A simplified experimental method to study conductivity percolation.

A simplified experimental setup and a measurement method adapted to the simplified data analysis proposed in previous works are described in detail. The setup is intended for investigation of the conductivity percolation in the water network formed on granular materials and studied in the course of dehydration. The results show that the new, easily accessible experimental setup enables determination of the principal percolation parameters with the same accuracy as the more sophisticated equipment employing an impedance analyzer. Mean critical exponents obtained from data collected for the samples of moisturized sand grains sized 0.6-0.8 mm by means of both experimental methods give the same results within the limit of measurement uncertainty.

Electric conductivity percolation in naturally dehydrating, lightly wetted, hydrophilic fumed silica powder.

In studying the dehydration of surface-moistened fumed silica Aerosil powders, we found a conductivity percolation transition at low hydration levels. Both the percolation exponent and the threshold are typical for correlated site-bond transitions in complex two-dimensional (2D) systems. The exponent values, 0.94-1.10, are indicative of severe heterogeneity in the conducting medium. The surface moisture at the percolation threshold takes on a universal value of 0.65 mg([H2O])/m(2)([silica]), independent of the silica grain size, and equivalent to twice the first hydration monolayer. This level is just sufficient to sustain a quasi-2D, hydrogen-bonded water network spanning the silica surface.

Phase equilibria in solutions of platelike particles: systems with steric and dispersive interactions between the platelets.

Our statistical thermodynamics model of solution of stiff, platelike, biaxial particles interacting solely via repulsion on contact (athermal limit) [Phys. Rev. E 62, 5011 (2000)] is extended to incorporate dispersion interactions between the particles. Dispersion forces between anisotropic particles are accounted for using the Imura-Okano approach. Numerical calculations specialized to solutions of either rods or disks show that besides the isotropic-nematic biphasic coexistence range, inclusion of attractive forces resulted in the appearance of nematic-nematic coexistence in both, disks and rods, solutions. The critical divergence of the difference between the order parameters and concentrations of the two nematics is observed while approaching the critical temperature. The minimum aspect ratio of rods or disks for the formation of the nematic phase is also discussed.

Water-network percolation transitions in hydrated yeast.

We discovered two percolation processes in succession in dc conductivity of bulk baker's yeast in the course of dehydration. Critical exponents characteristic for the three-dimensional network for heavily hydrated system, and two dimensions in the light hydration limit, evidenced a dramatic change of the water network dimensionality in the dehydration process.