We investigate the dynamics of spontaneous jumps of water drops from electrically charged superhydrophobic dielectric substrates during a sudden step reduction in gravity level. In the brief free-fall environment of a drop tower, with a non-homogeneous external electric field arising due to dielectric surface charges (with surface potentials 0.4-1.8 kV), body forces acting on the jumped drops are primarily supplied by polarization stress and Coulombic attraction instead of gravity. This electric body force leads to a drop bouncing behavior similar to well-known phenomena in 1-g0, though occurring for much larger drops (~0.5 mL). We show a simple model for the phenomenon, its scaling, and asymptotic estimates for drop time of flight in two regimes: at short-times close to the substrate when drop inertia balances Coulombic force due to net free charge and image charges in the dielectric substrate and at long-times far from the substrate when drop inertia balances free charge Coulombic force and drag. The drop trajectories are controlled primarily by the dimensionless electrostatic Euler number Eu, which is a ratio of inertial to electrostatic forces. To experimentally determine values of Eu we conduct a series of drop tower experiments where we observe the effects of drop volume, net free charge, and static surface potential of the superhydrophobic substrate on drop trajectories. We use a direct search optimization to obtain a Maximum Likelihood Estimate for drop net charge, as we do not measure it directly in experiment. For φEu/8π > 1 drops escape the electric field, where φ is a drop to substrate aspect ratio. However, we do not observe any escapes in our dataset. With an eye towards engineering applications we consider the results in light of the so-called low-gravity phase separation problem with a worked example.

This work examines electrohydrodynamic enhancement of capillary pump loops (CPL) for use in extraterrestrial nuclear applications. A capillary pump uses capillary action through a porous wick to transport heat and mass. The capillary pump is being considered as a method to improve heat transport in extraterrestrial nuclear applications. The work consists of a literature review of electrohydrodynamics, capillary pumped loops and space type nuclear reactors. Current CPLs are assessed for their performance and several design solutions are investigated using theoretical and analytical techniques. Experimental analysis is performed on an electrohydrodynamic gas pump to determine their suitability for implementation into the vapour leg of a capillary pump loop. The results suggest the EHD gas pumps could offer improved performance and it is recommended experiments should be performed in future work with an EHD gas pump in a CPL for verification. A new design for the electrohydrodynamic evaporator is also developed for enhanced performance. / UOIT

Thesis (Ph. D.)--University of Chicago, Department of Physics, December 2003. / CD-ROM contains entire thesis in PDF format. Includes bibliographical references. Also available on the Internet.

Thesis (M. Phil.)--University of Hong Kong, 1974. / Mimeographed.

In this thesis we report the results of investigations on the rheological response of emulsions to the application of the electric field. A front-tracking finite difference scheme is used in conjunction with Taylor-Melcher leaky dielectric theory to study the problem. The numerical results in different regions of the deformation-circulation map show that the structure formation in regions I and III can be hindered by the hydrodynamic effect. This is opposite to what is observed in the perfect dielectric cases and region II of the map. For perfect dielectric systems, where the electrohydrodynamics effects are absent, droplets form chain-like structures spanning the distance between the electrodes after the application of the electric field. Subsequently, the chains interact with each other to form columns comprising two or more chains. Point-dipole approximation is used to analyze the structure formation and it is shown that it is also applicable to region II where the hydrodynamic effect is weak and the behavior of the system is mainly governed by the dielectrophoretic forces. It is shown that the chain formation is not possible in regions I and III due to the competition between the dipolar force and torque on one side and hydrodynamic effect on the other side. In region I, the hydrodynamic torque prevents the chain formation by competing with the dipolar torque, which tends to align the drops with the electric field. On the other hand, in region III, the repulsive nature of the hydrodynamic effect opposes the attractive dipolar force and does not allow the particles to form stable chains.

Active colloids are small particles that can convert external energy supply into self-propulsion. Because of the existence of the energy current inside and across the system, active colloids exhibit behaviors that are far away from thermodynamic equilibrium. During the past decades, active colloids have been used to provide models for many different non-equilibrium system studies and have been designed to complete tasks on small scale. By tuning the particle size, shape, etc, or changing the actuation methods of the active colloid systems, people have developed a large number of different active colloid systems. Among all active colloid systems, the Quincke rotation system can effectively propel particles with rapid speed. This phenomenon refers to the spontaneous rolling of a dielectric sphere in a weakly conducting liquid under a DC electric field. Although the basic mechanism of a single Quincke roller has been well explained, some behaviors that occur in complex environments or with multiple Quincke particles are still mysteries. For example, one particle will move back and forth on the bottom electrode under a high electric DC field. This so-called Quincke Oscillation motion cannot be explained by the previous models well. So a new model is required. In this dissertation, we will focus on explaining this newly-discovered dynamic in the Quincke system. Then we will study the collective dynamics of multiple Quincke oscillators with designed experiments and models. In Chapter 1, the background and different actuation methods of active colloid systems are first introduced. Then the Quincke rotation system and its field-dependent dynamics are explained with a classic leaky dielectric model. The recent research results with Quincke systems are shortly reviewed afterward. In Chapter 2, we introduce the experimentally discovered Quincke Oscillation phenomenon. Then we reveal its dependency on liquid conductivity and particle size. This dynamic is finally explained by the asymmetric charging of the particle surface in the field-induced boundary layer near the electrode. This work opens the door to the study of the collective dynamics of Quincke oscillators. In Chapter 3, we first introduce a dynamical model considering the charge, dipole, and quadrupole moments of the sphere and predict its oscillatory motion under a non-uniform liquid conductivity environment. Then we study the behavior of two coupled Quincke oscillators with far-field hydrodynamic and electrostatic interactions. The numerical simulations predict the synchronization and alignment of two oscillators with fixed positions. We further develop a model based on weakly coupled oscillator assumptions by considering the relative phase and oscillating orientations of two oscillators. The model successfully explains the numerical simulation results and can be applied to other active colloid systems with multiple mobile oscillators. In Chapter 4, we show that the Quincke oscillators can assemble into a cluster and oscillate with high synchronization and alignment. This formation of the cluster can also increase the oscillation frequency of the oscillators. By considering the perfect contact rolling of the oscillators on the electrode, we develop a weakly coupled oscillator theory model. This model explains the tendency of particles to synchronize and align in a cluster and predicts the increase of the oscillation frequency when particles are in synchronized phases. The cluster is stabilized due to the existing phase waves observed in experiments and simulations. In Chapter 5, we introduce two other studies on Quincke rollers with different experimental designs. Particles of helical shape exhibit self-propulsion in the liquid bulk and highlight the role of shape in controlling particle dynamics. For multiple spheres in a height-confined system, the particles display a transition from a fluctuating state to an absorbing stable state depending on their density and the applied field strength. This work provides an experimental model for studying absorbing state. In Chapter 6, the development of the Quincke system study is reviewed and some future directions are suggested.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING / Includes bibliographical references. / by Mark C. Zaretsky. / M.S.

The effect of a non-uniform magnetic field on the coupled transport of momentum and heat is studied for the case of laminar pipe flow of a magnetically susceptible ferrofluid. The momentum and heat transport equations are complemented with the necessary electromagnetic terms and used to develop a computer simulation of the velocity profile and temperature distribution in the fluid. Two magnetic field configurations are studied. The first configuration is produced by a single short solenoid, located around the pipe. The magnetic field produced has both radial and axial components. For the second configuration, the electric current is inverted in one half of the solenoid, creating much stronger field gradients in both directions. The flow is laminar, driven by a constant pressure difference between the ends of the pipe. The apparent viscosity of the ferrofluid is modeled as dependent on temperature and magnetic field. In simulations involving heat transfer, a section of the pipe is maintained at higher constant temperature. The rest of the wall is adiabatic. A Visual-Basic code, FiRMa (Flow in Response to Magnetic field), was developed to perform the numerical simulations. For the water-based ferrofluid, results show reduction of average velocity and small deviations from the parabolic velocity profile as the result of vortex viscosity. Heat transfer calculations show a decrease in the heat transfer coefficient and an increase in the fluid exit temperature. These effects are due to the change in flow pattern and average velocity. Current research aims for the development of a stable liquid-metal based ferrofluid, because of the high electric and thermal conductivities. The FiRMa code is used to examine the expected response of a mercury-based ferrofluid to the magnetic fields under study. Results show that the electromagnetic effects on the liquid metal-based ferrofluid are much stronger, due to induced electric currents and the Lorentz force acting on them. / Graduation date: 2003

