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the "mushy zone." Another part of the difficulty is that there is relatively little effort in this area in the United States, especially in industrial laboratories.

3. Recent developments in scientific instrumentation, especially atomic-scale resolution in probe microscopy, plus extraordinary advances in computing power, mean that long-standing problems in solid mechanics should now be solvable. These are fundamentally challenging problems that involve non-equilibrium statistical physics, nonlinear dynamics, and the like. They are also, essentially without exception, directly relevant to modern technology. Among those problems are the following:

a. The origin of dynamic instabilities in brittle fracture;

b. The fundamental distinction between brittleness and ductility in both crystalline and amorphous solids;

c. The relation between molecular and mesoscopic structure and mechanical properties, especially fracture toughness, in composite materials containing, for example, varieties of polymeric constituents;

d. 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.

4. In all probability, the next major frontier for research in nonequilibrium physics will be in the area of biological materials and phenomena.

5. The same recent advances in scientific instrumentation and computing power that portend both major advances and major surprises in nonequilibrium materials research also force us to face fundamental issues in the physics of complex systems. The problem of understanding the limits of predictability in these systems must be addressed with every bit as much skill and objectivity as the more familiar problems of understanding specific properties of specific systems. These issues lie, not just at the interface between different scientific disciplines, but also at the interface between science and public affairs.



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