Wetting in Color: From photonic fingerprinting of liquids to optical control of liquid percolation

We provide an overview of our recent advances in the manipulation of wetting in inverse-opal photonic crystals. Exploiting photonic crystals with spatially patterned surface chemistry to confine the infiltration of fluids to liquidspecific spatial patterns, we developed a highly selective scheme for colorimetry, where organic liquids are distinguished based on wetting. The high selectivity of wetting, upon-which the sensitivity of the response relies, and the bright iridescent color, which disappears when the pores are filled with liquid, are both a result of the highly symmetric pore structure of our inverse-opal films. The application of horizontally or vertically orientated gradients in the surface chemistry allows a unique response to be tailored to specific liquids. While the generic nature of wetting makes our approach to colorimetry suitable for applications in liquid authentication or identification across a broad range of industries, it also ensures chemical non-specificity. However, we show that chemical specificity can be achieved combinatorially using an array of indicators that each exploits different chemical gradients to cover the same dynamic range of response. Finally, incorporating a photo-responsive polyelectrolyte surface layer into the pores, we are able to dynamically and continuously photo-tune the wetting response, even while the film is immersed in liquid. This in situ optical control of liquid percolation in our photonic-crystal films may also provide an error-free means to tailor indicator response, naturally compensating for batch-to-batch variability in the pore geometry.