Micro-seepage directed evaporation and salt nucleation of a brine droplet on oil-impregnated steel surfaces

Abstract

The lubricant/oil-impregnated surface was intensively investigated due to its versatile functionality in service environments. However, its environmentally sensitive failure caused by water droplets is still unclear. In this paper, we speculated that the evaporation/crystallization of a droplet depends on the microscopic interactions at the droplet/oil/substrate interface, which determines the failure of an oil-impregnated surface on reactive substrates. Experiments were conducted by placing a brine droplet on the oil-impregnated steel surface (OISS) and the oil-coated steel surface (OCSS) directly prepared on carbon steel. The evaporation and crystallization processes were systematically investigated by microscopic and spectroscopic methods. A dynamic polar coordinate was developed to qualitatively determine the location of the salt nucleus during evaporation. Our findings address the crucial role of the micro-scale interfacial actions affecting the macro-droplet behavior. The asymmetric edge pegging leads to a displacement of the center mass of a droplet, which alters the evaporation modes and rates. The micro-seepage of water causes the local rupture of oil layers and the droplet nailing, resulting in corrosion of the substrate structures, which constrains the nucleation of salt grains on OISS. The similar mechanism was verified by the droplet behavior on OCSS. Our findings are of great significance for deciphering the droplet behavior on engineering materials surfaces.