Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 150205

150205 - Fluorine-Free Liquid-Repellent Surfaces 

Solid surfaces that exhibit high repellency toward liquids with different properties have attracted tremendous interest due to their wide variety of practical applications, such as self-cleaning fabrics and drag-reduction coatings. The past two decades have witnessed significant progress in the development of such liquid-repellent surfaces. Enormous efforts have been devoted to understanding the design principles of these surfaces and discovering new materials and methods for fabricating liquid-repellent surfaces with unique functionalities. However, current liquid-repellent surfaces face several substantial obstacles, such as the use of per- and polyfluoroalkyl substances (PFAS) that are emerging contaminants of global concern and pose significantly adverse effects on the environment and humans, complex fabrication processes, and poor mechanical durability. 

Here we present our recent efforts toward developing novel strategies that have promising potential to overcome these challenges. We demonstrate that fluorine-free textured omniphobic surfaces, which are repellent to liquids with a wide range of surface tensions (e.g., water ~72 mN/m, hexadecane ~27 mN/m, and ethanol ~22 mN/m), can be rapidly fabricated using digital light processing (DLP)-based 3D printing. By tuning design and printing parameters, the unique doubly re-entrant surface textures that mimic the skin morphology of springtail can be created in the 3D printing process. The omniphobicity of the printed surfaces results from the combination of doubly re-entrant textures and moderately low solid surface energy obtained through surface chemistry modification using hydrocarbon silanes. Different liquids display high contact angles on the 3D printed fluorine-free omniphobic surfaces with doubly re-entrant textures, which possess better liquid repellency than the surfaces with re-entrant textures. Our results also show that ethanol droplet with low surface tension remains in the Cassie-Baxter state during the whole evaporation process. 

Furthermore, we present a new approach that allows easy sliding of liquid droplets on smooth solid surfaces. Unlike prior studies that typically rely on solid surface modification (e.g., creation of surface texture) to enable droplet sliding, our results show that droplets containing ionic surfactants such as anionic and cationic surfactants exhibit high mobility on smooth solid surfaces. The droplet sliding behavior is governed by surfactant properties (e.g., type of charge and concentration) as well as the electrostatic interaction between the solid surface and the surfactant. Ultra-low sliding angle can be achieved by increasing the surfactant concentration. This finding suggests that fluorine-free liquid repellency can be achieved without creating micro/nano surface textures and/or modifying surface chemistry through complex processes. We also demonstrate controlled manipulation of surfactant-laden droplets.

Presenting Author: Wei Wang University of Tennessee, Knoxville

Presenting Author Biography: Dr. Wei Wang is an Assistant Professor in the Department of Mechanical, Aerospace, and Biomedical Engineering at the University of Tennessee, Knoxville (UTK). Prior to joining the UTK in 2020, he was a postdoctoral research scholar in the Department of Mechanical and Aerospace Engineering at the North Carolina State University. He received his Ph.D. degree in Mechanical Engineering from Binghamton University in 2014. He is the leading author or co-author of over 30 peer-reviewed journal articles. He is the recipient of NSF CAREER Award and Ralph E. Powe Junior Faculty Enhancement Award. His current research interests include liquid-repellent surfaces, wetting phenomena on soft surfaces, and membranes with special wettability.

Authors: