Probing Liquid-Solid and Vapor-Liquid-Solid Interfaces of Hierarchical Surfaces Using High-Resolution Microscopy

Liquid-solid (LS) and vapor-liquid-solid (VLS) interfaces are important for the fundamental understanding of how surface chemistry impacts industrial processes and applications. Superhydrophobic surfaces, from structural hierarchies, were fabricated by coating flat smooth surfaces with hollow glass microspheres. These surfaces are referred to as structural hierarchical-modified microsphere surfaces (SHiMMs). Two-phase LS and three-phase VLS interfaces of water droplets on SHiMMs, with an apparent static contact angle (aSCA) of ∼160°, were probed at microscale using environmental scanning electron microscopy (ESEM) and high-resolution optical microscopy (OM). Both ESEM and OM confirmed the presence of air pockets in 3-150 μm range at the VLS triple-phase of the droplet peripheral contact line. The wetting characteristics of the LS interface in the interior of the water droplet were probed using energy-dispersive spectroscopy, which corroborated well with the VLS triple-phase observations, confirming the presence of both the microscale air pockets and fractional complete wetting of the SHiMMs. The superhydrophobic water droplets on the SHiMMs also exhibited relatively high adhesion to the SHiMMs-a tilt angle of 10°-40° was needed for detaching the droplets off the surfaces. Semiquantitative three-phase contact-line analysis and experimental data indicated high-water aSCA, and large adhesion on the microscale-roughened SHiMMs is attributed to pinning of the probe liquid both at the triple VLS and interior LS interfaces. The control over microroughness and surface chemistry of the SHiMMs will allow tuning of both the static and dynamic liquid-surface interactions.