Formation, dissolution and properties of surface nanobubbles

Surface nanobubbles are stable gaseous phases in liquids that form on solid substrates. While their existence has been confirmed, there are many open questions related to their formation and dissolution processes along with their structures and properties, which are difficult to investigate experimentally. To address these issues, we carried out molecular dynamics simulations based on atomistic force fields for systems comprised of water, air (N2 and O2), and a Highly Oriented Pyrolytic Graphite (HOPG) substrate. Our results provide insights into the formation/dissolution mechanisms of nanobubbles and estimates for their density, contact angle, and surface tension. We found that the formation of nanobubbles is driven by an initial nucleation process of air molecules and the subsequent coalescence of the formed air clusters. The clusters form favorably on the substrate, which provides an enhanced stability to the clusters. In contrast, nanobubbles formed in the bulk either move randomly to the substrate and spread or move to the water--air surface and pop immediately. Moreover, nanobubbles consist of a condensed gaseous phase with a surface tension smaller than that of an equivalent system under atmospheric conditions, and contact angles larger than those in the equivalent nanodroplet case. We anticipate that this study will provide useful insights into the physics of nanobubbles and will stimulate further research in the field by using all-atom simulations

Optimized transmission and selection designs in wireless systems

University of Technology Sydney. Faculty of Engineering and Information Technology.Most modern wireless communication systems are hierarchical complex systems which consist of many levels of design elements and are subject to limited resources (e.g. power or bandwidth). Thanks to numerous newly-introduced devices in different forms such as sensors and relays and the integration of multiple antennas, spectral efficiency and reliability of wireless transmission could be significantly improved. Nevertheless, it also becomes much more challenging to control the devices and allocate the limited resources in an optimal fashion in order to approach capacity gains. This dissertation is concerned with mixed-binary or combinatorial optimization problems to improve various service goals for a variety of interesting yet difficult wireless communication applications. These problems are highly prized for academic significance but remained open due to their mathematical challenges. We shall explore the hidden d.c. (difference of convex (or concave) functions) structure of the objective functions as well as the binary constraints. Further, we will prove such general d.c. programs can be equivalently converted into canonical d.c. programs with d.c. objective functions that are subject to convex and/or affine constraints only. Although global optimal algorithms are generally possible for such d.c. programs, they are normally very computation-intensive. Instead, we propose tailored path-following local-optimal d.c. algorithms with significantly reduced computational complexity. Through extensive simulation results, the designed d.c. decompositions of the problems are proven effective. The proposed algorithms are efficient and computationally affordable while locating outstanding solutions in comparison with other existing algorithms. In those more sophisticated problem scenarios, the d.c. algorithm appears to be the only suitable option thanks to the superior flexibility. In the first part of the thesis, we will consider a sensor network for spectrum sensing in the context of cognitive radios. To improve sensing quality and prolong the battery life of sensors, the least correlated subset of sensors needs to be selected. A new Bregman matrix deviation-based framework is shown applicable to all the concerned correlation measure functions. The second research investigates a relay-assisted multi-user wireless network. Besides the relay beamforming variables, we add into consideration a set of binary link variables which represent on/off operations of individual relays in relation to transmitter-receiver links. To achieve the maximin SNR or SINR capacity, certain relays may be optimally deactivated. This leads to reduced power consumption and complexity/ overhead of management. The relay assignment and beamforming design is a joint mixed combinatorial nonlinear program which is non-convex and non-smooth. Nonetheless, we show the it can be fit into a canonical d.c. optimization framework. Simulation results demonstrate the benefits of relay selection and beamforming. The last research stems from the study of conventional coordinated transmission design with respect to transmit covariance and precoding matrix/vector variables. Inspired by the well-known Han-Kobayashi message splitting method in 2-user SISO interference channels, we further extend the idea of message splitting to the MIMO interference networks. An innovative non-smooth rate formula is discovered which builds the foundation of the work. The design in common and private covariance matrices or beamforming vectors, as well as the pairing variables, is formulated as a joint combinatorial nonlinear program which is non-convex and non-smooth. Due to the great difficulty, it is not imminently possible to jointly handle both variables. Therefore, we first propose an intuitive heuristic pairing algorithm to find excellent pairing choices. Then, the non-convex optimization problems in covariance matrices or beamforming vector variables are dealt with in the d.c. optimization framework. Finally, simulation results reveal the great potential of the novel message splitting scheme in approaching rate capacity

