Study of the topography, morphology and the chemical composition of the coating

Topography and morphology of the urchin-like Al₂O₃ particles manufactured as discussed in “Manufacturing of “urchin”-like particles of Al₂O₃” and the eventual omniphobic coating, prepared as reported in “Manufacturing of the icephobic, omniphobic coating”, were studied with scanning electron microscope with auto-emission cathode (FE-SEM, FEI, Quanta 650FEG). Chemical composition of the samples was investigated with the energy dispersive X-ray spectrometry (EDS, EDAX Octane Elite). Typical results of the EDS spectroscopy of the reported surfaces are illustrated with Fig. 3.

EDS analysis of the icephobic, omniphobic surface is supplied. The pronounced fluorination of the surface is recognized.

Wetting characteristics of the coating (apparent contact angles and the contact angle hysteresis) were measured with the precise goniometric system (Kruss DSA-100). 8 μl de-ionized water droplets were used for the study of the wetting characteristics of the reported icephobic, omniphobic surfaces. The results were averaged across 10 measurements.

Contact angle hysteresis was established with the needle-syringe method. 5 μl droplet was placed on the surface and inflated with a syringe; the triple (contact) line was pinned to the substrate up to a certain volume of the droplet. When the triple line was pinned, the contact angle increased till a certain threshold value θ_A beyond which the triple line started to move. The threshold contact angle θ_A was registered as the advancing contact angle¹⁰. When a droplet was deflated with a syringe its volume was decreased to a certain limiting value; in parallel the contact angle decreases till a threshold value θ_R, registered as the receding contact angle, denoted θR. When θ = θ_R, the triple line suddenly moved. Both θ_A and θ_R are equilibrium, although metastable contact angles¹⁰. The difference between θ_A and θ_R, namely Δθ=θA-θR is known as the contact angle hysteresis¹⁰.

In parallel, sliding angles were established with the inclined (tilted) plane method. The minimal angle at which 5–10 µl water droplets started to slide along the studied surface was registered. The establishment of the contact angle hysteresis and the sliding angle were performed at the ambient conditions (t=25∘C).