Modeling of Unidirectional-Overloaded Transition in Catalytic Tubular Microjets

A numerical time-resolved model is presented for predicting the transition between unidirectional and overloaded motion of catalytic tubular microjets (Ti/Fe/Pt rolled-up microtubes) in an aqueous solution of hydrogen peroxide. Unidirectional movement is achieved by periodic ejection of gas bubbles from one end, whereas formation of multiple bubbles hinders microjet movement in overloaded regime. The influence of nucleation positions of bubbles, hydrogen peroxide concentration, liquid-platinum contact angle, microjet length, and cone angle on the bubble ejection frequency and microjet speed are investigated. We find agreement between the theoretical speeds of the microjet for a range of bubble nucleation positions (0.4L <= x(0) <= 0.6L) and our measurements (108 35 mu m/s) for unidirectional motion. In addition, we observe experimentally that transition to overloaded motion occurs for hydrogen peroxide concentration of 5%, whereas our model predicts this transition for concentrations above 2.5%.