Soft matter physics of the ground beneath our feet

The soft part of the Earth's surface – the ground beneath our feet – constitutes the basis for life and natural resources, yet a general physical understanding of the ground is still lacking. In this critical time of climate change, cross-pollination of scientific approaches is urgently needed to better understand the behavior of our planet's surface. The major topics in current research in this area cross different disciplines, spanning geosciences, and various aspects of engineering, material sciences, physics, chemistry, and biology. Among these, soft matter physics has emerged as a fundamental nexus connecting and underpinning many research questions. This perspective article is a multi-voice effort to bring together different views and approaches, questions and insights, from researchers that work in this emerging area, the soft matter physics of the ground beneath our feet. In particular, we identify four major challenges concerned with the dynamics in and of the ground: (I) modeling from the grain scale, (II) near-criticality, (III) bridging scales, and (IV) life. For each challenge, we present a selection of topics by individual authors, providing specific context, recent advances, and open questions. Through this, we seek to provide an overview of the opportunities for the broad Soft Matter community to contribute to the fundamental understanding of the physics of the ground, strive towards a common language, and encourage new collaborations across the broad spectrum of scientists interested in the matter of the Earth's surface

A phase-field study of ternary multiphase microstructures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.A diffuse-interface model for microstructures with an arbitrary number of components and phases was developed from basic thermodynamic and kinetic principles and applied to the study of ternary eutectic phase transformations. Gradients in composition and phase were included in the free energy functional, and a generalized diffusion potential equal to the chemical potential at equilibrium was defined as the driving force for diffusion. Problematic pair-wise treatment of phases at interfaces and triple junctions was avoided, and a cutoff barrier was introduced to constrain phase fractions to physically meaningful values. Parameters in the model were connected to experimentally measurable quantities. Numerical methods for solving the phase-field equations were investigated. Explicit finite difference suffered from stability problems while a semi-implicit spectral method was orders of magnitude more stable but potentially inaccurate. The source of error was found to be the rich temporal dynamics of spinodal decomposition combined with large timesteps and a first-order time integrator. The error was addressed with a second-order semi-implicit Runge-Kutta time integrator and adaptive timestepping, resulting in two orders of magnitude improvement in efficiency. A diffusion-limited growth instability in multiphase thin-film systems was discovered, highlighting how ternary systems differ from binary systems, and intricate asymmetries in the processes of solidification and melting were simulated. A nucleation barrier for solidification was observed and prompted development of a Monte-Carlo-like procedure to trigger nucleation. However when solid was heated from below the melting point, premelting was observed first at phase triple junctions and then at phase boundaries with stable liquid films forming under certain conditions. Premelting was attributed to the shape and position of the metastable liquid curve, which was found to affect microstructure by creating low energy pathways through composition space. Slow diffusivity in solid relative to liquid was shown to produce solutal melting of solid below the melting point. Finally, the multiphase method was used to produce the first reported simulation of the entire transient liquid phase bonding process. The model shows promise for optimizing the bonding process and for simulating non-planar solidification interfaces.by Daniel A. Cogswell.Ph.D

Spreading of viscous fluids and granular materials on slopes

Materials can flow down a slope in a wide range of geophysical and industrial contexts, including lava flows on volcanoes and thin films on coated surfaces. The aim of my research is to provide quantitative insight into these forms of motion and their dependence on effects of the topography, the volume and the rheology of the flowing structure. Numerous different problems are investigated through mathematical models, which are developed analytically and confirmed by laboratory experiments. The initial advance of long lava flows is studied by considering the flow of viscous fluid released on sloping channels. A scaling analysis, in agreement with analog experiments and field data, offers a practical tool for predicting the advance of lava flows and conducting hazard analysis. A simple and powerful theory predicts the structure of flows resulting from any time-dependent release of fluid down a slope. Results obtained by the method of characteristics reveal how the speed of the advancing front depends importantly on the rate of fluid supplied at an earlier time. Viscous flows on surfaces with different shapes are described by similarity solutions to address problems motivated by engineering as well as geophysical applications. Pouring viscous fluid out of a container can be a frustratingly slow process depending on the shape and the degree of tipping of the container. The discharge rate of the fluid is analysed in simple cases, shedding light on how containers can be emptied most quickly in cosmetic and food industries. In a separate study motivated by coating industries, thin films are shown to evolve with uniform thickness as they drain near the top of a horizontal cylinder or sphere. The leading edge eventually splits into rivulets as predicted theoretically and confirmed by experiments. Debris flows can develop levees and trigger avalanches which are studied by considering dense granular flows down a rough inclined plane. Granular materials released down a slope can produce a flowing structure confined by levees or trigger avalanches at regular intervals, depending on the steady rate of supply. The experimental results are discussed using theoretical ideas of shallow granular flows. Finally, materials flowing in long and slender ducts are investigated theoretically to better understand the digestive and urinary systems in biology. The materials are pumped in an elastic tube by translating waves of muscular contraction and relaxation. The deformation of the tube is predicted by solving a free-boundary problem, a similar mathematical exercise to predicting the moving boundaries of materials spreading on slopes

