The surface energy of graphene and its chemical derivatives governs fundamental interfacial interactions like molecular assembly, wetting, and doping. However, quantifying the surface energy of supported two-dimensional (2D) materials, such as graphene, is difficult because (1) they are so thin that electrostatic interactions emanating from the underlying substrate are not completely screened, (2) the contribution from the monolayer is sensitive to its exact chemical state, and (3) the adsorption of airborne contaminants, as well as contaminants introduced during transfer processing, screens the electrostatic interactions from the monolayer and underlying substrate, changing the determined surface energy. Here, we determine the polar and dispersive surface energy of bare, fluorinated, and hydrogenated graphene through contact angle measurements with water and diiodomethane. We accounted for many contributing factors, including substrate surface energies and combating adsorption of airborne contaminants. Hydrogenating graphene raises its polar surface energy with little effect on its dispersive surface energy. Fluorinating graphene lowers its dispersive surface energy with a substrate-dependent effect on its polar surface energy. These results unravel how changing the chemical structure of graphene modifies its surface energy, with applications for hybrid nanomaterials, bioadhesion, biosensing, and thin-film assembly.

Commercial aircraft operate over a wide range of atmospheric conditions, with temperatures exceeding 40 degrees C during takeoff and dropping below-50 degrees C at cruising altitudes near 10 km. An aircraft environmental control system (ECS) works to keep the crew and passengers comfortable by supplying them with conditioned air. The refrigeration unit of these aircraft, known as the air cycle machine (ACM), sees extreme conditions, ranging from engine bleed air exceeding 100 degrees C at the ACM compressor inlet to subzero temperatures at the ACM turbine outlet. These conditions can cause the ACM's condenser to frost, reducing the efficiency of the ECS. Engine bleed air can be used to defrost the condenser at the price of further reducing efficiency and cabin comfort. A larger condenser can alleviate frosting but will increase the cost and weight of the aircraft. An alternative is to apply a superhydrophobic coating to the condenser to provide increased resistance to frost growth with a minimal impact on nominal heat exchanger efficiency. In this work, we modified the surface wettability of an aluminum heat exchanger and tested it in a wind tunnel under a wide range of hot-side temperatures to study both frosting and defrosting performance. The superhydrophobic heat exchanger showed considerable improvement in system efficiency, sometimes completely eliminating frost growth and lowering the cycle normalized defrost time by up to 50%. A superhydrophobic coating on the ACM condenser of a turbine-powered aircraft has the potential to increase overall efficiency of the ECS and improve the energy efficiency of aircraft.(c) 2022 Elsevier Ltd. All rights reserved.

A water droplet impacting onto a supercooled surface is typically considered to freeze and adhere to the substrate. This ice accretion poses safety and economic threats to transportation infrastructure, power generation/transmission systems, and telecommunication facilities. Here we report the observation of ultra-low ice-substrate adhesion (0-50 kPa) and remarkable self-deicing during droplet-impact freezing on copper surfaces having medium to high supercooling (30 degrees C-80 degrees C). Mechano-thermo-hydraulic coupling during droplet-impact freezing governs the ice-substrate adhesion by gap. ping the droplet-substrate contact, enabling self-peeling facilitated by thermal-mechanical stress relaxation. We observe a strong adhesion region in the center of the frozen droplet, which determines the adhesion strength, and develop a regime map to delineate the dependence of adhesion/peeling on droplet inertia, substrate supercooling, and surface wettability. Our work demonstrates key mechanisms governing ice-substrate adhesion during impact icing and presents an approach to passive self-deicing.

* Corresponding author: Tel.: 617-981-9247; Email: nmiljkov@illinois.edu 1. Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA 2. Department of Electrical and Computer Engineering, University of Illinois, Urbana, Illinois 61801, USA 3. Frederik Seitz Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA 4. International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 8190395, Japan