An Optical Reporting Patch for Spatiotemporal Measurements of Water Activity in Complex Environments

Water activity, a_w, is an important state parameter in a wide variety of natural and technological contexts. Existing tools to measure water activity in situ and with spatial and temporal resolutions are limited. Here, we present the development of a composite silicone patch, AquaSheet, that reports water activity as a fluorescence signal. We proceed to demonstrate two modes of measurements: (i) spatially averaged, temporally resolved measurements with a commercial reflectance probe and spectrometer that is appropriate for measurements in both gases and liquids and (ii) spatially and temporally resolved measurements with a custom-built imaging system that allows for tracking the spatiotemporal evolution of a soil drying process. We conclude with a discussion of additional applications, limitations, and opportunities for improvement of the AquaSheet technology.