Near Interfacial Processes

Numerous models have been proposed for the processes occurring near the sediment-water interface in deep sea sediments that lead to a balance between dissolution and retention of calcium carbonate in these sediments. Investigation of these processes is currently one of the most active areas of research in the study of calcium carbonate behavior in the oceans. A major difficulty in studying and modeling these processes is that many of the most important changes take place over distances of only a few millimeters in a highly dynamic environment. 

Early models for the process of calcium carbonate dissolution from deep sea sediments (e.g., Takahashi and Broecker, 1977) were based on simple diagenetic models in which calcium carbonate dissolved into the pore waters of the sediments. 

A major benthic process, that had only casually been considered for its potential influence on carbonate accumulation in deep sea sediments, is the oxidation of organic matter. The general reaction for this process involving marine organic matter is (Emerson and Bender, 1981)  

Bottom Depth Depth (xlO6 mole cm2yr) EOrg-C Reference 

Emerson et al., 1982) have demonstrated that this approach will not work for studying the carbonate chemistry of deep sea sediment pore waters. The reason is that the solubility of carbonates changes substantially with temperature and pressure, and they are reactive enough to change the pore water chemistry when the cores are recovered. Consequently, most recent studies have relied on extracting pore waters in situ. The major difficulty with this technique is that it usually is not possible to obtain closely spaced samples or samples very near the sediment-water interface. This interface is, unfortunately, the region where most of the chemical changes associated with the carbonate-C02 system take place. 

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It is worthwhile mentioning that the interfacial potential created at the liquid-liquid interface is governed by single ionic or redox equilibrium only in the simple cases. The presence of various, often two, interfacial processes is a source of the steady-state potential, named also the mixed or the rest potential. Its value is situated between the two equilibrium potentials, near that one which corresponds to the higher exchange current... 

Fibers can be made by directly pulling some of the polymer from the melt. Similarly, fibers can be made using the interfacial process with fibers formed as a reaction of the two coreactants, which occurs at or near the interface. Neither instance has been used in industrial-scale fiber formation. 

Calculations such as those by Morse and Mackenzie (1990) indicate that the calcite saturation depth is generally —1 km greater than proposed by Berger (1977) and that it is much greater than R. It appears only loosely related to the FL. In the equatorial eastern Atlantic Ocean, FL is —600 m shallower than the saturation depth. If these observations are close to correct, the long cherished idea of a tight relation between seawater chemistry and carbonate depositional facies must be reconsidered (Mekik et al., 2002). The influence of near interfacial diagenetic processes on these relationships is discussed in the next section. 

Surfactants have very special qualities that make them invaluable to the petroleum industry. The relevance of various interfacial phenomena, such as adsorbed surfactant films, self-assembly, contact angle, wetting, foams and emulsions, in nearly every process in the industry has been discussed. In addition, this chapter summarized the importance of the adsorption and aggregation behaviour of surfactants with regard to drilling, enhanced oil recovery, antifoaming, corrosion inhibition, oil spill clean-up, oil/water separation and fluidization of highly viscous materials. 

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

The presentation in this paper concentrates on the use of large-scale numerical simulation in unraveling these questions for models of two-dimensional directional solidification in an imposed temperature gradient. The simplest models for transport and interfacial physics in these processes are presented in Section 2 along with a summary of the analytical results for the onset of the cellular instability. The finite-element analyses used in the numerical calculations are described in Section 3. Steady-state and time-dependent results for shallow cell near the onset of the instability are presented in Section 4. The issue of the presence of a fundamental mechanism for wavelength selection for deep cells is discussed in Section 5 in the context of calculations with varying spatial wavelength. 

Interfacial transfer of chemicals provides an interesting twist to our chemical fate and transport investigations. Even though the flow is generally turbulent in both phases, there is no turbulence across the interface in the diffusive sublayer, and the problem becomes one of the rate of diffusion. In addition, temporal mean turbulence quantities, such as eddy diffusion coefficient, are less helpful to us now. The unsteady character of turbulence near the diffusive sublayer is crucial to understanding and characterizing interfacial transport processes. 

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