional particle imaging velocimetry (PIV) system. The motion of the bubbles is monitored by using a high speed video camera. It has been found that, if the core bubbles are all of the same size, a bubble ring forms at the center of the vortex such that bubbles are azimuthally uniformly distributed. There is a saturation number (N[subscript s]) of bubbles in the ring, such that the addition of one more bubble leads eventually to a coalescence and a subsequent complicated evolution. Ns increases with increasing rotation speed and decreasing bubble size. For bubbles of non-uniform size, small bubbles and large bubbles in nearly the same orbit can be observed to cross due to their different circulating speeds. The wall bubbles, however, do not become uniformly distributed, but instead form short bubble-chains which might eventually evolve into large bubbles. The motion of droplets and particles in a Taylor vortex was also investigated. As with bubbles, droplets and particles align into a ring structure at low rotation speeds, but the saturation number is much smaller. Moreover, at high rotation speeds, droplets and particles exhibit a characteristic periodic oscillation in the axial, radial and tangential directions due to their inertia. In addition, experiments with non-spherical particles show that they behave rather similarly. This study provides a better understanding of particulate behavior in vortex flow structures. / Singapore-MIT Alliance (SMA)

An experimental investigation was performed to study the influence of the bubble size on an effervescent atomization. Experiments were conducted in horizontal facility with a 25.4mm diameter feeding pipe using water and air as the working fluids that were sprayed through an effervescent nozzle. Water flow rates from 113 to 189 kg/min and air to liquid mass ratios from 1% to 4% were selected. High speed photographs, of the bubbles in the feeding conduit and of the resulting droplets on the spray, were taken to use the particle projected areas to estimate their sizes. A monotonic positive correlation was found between the bubble size and the droplet size, in a fairly narrow range of feed flow void fractions. A bubble size sensitivity parameter was defined. Knowledge of the droplet behaviour provides data to enhance the design and operating conditions of the atomization process and a means to control droplet size.

An experimental investigation was performed to study the influence of the bubble size on an effervescent atomization. Experiments were conducted in horizontal facility with a 25.4mm diameter feeding pipe using water and air as the working fluids that were sprayed through an effervescent nozzle. Water flow rates from 113 to 189 kg/min and air to liquid mass ratios from 1% to 4% were selected. High speed photographs, of the bubbles in the feeding conduit and of the resulting droplets on the spray, were taken to use the particle projected areas to estimate their sizes. A monotonic positive correlation was found between the bubble size and the droplet size, in a fairly narrow range of feed flow void fractions. A bubble size sensitivity parameter was defined. Knowledge of the droplet behaviour provides data to enhance the design and operating conditions of the atomization process and a means to control droplet size.

The most popular way to achieve accelerated expansion of the universe is by introducing a scalar field in which motion of state varies with time. The accelerated expanded universe was first observed by Type Ia supernovae and future confirmed by the latest of CMB (Cosmic Microwave Background). The reason for the accelerated universe is the existence of dark energy. In this dissertation, we discuss the relationship between dark matter, dark energy, reheating and the standard model, and we find that it is possible for us to unify dark energy, dark matter and a reheating field into one scalar field. There is a very important stage called inflationary, and we find that the residue of the inflationary field, which is also described by a scalar field, can form bubbles in our universe due to the gravity force. We discuss that these bubbles are stable since they are trapped in their potential wells, and the bubbles can be a candidate for dark matter. We also discuss the scalar singlet filed, with the simplest interaction with the Higgs field, and we find that a static, classical droplet can be formed. The physics picture of the droplet is natural, and it is almost the same as the formation of an oil droplet in water. We show that the droplet is absolutely stable. Due to the very weak interaction with the Standard Model particles, the droplet becomes a very promising candidate for dark matter.

Spray cooling is a promising technique which is used to remove large amounts of heat from surfaces. It is characterized by uniform heat removal, low droplet impact velocity and better cooling efficiency when compared to other cooling schemes. It can be used in electronic cooling, and other applications. However, due to the multiple impacts of droplets, the film fluid dynamics and morphology are quite complicated. Moreover, the effect of heat transfer under spray cooling is not well understood due to the large number of interdependent variables such as impact spacing, impact angle, droplet diameter, droplet velocity and droplet frequency to name a few. An experimental approach is proposed and used to minimize and control key independent variables to determine their effects on surface temperature and heat transfer cooling mode. The effects of droplet impact angle and spacing on different heat flux conditions are studied. The film thickness is also obtained to further investigate the relationship between the independent variable and the observed heat transfer mechanism. The study of coherent droplet impingement on an open surface is experimentally characterized using high speed imaging and infrared thermography. Single stream droplet impingent cooling with different impact angle is also studied. Temperature distribution and impact crater morphology are obtained under different heat flux conditions. Film thickness inside droplet impact craters is measured to understand the relationship between minimum surface temperature and film thickness. Next, double streams droplet impingement cooling with different spacings and impact angles are investigated. The optimum spacing is found to reduce the droplet-to-droplet collision and to minimize splashing, resulting in enhanced heat transfer and better use of the cooling fluid. The film thickness is also measured to understand the relationship between the heat transfer results and the controllable independent variables. The results and conclusions of this study are useful in understanding the physics of spray cooling and can be applied to design better spray cooling systems.