Morphology and dynamics of ice crystals and the effect of proteins

286 p.La tesis "Morfología y dinámica de los cristales de hielo y su efecto en las proteínas" se basa en una amplia gama de temas que abarcan el fundamento de la estructura del hielo (tanto su superficie como su morfología), la interacción de las proteínas con el hielo y las ciencias ambientales (formación de las nubes y la dinámica de los glaciares). El enfoque se centra en las interfaces hielo/vapor e hielo/agua.Mediante el microscopio electrónico de barrido ambiental (ESEM) a temperaturas inusualmente bajas, se ha logrado acceder a estudiar la morfología del hielo in-situ en diversas áreas del diagrama de fases (presión-temperatura). Además de reproducir las morfologías ya conocidas de cristales individuales e hielo poli-cristalino, se han observado formas ya conocidas de hielo, así como hielo poli-cristalino. Nuevas geometrías llamadas ¿pools¿, formas circulares de ¿m de diámetro, fueron encontradas en los límites del grano de la superficie del hielo poli-cristalino durante procesos de sublimación lento.Además, se estudiaron ocho soluciones diferentes de proteínas en condiciones de súper-enfriamiento mediante las técnicas de congelación de gota (drop freezing technique) y calorimetría diferencial de barrido (DSC). Únicamente, la (apo)ferritina y la ferritina han mostrado buenas características para la nucleación de hielo, es decir, la congelación bajo pequeño súper-enfriamiento (solo algunos grados debajo de 0 ºC), mientras que la mayoría de las soluciones de proteínas estudiadas se congelan por debajo de -15 ºC, como el agua pura.CIC NanoGUNE:nanoscience cooperative research center CFM: materials physics center ETH Zürich: Institute for Atmospheric and Climate Scienc

Phase-field-crystal models for condensed matter dynamics on atomic length and diffusive time scales: an overview

Here, we review the basic concepts and applications of the phase-field-crystal (PFC) method, which is one of the latest simulation methodologies in materials science for problems, where atomic- and microscales are tightly coupled. The PFC method operates on atomic length and diffusive time scales, and thus constitutes a computationally efficient alternative to molecular simulation methods. Its intense development in materials science started fairly recently following the work by Elder et al. [Phys. Rev. Lett. 88 (2002), p. 245701]. Since these initial studies, dynamical density functional theory and thermodynamic concepts have been linked to the PFC approach to serve as further theoretical fundaments for the latter. In this review, we summarize these methodological development steps as well as the most important applications of the PFC method with a special focus on the interaction of development steps taken in hard and soft matter physics, respectively. Doing so, we hope to present today's state of the art in PFC modelling as well as the potential, which might still arise from this method in physics and materials science in the nearby future.Comment: 95 pages, 48 figure

Large-Scale Patterning and Dynamics of Topological Solitons In Chiral Nematic Liquid Crystals

