Pore structure and function of synthetic nanopores with fixed charges: tip shape and rectification properties

We present a complete theoretical study of the relationship between the structure (tip shape and dimensions) and function (selectivity and rectification) of asymmetric nanopores on the basis of previous experimental studies. The theoretical model uses a continuum approach based on the Nernst-Planck equations. According to our results, the nanopore transport properties, such as current-voltage (I-V) characteristics, conductance, rectification ratio, and selectivity, are dictated mainly by the shape of the pore tip (we have distinguished bullet-like, conical, trumpet-like, and hybrid shapes) and the concentration of pore surface charges. As a consequence, the nanopore performance in practical applications will depend not only on the base and tip openings but also on the pore shape. In particular, we show that the pore opening dimensions estimated from the pore conductance can be very different, depending on the pore shape assumed. The results obtained can also be of practical relevance for the design of nanopores, nanopipettes, and nanoelectrodes, where the electrical interactions between the charges attached to the nanostructure and the mobile charges confined in the reduced volume of the inside solution dictate the device performance in practical applications. Because single tracks are the elementary building blocks for nanoporous membranes, the understanding and control of their individual properties should also be crucial in protein separation, water desalination, and bio-molecule detection using arrays of identical nanopores.     

Transverse vibrations of a beam elastically restrained at one end and with a mass and spring at the other subjected to an axial force

This paper deals with the exact solution of the title problem. The classical theory of vibrations of beams is used and a tabulation of eigenvalues is presented as a function of some of the governing mechanical parameters.The problem is of basic interest in several areas of engineering, specially to nuclear reactor systems designers since in many instances pumps or other electromechanical arrangements are placed at the end of a clamped beam.     

A branch-and-cluster coordination scheme for selecting prison facility sites under uncertainty

A multi-period stochastic model and an algorithmic approach to location of prison facilities under uncertainty are presented and applied to the Chilean prison system. The problem consists of finding locations and sizes of a preset number of new jails and determining where and when to increase the capacity of both new and existing facilities over a time horizon, while minimizing the expected costs of the prison system. Constraints include maximum inmate transfer distances, upper and lower bounds for facility capacities, and scheduling of facility openings and expansion, among others. The uncertainty lies in the future demand for capacity, because of the long time horizon under study and because of the changes in criminal laws, which could strongly modify the historical tendencies of penal population growth. Uncertainty comes from the effects of penal reform in the capacity demand. It is represented in the model through probabilistic scenarios, and the large-scale model is solved via a heuristic mixture of branch-and-fix coordination and branch-and-bound schemes to satisfy the constraints in all scenarios, the so-called branch-and-cluster coordination scheme. We discuss computational experience and compare the results obtained for the minimum expected cost and average scenario strategies. Our results demonstrate that the minimum expected cost solution leads to better solutions than does the average scenario approach. Additionally, the results show that the stochastic algorithmic approach that we propose outperforms the plain use of a state-of-the-art optimization engine, at least for the three versions of the real-life case that have been tested by us.     

Migration and regional labor market adjustment: Chile 1977–1982 and 1987–1992

The main objective of this paper is to evaluate the migration processes that have been occurring in Chile between 1977–1982 and 1987–1992, as a market mechanism to re-allocate labor among regions. Using traditional consumer theory, a model is developed for a migrant who is evaluating migration. Secondly, this model is estimated, with cross section aggregate data, for both periods using a logit formulation. The results indicate that there is a strong force in the Chilean regional labor market, which serves to concentrate the workforce around the largest populated region of the country. Finally, regional labor markets are simulated to show that migration forces are very weak to arbitrage regional wages and unemployment rate and specific policy is required to promote balanced development across Chilean regions. Received: January 1999/Accepted: August 2001     

Lactose-induced diarrhea increases oxidative stress and it is more severe in rats deficient in vitamin E

Diarrhea is the disease with high incidence in the world and causes infant mortality and malnutrition in the developing world. This justifies the study of nutrition and diarrhea. Due to ethical and financial considerations it is difficult to study nutrition and diarrhea in children thus animal models have become a convenient alternative. In previous studies it was shown that lactose induced diarrhea in rats was associated with a reduction in tissue levels of vitamin E and also with evidence of an inflammatory response of the intestine. Accordingly, in this study, in order to determine the effect of this type of diarrhea on the level of oxidative stress, diarrhea was induced in vitamin E sufficient and deficient rats. The results showed that after 23 days the tissue concentration of vitamin E decreased in all the rats with diarrhea but this reduction was substantially greater in the vitamin E deficient group. Moreover, diarrhea was 60% more severe in the vitamin E deficient rats than in the vitamin E sufficient group that also had diarrhea. Both diarrhea and vitamin E deficiency altered malonaldehyde and superoxide dismutase levels in various tissues. However, the most outstanding changes associated with diarrhea were a 100% increment in plasma malonaldehyde and erythrocyte superoxide dismutase activities which were 8 to 11 times higher than those seen in the rats without diarrhea. These non-invasive changes correlated well with the severity of diarrhea. The study shows that vitamin E deficiency results in diarrheas which are more severe and that lactose induced diarrhea is associated with higher levels of oxidative stress.     

Simultaneous correlation of complementary set of sequences using a transpose generation approach

Complementary sets of sequences are currently being applied to signal coding, radar, and multi-user systems, among others. Their particular mathematical properties make them adequate for multi-emission and noisy environments. Nowadays sustained efforts are being devoted to reduce the calculations involved in the generation and/or correlation of these signals by means of recursive algorithms. Some authors have proposed efficient algorithms that are based on modular architectures made up of adders, multipliers and delays. This work introduces a new approach to correlation algorithms of complementary sets of sequences, which is based on a transposition of the generation process. This approach allows to notoriously reduce calculations, and enables the simultaneous correlation of M sequences, without adopting time multiplexing schemes or complex parallel implementations. The correlation algorithm is theoretically demonstrated and its calculation performance is evaluated in a hardware reconfigurable platform. A comparison with other algorithms is included, considering the amount of calculations as a function of the length of the sequences.     

Multivariable control of grinding plants: a comparative simulation study

In this paper five multivariable adaptive and classical control strategies have been studied and implemented in a simulator of the copper grinding plant of CODELCO-Andina. The strategies presented were compared and, according to theory, exhibit good behavior. The extended horizon, pole-placement and model reference multivariable adaptive control strategies were formulated in discrete-time and use a model of the plant whose parameters are updated on line using the recursive least squares method along with UD factorization of the covariance matrix and variable forgetting factor. The direct Nyquist array and sequential loop closing techniques were also studied and simulated. The two-by-two multivariable system chosen to represent the grinding plant has the percentage of solids (density) of the pulp fed to the hydrocyclones (which is highly correlated with the percentage of +65 mesh in the overflow of hydrocyclones) and the sump level as output (controlled) variables. The water flow added to the sump and the speed of the pump are its input (manipulated) variables. All the algorithms tested by simulation exhibited good performance and were able to control the grinding plant in a stable fashion. Adaptive algorithms showed better performance than classical techniques, with the extended horizon and pole-placement algorithms proving to be the best. The fact that adaptive algorithms continuously adjust their parameters renders such controllers superior to those based on fixed parameters.