AN ANALYSIS OF THE COMPETITIVENESS OF SOUTHEASTERN FRESH VEGETABLE CROPS USING QUADRATIC PROGRAMMING

This study determined the competitive potential of the temperate southeastern U.S. region to produce selected fresh vegetables for the national market. Results indicated that the region may be competitive in the production and marketing of snap beans, cucumbers, bell peppers, and spring tomatoes. The region would also be competitive in the production and marketing of broccoli, summer and fall tomatoes, and spinach, given cost reductions of 10 to 15 percent. Major cost reductions were required for the region to be competitive in the production and interregional marketing of sweet onions in the absence of effective product differentiation.Crop Production/Industries,

Determination of the stiffness of the nuclear symmetry energy from isospin diffusion

With an isospin- and momentum-dependent transport model, we find that the degree of isospin diffusion in heavy ion collisions at intermediate energies is affected by both the stiffness of the nuclear symmetry energy and the momentum dependence of the nucleon potential. Using a momentum dependence derived from the Gogny effective interaction, recent experimental data from NSCL/MSU on isospin diffusion are shown to be consistent with a nuclear symmetry energy given by $E_{\text{sym}}(\rho)\approx 31.6(\rho /\rho_{0})^{1.05}$ at subnormal densities. This leads to a significantly constrained value of about -550 MeV for the isospin-dependent part of the isobaric incompressibility of isospin asymmetric nuclear matter.Comment: 4 pages, 4 figures, 1 table, revised version, to appear in PR

Strategic Judgment Proofing

A liquidity-constrained entrepreneur raises capital to finance a business activity that may harm bystanders. The entrepreneur raises senior (secured) debt to shield assets from the tort victims in bankruptcy. For a fixed level of borrowing, senior debt creates better incentives for precaution taking than either junior debt or outside equity. The entrepreneur's level of borrowing is, however, socially excessive. Giving tort victims priority over senior debtholders in bankruptcy prevents overleveraging but leads to suboptimal incentives. Lender liability exacerbates the incentive problem even further. A limited seniority rule dominates these alternatives. Shareholder liability, mandatory liability insurance, and punitive damages are also discussed

The effect of in-plane magnetic field on the spin Hall effect in Rashba-Dresselhaus system

In a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit couplings, there are two spin-split energy surfaces connected with a degenerate point. Both the energy surfaces and the topology of the Fermi surfaces can be varied by an in-plane magnetic field. We find that, if the chemical potential falls between the bottom of the upper band and the degenerate point, then simply by changing the direction of the magnetic field, the magnitude of the spin Hall conductivity can be varied by about 100 percent. Once the chemical potential is above the degenerate point, the spin Hall conductivity becomes the constant $e/8\pi$, independent of the magnitude and direction of the magnetic field. In addition, we find that the in-plane magnetic field exerts no influence on the charge Hall conductivity.Comment: 11 pages, 3 figures, to be published on Phys. Rev.

Thermal Charm Production in Quark-Gluon Plasma at LHC

Charm production from the quark-gluon plasma created in the midrapidity of central heavy ion collisions at the Large Hadron Collider (LHC) is studied in the next-to-leading order in QCD. Using a schematic longitudinally boost-invariant and transversally expanding fire-cylinder model, we find that charm production could be appreciably enhanced at LHC as a result of the high temperature that is expected to be reached in the produced quark-gluon plasma. Sensitivities of our results to the number of charm quark pairs produced from initial hard scattering, the initial thermalization time and temperature of the quark-gluon plasma, and the charm quark mass are also studied.Comment: 8 pages, 9 figures; adding a figure and relevant discussion on the sensitivity of our results to the number of charm quark pairs produced from initial hard scattering. Version accepted for publication in PR

Effects of Cutoff Functions of Tersoff Potentials on Molecular Dynamics Simulations of Thermal Transport

Past molecular dynamics studies of thermal transport have predominantly used Stillinger-Weber potentials. As materials continuously shrink, their properties increasingly depend on defect and surface effects. Unfortunately, Stillinger-Weber potentials are best used for diamond-cubic-like bulk crystals. They cannot represent the energies of many metastable phases, nor can they accurately predict the energetics of defective and surface regions. To study nanostructured materials, where these regions can dominate thermal transport, the accuracy of Tersoff potentials in representing these structures is more desirable. Based upon an analysis of thermal transport in a GaN system, we demonstrate that the cutoff function of the existing Tersoff potentials may lead to problems in determining the thermal conductivity. To remedy this issue, improved cutoff schemes are proposed and evaluated