Time-resolved study of optical properties and microscopic dynamics during the drying of TiO2 films by spectral diffusing wave spectroscopy.

Abstract

We present a combined experimental, theoretical, and numerical study of photon transport and microscopic dynamics in rigid and drying turbid thin films. Our setup is based in multispeckle diffusing wave spectroscopy and is adapted for frequency sweep of the illuminating source. We apply our approach to simultaneously monitor the changes in optical properties and microscopic dynamics of turbid thin films of rutile TiO2 powder dispersed in ethanol during the full drying process. Accordingly, we introduce an extension of the photon diffusion model for spectral speckle intensity correlations to account for system microscopic dynamics. We find that our results are well described by the model, where parameters required as the time-dependent sample thickness and transport mean free path are obtained from experiments. Furthermore, our findings are validated by numerical simulations of speckle dynamics based on the copula scheme. We consider that our scheme could be useful in time-resolved physical characterization of time-evolving turbid thin systems.