Observation of the aggregation behavior of silica sols using laser speckle contrast measurements

We report on a new optical method to observe the onset of aggregation in alcoholic tetraethoxysilane (TEOS) sols using laser speckle contrast measurements. The contrast in a speckle image produced by coherent light provides information about the internal contrast of the medium being studied. For silica sols, changing the amount of acid or base catalyst was the most important factor in determining the aggregation behavior of the sol. We investigated this effect by varying the TEOS/base ratio by a factor of 6. This shifted the onset of aggregation as determined by speckle contrast from ∼46% of the gel time for the lowest amount of base to ∼74% for the highest. Conversely, varying TEOS/acid ratio by a factor of 3, shifted the onset of aggregation from ∼74% of the gel time for the lowest amount of acid to ∼64% for the highest. Measurements of this type provide information that can be used to test models of sol aggregation and gel formation.