Slippery or sticky nano-porous silica coatings impregnated with squalane: The role of oil over-layer

Surface designs and wetting mechanisms of Nepenthes pitcher plant provide useful guiding principles for achieving control over the wettability of solid surfaces as mimicked in slippery liquid-infused porous surfaces (SLIPS). Here, the effect of the over-layer thicknesses was investigated by gradual impregnation of porous silica coatings with squalane. Characterisation and wettability of the coatings at various stages of the impregnation were studied using fluorescence microscopy, gravimetric analysis and water contact/sliding angle measurements. The technique allowed us to progressively generate variable thicknesses of the oil over-layers and systematically tune the wetting behaviour of the coatings. The results clarify that very thin oil over-layer may not lead to slippery surface and the slipperiness of the coated surfaces was observed to increase with increase in the thickness of the over-layer. It is suggested that fabricated SLIPSs be accompanied with specified workable thickness of the oil over-layer in order to clearly evaluate their efficiencies.