A Dip-Decoating Process for Producing Transparent Bi-Superhydrophobic and Wrinkled Water Surfaces

As mechanical strength is required in most applications using superhydrophobic surfaces, much effort has been applied toward enhancing this property. However, mechanical weakness can also be useful, but little attention has been focused on this seemingly undesirable feature. Here, using a sol-gel SiO2 coating as a weak surface, a dip-decoating phenomenon is demonstrated to occur when immersing and withdrawing the coating from water. As a result, part of the coating's outer layer of nanoparticles (NPs) is peeled off by shearing motions of the three-phase line, generating a bi-superhydrophobic surface constructed by adjacent pristine and dipped areas, with different colors, adhesive forces, and compositional stabilities. Such a Janus surface, which also features ultrahigh transmittance due to antireflection, can be used as a peculiar multifunctional platform for droplet manipulations. The dip-decoating technique also works as a distinctive way for introducing particles onto water surface, in which the density and distribution of peeled NPs can be adjusted by multi-dipping processes. The wrinkled water surface, caused by high NP density, can be used with the Langmuir-Blodgett coating technique and have potential in fundamental studies, such as the evaporation of coated liquids.