5,615 research outputs found
Stokesian Dynamics
Particles suspended or dispersed in a fluid medium occur in a wide variety of natural and man-made settings, e.g. slurries, composite materials, ceramics, colloids, polymers, proteins, etc. The central theoretical and practical problem is to understand and predict the macroscopic equilibrium and transport properties of these multiphase materials from their microstructural mechanics. The macroscopic properties might be the sedimentation or aggregation rate, self-diffusion coefficient, thermal conductivity, or rheology of a suspension of particles. The microstructural mechanics entails the Brownian, interparticle, external, and hydrodynamic forces acting on the particles, as well as their spatial and temporal distribution, which is commonly referred to as the microstructure. If the distribution of particles were given, as well as the location and motion of any boundaries and the physical properties of the particles and suspending fluid, one would simply have to solve (in principle, not necessarily in practice) a well-posed boundary-value problem to determine the behavior of the material. Averaging this solution over a large volume or over many different configurations, the macroscopic or averaged properties could be determined. The two key steps in this approach, the solution of the many-body problem and the determination of the microstructure, are formidable but essential tasks for understanding suspension behavior.
This article discusses a new, molecular-dynamics-like approach, which we have named Stokesian dynamics, for dynamically simulating the behavior of many particles suspended or dispersed in a fluid medium. Particles in suspension may interact through both hydrodynamic and nonhydrodynamic forces, where the latter may be any type of Brownian, colloidal, interparticle, or external force. The simulation method is capable of predicting both static (i.e. configuration-specific) and dynamic microstructural properties, as well as macroscopic properties in either dilute or concentrated systems. Applications of Stokesian dynamics are widespread; problems of sedimentation, flocculation, diffusion, polymer rheology, and transport in porous media all fall within its domain. Stokesian dynamics is designed to provide the same theoretical and computational basis for multiphase, dispersed systems as does molecular dynamics for statistical theories of matter.
This review focuses on the simulation method, not on the areas in which Stokesian dynamics can be used. For a discussion of some of these many different areas, the reader is referred to the excellent reviews and proceedings of topical conferences that have appeared (e.g. Batchelor 1976a, Dickinson 1983, Faraday Discussions 1983, 1987, Family & Landau 1984). Before embarking on a description of Stokesian dynamics, we pause here to discuss some of the relevant theoretical literature on suspensions, and dynamic simulation in general, in order to put Stokesian dynamics in perspective
Ethical issues involving long-term land leases: a soil sciences perspective
As populations grow and arable land becomes increasingly scarce, large-scale long- term land leases are signed at a growing rate. Countries and investors with large amounts of financial resources and a strong agricultural industry seek long-term land leases for agricultural exploitation or investment purposes. Leaders of financially poorer countries often advertise such deals as a fast way to attract foreign capital. Much has been said about the short-term social costs these types of leases involve, however, less has been said about the normative dimension of their long-term environmental impact. We therefore will focus on the likely impact such deals have for soil conservation, by (1) briefly introducing the basics of long-term leasing arrangements by comparing land leases to the renting of buildings, (2) explaining from a soil sciences perspective the difficulties in assessing the current value of an estate and in calculating the damages of soil erosion and degradation, and (3) show how difficult it is to incentivize the conservation of soil quality when one cannot sufficiently and cost-effectively valorize existing environmental capital and eventual future damages. Attempting to oblige tenants through contracts to invest in sustainable stewardship has limited potential when liability payments do not reflect true costs and are hard to enforce
Hurricane María: An Agroecological Turning Point for Puerto Rico?
