Condensed-Matter and Materials Physics Basic Research for Tomorrow's Technology (1999) / Chapter Skim
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4 Nonequilibrium Physics
4 Nonequilibrium Physics
Pages 168-193
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From page 168... ...
Examples include flowing fluids driven by thermal or pressure gradients, solid materials deforming or breaking under the influence of external stresses, or quantum systems atomic spins, perhaps being driven by oscillating electromagnetic fields. As a field of research, nonequilibrium physics is simultaneously very new and very old.
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From page 169... ...
The first of these topics is pattern formation and turbulence in fluid flow. The next two are in the areas of processing and performance of structural materials, specifically, microstructural pattern formation in solidification and a group of topics in solid mechanics: friction, fracture, granular materials, and polymers and adhesives.
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From page 170... ...
We shall have to understand the importance of, and the impediments facing, our efforts to bring new science to bear on technology and to take advantage of new technologies to advance basic science. PATTERN FORMATION AND TURBULENCE IN FLUID DYNAMICS Nonequilibrium Phenomena in Fluids When a fluid system is driven far from mechanical or thermodynamic equilibrium, it has a spontaneous tendency to form patterns and defects.
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From page 171... ...
Specific examples will continue to provide useful insights, and the methods of analysis that they generate will find broad utility. What is unclear is whether a deep general theory will emerge from the knowledge acquired by studying special systems; whether nonequilibrium phenomena, like thermodynamic critical phenomena, fall into a small number of universality classes; and whether a broadbased understanding will eventually enable us to predict and control complex, technologically important processes.
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From page 172... ...
Although a suitable integral of the amplitude function and its gradient serves as an approximate free energy in some cases, this is not a viable procedure in general. A class of pattern formation problems that has not been fully explored is the nucleation and growth of turbulent "spots" in boundary layers (or "slugs" and "puffs" in pipe and channel flows)
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From page 173... ...
A quantitative theory of turbulence is likely to be valuable in the study of other nonequilibrium phenomena. This is why turbulence merits some attention and discussion here; indeed, until the 1960s, fluid turbulence was the clearest example of a phenomenon in which a large range of length scales are simultaneously important (see Box 4.1~.
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From page 174... ...
There is no apparent self-similarity and, in contrast to earlier ideas, individual structures do not become more isotropic at smaller length scales. Note, however, that the anisotropy of individual structures does not necessarily preclude statistical isotropy.
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From page 175... ...
The stresses may be applied by mechanical, thermal, or other means. The changes include instabilities, bifurcations, temporal chaos, pattern formation, phase modulations, defects, growth of localized structures, interactions among dissimilar length scales and timescales, universal and anomalous scaling, intermittence, anomalous transport, and the like.
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From page 176... ...
Only in the past few years have we finally learned how these elegant dendritic crystals emerge literally out of thin air, and why they occur with such diversity that no two seem to be exactly alike. Much of the research on dendritic crystal growth has been driven not only by our natural curiosity about such phenomena, but also by the need to understand and control metallurgical microstructures.
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From page 177... ...
We know that the mushy zone has its own collective instabilities that can produce fatal structural defects in the solidified materials. The situation in the real world is even more complicated.
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From page 178... ...
The term "singular" means that the perturbation, in this case the surface tension at the solidification front, completely changes the mathematical nature of the problem whenever it appears, no matter how infinitesimally weak it might be. This theory has been worked out in detail for many relevant situations, such as the xenon dendrite shown here (Figure 4.2.1)
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From page 179... ...
Perhaps the most important reason for this change is that many of the most challenging modern problems in solid mechanics involve nonequilibrium phenomena and therefore pose novel fundamental questions. Another important reason is that, for the first time in history, we have the experimental and computational tools needed to answer those questions.
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From page 180... ...
Questions of this kind will become even more urgent and difficult when, later in this chapter, we talk about granular materials, biomaterials, materials in constrained geometries, or a variety of other states of matter whose unconventional mechanical properties have yet to be explored and understood. Instabilities in Dynamic Fracture The field of fracture mechanics is among the most elegant and important in all of the engineering sciences.
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From page 181... ...
The term "vortex glass" refers to the tangle of field-carrying vortices that occurs in certain kinds of superconductors. For ordinary practical purposes, glassy materials become solids when cooled below their so-called "glass temperatures." They acquire immeasurably large viscosities and they support shear stresses.
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From page 182... ...
Since our ancestors first made stone tools and later learned how to "cut" diamonds, it has been clear that sharp, smooth fracture surfaces can be made by producing cleavage cracks in glassy or crystalline solids. Apparently the trajectories of those cracks are stable.
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From page 183... ...
Polymers and Adhesives As discussed elsewhere in this report, an increasing fraction of the structural materials used in modern technological applications are polymers or polymeric composites. Nonequilibrium phenomena are involved both in the chemical and thermal processing of these materials and in the way they respond to stresses during use.
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From page 184... ...
Outstanding questions include, How long do polyethylene connectors at an interface have to be to allow a zone of plastic deformation to form ahead of a crack tip? If parts of such connectors are incorporated into crystals, is that sufficient to lock them into place, or do they have to be both entangled and run through the adjacent crystals for effective anchoring?
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From page 185... ...
Although such models still must include assumptions about irreversible behavior, they are relatively well posed and, in some cases, they are now beginning to produce credible agreement between theory and experiment. The other two categories of friction problems are fundamentally more challenging because they involve two or more widely separated length scales and timescales.
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From page 186... ...
They are triggered when some piece of a fault is brought to its slipping threshold by the tectonic forces in the Earth's crust. They have the additional features that they occur on large length scales and have an extremely broad range of sizes, even on single fault segments.
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From page 187... ...
Their free surfaces spontaneously form regular patterns when shaken in special ways; their internal stresses organize themselves into chain-like structures under certain kinds of loading; flow patterns sometimes look roughly like localized shear bands. Granular materials are only the simplest examples of states of matter that are unfamiliar and relatively unexplored from a fundamental point of view, yet appear in many ordinary circumstances.
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From page 188... ...
when we try to understand the nonequilibrium properties of these materials, we find ourselves in uncharted territory. We find ourselves even further afield when we consider the nonequilibrium physics of yet more dynamically complex materials such as foams, or the colloidal suspensions, gels, and so on discussed in Chapter 5.
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From page 189... ...
But it is only very recently, with the advent of new experimental techniques, that the outstanding questions in this immense field are beginning to become well-posed problems in nonequilibrium materials physics. Length Scales, Complexity, and Predictability There is a growing consensus among seismologists that it is impossible, even in principle, to predict earthquakes.
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From page 190... ...
If the analogy of earthquake prediction is accurate, then many of these problems will have to be solved in ways that are not now familiar to us. FURTHER PROSPECTS FOR THE FUTURE Nonequilibrium Phenomena in the Quantum Domain Modern technological advances in mesoscopic and atomic systems, described elsewhere in this report, have made it imperative that we extend the study of nonequilibrium phenomena to the quantum domain.
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From page 191... ...
The most obvious difference is that they are highly complex in their basic ingredients. Even the simplest biological materials are composed of large multicomponent molecules that, individually, perform specific chemical and mechanical functions.
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From page 192... ...
2. Very significant progress has been made in the last decade in understanding dendritic pattern formation in crystal growth.
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From page 193... ...
The relation between molecular and mesoscopic structure and the dynamics of friction in an extremely wide variety of situations, ranging from atomically flat surfaces interacting across molecularly thin layers of lubricants, to tectonic plates interacting across earthquake faults; and e. The relation between elementary interactions between grains and the macroscopic mechanical behavior of granular materials.
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