Stimuli-Responsive Wettability of Nonplanar Substrates: pH-Controlled Floatation and Supporting Force

We have prepared pH-responsive planar and nonplanar substrates by chemical modification of a rough gold surface with pH-responsive 2-(11-mercaptoundecanamido)benzoic acid (MUABA). The rough surface exhibits a pH-responsive behavior with an exceptionally large change in contact angle (CA) as a function of the pH, from nearly superhydrophobic (CA of about 144°) to superhydrophilic (CA of near 0°). The relationship between pH and the supporting force of gold threads coated by MUABA has been investigated. The pH-responsive coating on the gold thread can provide bigger supporting forces at low pH than at high pH due to the hydrophobicity in the former case. In addition, the change of surface wettability can influence its floatation as well, therefore providing a new approach for controlling the motion of gold threads on water.