Anisotropy of silica aerogels induced by small strain

We have performed optical birefringence measurements on isotropic aerogel samples grown with varying amounts of ammonia catalyst to determine the relationship between stress, strain, and transmitted light intensity, focusing on the regime of relatively small axial strain. We find that our samples exhibit a delayed onset of birefringence at ∼3 % strain, beyond which the transmitted intensity increases linearly, with a faster rate of increase for samples prepared with larger catalyst concentrations. We have also calculated the sound velocity from the slope of the stress-strain curve, and found an approximately linear increase with catalyst solution pH for samples of similar density. Samples of similar density also appear to exhibit a universal relationship between applied stress and transmitted intensity. We are not able to identify the mechanism for this unique behavior.