title: Octagon to Square Wetting Area Transition of Water-Ethanol Droplets on a Micropyramid Substrate by Increasing Ethanol Concentration.
abstract: The wettability and evaporation of water-ethanol binary droplets on the substrate with micropyramid cavities are studied by controlling the initial ethanol concentrations. The droplets form octagonal initial wetting areas on the substrate. As the ethanol concentration increases, the side ratio of the initial wetting octagon increases from 1.5 at 0% ethanol concentration to 3.5 at 30% ethanol concentration. The increasing side ratio indicates that the wetting area transforms from an octagon to a square if we consider the octagon to be a square with its four corners cut. The droplets experience a pinning-depinning transition during evaporation. The pure water sessile droplet evaporation demonstrates three stages from the constant contact line (CCL) stage, and then the constant contact angle (CCA) stage, to the mixed stage. An additional mixed stage is found between the CCL and CCA stages in the evaporation of water-ethanol binary droplets due to the anisotropic depinning along the two different axes of symmetry of the octagonal wetting area. Droplet depinning occurs earlier on the patterned surface as the ethanol concentration increases.
authors.affiliation: School of Mechanical and Aerospace Engineering, Nanyang Technological University , 50 Nanyang Avenue, Singapore 639798, Singapore.
authors.forename: Huicheng
authors.identifier: 0000-0002-2047-2406
authors.initials: H
authors.lastname: Feng
authors.affiliation: Institute of Materials Research and Engineering , A*Star, 2 Fusionopolis Way, Innovis, Level 9, Singapore 138634, Singapore.
authors.forename: Karen Siew-Ling
authors.initials: KS
authors.lastname: Chong
authors.affiliation: Institute of Materials Research and Engineering , A*Star, 2 Fusionopolis Way, Innovis, Level 9, Singapore 138634, Singapore.
authors.forename: Kian-Soo
authors.initials: KS
authors.lastname: Ong
authors.affiliation: School of Mechanical and Aerospace Engineering, Nanyang Technological University , 50 Nanyang Avenue, Singapore 639798, Singapore.
authors.forename: Fei
authors.identifier: 0000-0002-7469-7184
authors.initials: F
authors.lastname: Duan