Influence of Viscoelasticity and Permeability on the Stress Response of Silica Gel

Abstract

This review examines the chemical and physical factors that control the response of silica gel to stimuli such as aging, heating, mechanical deformation, and drying. When the liquid phase is chemically aggressive, the gel exhibits spontaneous shrinkage (syneresis) and viscoelasticity; in an inert medium, the network of the gel is purely elastic. The rate of the responses to mechanical loads and changes in temperature depends on the permeability, as well as the modulus, of the network. The properties of the gel change by orders of magnitude as the network contracts during drying, and this has a dramatic effect on the drying stresses.

Characterization of saturated porous bodies

“…For this gel, 1 -,4 = 2(1 +v p) / 3 ~ 0.8, so v p ~ 0.2, which is typical of silica gels [11,35]; the bulk modulus of the drained network is found to be 4.0 MPa, whereas K s is expected to be some GPa, so we are well within the "gel limit". The shape of the relaxation curve is perfectly described by Equations (25) and (30).…”

“…As the pore pressure equilibrates, the force required to sustain a fixed deflection of the beam decreases with time, and analysis of the kinetics of relaxation yields the permeability-. This method has been applied to gels [9][10][11], porous glass [12], and cement paste [13][14][15][16]. It permits measurement of very low permeabilities in minutes or hours; for example, a permeability of~10 "21 m e = 10 -14 mJs) can be measured in less than an hour [13,16].…”

“…As a result, the expansion is anomalously high during heating, and relaxation occurs during an isothermal hold as the liquid drains from the pores and the solid contracts to a dimension determined by its own thermal expansion coefficient. This method has been used to measure the permeability of gels [11,17] and cement paste [18], and it also yields permeabilities on the order of 10 "2~ m 2 in a few hours. The disadvantage of the method is that it requires an independent measurement of the elastic properties of the porous body.…”

Erratum

“…In the gel limit, the constants b, , and all reduce to unity, so there is no change in the results reported previously for gels (for example, in Ref. 7 and the papers cited therein).…”

Dynamics of poroelastic filaments

“…3λ+2µ for a circular rod) is the dimensionless torsional rigidity (normalised by the bending contribution to M (1) ), τ a is the dimensionless twist strain, and the λ n are determined by solving equation (18). We note that the twisting moment has no poroelastic contribution because it is purely a shear deformation, and poroelastic effects arise only from volumetric deformations as seen in equation (5).…”