The coexistence of order and fluidity in soft condensed matter often mimics that found in biological cells, which allows for complex collective dynamics and for highly technological applications, like displays and sensors. In active soft matter, different forms of emergent order can even arise because of this out-of-equilibrium dynamic behavior, powered by local energy conversion. We show that this emergent ordering can mimic behavior of familiar living systems with coherent motion, like crowds of people and schools of fish. Most active matter systems are biological in origin, although the discovery of inanimate, purely synthetic particles capable of such emergent behavior would enable new breeds of materials and nanomachines. Such examples are limited, typically with chemical or mechanical agitation sources of energy. We show that thousands of particle-like topological solitons can exhibit collective dynamics while either (1) each converting macroscopically-supplied electric energy into motion along spontaneously-selected directions uncorrelated with the direction of electric field, (2) responding to photo-manipulation of the material&rsquo;s elastic free energy landscape by changing shape or moving away from the light, or (3) a combination of the two. We demonstrate that these soliton dynamics occur in the absence of backflows, have the ability to carry cargo, display tunable dynamic self-assembly with their neighbors, possess long-range repulsive interactions, and exhibit emergent order and giant-number fluctuation scaling consistent with active systems such as schooling fish. We uncover how these topological solitons react to changes in the local and global elastic free energy, both in samples with flat geometries and spherical liquid crystal shells. We further show that skyrmions in motion can pack together as dense quasi-hexagonal crystallites and exhibit crowding and jamming behavior similar to crowds of people moving around obstacles. Uniquely to our system, this plethora of emergent active behavior occurs in the absence of physical or biological particles and material flows, revealing the defining characteristics of the new field of solitonic active matter and promising many technological uses.</p

Mathematical Models of ice stream dynamics and supraglacial drainage

Patterning is a recurrent feature of glacial systems, which characterizes as much subglacial and supraglacial environments as the flow of ice itself. Some examples include bedforms developing at the contact between ice and bed, spatial organization in subglacial and supraglacial drainage networks, the narrow corridors of fast flowing ice known as ice streams that form the arterial drainage system of large ice sheets, and temporal switches between slow and fast flow regimes in glacier and ice stream flow. This thesis focusses on two types of glacial patterns, namely ice streams and channelization in supraglacial drainage networks. Ice flow within ice sheets is far from uniform, with the narrow bands known as ice streams flowing at velocity two order of magnitude larger than the rest of the ice sheet. In the Siple Coast region of West Antarctica ice streams experiance weak topographic confinement, thus suggesting that they may originate spontaneously from an otherwise uniform flow as a fingering instability. Motivated by observations suggesting that the marked contrast in velocity between ice streams and surrounding ice is due to a transition from frozen, thus sticky bed underneath slow flowing regions, to molten, thus well lubricated bed under ice streams, we investigate the role of basal thermal transitions in relation to the onset of ice streams. Our findings suggest that basal transitions from frozen to molten bed (or vice versa) can undergo an instability potentially leading to the onset of streaming. An asymptotic analysis for short wavelenght perturbations shows that, at wavelengths of few ice thicknesses, such instability is controlled by the interplay between strain heating and heat advection from the region upstream of the transition. We also find that the background structure of the ice sheet is key to pattern formation. In particular, in the case of ice flowing from molten to frozen regions we find an instability at the ice sheet thickness scale or smaller, which is not resolved by most ice sheet models. Observations reveal that ice streams experience significant temporal variability on a variety of time scales, ranging from decadal to multi-millennial ones. As much as spatial patterning, such variability holds implications for the future of ice sheets, sea level change, and the interpretation of geological records. Recent work \citep{robel} shows that the switch between steady streaming conditions and self-sustained oscillations with multi-millennial periodicity can be understood as a Hopf bifurcation. Little is presently known about shorter scale variability, which however appears more likely to originate from external forcing. In chapter \ref{ch:stoch} we explore the effects of a specific type of forcing, i.e. stochastically-varying climatic conditions, on the temporal dynamics of ice stream flow. We find that data-based climate fluctuations alter the deterministic dynamics substantially, and are capable of introducing widespread, short-scale oscillations even in ranges of the parametric regime where the deterministic dynamics predict steady streaming. We thus conclude that noise-induced transitions may play a role in the observed temporal dynamics of ice stream flow. In part \ref{drain} we turn to patterning in drainage networks on the surface of glaciers. Supraglacial drainage networks route meltwater originating on the surface of glaciers towards moulins and crevasses, through which it eventually reaches the base of the ice. Therefore, understanding the physical controls on the structure of the drainage network has implications for how surface melt influences the motion of ice. Here we focus on the physical controls on the formation of evenly spaced channels on the surface of glaciers. In particular, we find that the flow of meltwater on bare ice is capable of carving evenly spaced channels as a result of a morphological instability. We show that in certain conditions the network is shaped solely by the hydrodynamics of meltwater regardless of ice thermal conditions, which justifies widely-observed regular patterns in drainage networks. Finally, comparison of our results with the geometrical feature of supraglacial networks reported in the literature shows good agreement between model's predictions and observations

