Basic Research Needs for Geosciences - Energetics Meetings and ...

CROSSCUTTING ISSUE:THE MICROSOPIC BASIS OF MACROSCOPIC COMPLEXITYdynamic nature of the paths themselves. There is a possibility of extreme events—i.e.,breakthrough phenomena—not only in the approach to steady state, but possibly also within it.Also poorly understood is the effect of inhomogeneous wetting. If the inhomogeneities aremicroscopic (smaller than a grain) and uncorrelated, then they would merely manifestthemselves in the average wetting properties. However the inhomogeneities are more likely tomanifest themselves from grain-to-grain, and may even cluster. In this case there would beeffects due to their spatial correlations.Chemical kinetics can also be altered by disorder. For example, phenomena as diverse as theweathering of silicate minerals, the recrystallization of carbonates (Fantle and DePaolo 2006),and the degradation of organic matter in soils and sediments (Middelburg 1989) have been foundto exhibit apparent reaction rate constants that decay inversely with time, like 1/t, at time scalesranging from less than a year to more than millions of years. The reasons underlying such timedependentreactivity are unclear, but they probably depend on a combination of inhomogeneitiesof the reactive substrate and differences in the accessibility of the substrate to reactants insolution. That the decay of reactivity is so simply expressed as a power of time suggests theexistence of a simple mechanism common to all such problems.RESEARCH APPROACHESThe essential need is to develop a scientific basis of "emergent" phenomena relevant topredicting geological processes at a technologically relevant scale. To reach this goal, we mustfirst develop a fundamental understanding of physical and chemical processes at the molecularscale. Such small scales are important because of the unusual mineral-fluid interactions that canbe expected to occur in geologic environments impacted by human intervention.Conversely, there is a need to develop methods that link changing macroscopic properties (e.g.,chemical concentrations and flow properties) to changes at mineral-fluid interfaces. Thus weexpect that mineral surfaces will be the most important point of contact between microscopic andmacroscopic dynamics. Changes in chemistry can influence surface properties (roughness andsurface area), and the surface properties in turn can influence flow (e.g., by clogging ordissolution).Progress with such coupled problems requires a hierarchy of approaches. One is theoretical andincludes the use of computer simulation. From basic considerations of physical dynamics andstatistical mechanics, one seeks simple idealized models that show how macroscopic observablescan be related to microscopic dynamics. Similarly it is appropriate to construct laboratoryexperiments that explore the effects of microscopic disorder on macroscopic dynamics. Ourunderstanding of immiscible two-phase flow in porous media has greatly benefited from thisapproach. The challenge now is to construct similarly useful experimental analogs that show howmineral-fluid interactions are influenced by molecular scale disorder, and how changing surfaceproperties influence flow. Experiments, theory, and simulation should be aimed at developing apredictive theory that directly connects microscopic disorder to macroscopic properties.156 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems