Ice-lens formation and geometrical supercooling in soils and other colloidal materials

We present a new, physically-intuitive model of ice-lens formation and growth during the freezing of soils and other dense, particulate suspensions. Motivated by experimental evidence, we consider the growth of an ice-filled crack in a freezing soil. At low temperatures, ice in the crack exerts large pressures on the crack walls that will eventually cause the crack to split open. We show that the crack will then propagate across the soil to form a new lens. The process is controlled by two factors: the cohesion of the soil, and the geometrical supercooling of the water in the soil; a new concept introduced to measure the energy available to form a new ice lens. When the supercooling exceeds a critical amount (proportional to the cohesive strength of the soil) a new ice lens forms. This condition for ice-lens formation and growth does not appeal to any ad hoc, empirical assumptions, and explains how periodic ice lenses can form with or without the presence of a frozen fringe. The proposed mechanism is in good agreement with experiments, in particular explaining ice-lens pattern formation, and surges in heave rate associated with the growth of new lenses. Importantly for systems with no frozen fringe, ice-lens formation and frost heave can be predicted given only the unfrozen properties of the soil. We use our theory to estimate ice-lens growth temperatures obtaining quantitative agreement with the limited experimental data that is currently available. Finally we suggest experiments that might be performed in order to verify this theory in more detail. The theory is generalizable to complex natural-soil scenarios, and should therefore be useful in the prediction of macroscopic frost heave rates.Comment: Submitted to PR

Front propagation in a phase field model with phase-dependent heat absorption

Copyright © 2006 Elsevier. NOTICE: This is the author’s version of a work accepted for publication by Elsevier. Changes resulting from the publishing process, including peer review, editing, corrections, structural formatting and other quality control mechanisms, may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Physica D, Vol 215, Issue 2, 2006, DOI: 10.1016/j.physd.2006.01.024We present a model for the spatio-temporal behaviour of films exposed to radiative heating, where the film can change reversibly between amorphous (glassy) and crystalline states. Such phase change materials are used extensively in read-write optical disk technology. In cases where the heat absorption of the crystal phase is less than that in the amorphous state we find that there is a bi-stability of the phases. We investigate the spatial behaviours that are a consequence of this property and use a phase field model for the spatio-temporal dynamics in which the phase variable is coupled to a suitable temperature field. It is shown that travelling wave solutions of the system are possible and, depending on the precise system parameters, these waves can take a range of forms and velocities. Some examples of possible dynamical behaviours are discussed and we show, in particular, that the waves may collide and annihilate. The longitudinal and transverse stability of the travelling waves are examined using an Evans function method which suggests that the fronts are stable structures

What drives long-run biodiversity change? New insights from combining economics, palaeoecology and environmental history

This paper presents a new approach to understanding the effects of economic factors on biodiversity change over the long run. We illustrate this approach by studying the determinants of biodiversity change in upland Scotland from 1600-2000. The measure of biodiversity used is a proxy for plant species diversity, constructed using statistical analysis of paleoecological (pollen) data. We assemble a new data set of historical land use and prices over 11 sites during this 400 year period; this data set also includes information on changes in agricultural technology, climate and land ownership. A panel model is then estimated, which controls for both supply and demand shifts over time. A main result is that prices, which act in our model as a proxy for livestock numbers, do indeed impact on biodiversity, with higher prices leading to lower biodiversity

Unsettling sustainability: the poetics of discomfort

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Laboratory observations of double-diffusive convection using high-frequency broadband acoustics

Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Experiments in Fluids 46 (2009): 355-364, doi:10.1007/s00348-008-0570-9.High-frequency broadband (200-300 kHz) acoustic scattering techniques have been used to observe the diffusive regime of double-diffusive convection in the laboratory. Pulse compression signal processing techniques allow 1) centimetre-scale interface thickness to be rapidly, remotely, and continuously measured, 2) the evolution, and ultimate merging, of multiple interfaces to be observed at high-resolution, and 3) convection cells within the surrounding mixed layers to be observed. The acoustically measured interface thickness, combined with knowledge of the slowly-varying temperatures within the surrounding layers, in turn allows the direct estimation of double-diffusive heat and buoyancy fluxes. The acoustically derived interface thickness, interfacial fluxes and migration rates are shown to support established theory. Acoustic techniques complement traditional laboratory sampling methods and provide enhanced capabilities for observing the diffusive regime of double-diffusion in the ocean.Funding for this project was provided by the Ocean Acoustics program at the Office of Naval Research, and by the WHOI Cecil and Ida Greene Technology Award

Space, state-building and the hydraulic mission: crafting the Mozambican State

This article explores the role of large-scale water infrastructure in the formation of states in sub-Saharan Africa. We examine this through a focus on government agents and their shifting hydro-developmental visions of the state in colonial and post-colonial Mozambique. Over time, the focus, underlying principles, and goals of the hydraulic mission shifted, triggered by contextual factors and historical developments within and outside the country. We identify the making of three hydraulic paradigms, fostering different imaginaries of ‘the state’ and social and spatial engineering of the territory: the ‘Estado Novo’ (1930 - 1974), the socialist post-independence state-space (1974 - 1987) and the neoliberal state (1987 - present). We then conclude by discussing how the shifting discursive justifications for infrastructure projects consolidate different state projects and link these to material re-patterning of hydrosocial territories, showing that whilst promoted as a rupture with the past, emerging projects tend to reaffirm, rather than redistribute, power and water within the country

Effect of Magnetic-Field on the Microstructure and Macrosegregation in Directionally Solidified Pb-Sn Alloys

An investigation into the influence of a transverse magnetic field (0.45 T) on the mushy zone morphology and macrosegregation in directionally solidified hypoeutectic Pb-Sn alloy shows that the field has no influence on the morphology of dendritic arrays. The field does, however, cause severe distortion in the cellular array morphology. Cellular arrayed growth with the magnetic field results in an extensive channel formation in the mushy zone, as opposed to the well-aligned and uniformly distributed cells formed in the absence of the field. The channels are produced due to the anisotropy in the thermosolutal convection caused by the magnetic field. Macrosegregation, however, along the length of the directionally solidified samples is not influenced by this magnetic field for either the cellular or dendritic arrays

Laboratories, laws, and the career of a commodity

Unlike most foods, milk is produced fresh at least twice every day, thus recreating, over 700 times a year, a commodity ‘designed’ by the combination of nature, commerce, and law. The paper is a study of the ontogenesis of this commodity in Britain since 1800, stressing the emergence of two new objectivities: dairy science and the law on adulteration. In the words of Christopher Hamlin, what mattered was the “manufacture of certainty, however flimsy that certainty might later be shown to be.'' This was achieved by the collection of samples, the generation of facts by the deployment of the laboratory technologies of physics and chemistry, and a semimonopoly over the truth-power of dairy science that was gradually built up by the large commercial companies. A foundation of state-sponsored regulation provided an official legitimation of compositional standards that suited the interests of capital but ignored ‘natural’ variations in quality and often pilloried innocent producers. The public eventually became accustomed to the regulated quality of the milk in its ‘pinta’ and assumed it to be natural. Even the standardization of composition since 1993 has caused very little disquiet among the consuming public, although milk is now a fully constructed commodity like any other dairy product. Mechanical modernity has at last triumphed over a century of ‘milk as it came from the cow’