Wetting as a basis for a highly selective colorimetric indicator for organic liquids*

We present a colorimetric indicator for organic liquids that couples distinct macroscopic color patterns to minute differences in liquids' intrinsic wettability to a surface. We find that when a liquid percolates through the pores of large-area, defect-free silica inverse-opal films, a highly consistent re-entrant geometry leads to sharply defined threshold wettability for liquid infiltration, occurring at intrinsic contact angles near 20\r{ }. The structure also acts as a 3D photonic crystal, producing bright iridescent color that disappears when infiltrated with liquid, coupling the highly selective wetting observed to an easy-to-visualize colorimetric response. Combining a percolation model and FDTD optical simulations, we estimate the selectivity of the colorimetruic response. In addition, we present a technique to generate precisely controlled spatial patterns of surface chemistry throughout the porous network. This lets us tailor the wettability threshold to specific liquids across a continuous range. Using these techniques, we demonstrate the applicability of this indicator to colorimetrically distinguish: i) ethanol-water mixtures varying by only 2.5{\%} in concentration; ii) hexane, heptane, octane, nonane, and decane; and iii) samples of gasoline (regular unleaded) and diesel.