Optimization of superamphiphobic layers based on candle soot

Liquid repellent layers can be fabricated by coating a fractal-like layer of candle soot particles with a silicon oxide layer, combusting the soot at 600 degrees C and subsequently silanizing with perfluoroalkylsilanes. Drops of different liquids deposited on these so called "superamphiphobic" layers easily roll off thanks to the low liquid-solid adhesion. The lower value of the surface tension of liquids that can be repelled depends on details of the processing. Here, we analyze the influence of the soot deposition duration and height with respect to the flame on the structure and wetting properties of the superamphiphobic layer. The mean diameter of the soot particles depends on the distance from the wick. Close to the wick, the average diameter of the particles varies between 30 and 50 nm as demonstrated by scanning electron microscopy (SEM). Close to the top of the flame, the particles size decreases to 10-20 nm. By measuring the mass of superamphiphobic layers and their thickness by laser scanning confocal microscopy (LSCM) in reflection mode, we could determine that the average porosity is 0.91. The height-dependent structural differences affect the apparent contact and roll-off angles. Lowest contact angles are measured when soot is deposited close to the wick due to wax that is not completely burnt, smearing out the required overhanging structures. The small particle size close to the top of the flame also reduces contact angles, again due to decreasing size of overhangs. Sooting in the middle of the flame led to optimal liquid repellency. Furthermore, for sooting times longer than 45 s the properties of the layer did not change with sooting time, verifying the self-similarity of the layer.