CROSSOVER FROM SCALAR TO VECTORIAL PERCOLATION IN SILICA GELATION

We present a rheological study of the gel transition in silica gels resulting from the hydrolysis-condensation of a silicon alkoxide. The complex shear modulus is measured in a large frequency range (10(-2)-10(2) rad/s). The exponents for the viscosity s = 0.7+/-0.1, for the elasticity t = 2.0+/-0.1, for the dynamic crossover frequency z = 2.9+/-0.2, and for the scaling-frequency power law DELTA = 0.72+/-0.03 are in agreement with the scalar percolation model in a restricted region around the gelation time (3%). Beyond, there is a crossover to a vectorial percolation regime characterized by a much higher elasticity exponent t' = 3.6+/-0.1.