Analysis of Wetting Behavior and Solidification Process of Molten Urea on a Superhydrophobic Surface and Its Application in Large Granular Urea Production

The wetting behavior of polar high-temperature melt on superhydrophobic surfaces is rarely studied, although water wetting process under normal temperature has been widely investigated. In this work, molten urea was considered as the typical polar melt substance, and its wetting behavior and solidifying process on a polytetrafluoroethylene (PTFE)-coated superhydrophobic stainless steel surface (PSSSS) were investigated. The results confirm the super-repellency of PSSSS on molten urea droplets with a static angle of over 155 degrees and a rolling angle of 3.5 +/- 1 degrees, which is consistent with the Cassie-Baxter state. Such a superurea-melt-phobic state is ascribed to the high roughness of the PTFE-coated surface and high cohesive energy density difference between the urea and PTFE. The solidification process of the urea melt on PSSSS occurred from the outside to inside in 44 s at 18 degrees C to form a compact urea granule of large size and high mechanical strength. This occurrence provides a feasible and promising granulation strategy to produce qualified large urea granules using a green, simple, and cost-effective process.