arXiv subjects

Signe Kjelstrup

Publications and source records attributed to Signe Kjelstrup.

24 records · Page 2Linked to original sources

arXiv2016

A Monte Carlo Algorithm for Immiscible Two-Phase Flow in Porous Media

We present a Markov Chain Monte Carlo algorithm based on the Metropolis algorithm for simulation of the flow of two immiscible fluids in a porous medium under macroscopic steady-state conditions using a dynamical pore network model that tracks the motion of the fluid interfaces. The Monte Carlo algorithm is based on the configuration probability, where a configuration is defined by the positions of all fluid interfaces. We show that the configuration probability is proportional to the inverse of the flow rate. Using a two-dimensional network, advancing the interfaces using time integration the computational time scales as the linear system size to the fourth power, whereas the Monte Carlo computational time scales as the linear size to the second power. We discuss the strengths and the weaknesses of the algorithm.

physics.flu-dyn↗

arXiv2016

Ensemble Distribution for Immiscible Two-Phase Flow in Porous Media

We construct an ensemble distribution to describe steady immiscible two-phase flow of two incompressible fluids in a porous medium. The system is found to be ergodic. The distribution is used to compute macroscopic flow parameters. In particular, we find an expression for the overall mobility of the system from the ensemble distribution. The entropy production at the scale of the porous medium is shown to give the expected product of the average flow and its driving force, obtained from a black-box description. We test numerically some of the central theoretical results.

physics.flu-dyn↗

arXiv2016

Entropy Production in Mesoscopic Stochastic Thermodynamics: Nonequilibrium Kinetic Cycles Driven by Chemical Potentials, Temperatures, and Mechanical Forces

Nonequilibrium thermodynamics (NET) investigates processes in systems out of global equilibrium. On a mesoscopic level, it provides a statistical dynamic description of various complex phenomena such as chemical reactions, ion transport, diffusion, thermochemical, thermomechanical and mechanochemical fluxes. In the present review, we introduce a mesoscopic stochastic formulation of NET by analyzing entropy production in several simple examples. The fundamental role of nonequilibrium steady-state cycle kinetics is emphasized. The statistical mechanics of Onsager's reciprocal relations in this context is elucidated. Chemomechanical, thermomechanical, and enzyme-catalyzed thermochemical energy transduction processes are discussed. It is argued that mesoscopic stochastic NET provides a rigorous mathematical basis of fundamental concepts needed for understanding complex processes in chemistry, physics and biology, and which is also relevant for nanoscale technological advances.

cond-mat.stat-mech↗

arXiv2014

Silver catalyzed Fluorouracil degradation; a promising new role for graphene

Chemotherapy treatment usually involves the delivery of fluorouracil (5-Fu) together with other drugs through central venous catheters. Catheters and their connectors are increasingly coated (or impregnated) with silver or argentic alloys/compounds. Complications such as broken catheters are common, leading to additional suffering for patients and increased medical costs. Here, we uncover a likely cause of such failure through a study of the surface chemistry relevant to chemotherapy drug delivery, i.e. between 5-Fu and silver. We show that silver catalytically decomposes 5-Fu, releasing HF as a product. This reaction compromises the efficacy of the treatment, and at the same time, releases HF which is damaging to both patient and catheter. Our study not only reveals an important reaction which has so far been overlooked, but additionally allows us to propose that graphene coatings inhibit such a reaction and offer superior performance for cancer treatment applications.

cond-mat.mtrl-sci↗

arXiv2012

Effective Rheology of Bubbles Moving in a Capillary Tube

We calculate the average volumetric flux versus pressure drop of bubbles moving in a single capillary tube with varying diameter, finding a square-root relation from mapping the flow equations onto that of a driven overdamped pendulum. The calculation is based on a derivation of the equation of motion of a bubble train from considering the capillary forces and the entropy production associated with the viscous flow. We also calculate the configurational probability of the positions of the bubbles.

physics.flu-dyn↗

arXiv2011

Adsorption of CO$_2$ and CH$_4$ and their mixtures in gas hydrates

We report results from grand-canonical Monte Carlo simulations of methane and carbon dioxide adsorption in structure sI gas hydrates. Simulations of pure component systems show that all methane sites are equivalent, while carbon dioxide distinguishes between two types of sites, large or small. The adsorbed mixture can be regarded as ideal, as long as only large sites are occupied. A strong preference is demonstrated for methane, when the smaller sites become filled. The molar heat of adsorption of methane decreases with composition, while the molar heat of adsorption for carbon dioxide passes an extremum, essentially in accordance with the observation on the site sizes. The Helmholtz energies of the hydrate with CO$_2$-CH$_4$ gas mixture for temperatures between 278 and 328 K and pressures between 10$^4$ and 10$^9$ Pa indicate that certain mixtures are more stable than others. The results indicate that a thermodynamic path exists for conversion of a pure methane hydrate into a pure carbon dioxide hydrate without destroying the hydrate structure.

cond-mat.mtrl-sci↗