Investigations of silica alcogel aging using coherent light

Light scattering methods have previously been used to monitor the formation of gels. In this report we present new light scattering techniques to study the properties of silica alcogels during the aging process. Monitoring one particular polarization transformation of scattered light with time reveals a clear increase in internal strain in standing alcogels with time. The stress birefringence coefficient of an acid-catalyzed SiO₂ gel was found to be 1.3 Brewsters. Additionally, the evolution of the stiffness of alcogels was investigated using laser speckle methods. Specifically, image analysis of specklegrams obtained during multi-frequency acoustic excitation of aging gels was used to non-destructively measure the hardening of alcogels. For an acid-catalyzed gel with a theoretical density of ∼0.05 g/cm³ SiO₂, the rate of hardening is found to be greatest between gelation and 2× the gel time, and drops considerably thereafter. The Young's modulus of the gel can be monitored over time with this method and was found to range from 6.2 x 10³ N/m² after 6 h to 2.2 x 10⁵ N/m² after 24 h for acid-catalyzed silica gels.