The initial stages of cave formation: Beyond the one-dimensional paradigm

The solutional origin of limestone caves was recognized over a century ago, but the short penetration length of an undersaturated solution made it seem impossible for long conduits to develop. This is contradicted by field observations, where extended conduits, sometimes several kilometers long, are found in karst environments. However, a sharp drop in the dissolution rate of CaCO_3 near saturation provides a mechanism for much deeper penetration of reactant. The notion of a "kinetic trigger" - a sudden change in rate constant over a narrow concentration range - has become a widely accepted paradigm in speleogenesis modeling. However, it is based on one-dimensional models for the fluid and solute transport inside the fracture, assuming that the dissolution front is planar in the direction perpendicular to the flow. Here we show that this assumption is incorrect; a planar dissolution front in an entirely uniform fracture is unstable to infinitesimal perturbations and inevitably breaks up into highly localized regions of dissolution. This provides an alternative mechanism for cave formation, even in the absence of a kinetic trigger. Our results suggest that there is an inherent wavelength to the erosion pattern in dissolving fractures, which depends on the reaction rate and flow rate, but is independent of the initial roughness. In contrast to one-dimensional models, two-dimensional simulations indicate that there is only a weak dependence of the breakthrough time on kinetic order; localization of the flow tends to keep the undersaturation in the dissolution front above the threshold for non-linear kinetics.Comment: to be published in Earth and Planetary Science Letter

A new model for simulating colloidal dynamics

We present a new hybrid lattice-Boltzmann and Langevin molecular dynamics scheme for simulating the dynamics of suspensions of spherical colloidal particles. The solvent is modeled on the level of the lattice-Boltzmann method while the molecular dynamics is done for the solute. The coupling between the two is implemented through a frictional force acting both on the solvent and on the solute, which depends on the relative velocity. A spherical colloidal particle is represented by interaction sites at its surface. We demonstrate that this scheme quantitatively reproduces the translational and rotational diffusion of a neutral spherical particle in a liquid and show preliminary results for a charged spherical particle. We argue that this method is especially advantageous in the case of charged colloids.Comment: For a movie click on the link below Fig

The surface morphology of a small drainage basin in the North Dakota Badlands

To describe the surface morphology of a fourth order drainage basin located in the North Dakota Badland,, the writer made quanti tative application• of known descriptive techniques and morphological laws. The surface configuration is a function of the study area\u27s linear, areal, and relief properties and surface elements. The linear and areal properties are determined from maps showing the drainage basin \u27s surface geometry, the relief properties are determined from large-scale topographic maps of the area, and the surface elements are determined by direct field measurements of the ground slopes, surface materials, and vegetal coverage. The surface geometry of the drainage basin 1B expressed by the relationship between the number of streams, stream lengths, basin areas, and stream order nwnbere1 while, the characteristics of its erosional topography are defined by the elongation ratio, constant of channel maintenance, texture ratio, drainage density, stream frequency values, and a hypsometric analysis. Maps of the surface elements and their associated frequency distributions represent the areal location and relative magnitude of the different classes c0111poaing each component of the surface elements. The descriptive forms representing the study area are analogous to many of those revealed in comparative studies of other badland area; however, some of the study area\u27s topographic aspects appear to be unique to the North Dakota region

Groundwater flow, hydrochemistry, and uranium deposition in the Powder River Basin, Wyoming

The relation between regional groundwater flow, hydrochemistry, and uranium distribution in the Powder River Basin indicates that uranium was deposited during the Tertiary Period in groundwater recharge areas where the groundwater changed from a sulfate-bicarbonate water to a bicarbonate-rich water. The regional recharge and discharge areas of present-day groundwater-flow systems have about the same location as the recharge and discharge areas of the Tertiary groundwater-flow systems. The present-day groundwater is recharged in the eastern, western, and especially in the southern margins of the basin and is discharged in the valley of the Powder River, especially in the north. Flow nets for the groundwater were constructed on the basis of piezometric data from existing water wells in the Powder River Basin. The Tertiary groundwater-flow systems had a larger longitudinal flow component than present-day groundwater-flow systems. The present-day topography causes a large lateral and vertical groundwater-flow component toward the large river valleys. The topography during the Tertiary was a result of deposition and had little relief, whereas, the present-day topography is a result of erosion and has large relief. The groundwater chemistry of this area during the Tertiary was probably similar to that of today because the groundwater flowed through the same sediment as present-day groundwater. Anions in the present-day groundwater undergo the following sequence of hydrochemical changes along the regional flow path from the southern recharge area to the northern discharge area: HCO3- + SO4-- à SO4--+ HCO3-àHCO3-. An abrupt decrease in sulfate concentration occurs where the sulfate is removed from solution in the transition zone between the SO4-- + HCO3-and HCO3- facies. The most likely cause of this decrease is a strong reducing environment, which may be caused by sulfate-reducing bacteria. Cations in the present-day groundwater undergo the following sequence of hydrochemical changes along a flow path from shallow to deep groundwater in recharge areas: Ca++ + Na+ àNa+ + Ca++ àNa+. The decrease in calcium and increase in sodium with depth is caused by base exchange and the precipitation of calcite cement. Major unoxidized uranium deposits in the Powder River Basin occur near the transition zone between the SO4-- + HCO3-and HCO3- facies. The uranium is transported in solution by groundwater in the HCO3- + SO4-- and the SO4-- + HCO3- facies and precipitated in the transition zone between the SO4-- + HCO3-and HCO3- facies. Precipitation occurs where strong reducing conditions exist around abundant organic material in which sulfate-reducing bacteria may live and multiply