Since boiling heat transfer affords a very effective means to transfer heat, it is implemented in numerous technologies and industries ranging from large power generation plants to micro-electronic thermal management. Although having been a subject of research for several decades, an accurate prediction of boiling heat transfer is still challenging due to the complexity of the coupled mechanisms involved. It appears that the boiling heat transfer coefficient is intimately related to bubble dynamics (i.e. bubble nucleation, growth and detachment) as well as factors such as nucleation site density and interaction between neighbouring and successive bubbles. In order to contribute to the understanding of the boiling phenomenon, an experimental investigation of saturated pool boiling from a single or two neighbouring artificial nucleation sites on a polished copper surface has been performed. The bubble growth dynamics has been characterized for different wall superheats and a experimental growth law has been established. The interaction between successive bubbles from the same nucleation site has been studied, showing the bubble shape oscillations that can be caused by these interactions. The forces acting on a growing bubble has been reviewed, and a complete momentum balance has been made for all stages of bubble growth. The curvature along the interface has been measured, and indications concerning the mechanism of bubble detachment have been suggested. The rise of bubble after detachment has been investigated, and the maximum velocity reached before a change of direction has been estimated and compared to existing models from the literature. The interaction between bubbles growing side by side has been studied: the generation and propagation of a wave front during the coalescence of two bubbles has been highlighted. As boiling heat transfer enhancement techniques are being imagined and developed, this study also focuses on the electrohydrodynamic enhancement technique. Boiling experiments have been performed in the presence of electric fields, and their effects on heat transfer and bubble dynamics have been characterized. Although the volume of the bubbles at detachment and the relationship between the bubble frequency and the wall superheat were not affected, the bubble growth curve was modified. The bubbles were elongated in the direction of the electric field, and this elongation was estimated and compared to other studies from the literature. The rising velocity of the bubble was reduced in the presence of electric field, and the behaviour of bubbles growing side by side was modified, the electric field causing the bubbles to repeal each other. These results, obtained in a fully controlled environment, provide compelling evidence that electric fields can be implemented to alter the bubble dynamics and subsequently heat transfer rates during boiling of dielectric fluids.

The effect of surface modification and high intensity electric field (uniform and non – uniform) acting separately or in combination on pool boiling of R-123 is presented in this thesis. The effect of surface modification was investigated on saturated pool boiling of R-123 for five horizontal copper surfaces modified by different treatments, namely: an emery polished surface, a fine sandblasted surface, a rough sandblasted surface, an electron beam (EB) enhanced surface and a sintered surface. Each 40 mm diameter heating surface formed the upper face of an oxygen-free copper block, electrically heated by embedded cartridge heaters. The experiments were performed from the convective heat transfer regime to the critical heat flux, with both increasing and decreasing heat flux, at 1.01 bar, and additionally at 2 bar and 4 bar for the emery polished surface. Significant enhancement of heat transfer with increasing surface modification was demonstrated, particularly for the EB enhanced and sintered surfaces. The emery polished and sandblasted surface results are compared with nucleate boiling correlations and other published data. The effect of uniform and non-uniform electric fields on saturated pool boiling of R-123 at 1.01 bar pressure was also examined. This method of heat transfer enhancement is known as electrohydrodynamic abbreviated as EHD-enhancement. A high voltage potential was applied at the electrode located above the heating surface, which was earthed. The voltage was varied from 0 to 30 kV. The uniform electric field was provided through a 40 mm diameter circular electrode of stainless steel 304 wire mesh having an aperture of 5.1 mm, while the non-uniform electric field was obtained by using a 40 mm diameter circular rod electrode with rods 5 and 8 mm apart. The effect of uniform electric field was investigated using all five modified surfaces, i.e. emery polished, fine sandblasted, rough sandblasted, EB enhanced and sintered surfaces, while non – uniform electric field was tested using the emery polished, fine sandblasted, EB enhanced and sintered surfaces. The effect of pressure on EHD enhancement was also examined using emery polished surface at saturation pressure of 2 and 4 bars while the electric field was fix at 20 kV corresponding to 2 MV/m. Further, the bubble dynamics is presented for the emery polished surface obtained using a high-speed high – resolution camera.