RESUMO
Water beneath a layer of oil in a producing reservoir may rise and form a bell-shaped cone in the vicinity of the well. We discuss how the dynamics of cone formation in two dimensions depends on the gravitational contrast, the interfacial tension, and the flow rate of oil. For a constant flow rate below a critical rate Q(c), stable cones are formed. At rates above Q(c), two dynamical regimes are expected. These are slow initial formation and fast breakthrough to the well. Quasi-two-dimensional transparent porous models were used to perform cone formation experiments. Effective acceleration due to gravity was systematically varied. The experiments were simulated using a stochastic model based on invasion percolation in which capillary forces were explicitly taken into account. We find agreement between experiments and simulations, and consistency with the theoretical predictions.
RESUMO
The translational diffusion coefficient D020,w, of monomeric human immunoglobulin G (IgG) has been studied by photon-correlation spectroscopy as a function of pH and protein concentration. At pH 7.6, we find D020,w = 3.89 x 10(-7) cm2/sec, in good agreement with the value determined by classic methods. This value corresponds to an effective hydrodynamic radius R, of 55.1 +/- 0.3 A. As pH is increased to 8.9; with the same ionic strength, the molecule appears to expand slightly (3.5% increase in hydrodynamic radius). The concentration dependence of the IgG diffusion constant is interpreted in terms of solution electrostatic effects and shows that long-range repulsive interactions are negligible in the buffer used. The diffusion coefficient for dimeric IgG has also been determined to be D20,w = 2.81 x 10(-7) +/- 0.04 cm2/sec at 1.6 mg/ml, which corresponds to a hydrodynamic radius of 75 A. For light-scattering studies of protein molecules in the dimension range of 5-10 nm (Mr approximately 10(5)-10(7] we find monomeric horse spleen ferritin well suited as a reference standard. Ferritin is a spherical molecule with a hydrodynamic radius R of 6.9 +/- 0.1 nm and is stable for years in our standard Tris-HCl-NaCl buffer even at room temperature.
