On the normal modes of weak colloidal gels

The normal modes and relaxation rates of weak colloidal gels are investigated in computations employing different models of the hydrodynamic interactions between colloids. The eigenspectrum is computed for freely draining, Rotne-Prager-Yamakawa and Accelerated Stokesian Dynamics approximations of the hydrodynamic mobility in a normal mode analysis of a harmonic network representing the gel. The spatial structure of the normal modes suggests that measures of collectivity and energy dissipation in the gels are fundamentally altered by long-ranged hydrodynamic interactions, while hydrodynamic lubrication affects only the relaxation rates of short wavelength modes. Models accounting for long-ranged hydrodynamic interactions exhibit a microscopic relaxation rate for each normal mode, $\lambda$ that scales as $\lambda\sim l^{-2}$, where $l$ is the spatial correlation length of the mode. For the freely draining approximation, $\lambda\sim l^\gamma$, where $\gamma$ varies between 3 and 2 with increasing $\phi$. A simple phenomenological model of the internal elastic response to normal mode fluctuations is developed, which shows that long-ranged hydrodynamic interactions play a central role in the viscoelasticity of the gel network. Dynamic simulations show that the stress decay as measured by the time-dependent shear modulus matches the normal mode predictions and the phenomenological model. Analogous to the Zimm model in polymer physics, our results indicate that long-ranged hydrodynamic interactions play a crucial role in determining the microscopic dynamics and macroscopic properties of weak colloidal gels

Anisotropic diffusion in confined colloidal dispersions: The evanescent diffusivity

We employ an analogy to traditional dynamic light scattering to describe the inhomogeneous and anisotropic diffusion of colloid particles near a solid boundary measured via evanescent wave dynamic light scattering. Following this approach, we generate new expressions for the short-time self- and collective diffusivities of colloidal dispersions with arbitrary volume fraction. We use these expressions in combination with accelerated Stokesian dynamics simulations to calculate the diffusivities in the limit of large and small scattering wave numbers for evanescent penetration depths ranging from four particle radii to one-fifth of a particle radius and volume fractions from 10% to 40%. We show that at high volume fractions, and larger penetration depths, the boundaries have little effect on the dynamics of the suspension parallel to the wall since, to a first approximation, the boundary acts hydrodynamically much as another nearby particle. However, near and normal to the wall, the diffusivity shows a strong dependence on penetration depth for all volume fractions. This is due to the lubrication interactions between the particles and the boundary as the particle moves relative to the wall. These results are novel and comprehensive with respect to the range of penetration depth and volume fraction and provide a complete determination of the effect of hydrodynamic interactions on colloidal diffusion adjacent to a rigid boundary

Particle motion between parallel walls: Hydrodynamics and simulation

The low-Reynolds-number motion of a single spherical particle between parallel walls is determined from the exact reflection of the velocity field generated by multipoles of the force density on the particle’s surface. A grand mobility tensor is constructed and couples these force multipoles to moments of the velocity field in the fluid surrounding the particle. Every element of the grand mobility tensor is a finite, ordered sum of inverse powers of the distance between the walls. These new expressions are used in a set of Stokesian dynamics simulations to calculate the translational and rotational velocities of a particle settling between parallel walls and the Brownian drift force on a particle diffusing between the walls. The Einstein correction to the Newtonian viscosity of a dilute suspension that accounts for the change in stress distribution due to the presence of the channel walls is determined. It is proposed how the method and results can be extended to computations involving many particles and periodic simulations of suspensions in confined geometries

Simulation of hydrodynamically interacting particles near a no-slip boundary

The dynamics of spherical particles near a single plane wall are computed using an extension of the Stokesian dynamics method that includes long-range many-body and pairwise lubrication interactions between the spheres and the wall in Stokes flow. Extra care is taken to ensure that the mobility and resistance tensors are symmetric, positive, and definite—something which is ineluctable for particles in low-Reynolds-number flows. We discuss why two previous simulation methods for particles near a plane wall, one using multipole expansions and the other using the Rotne-Prager tensor, fail to produce symmetric resistance and mobility tensors. Additionally, we offer some insight on how the Stokesian dynamics paradigm might be extended to study the dynamics of particles in any confining geometry

