Wetting properties of protective coatings based on structured surfaces

Extreme water repellency observed on the surface of lotus leaf and the mechanism behind it is well studied and used for the fabrication of self-cleaning materials. Recently, the slippery mechanism of the rim of Nepenthes pitcher plant was revealed and mimicked in the so called Slippery Liquid-Infused Porous Surfaces (SLIPS). Here, we have fabricated silica nano-porous coatings using hydrophobic fumed silica. The partial impregnations of the coatings with different amounts of non-volatile oil (squalane) allows us to gradually tune its wetting behaviour from a superhydrophobic state in the absence of oil, through various intermediate wetting states, to slippery surfaces at high loading of oil. Superhydrophobic and slippery surfaces produced were found to possess low retention of water drops and good anti-biofouling characteristics towards algae cells. The intermediate coatings between the superhydrophobic and to slippery surfaces were found to exhibit different degrees of water and algae cells adhesions in similar trends. A simplified one-step deposition for large scale production of the coatings has been developed from a single formulated dispersion. In order to understand the wetting mechanism of the squalane-impregnated surfaces, the coatings were duplicated into solid states by similar partial impregnation using curable polydimethylsiloxane, PDMS (Sylgard 184). In these forms, the SEM images have clearly revealed the morphology of these gradual oil-impregnated coatings to spans from hierarchical nano/micro-structured coatings through larger scales of roughness to smoother coatings. Studies with curable Sylgard have led to the development of a simple and environmentally friendly method to grow ultra-thin polymeric films on solid surfaces for the fabrication of a more stable coatings and the modification of structured materials such as silica/aluminium oxide coatings as well as different metals and paper materials. In-depth characterisation of the fabricated substrates including, the porosity and thickness of the coating as well as the distribution and mass of the impregnated oil have been determined using capillary-driven impregnation, Dektak profiling system, fluorescence microscopy, and gravimetric analysis, respectively. Wetting properties of the coatings such as contact angles, sliding angles, sliding forces, contact angle hysteresis and adhesion of algae cells have been investigated and are discussed in term of the resulted changes in the surface architectural structures and surface chemistry following the gradual oil impregnation