Realization of Superhydrophobic Surface for the Detection of Residual Detergent Concentration

A superhydrophobic surface was realized on a glass substrate and on the inner wall of a capillary tube, employing a combination of ZnO nanorod arrays and stearic acid. The vertical ZnO nanorod arrays were grown by a unique two-step solution method consisting of a seed formation step and nanorod growth step. Stearic acid appeared to be coated over the tips of ZnO nanorod arrays, increasing the possibility of air trapping in the nanorod forest. A water contact angle of 156.3 degrees was demonstrated on the surface-modified glass, and it was attributed mainly to the air-trapping effect. The surface-modified glass substrate was used for contact angle measurements of detergent solutions, and the contact angle was found to monotonically decrease with increasing detergent concentration. Similarly, it was disclosed that the capillary height consistently decreased as a function of detergent concentration. These results indicate that contact angle measurement or capillary rise measurement may be a simple and practical way to detect the residual detergent concentration in water.