Effect of surfactant on bubble formation and bubble necking on superhydrophobic surfaces

We experimentally studied the effect of surfactant on bubble formation and bubble necking on a superhydrophobic surface (SHS). The bubble was created by injecting gas through an orifice at a constant flow rate in the quasi-static regime. The surfactant, 1-pentanol, was mixed with water at a concentration C ranging from 0 to 0.08 mol/L, corresponding to a surface tension 𝜎 ranging from 72 to 43 mN/m. We found that as C increased, bubble detachment volume (V_d) and maximum bubble base radius (R_d^max) decreased. For low surfactant concentration, the static contact angle 𝜃₀ remained nearly constant, and V_d and R_d^max decreased due to lower surface tensions, following the scaling laws: R_d^max~𝜎^1/2 and V_d~𝜎^3/2. The bubble shapes at different concentrations were self-similar and scaled with the capillary length. For high surfactant concentration, however, 𝜃₀ was greatly reduced, and V_d and R_d^max decreased due to the combined effects of reduced 𝜃₀ and smaller 𝜎. Last, we found that 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. Our results provide a better understanding of bubble formation on complex surfaces in complex liquids.