Hygroscopicity promotes condensation

Abstract Many substances in nature and human society exhibit hygroscopicity. Nowadays, these hygroscopic substances have been broadly used in various domains, e.g., suppressing condensation and icing, collecting water vapor, electric power generation, indoor dehumidification, vapor-fed photocatalytic water splitting and hydrogen production. Due to its hygroscopicity, it has been believed for hundreds of years that hygroscopic substances reduce surrounding humidity and inhibit condensation. Here, we report a distinct phenomenon where hygroscopic liquids, particularly ethylene glycol, a typical hygroscopic solution, induce spontaneous condensation of surrounding water vapor in confined spaces, forming pure condensed droplet patterns. On hydrophobic surfaces, ethylene glycol droplets spontaneously generate a ring of condensed microdroplets around them, presenting a typical structure of Saturn-ring droplets. On smooth surfaces, a filter paper strip soaked in ethylene glycol can cause obvious condensation on the surface when it approaches the surface and within a certain distance, through the confined hygroscopic condensation effect. By utilizing this phenomenon, we achieve water vapor condensation screen-printing of specific shapes on a fixed surface, which we define as Screen Printing of Condensation Patterns (SPCP). This provides an important approach for controlling water vapor condensation and preparing microstructure of moisture sensitive materials. Additionally, this phenomenon can be used for moisture harvesting without the need for specially designed structured surfaces or complex adsorption and desorption processes, achieving a maximum theoretical condensation rate of 20.7 g/m²·min at 80% relative humidity. This unusual phenomenon not only challenge the traditional understanding of hygroscopic substances, but also introduce a new strategy for precisely controlling water vapor condensation, and exploring various novel applications based on controllable condensation patterns.