Path Crossing Exponents and the External Perimeter in 2D Percolation

2D Percolation path exponents $x^{\cal P}_{\ell}$ describe probabilities for traversals of annuli by $\ell$ non-overlapping paths, each on either occupied or vacant clusters, with at least one of each type. We relate the probabilities rigorously to amplitudes of $O(N=1)$ models whose exponents, believed to be exact, yield $x^{\cal P}_{\ell}=({\ell}^2-1)/12$. This extends to half-integers the Saleur--Duplantier exponents for $k=\ell/2$ clusters, yields the exact fractal dimension of the external cluster perimeter, $D_{EP}=2-x^{\cal P}_3=4/3$, and also explains the absence of narrow gate fjords, as originally found by Grossman and Aharony.Comment: 4 pages, 2 figures (EPSF). Revised presentatio

Viscous stabilization of 2D drainage displacements with trapping

We investigate the stabilization mechanisms due to viscous forces in the invasion front during drainage displacement in two-dimensional porous media using a network simulator. We find that in horizontal displacement the capillary pressure difference between two different points along the front varies almost linearly as function of height separation in the direction of the displacement. The numerical result supports arguments taking into account the loopless displacement pattern where nonwetting fluid flow in separate strands (paths). As a consequence, we show that existing theories developed for viscous stabilization, are not compatible with drainage when loopless strands dominate the displacement process.Comment: The manuscript has been substantially revised. Accepted in Phys. Rev. Let

A New Exponent Characterizing the Effect of Evaporation on Imbibition Experiments

We report imbibition experiments investigating the effect of evaporation on the interface roughness and mean interface height. We observe a new exponent characterizing the scaling of the saturated surface width. Further, we argue that evaporation can be usefully modeled by introducing a gradient in the strength of the disorder, in analogy with the gradient percolation model of Sapoval {\it et~al.}. By incorporating this gradient we predict a new critical exponent and a novel scaling relation for the interface width. Both the exponent value and the form of the scaling agree with the experimental results.Comment: 12 pages, REVTeX 3.0, figures on request (accepted for PRL

Three dimensional hysdrodynamic lattice-gas simulations of binary immiscible and ternary amphiphilic flow through porous media

We report the results of a study of multiphase flow in porous media. A Darcy's law for steady multiphase flow was investigated for both binary and ternary amphiphilic flow. Linear flux-forcing relationships satisfying Onsager reciprocity were shown to be a good approximation of the simulation data. The dependence of the relative permeability coefficients on water saturation was investigated and showed good qualitative agreement with experimental data. Non-steady state invasion flows were investigated, with particular interest in the asymptotic residual oil saturation. The addition of surfactant to the invasive fluid was shown to significantly reduce the residual oil saturation.Comment: To appear in Phys. Rev.

Scaling properties of driven interfaces in disordered media

We perform a systematic study of several models that have been proposed for the purpose of understanding the motion of driven interfaces in disordered media. We identify two distinct universality classes: (i) One of these, referred to as directed percolation depinning (DPD), can be described by a Langevin equation similar to the Kardar-Parisi-Zhang equation, but with quenched disorder. (ii) The other, referred to as quenched Edwards-Wilkinson (QEW), can be described by a Langevin equation similar to the Edwards-Wilkinson equation but with quenched disorder. We find that for the DPD universality class the coefficient $\lambda$ of the nonlinear term diverges at the depinning transition, while for the QEW universality class either $\lambda = 0$ or $\lambda \to 0$ as the depinning transition is approached. The identification of the two universality classes allows us to better understand many of the results previously obtained experimentally and numerically. However, we find that some results cannot be understood in terms of the exponents obtained for the two universality classes {\it at\/} the depinning transition. In order to understand these remaining disagreements, we investigate the scaling properties of models in each of the two universality classes {\it above\/} the depinning transition. For the DPD universality class, we find for the roughness exponent $\alpha_P = 0.63 \pm 0.03$ for the pinned phase, and $\alpha_M = 0.75 \pm 0.05$ for the moving phase. For the growth exponent, we find $\beta_P = 0.67 \pm 0.05$ for the pinned phase, and $\beta_M = 0.74 \pm 0.06$ for the moving phase. Furthermore, we find an anomalous scaling of the prefactor of the width on the driving force. A new exponent $\varphi_M = -0.12 \pm 0.06$, characterizing the scaling of this prefactor, is shown to relate the values of the roughnessComment: Latex manuscript, Revtex 3.0, 15 pages, and 15 figures also available via anonymous ftp from ftp://jhilad.bu.edu/pub/abms/ (128.197.42.52

