Showing papers by "Simón Bolívar University published in 1990"

Orientation cues used by ants

Abstract: The cues used for orientation during homing were examined in ants from 3 subfamilies, i.e.Atta laevigata, Acromyrmex octospinosus, Acromyrmex rugosus, Trachymyrmex urichi; Ectatomma ruidum andPseudomyrmex termitarius, by means of a Y-maze experiment. The results showed that ants use various cues for orientation through a maze. For each ant species, the cues may be classified in a kind of species-specific hierarchy according to their importance during homing. In addition to visual and chemical orientation, empirical evidence supporting the existence of a topochemical is presented.P. termitarius uses a kinesthetic sense for orientation. Learning of the cues in the laboratory is not directly related to the respective hierarchy used for orientation through a maze by each species, but seems to be inversely correlated with the social complexity of the species. The number of cues used for orientation correlates directly with the social complexity of the species. A divergent evolutionary development of orientation strategies, even among closely related species, is proposed.

The equilibrium and stability of menisci between touching spheres under the effect of gravity

Abstract: The equilibrium shapes and stability of menisci formed at the contact point between two vertically aligned spheres were theoretically studied. The equilibrium configurations were determined as solutions of the equation of Young—LaPlace. The stability of the equilibrium shapes was determined by means of a perturbation analysis of the three-dimensional form of the equation of Young—LaPlace. It was found that there is a maximum amount of liquid that can be retained at the contact point, which is determined by geometrical considerations when gravitational effects are important, and by the onset of instability when gravitational effects are negligible. The maximum amount of liquid diminishes as the gravitational forces become stronger with respect to surface tension forces. In the case of small contact angles, an increase in the contact angle results in an increase in the maximum liquid retention, whereas, when the contact angle is large, this trend is reversed.

An experimental study of gas holdup in two-phase bubble columns with foaming liquids

Abstract: Gas-liquid upward flow experiments have been performed in two bubble columns of different diameters (0.10 and 0.29 m,) using air as gas phase and several liquids: water, aqueous solutions of ethanol and glycerine, kerosene, and a solution of a surfactant in kerosene. The main goal of the study is the analysis of foaming systems, including the comparison of their behavior with respect to non-foaming systems. The gas holdup was determined experimentally as a function of the gas and liquid superficial velocities in bubbling, churn-turbulent and foaming regimes. It was found that, for foaming systems, semi-batch operation enhances foam formation, yielding higher holdups than those obtained in continuous operation at very low liquid velocities. Opposite to what is observed in non-foaming systems, the liquid superficial velocity affects the gas holdup appreciably in foaming systems. An increase in column diameter results in a decrease in gas holdup for all the systems studied. In aqueous foaming system...

Mixing expansivities and Grashof numbers in supercritical fluids using cubic equations-of-state

Abstract: The isothermal, isobaric expansivity upon mixing or, simply, mixing expansivity, to which Grashof numbers are proportional, is hereby defined. A general expression for this variable has been developed by using a generalized four-parameter cubic equation-of-state (EOS) of the Schmidt and Wenzel type. The mixing expansivity was calculated for several solid-SCF binary systems (including all the combinations of five selected solvents and 11 solutes) using the Peng-Robinson EOS. Results show that the mixing expansivity becomes very high as the system approaches the critical point of the solvent. The dependence of the mixing expansivity on composition proved to be significant, reaching a maximum value at infinite dilution. No definite trends in terms of the properties of the solute or solvent was found. The increase in mixing expansivity added to the reduction in kinematic viscosity may account for an enhancement up to 107 in the calculation of Grashof numbers. The results confirm experimental evidence that natural convection forces play an important role in supercritical processes, and should be taken into account when designing either an industrial process or an experimental setup to operate in the vicinity of the critical point.

Radioisotope X-ray fluorescence analysis of vanadium in petroleum coke samples

Abstract: Radioisotope X-ray fluorescence spectrometry, RIXRF, was used for the determination of vanadium in Venezuelan petroleum coke samples using a simple standardless method and fundamental parameter approach. The values obtained by this method were in agreement with values measured by graphite furnace atomic absorption spectrometry, GFAAS.

Reproduction in the toad Colostethus trinitatus (Anura : Dendrobatidae) in a northern Venezuela seasonal environment

Abstract: Various aspects of the reproductive biology of a montane population of Colostethus trinitatus were quantified during the transition from a prolonged dry season to a wet season with abnormally low rainfall. One-third to three-quarters of the females collected in the dry period (March to June) had mature ovarian follicles. The proportion of ovigerous females did not increase in July or August when the number of rainy days was greater. However, breeding and egg laying were only observed in the latter season. The principal environmental change that stimulated oviposition was frequent daily rain which increased humidity in the terrestrial nest sites. Pair formation, nests, and males carrying larvae are reliable criteria that indicate the breeding period of Colostethus trinitatus. The average number of mature ovules (> 2 mm) was greater in the wet season (x= 13.1) than during the dry months (x = 11.3). Female standard length and the number of yolked follicles was positively correlated. Five nests contained an average of 13.6 eggs, and males carried only slightly fewer larvae (x = 11.5). One female and one individual of unknown sex were found with eggs at two different nests. Five nests were unattended.

Fundamental solutions of the streamfunction–vorticity formulation of the Navier–Stokes equations

Abstract: A new boundary element procedure is developed for the solution of the streamfunction-vorticity formulation of the Navier-Stokes equations in two dimensions. The differential equations are stated in their transient version and then discretised via finite differences with respect to time.

