Tunable Visual Detection of Dew by Bare Artificial Opals

The photonic properties of artificial opals directly depend on the atmosphere (air, usually) filling the voids in the constitutive dielectric material. Here, a novel use of bare artificial silica opals (direct and inverse) is presented for straightforward water dew detection, in which the stop‐band exhibits a drastic change upon soaking of the voids. In particular, the opal reflectance drops to one‐fifth within 100 ms scale, so the fading of the opal structural color upon dew formation is evident for the naked eye. Moreover, the stop‐band redshifts up to 60 nm, leading, by convenient selection of the stop‐band wavelength, to perceptible color change. Due to the porous nature of the opal, the dew formation in the opal voids is shifted to lower saturation pressures compared to an open system. This is experimentally demonstrated by controlled reduction of the pores size, so that the advancement of the dew point temperature is tuned between 2.5° and 6.3°. The outstanding color change together with the easy fabrication (without postprocessing) and selectable size (from miniature to large‐area) make artificial opals cost‐effective and versatile visual dew detectors. The tunable advance of the opal response over the actual dew point is practical to adequately anticipate and prevent dew formation in neighboring surfaces. Furthermore, opals might serve as useful probe systems for the study of fundamental phenomena such as condensation of vapors in porous and particulate media.