The problem of calcified ascending aorta during aortocoronary bypass

Heavily calcified ascending aorta significantly increased morbidity and lethality during open-heart surgery. Cannulation and clamping (partial or total) of severely atherosclerotic ascending aorta can easily cause damage and rupture of aortic wall, with consequential distal (often fatal) embolization with atheromatous debris (brain, myocardium). From June 1998. until June 2000, 11 of 2 136 (0.5%) patients who underwent coronary artery bypass grafting were with the severe atheromatous ascending aorta. The site of cannulation was in the aortic arch in three patients (aorta was occluded with Foley catheter in one case, and single clamp technique was used in the other two cases). The femoral artery was the cannulation site in other five cases. Profound hypothermia, ventricular fibrillation, and circulatory arrest, with no cross-clamping or cardioplegia, were used in three patients. Two patients were operated on with extracorporeal circulation, one in normothermia, on the beating heart, the other in moderate hypothermia, on fibrillating heart. In three patients myocardial revascularization was performed on the beating heart, in normothermia, without extracorporeal circulation. Postoperative course was uneventful in all 11 patients. Neither atheroembolism in the peripheral organs, nor atheroembolism of the extramities occurred. The proposed surgical approaches have the potential to reduce the prevalence of stroke and systemic embolization associated with coronary artery bypass grafting in patients with heavily calcified ascending aorta. This result was achieved due to the applied modifications of standard cardiosurgical technique

Integrated natural radiation exposure studies in stable Yugoslav rural communities

The results of field investigations of natural radiation exposures of the general population in two stable rural communities in Yugoslavia are presented. The principal emphasis was on exposures to contemporary indoor radon, but measurements of external penetrating radiation absorbed dose rates in air were carried out in the majority of cases. In addition, in a limited number of dwellings, measurements of thoron gas concentrations were made. By means of making a series of sequential 3-month radon measurements, both seasonal variations and annual average radon levels in the dwellings were determined. Using passive alpha track detectors, individual radon and thoron indoor concentrations as high as 9591 Bq m(-3) and 709 Bq m(-3), respectively, were detected while absorbed dose rates in air in the dwellings as high as 430 nGy h(-1) were recorded. On the basis of these different types of measurements, assessments could be made of the integrated natural radiation exposures being received by the populations. In addition to contemporary radon measurements, retrospective radon exposure assessments in most of the dwellings were made on the basis of measurements of Po-210 concentrations in both surface (glass) traps and in volume (porous materials) traps. A description is given of the sampling strategics and protocols used in this field work. It is shown that at least one stable rural community receiving high natural radiation exposures, has been clearly identified and plans for future health investigations of the population there are outlined. (C) 2001 Elsevier Science B.V. All rights reserved.Scientific Workshop on Radon in the Living Environment, Apr, 1999, Athens, Greec

Radon survey in the high natural radiation region of Niska Banja, Serbia

A radon survey has been carried out around the town of Niska Banja (Serbia) in a region partly located over travertine formations, showing an enhanced level of natural radioactivity. Outdoor and indoor radon concentrations were measured seasonally over the whole year, using CR-39 diffusion type radon detectors. Outdoor measurements were performed at 56 points distributed over both travertine and alluvium sediment formations. Indoor radon concentrations were measured in 102 living rooms and bedrooms of 65 family houses. In about 50% of all measurement sites, radon concentration was measured over each season separately, making it possible to estimate seasonal variations, which were then used to correct values measured over different periods, and to estimate annual values. The average annual indoor radon concentration was estimated at over 1500 Bq/m(3) and at about 650 Bq/m(3) in parts of Niska Banja located over travertine and alluvium sediment formations, respectively, with maximum values exceeding 6000 Bq/m(3). The aver age value of outdoor annual radon concentration was 57 Bq/m(3), with a maximum value of 168 Bq/m(3). The high values of indoor and outdoor radon concentrations found at Niska Banja make this region a high natural background radiation area. Statistical analysis of our data confirms that the level of indoor radon concentration depends primarily on the underlying soil and building characteristics. (c) 2006 Elsevier Ltd. All rights reserved