When Hurricane María tore through Puerto Rico on September 20, 2017, it left 17 dead, 11,000 seeking shelter, and the island’s 3.4 million people without power, water, or fresh food supplies.i It also ripped off the democratic veneer of the US’ “commonwealth,” revealing the structural vulnerability of an island that has been colonized for over half a millennium. Disasters tend to unmask both unsustainable practices and inequitable relations of power. But they can also unleash the power of solidarity and self-governance as communities—abandoned by their governments and preyed upon by disaster capitalists—come together in unexpected ways. In the aftermath of Puerto Rico’s worst social, economic and environmental catastrophe, the Puerto Rican food sovereignty movement is using agroecology to reconstruct the island’s beleaguered food system
Stokesian Dynamics simulation of Brownian suspensions
The non-equilibrium behaviour of concentrated colloidal dispersions is studied by Stokesian Dynamics, a general molecular-dynamics-like technique for simulating particles suspended in a viscous fluid. The simulations are of a suspension of monodisperse Brownian hard spheres in simple shear flow as a function of the Péclet number, Pe, which measures the relative importance of shear and Brownian forces. Three clearly defined regions of behaviour are revealed. There is first a Brownian-motion-dominated regime (Pe ≤ 1) where departures from equilibrium in structure and diffusion are small, but the suspension viscosity shear thins dramatically. When the Brownian and hydrodynamic forces balance (Pe ≈ 10), the dispersion forms a new ‘phase’ with the particles aligned in ‘strings’ along the flow direction and the strings are arranged hexagonally. This flow-induced ordering persists over a range of Pe and, while the structure and diffusivity now vary considerably, the rheology remains unchanged. Finally, there is a hydrodynamically dominated regime (Pe > 200) with a dramatic change in the long-time self-diffusivity and the rheology. Here, as the Péclet number increases the suspension shear thickens owing to the formation of large clusters. The simulation results are shown to agree well with experiment
Theory of light-enhanced phonon-mediated superconductivity
We investigate the dynamics of a phonon-mediated superconductor driven out of
equilibrium. The electronic hopping amplitude is ramped down in time, resulting
in an increased electronic density of states. The dynamics of the coupled
electron-phonon model is investigated by solving Migdal-Eliashberg equations
for the double-time Keldysh Green's functions. The increase of the density of
states near the Fermi level leads to an enhancement of superconductivity when
the system thermalizes to the new state at the same temperature. We provide a
time- and momentum-resolved view on this thermalization process, and show that
it involves fast processes associated with single-particle scattering and much
slower dynamics associated with the superconducting order parameter. The
importance of electron-phonon coupling for the rapid enhancement and the
efficient thermalization of superconductivity is demonstrated, and the results
are compared to a BCS time-dependent mean-field approximation.Comment: 12 pages, 8 figure
Dynamic simulation of hydrodynamically interacting suspensions
A general method for computing the hydrodynamic interactions among an infinite suspension of particles, under the condition of vanishingly small particle Reynolds number, is presented. The method follows the procedure developed by O'Brien (1979) for constructing absolutely convergent expressions for particle interactions. For use in dynamic simulation, the convergence of these expressions is accelerated by application of the Ewald summation technique. The resulting hydrodynamic mobility and/or resistance matrices correctly include all far-field non-convergent interactions. Near-field lubrication interactions are incorporated into the resistance matrix using the technique developed by Durlofsky, Brady & Bossis (1987). The method is rigorous, accurate and computationally efficient, and forms the basis of the Stokesian-dynamics simulation method. The method is completely general and allows such diverse suspension problems as self-diffusion, sedimentation, rheology and flow in porous media to be treated within the same formulation for any microstructural arrangement of particles. The accuracy of the Stokesian-dynamics method is illustrated by comparing with the known exact results for spatially periodic suspensions
Sound Velocity Anomaly at the Mott Transition: application to organic conductors and V2O3
Close to the Mott transition, lattice degrees of freedom react to the
softening of electron degrees of freedom. This results in a change of lattice
spacing, a diverging compressibility and a critical anomaly of the sound
velocity. These effects are investigated within a simple model, in the
framework of dynamical mean-field theory. The results compare favorably to
recent experiments on the layered organic \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl
conductor . We predict that effects of a similar magnitude are expected for
V2O3, despite the much larger value of the elastic modulus of this material.Comment: New discussion of the relation between the sound-velocity and the
compressibility has been adde
Slash-and-mulch: Exploring the role of shrub-based agroforestry systems for smallholder farmers in the Sahel
In Burkina Faso, degraded soils where no crop production is possible affect more than 1 million farmers (Bai et al., 2008). If restored, these soils could potentially contribute to increased local food provision and to climate change mitigation via global carbon sequestration. Advancements toward the imitation of dryland forest floors to restore agricultural soils in the Sahel, a form of ‘ecosystem mimicry’ (Ewel, 1999), can be achieved via the intensive application of carbon-rich mulches. Therefore, strategies are needed to restore and maintain soil productivity via greater inputs of organic matter, greater water retention, reduced evaporation and runoff, increased soil biological diversity, and nutrient cycling and availability. Indigenous forms of agroforestry based on the use of local shrubs, as practiced by farmers, offer opportunities to design sustainable farming systems that are based on agroecological principles and can contribute to building resilience and adaptability in the face of climate change
Dynamical Mean-Field Theory - from Quantum Impurity Physics to Lattice Problems
Since the first investigation of the Hubbard model in the limit of infinite
dimensions by Metzner and Vollhardt, dynamical mean-field theory (DMFT) has
become a very powerful tool for the investigation of lattice models of
correlated electrons. In DMFT the lattice model is mapped on an effective
quantum impurity model in a bath which has to be determined self-consistently.
This approach lead to a significant progress in our understanding of typical
correlation problems such as the Mott transition; furthermore, the combination
of DMFT with ab-initio methods now allows for a realistic treatment of
correlated materials. The focus of these lecture notes is on the relation
between quantum impurity physics and the physics of lattice models within DMFT.
Issues such as the observability of impurity quantum phase transitions in the
corresponding lattice models are discussed in detail.Comment: 18 pages, 5 figures, invited paper for the Proceedings of the "3rd
International Summer School on Strongly Correlated Systems, Debrecen, 2004
- …