Equivalence Principle Measurements

Over the centuries there have been many experimental tests of the universality of free-fall. To date, these measurements have established the equivalence between inertial and gravitational mass to high precision, justifying its use as a foundation stone of general relativity by Einstein. There is a surprising richness in the variety of techniques and choice of test bodies which have been used so far, and a brief review is presented. However, future space experiments promise much better precision in this measurement and STEP is presented in some detail as one of a number of such missions. Using pairs of concentric free-falling proof-masses, STEP will be able to test the Equivalence Principle (EP) to a sensitivity at least five orders of magnitude better than currently achievable on ground. The EP is a founding principle of general relativity and STEP is the most sensitive experiment of this type planned so far, aiming at 1 part in 10¹⁸.     

SURFACE POTENTIAL AND PHOTO-ADSORPTION ONTO SEMICONDUCTORS

Abstract— A theory of photo-adsorption onto semiconductors is developed using the kinetic expressions and the relations for steady state carrier concentrations in terms of the quasien Fermi levels. Expressions are derived which emphasize the importance of the surface potential. In particular it is shown that a significant quantity is the ratio of the surface potential to a critical quantity, Y_sc . An expression for the latter is given and it is shown that photoadsorption occurs if the ratio is greater than unity while desorption occurs if the ratio is less than unity. The theory also suggests a useful graphical representation of the effects of impurities and surface potential on adsorption.     

The edge nucleus of a point-determining graph

In this paper we investigate the edge nucleus E⁰(G) of a point-determining graph G. We observe several relationships between E⁰(G) and the nucleus G⁰ = {v ∈ V(G)∣ G ? v is point determining} and use these relationships to prove several properties of E⁰(G). In particular, we show that there are only a finite number of graphs with a given edge nucleus and we determine those graphs G for which |E⁰(G)| ≤ 2. We also show that an n-clique of a point-determining graph G contains at least n?2 edges of E⁰(G) and if G is totally point determining, then every odd cycle of G meets E⁰(G).     

Developing counter current chromatography to meet the needs of pharmaceutical discovery

Experiments have been carried out to evaluate Counter Current Chromatography (CCC) as an alternative purification technique to preparative Reverse Phase High Performance Liquid Chromatography (RP-HPLC) for small molecule pharmaceuticals. The major drawback of CCC is the extensive time required in selecting the solvents to perform the separation. This is equivalent to choosing both the stationary phase and the mobile phase at the same time. In RP-HPLC it is a simple matter of deciding on the gradient, most samples can be purified on a C18 column with a water:acetonitrile gradient. The majority of the initial work was based on a standard test set of commercially available compounds, developed within our group to evaluate the performance of the HPLC apparatus and the column prior to the start of work each day. The work carried out on CCC has shown that the technique offers similar capabilities and can be carried out using similar protocols to RP-HPLC. CCC also has some advantages over RP-HPLC and can be regarded as a valuable addition to the chromatography toolbox.     

Predicting particle trajectories on an electrodynamic screen – Theory and experiment

The electrodynamic screen, or EDS, was first introduced to the electrostatics community in the early 1970's. Since that time, it has been studied by several research groups who have investigated its use as a means to remove unwanted particles from insulator surfaces. In the typical EDS, interdigitated electrodes are embedded or deposited on the surface, then energized by three-phase square or sinusoidal voltages of magnitude 500–1200 V at frequencies between 5 and 200 Hz. The resulting electric field entrains previously deposited particles and moves them laterally across the surface. Electrodynamic screens have been proposed for use on solar cells in Mars and Moon space missions, as well as the removal of dust from renewable-energy solar collectors such as photovoltaic panels, solar reflectors, and mirrors. Though often considered merely a nuisance, dust has the potential to partially or totally obscure the solar flux incident on any solar collector.Development of EDS technology has evolved mostly from a “trial and error” approach to choosing such parameters as electrode spacing, depth, and width, as well as voltage magnitude and frequency. Though some theoretical studies may be found in the literature, a more detailed understanding would be valuable in optimizing EDS performance for a particular application and composition of ambient dust, which may vary with geographical location with respect to size, permittivity, and conductivity. A theoretical understanding of how particles are entrained and made to move on an EDS will facilitate proper choice of relevant parameters in future EDS systems. Our work thus attempts to analyze the motion of dust particles subject to all the forces encountered at the surface of an EDS, including the coulomb and dielectrophoretic forces, as well as aerodynamic drag, gravity, and friction. Previous work has shown that particles entrained in the traveling-wave electrostatic field of an EDS exhibit one of two behaviors: ordered, lateral motion; and semi-chaotic motion. These two behaviors have been found both experimentally and in theoretical simulations. This paper attempts to correlate trajectory computations with video observations of these behaviors as recorded in a laboratory setting.