Cohesion development in disrupted soils as affected by clay and organic matter content and temperature

Soils were dispersed and separated into sand, silt, and clay fractions that were reconstituted to give mixtures of each soil with 5 to 40% clay. In the range from 0 to 35% clay, higher clay contents resulted in greater stability. Rate of cohesion recovery was over 10 times as fast at 90°C as it was at 23°C, showing that the processes Involved are physical-chemical rather than biological. Maximum rates of cohesion recovery occurred at moderate soil water tensions, probably because some tension is needed to pull the particles into direct contact, but a continuous water phase is also essential to allow diffusion of bonding agents to the contact points. Since diffusion rates in water increase 300%, while rate of cohesion recovery increased 1000% when temperature was raised from 23 to 90°C, other factors, such as higher Mobilities at higher temperatures of compounds contributing hooding ions to the solution. probably play a role In the rate of cohesion recovery. Recovery of cohesion was more rapid in the soil with organic C contents of 0.004 kg/kg than in the soil with 0.012 kg/kg. When the organic matter was removed with H2O2 from the soil with 0.012 kg C/kg, its rate of cohesion recovery increased. Rate of cohesion recovery of this high organic matter soil was also increased by aging it at 0.1 kg H2O/kg soil compared to 0.2 kg/kg. A possible explanation is that organic coatings, tending to prevent direct contact and bonding of adjacent projections of mineral surfaces, are forced away from contact points by extremely strong forces that pull the adjacent minerals together when soil water tensions are high. When the higher organic matter soil had been consolidated by air-drying and rehydrated, its rate of cohesion recovery was just as rapid as that of the soil with low organic matter

Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets

The spins of ten stellar black holes have been measured using the continuum-fitting method. These black holes are located in two distinct classes of X-ray binary systems, one that is persistently X-ray bright and another that is transient. Both the persistent and transient black holes remain for long periods in a state where their spectra are dominated by a thermal accretion disk component. The spin of a black hole of known mass and distance can be measured by fitting this thermal continuum spectrum to the thin-disk model of Novikov and Thorne; the key fit parameter is the radius of the inner edge of the black hole's accretion disk. Strong observational and theoretical evidence links the inner-disk radius to the radius of the innermost stable circular orbit, which is trivially related to the dimensionless spin parameter a_* of the black hole (|a_*| < 1). The ten spins that have so far been measured by this continuum-fitting method range widely from a_* \approx 0 to a_* > 0.95. The robustness of the method is demonstrated by the dozens or hundreds of independent and consistent measurements of spin that have been obtained for several black holes, and through careful consideration of many sources of systematic error. Among the results discussed is a dichotomy between the transient and persistent black holes; the latter have higher spins and larger masses. Also discussed is recently discovered evidence in the transient sources for a correlation between the power of ballistic jets and black hole spin.Comment: 30 pages. Accepted for publication in Space Science Reviews. Also to appear in hard cover in the Space Sciences Series of ISSI "The Physics of Accretion onto Black Holes" (Springer Publisher). Changes to Sections 5.2, 6.1 and 7.4. Section 7.4 responds to Russell et al. 2013 (MNRAS, 431, 405) who find no evidence for a correlation between the power of ballistic jets and black hole spi

Uso do índice S na determinação da condutividade hidráulica não-saturada de solos do cerrado brasileiro

O emprego de modelos numéricos permite determinar a condutividade hidráulica não-saturada, K(h), de modo mais rápido e a um custo menor que a medição em campo; para tanto, é necessário o desenvolvimento de modelos cada vez mais acurados. Propôs-se, neste trabalho, verificar se o modelo baseado no índice S (declividade da curva característica de retenção da água do solo em seu ponto de inflexão) e na tensão da água nesse ponto de inflexão (hi) é adequado para a determinação da condutividade hidráulica não-saturada de solos de diferentes classes texturais. O estudo foi realizado sobre 2166 amostras de solo, das quais constavam informações sobre textura, condutividade hidráulica saturada e retenção da água do solo, registradas nos bancos de dados dos Laboratórios de Solo das Embrapa Arroz e Feijão e Embrapa Cerrados, abrangendo solos sob cerrado das regiões CO, N e NE do Brasil. Verificou-se que a condutividade hidráulica não-saturada no ponto de inflexão da curva de retenção (Ki) pode ser determinada a partir do índice S e de hi, independentemente da textura do solo, e que determinações de K(h) feitas com base em Ki apresentaram altas correlações com valores medidos. O uso do índice S e de hi na determinação de K(h) deve ser mais testado para determinar seu poder de predição.Unsaturated hydraulic conductivity, K(h), can be determined rapidly and economically using numerical models in comparison to field measurements. For this is necessary to develop more accurate models. The objective of this paper was to verify if the model based upon S index (the slope of the soil-water retention curve at its inflection point) and soil-water tension at this inflection point (hi) is suitable to determine unsaturated hydraulic conductivity of soils of different textural classes. The study was done on 2166 samples covering 'Cerrado' soils of Mid-West, North and Northeast regions of Brazil, of which there was information about soil texture, saturated hydraulic conductivity, and soil-water retention in data base of Soil Laboratories of Embrapa Arroz e Feijão and Embrapa Cerrados. It was observed that unsaturated hydraulic conductivity at the inflection point of soil-water retention curve (Ki) can be determined from S index and hi, independently of soil texture. Determinations of K(h) based upon Ki showed a high correlation to measured values. The use of S index and hi to determine K(h) should be further tested to determine its predictive power