Segregation Of Binary Mixtures Research Articles

Avalanche Segregation of Binary Mixtures of Granular Media

ABSTRACTWhen an initially homogeneous binary mixture of granular media such as fine and coarse sand is poured near the closed edge of a “quasi-two-dimensional” Hele-Shaw cell consisting of two vertical transparent plates held a narrow distance apart, the mixture spontaneously forms alternating segregated layers. Using digital images taken with a CCD camera and Fourier transformed to obtain the structure function of the banding pattern, the wavelength selection mechanism and degree of segregation are studied as the plate separation of the Hele-Shaw cell is systematically increased. For a given flow rate, the degree of segregation, as reflected in the amplitude of the peak in the structure function, decreases as the plate separation increases. For wider plate spacings the peak in the structure function shifts to lower wavelengths. These results are compared to numerical simulations of the avalanche segregation using a stochastic sandpile model.

Rotationally induced segregation of granular materials.

We have studied segregation in binary mixtures of different granular media subjected to rotation in horizontal tubes. Mixtures separate into bands of different relative concentrations arranged along the axis of the tubes. Axial modulations of the tube radius lock bands in space and induce segregation in otherwise nonsegregating mixtures of glass beads. The segregation results from an instability analogous to spinodal decomposition, and we present a model which describes this instability.

Statistical and structural effects during solute-atom segregation to grain boundaries

Mean field theory of a rigid grain boundary segregation in binary mixture is developed on the basis of local equilibrium and geometrical considerations. The application of the Guggenheim adsorption isotherm equation to the problem of internal interfaces is proved to be valid for the case of regular substitutional solid solution. The segregation coefficient is found to be a function of a new interaction constant which yields the relative stability of an interface between a crystal made of pure solute and a crystal of pure solvent, with respect to the stability of grain boundaries in solvent and solute materials. Three typical structures are obtained when a local miscibility gap is formed and two other structures when the bulk tends to order. Segregation can stabilize new boundary structures which may involve changes in the intergranular translation, faceting, etc.