System Design for a Long-Line Quantum Repeater

We present a new control algorithm and system design for a network of quantum repeaters, and outline the end-to-end protocol architecture. Such a network will create long-distance quantum states, supporting quantum key distribution as well as distributed quantum computation. Quantum repeaters improve the reduction of quantum-communication throughput with distance from exponential to polynomial. Because a quantum state cannot be copied, a quantum repeater is not a signal amplifier, but rather executes algorithms for quantum teleportation in conjunction with a specialized type of quantum error correction called purification to raise the fidelity of the quantum states. We introduce our banded purification scheme, which is especially effective when the fidelity of coupled qubits is low, improving the prospects for experimental realization of such systems. The resulting throughput is calculated via detailed simulations of a long line composed of shorter hops. Our algorithmic improvements increase throughput by a factor of up to fifty compared to earlier approaches, for a broad range of physical characteristics.Comment: 12 pages, 13 figures. v2 includes one new graph, modest corrections to some others, and significantly improved presentation. to appear in IEEE/ACM Transactions on Networkin

Young children's interpersonal trust consistency as a predictor of future school adjustment

Young children’s interpersonal trust consistency was examined as a predictor of future school adjustment. One hundred and ninety two (95 male and 97 female, M age = 6 years 2 months, SD age = 6 months) children from school years 1 and 2 in the United Kingdom were tested twice over one-year. Children completed measures of peer trust and school adjustment and teachers completed the Short-Form Teacher Rating Scale of School Adjustment. Longitudinal quadratic relationships emerged between consistency of children’s peer trust beliefs and peer-reported trustworthiness and school adjustment and these varied according to social group, facet of trust, and indictor of school adjustment. The findings support the conclusion that interpersonal trust consistency, especially for secret-keeping, predicts aspects of young children’s school adjustment

Electrophoretic mobility of a charged colloidal particle: A computer simulation study

We study the mobility of a charged colloidal particle in a constant homogeneous electric field by means of computer simulations. The simulation method combines a lattice Boltzmann scheme for the fluid with standard Langevin dynamics for the colloidal particle, which is built up from a net of bonded particles forming the surface of the colloid. The coupling between the two subsystems is introduced via friction forces. In addition explicit counterions, also coupled to the fluid, are present. We observe a non-monotonous dependence of the electrophoretic mobility on the bare colloidal charge. At low surface charge density we observe a linear increase of the mobility with bare charge, whereas at higher charges, where more than half of the ions are co-moving with the colloid, the mobility decreases with increasing bare charge.Comment: 15 pages, 8 figure

An investigation of the impact of young children's self-knowledge of trustworthiness on school adjustment: a test of the realistic self-knowledge and positive illusion models

The study aimed to examine the relationship between self-knowledge of trustworthiness and young children’s school adjustment. One hundred and seventy-three (84 male and 89 female) children from school years 1 and 2 in the United Kingdom (mean age 6 years 2 months) were tested twice over one year. Children’s trustworthiness was assessed using: (a) self-report at Time 1 and Time 2, (b) peers’ reports at Time 1 and Time 2, and (c) teacher-reports at Time 2. School adjustment was assessed by child-rated school-liking and the Short-Form Teacher Rating Scale of School Adjustment. Longitudinal quadratic relationships were found between school adjustment and children’s self-knowledge, using peer-reported trustworthiness as a reference: more accurate self-knowledge of trustworthiness predicted increases in school adjustment. Comparable concurrent quadratic relationships were found between teacher-rated school adjustment and children’s self-knowledge, using teacher-reported trustworthiness as a reference, at Time 2. The findings support the conclusion that young children’s psychosocial adjustment is best accounted for by the realistic self-knowledge model (Colvin & Block, 1994)

Numerical Simulations of Particulate Suspensions via a Discretized Boltzmann Equation Part II. Numerical Results

A new and very general technique for simulating solid-fluid suspensions has been described in a previous paper (Part I); the most important feature of the new method is that the computational cost scales with the number of particles. In this paper (Part II), extensive numerical tests of the method are described; for creeping flows, both with and without Brownian motion, and at finite Reynolds numbers. Hydrodynamic interactions, transport coefficients, and the short-time dynamics of random dispersions of up to 1024 colloidal particles have been simulated.Comment: Text and figures in uuencode-tar-compressed postcript Email [email protected]