Hydrodynamics of a semibatch slurry bubble column with a foaming liquid

Abstract: We present an experimental study of solids distribution in a semi-batch slurry bubble column in the presence of foam formation. The solids concentration profiles are analyzed by means of a modification of the sedimentation-dispersion model.

Symmetry properties of the Rayleigh-type optical mixing signal in frequency space

Abstract: The dependence of the Rayleigh-type optical mixing signal intensity on the incident pump and probe frequencies for different values of T1/T2 is shown in a three-dimensional plot for a homogeneously broadened line. The symmetry properties of the signal in this space and of the nonlinear process that generates the line are related. These properties are used to show the dependence of the measured spectra on the way in which the frequencies of the incident fields are varied. Furthermore, this plot in frequency space yields information that may be used for the determination of the relaxation times T1 and T2.

Micellar effects upon the basic hydrolysis of 2,7-bis(diethylamino)phenazoxonium chloride

Abstract: The effect of cationic and anionic surfactants on the reaction of Basic Blue 3 (2,7-bis(diethylamino)phenazoxonium chloride, 1) with hydroxide ion has been studied. Cetyltrimethylammonium bromide (CTAB), cetyl and tetradecyltrimethylammonium chloride (CTAC and MTAC) enhance the rate of basic hydrolysis whereas sodium dodecyl sulfate (SDS) has an inhibitory effect. The extent of micellar catalysis is reduced by the addition of organic solvents. The results have been analyzed on the basis of the pseudophase ion-exchange model [1–3].

Amino acids and memory consolidation in the cricket. II: Effect of injected amino acids and opioids on memory.

Abstract: The effect of injections of selected amino acids on memory, given before a maze-learning, was investigated. Thirsty crickets (Pteronemobius sp.) were trained to turn only to one side of a symmetrical Y-shaped maze using reinforcements with water. The insects retained the learned task 24 hr later. N2 anoxia applied immediately after training produced retrograde amnesia. Injections of Ala, Arg, Gln or morphine prior to training blocked the amnesic action of anoxia, whereas those of Cys, Met, Pro, Orn, octapamine or naloxone did not. Naloxone blocked long-term memory formation, but not learning, whereas Pro and Orn blocked both. The antiamnesic effect of morphine and Arg, but not that of Ala, was blocked by naloxone. A hypothesis assigning a neuromodulatory role to some amino acids is put forward.

Semiempirical study of electronic and bonding properties of iron silicide clusters

Abstract: Molecular orbital calculations of iron, silicon, and iron silicide clusters have been carried out using the UHF-MINDO/SR method. The nature of the bonding in these compounds has been investigated by analyzing the importance of bonding indexes and diatomic components of the total energy. It has been found that in iron silicide the strongest bond is formed between Fe-Si and that it arises mainly as the result ofsp-sp type orbital interactions. Althoughd orbitals show very little overlap withs-p orbitals, they do contribute significantly to bonding through electrostatic type diatomic interactions. By means of a detailed analysis ofsp, andd orbitals and total density of states (DOS) of Fe7Si7, Si7Fe7, Fe15, and Si17 clusters, the present calculations have permitted us to explain the origin of the iron silicide UPS experimental peaks.

Amino acids and memory consolidation in the cricket. I: Changes in the titer of free amino acids in nervous tissue after learning.

Abstract: The involvement of certain amino acids in the memory consolidation process was investigated in the cricket Pteronemobius sp. Thirsty crickets were trained to constantly turn to one side of a symmetrical Y-shaped maze using reinforcement with water. Controls were trained to turn to both sides of the maze according to a random program. Animals were sacrificed immediately after training and free amino acid fractions were isolated from whole brain, subesophagic, prothoracic, mesothoracic and metathoracic ganglia homogenates and analyzed by high pressure liquid chromatography. A complex pattern of variation in the titer of amino acids emerged after learning, where the changes differed among the various ganglia. The most conspicuous change was an increase in the levels of urea and an amino acid-like compound related to the urea cycle, in all ganglia except the subesophagic one, if compared to controls. Arginine increased in the subesophagic ganglion, but decreased significantly in the metathoracic ganglion. The variation of ganglionic amino acid levels and its possible relation to mnemonic processes is discussed.

Real-time Detection And Quantification Of Ischemic ECG Changes

Abstract: A real-time algorithm, for one lead ST-segment analysis, that can be implemented in intensive coronary care bedside monitors, has been developed. The algorithm allows QRS detection, beat labeling and alignment, averaging, b ase-line correction, and quantification of the ST-segment. A trend graphic plot of ST-deviation or ST-slope vs. time is generated as the output of the algorithm. In the absence of an adequate data base, we have tested the algorithm with selected exercise ECG recordings, records obtained during angioplasty, and four strips f rom the lnstituto d i Fisiologia Clinica di Pisa data base, with very encouraging results.

ECG Data Compression Through Adaptive Sampling And Arithmetic Coding

Abstract: ECG data compression is necessary in many applications. Especially in continuously storing 24-hour ambulatory ECG's. There is a current interest in developing new methods and algorithms that can be used in the design of digital H olter devices. This paper presents a novel approach to this problem. ECG data compression is achieved in two stages. The first performs compression by adaptive sampling using, both, the Turning Point and FAN (SAPA) algorithms. The second part is an implementation of a modified arithmetic coding algorithm that does not introduces changes in the data, being completely reversible. Using a test sample composed of 126 extracts of 20 secs, 8-bit resolution, selected from the MIT/BIH arrhythmia database, an average compression of 1 1.29: 1 has been obtained.