Detecting sol–gel transition using light transmission

This paper describes a novel light transmission technique for detecting sol–gel transition. The technique uses a fluorescent light source, a CCD camera and a clear borosilicate mold to measure the light transmission characteristics of liquid aerogel precursors as they gel. We used tetramethoxysilane-based aerogel precursors (TMOS:MeOH:H2O:NH4OH). Some of the precursor mixtures were doped with strongly absorbing probes in order to facilitate visual observation of the gelation process and improve image quality, whereas others were not doped. Solutions were placed in the mold and heated under pressure in a hot-press. In all cases, we observed that the solutions darkened in appearance and recorded this phenomenon with the CCD camera. To quantify the degree of darkening we converted the RGB images to a hue-saturation-intensity color space. Plots of hue and intensity versus temperature were characterized by a decrease at 95–115 °C. Visual inspection confirmed that the solutions started to gel in this temperature range, resulting in an increase in particle size, increase in extent of light scattering, and subsequent decrease in transmitted light.