Influence of Seed Particle Material, Preparation, and Dynamics on Nanowire Growth

Semiconducting nanowires have attracted scientific attention for more than 20 years due to their potential applications in electronic devices, as sensors, and in solid state lighting. These applications require high quality nanowires to begin with. Achieving such good control over the growth of nanowires is not trivial and requires profound understanding of the underlying processes. In this thesis, nanowires of different materials and combinations thereof have been grown with the help of seed particles by metal-organic vapor phase epitaxy (MOVPE). The focus of the investigations lies on the influence of several seed particle properties on nanowire growth. First, we compared six particle preparation and deposition methods for the most common seed particle material – gold - and their influence on the growth of GaAs nanowires. We observed only small differences, mainly in incubation times, which did not have a significant effect on the nanowire length after some growth time, though. The optical properties, however, varied between nanowires seeded by different particle types. Further, copper as seed particle material for growth of InP nanowires and InP-InAs heterostructures was investigated. The aim was to get a deeper understanding of which properties or combination of properties determine a “good” seed particle material. InP nanowire growth from Cu particles differs a lot from nanowire growth from Au seed particles in terms of temperature range and precursor molar fractions. Furthermore, growth from two types of particles – Cu-rich and In-rich – occurs simultaneously at low V/III ratios. The investigations of InP-InAs heterostructures showed that it is indeed possible to grow straight heterostructures, but we observed unusual layer formation of the InAs segments. Finally, we used the possibility of in situ TEM to investigate nanowire growth at the IBM T.J. Watson Research Center. We combined group IV and group III/V materials and investigated the particle dynamics that may lead to kinking. In addition, we investigated the instantaneous kinetics of GaP growth

Morphology and dynamics of ice crystals and the effect of proteins

286 p.La tesis "Morfología y dinámica de los cristales de hielo y su efecto en las proteínas" se basa en una amplia gama de temas que abarcan el fundamento de la estructura del hielo (tanto su superficie como su morfología), la interacción de las proteínas con el hielo y las ciencias ambientales (formación de las nubes y la dinámica de los glaciares). El enfoque se centra en las interfaces hielo/vapor e hielo/agua.Mediante el microscopio electrónico de barrido ambiental (ESEM) a temperaturas inusualmente bajas, se ha logrado acceder a estudiar la morfología del hielo in-situ en diversas áreas del diagrama de fases (presión-temperatura). Además de reproducir las morfologías ya conocidas de cristales individuales e hielo poli-cristalino, se han observado formas ya conocidas de hielo, así como hielo poli-cristalino. Nuevas geometrías llamadas ¿pools¿, formas circulares de ¿m de diámetro, fueron encontradas en los límites del grano de la superficie del hielo poli-cristalino durante procesos de sublimación lento.Además, se estudiaron ocho soluciones diferentes de proteínas en condiciones de súper-enfriamiento mediante las técnicas de congelación de gota (drop freezing technique) y calorimetría diferencial de barrido (DSC). Únicamente, la (apo)ferritina y la ferritina han mostrado buenas características para la nucleación de hielo, es decir, la congelación bajo pequeño súper-enfriamiento (solo algunos grados debajo de 0 ºC), mientras que la mayoría de las soluciones de proteínas estudiadas se congelan por debajo de -15 ºC, como el agua pura.CIC NanoGUNE:nanoscience cooperative research center CFM: materials physics center ETH Zürich: Institute for Atmospheric and Climate Scienc