Effect of Surfactant on Bubble Formation on Superhydrophobic Surface in Quasi-Static Regime

We experimentally studied the effect of a surfactant on bubble formation on a superhydrophobic surface (SHS). The bubble was created by injecting gas through an orifice on the SHS at a constant flow rate in the quasi-static regime. The surfactant, 1-pentanol, was mixed with water at concentration <i>C</i> ranging from 0 to 0.08 mol/L, corresponding to surface tension <i>σ</i> ranging from 72 to 43 mN/m. We found that as <i>C</i> increased, the bubble detachment volume (<i>V</i>_d) and maximum bubble base radius (<i>R</i>_d^max) decreased. For a low surfactant concentration, the static contact angle <i>θ</i>₀ remained nearly constant, and <i>V</i>_d and <i>R</i>_d^max decreased due to lower surface tensions, following the scaling laws <i>R</i>_d^max~<i>σ</i>^1/2 and <i>V</i>_d~<i>σ</i>^3/2. The bubble shapes at different concentrations were self-similar. The bubble height, bubble base radius, radius at the bubble apex, and neck radius all scaled with the capillary length. For high surfactant concentrations, however, <i>θ</i>₀ was greatly reduced, and <i>V</i>_d and <i>R</i>_d^max decreased due to the combined effects of reduced <i>θ</i>₀ and smaller <i>σ</i>. Lastly, we found that the surfactant had a negligible impact on the forces acting on the bubble, except for reducing their magnitudes, and had little effect on the dynamics of bubble pinch-off, except for reducing the time and length scales. Overall, our results provide a better understanding of bubble formation on complex surfaces in complex liquids.