title: Oil droplet self-transportation on oleophobic surfaces.
abstract: Directional liquid transportation is important for a variety of biological processes and technical applications. Although surface engineering through asymmetric chemical modification or geometrical patterning facilitates effective liquid manipulation and enables water droplet self-transportation on synthetic surfaces, self-transportation of oil droplets poses a major challenge because of their low surface tension. We report oil droplet self-transportation on oleophobic surfaces that are microtextured with radial arrays of undercut stripes. More significantly, we observe three modes of oil motion on various sample surfaces, namely, inward transportation, pinned, and outward spreading, which can be switched by the structure parameters, including stripe intersection angle and width. Accompanying theoretical modeling provides an in-depth mechanistic understanding of the structure-droplet motion relationship. Finally, we reveal how to optimize the texture parameters to maximize oil droplet self-transportation capability and demonstrate spontaneous droplet movement for liquids down to a surface tension of 22.4 mN/m. The surfaces presented here open up new avenues for power-free liquid transportation and oil contamination self-removal applications in various analytical and fluidic devices.
authors.affiliation: Department of Materials Science and Engineering, Aalto University School of Chemical Technology, Tietotie 3, 02150 Espoo, Finland.
authors.forename: Juan
authors.initials: J
authors.lastname: Li
authors.affiliation: Department of Applied Physics, Aalto University School of Science, Puumiehenkuja 2, 02150 Espoo, Finland.
authors.forename: Qi Hang
authors.identifier: 0000-0002-4569-0274
authors.initials: QH
authors.lastname: Qin
authors.affiliation: Department of Micro- and Nanosciences, Aalto University School of Electrical Engineering, Tietotie 3, 02150 Espoo, Finland.
authors.forename: Ali
authors.initials: A
authors.lastname: Shah
authors.affiliation: Department of Applied Physics, Aalto University School of Science, Puumiehenkuja 2, 02150 Espoo, Finland.
authors.forename: Robin H A
authors.identifier: 0000-0002-2076-242X
authors.initials: RH
authors.lastname: Ras
authors.affiliation: Department of Applied Physics, Aalto University School of Science, Puumiehenkuja 2, 02150 Espoo, Finland.
authors.forename: Xuelin
authors.initials: X
authors.lastname: Tian
authors.affiliation: Department of Materials Science and Engineering, Aalto University School of Chemical Technology, Tietotie 3, 02150 Espoo, Finland.
authors.forename: Ville
authors.initials: V
authors.lastname: Jokinen
mesh_terms: D057927:Hydrophobic and Hydrophilic Interactions; D008855:Microscopy, Electron, Scanning; D008956:Models, Chemical; D009821:Oils / Q000737:chemistry*; D013499:Surface Properties; D059008:Time-Lapse Imaging
chemical_list: D009821:Oils
keywords: anisotropic surfaces; anti-contamination; geometrical gradient; micro- and nanodevices; microfludics; oil droplet; oleophobic; self-transportation; undercut structure; wetting