Black and White: Structural and Functional Aspects of Dermal Chromatophores of the Marbled Whiptail Lizard (Cnemidophorus Marmoratus)

The skin of reptiles is a complex organ with many sensory, regulatory and behavioral functions. Desert reptiles face a suite of challenges as their skin contacts hot-dry environmental surfaces. The marbled whiptail lizard, Cnemidophorus marmoratus (Lacertilia: Teiidae), is found in hot deserts of southeastern New Mexico and western Texas and is often above ground during the hottest times of the day. When active, these lizards encounter a broad range and intensity of visible and infrared wavelengths. The role of the integument in temperature regulation, although poorly understood, is critically important for these diurnal animals. Albedo, coloration, pattern, and in some circumstances color change, may be determined by placement and orientation of three distinct types of dermal chromatophores (xanthophores, iridophores, and melanophores). Here electron microscopy, brightfield microscopy, and reflectance spectroscopy is used to examine some of the structural-functional relationships of the chromatophores of these lizards with particular attention to the crystal containing iridophore

AnswerTree – a hyperplace-based game for collaborative mobile learning

In this paper we present AnswerTree, a collaborative mobile location-based educational game designed to teach 8-12 year olds about trees and wildlife within the University of Nottingham campus. The activity is designed around collecting virtual cards (similar in nature to the popular Top TrumpsTM games) containing graphics and information about notable trees. Each player begins by collecting one card from a game location, but then he or she can only collect further cards by answering questions – whose solutions are obtainable through sharing knowledge with other cardholders. This ostensibly allows each player to become a subject expert at the start of the game, encouraging collaborative interaction for the game to be successfully completed. In this initial paper we will outline the structure and background of this location based game. AnswerTree has been authored within the Hyperplace framework, and is a first implementation of a wider process to develop a flexible, multi-purpose platform for both individual and group location-based mobile learning

TWILIGHT OF NEWHAVEN: THE TRANSFORMATION OF AN ANCIENT FISHING VILLAGE INTO A MODERN NEIGHBORHOOD

In 1504, King James IV of Scotland founded the village of Newhaven, three miles north of Edinburgh on the shores of the Firth of Forth. Newhaven rose to prominence as the most well-known of Scotland’s fishing villages and reached its zenith in 1928 with the launching of its last ship, the Reliance. It was the beginning of the end of the Newhavener way of life, their twilight. This is the story of decline and domicide as economic forces and the City of Edinburgh Council transformed the ancient village of Newhaven into a modern neighborhood. This small fishing community, with its own unique culture and traditions, such as its famous fishwives, became just another tourist attraction in the Scottish capital. Newhaven began experiencing decline around 1928 due to four main factors: technological advances in fishing, overfishing, extreme pollution, and generational disinterest in perpetuating the Newhavener way-of-life. The City of Edinburgh Council’s urban renewal program forced the modernization of Newhaven between 1958 and 1978. This urban renewal program, together with the Scottish Presbyterian Church’s involuntary amalgamation of Newhaven’s two churches in 1974, ensured Newhaven’s destruction by joining with the decline of fishing to end the village’s distinctive economic, social, and political patterns. My research concludes with the efforts of the inhabitants of Newhaven the neighborhood to forge a new community in the post-1978 years and preserve a legacy of their past for future generations to enjoy. Newhaven joins the ranks of many other small places cleared away by those in power, proving that the Newhavens of the world are “especially vulnerable to extinction.” Learning from Newhaven’s pattern of destruction will help prevent future injustices against small communities. My research preserves Newhaven’s memory and documents the nature of its struggles through the use of oral histories, primary and secondary sources, and preserved media

The Conference on the Law of the Sea: A Report

From the viewpoint of the United States, far too much attention was given by many delegates to the political aspects of the articles and too little attention to the legal. Many of the new and the underdeveloped States adopted the position that rules established before they were able to influence their formulation should be changed as a matter of progress. They viewed some aspects of freedom of the high seas as a fiction invented by the maritime nations to rob them of their living resources off their coasts