Surface roughness analysis of the hydrophilic SiO₂/TiO₂ nano bi-layers by Level crossing approach

The effect of etching time on the statistical properties of the hydrophilic surface of SiO_2/TiO_2/Glass nano bi-layer has been studied using Atomic Force Microscopy (AFM) and stochastic approach based on the level crossing analysis. We have created a rough surface of the hydrophilic SiO_2/TiO_2 nano bi-layer system by using 26% Potassium Hydroxide (KOH) solution. Measuring the average apparent contact angle assessed the degree of hydrophilicity and the optimum condition was determined at 10 min etching time. Level crossing analysis based on AF images provided deeper insight into the microscopic details of the surface topography. For different etching time, it has been shown that the average frequency of visiting a height with positive slope behaves Gaussian for heights near the mean value and obeys power law for the heights far away from the mean value. Finally, by applying the generalized total number of crossings with positive slope, it was found that the both high heights and deep valleys of the surface are extremely effective in hydrophilic degree of the SiO_2/TiO_2/Glass nano bi-layer investigated system.