Extending Lifetime of Water Droplets Using Mirror‐Nanoporous Surfaces

Abstract The growing use of droplets in a broad range of technologies demands strategies to reduce the evaporation rate, in order to extend the droplets’ lifetime, especially in bioapplications that require operations at physiological temperature. These applications would also require minimum external intervention and, mainly, avoiding the addition of chemical agents. Here, a simple method that yields long lasting droplets at relatively high temperatures is presented: the confinement between mirror‐nanoporous surfaces. This approach takes advantage of the locally high humidity generated by the droplets in contact with nanoporous thin films. Both surfaces in contact with the liquid spontaneously form wet annuli surrounding the contact line of the squeezed droplet, which raise the vapor pressure in the confined space and thus decrease the overall droplet evaporation. A model in the framework of classic transport phenomena is derived to rationalize the experimental design. The proposed method is indeed performed in open environments to preserve microliter droplets at temperatures even above the physiological range, and therefore overcomes the limitations of the usual solutions that require humidity chambers or oil‐immersed droplets. The results bring a fresh perspective for the development of versatile platforms for a variety of droplet‐based technologies.