Single and pairwise motion of particles near an ideally polarizable electrode

Single-particle longitudinal motion and pairwise lateral motion was investigated while the particles were excited by an oscillating electric field directed normally to an electrode proximate to the particles. The electrode was polarized over a range of potential insufficient to drive electrochemical reactions, a range called the "ideally polarizable region". The particles' motion was qualitatively dependent on the choice of electrolyte despite the absence of electrochemical reactions. As when electrochemical reactions were not explicitly excluded, the phase angle θ between particle height and electric field correlated with the particles' separation or aggregation during excitation. A simple harmonic oscillator model of the particles' response, including colloidal and hydrodynamic forces and including the Basset force not previously cited in this context, showed how θ can increase from 0° at low frequencies, cross 90° at ～100 Hz, and then increase to 180° as frequency was increased. The model captured the essence of experimental observations discussed here and in earlier works. This is the first a priori prediction of θ for this problem.     

Least squares problems with inequality constraints as quadratic constraints

Linear least squares problems with box constraints are commonly solved to find model parameters within bounds based on physical considerations. Common algorithms include Bounded Variable Least Squares (BVLS) and the Matlab function lsqlin. Here, the goal is to find solutions to ill-posed inverse problems that lie within box constraints. To do this, we formulate the box constraints as quadratic constraints, and solve the corresponding unconstrained regularized least squares problem. Using box constraints as quadratic constraints is an efficient approach because the optimization problem has a closed form solution. The effectiveness of the proposed algorithm is investigated through solving three benchmark problems and one from a hydrological application. Results are compared with solutions found by lsqlin, and the quadratically constrained formulation is solved using the L-curve, maximum a posteriori estimation (MAP), and the χ² regularization method. The χ² regularization method with quadratic constraints is the most effective method for solving least squares problems with box constraints.     

Lidar detection using a dual-frequency source

A new technique of dual-frequency Doppler-lidar measurement is investigated. This technique is based on the use of a coherently locked, tunable, dual-frequency laser source and is shown to accurately measure velocities as small as 26 mum/s. It is generated by exploiting the nonlinear dynamics of a semiconductor laser through a proper combination of optical injection and operating conditions.     

Aniline incorporated silica nanobubbles

We report the synthesis of stearate functionalized nanobubbles of SiO₂ with a few aniline molecules inside, represented as C₆H₅NH₂@SiO₂@stearate, exhibiting fluorescence with red-shifted emission. Stearic acid functionalization allows the materials to be handled just as free molecules, for dissolution, precipitation, storage etc. The methodology adopted involves adsorption of aniline on the surface of gold nanoparticles with subsequent growth of a silica shell through monolayers, followed by the selective removal of the metal core either using sodium cyanide or by a new reaction involving halocarbons. The material is stable and can be stored for extended periods without loss of fluorescence. Spectroscopic and voltammetric properties of the system were studied in order to understand the interaction of aniline with the shell as well as the monolayer, whilst transmission electron microscopy has been used to study the silica shell.     

Kinetics of methane and tar evolution during coal pyrolysis

The first objective of this work was to compare the pyrolysis behavior of coals coming from different geographic locations (South Africa, South America, Europe, Australia, and North America). This preliminary study was limited to the kinetics of methane and tar evolution, with data on additional species to be reported in a separate publication. The second objective was to examine the possible relationship between tar and methane evolution during pyrolysis. This study was done by employing a thermogravimetric analyzer coupled with a Fourier-transform infrared spectrometer (TG-FTIR). The evolution curves for 35 coals of different elemental compositions were measured at three different heating rates (10, 30, and 100 K/min). Pyrolysis kinetics were described using a simple first-order reaction model. The technique, first proposed by Kissinger, is based on the variation of the temperature at which a volatile species evolution rate is a maximum (T_max) as a function of the heating rate. The TG-FTIR data for tar evolution reveal a generally consistent behavior for coals from different parts of the world, showing increasing activation energies with increasing coal rank. The same correlation is also true for methane, although the slope of the activation energy versus carbon content curve is rather flat, at least up to about 90% carbon content. The values of activation energies for methane evolution were found to be lower in the case of the Argonne coals, as compared with the non-US coals. A study of the temperatures at which the evolution of methane and tar begins (T_ini), and the temperatures at which the evolution rates reach a maximum (T_max), reveals a correlation between the T_ini for methane and T_max for tar. This may be due to the fact that both tar and methane evolve as a result of similar reactions involved in the breakup and recombination of the coal macromolecular network.     

Heat pulse propagation in a crystal: A molecular dynamical calculation

The propagation of a heat pulse into a perfect bcc crystal is studied by means of molecular dynamical calculations. We observe second sound waves associated with the heat pulse as well as with longitudinal and transverse elastic pulses. Our results explain a number of features observed in second sound experiments and suggest that second sound is a phenomenon of general occurrence.