Wetting and Capillarity Effects on Bubble Formation From Orifice Plates Submerged in Pools of Water

An experimental study of bubble growth from submerged orifice plates in pools of water is carried out to scale and correlate the effects of surface wettability and orifice diameter D₀ on ebullience. Measurements of bubble growth on surfaces with nine different contact angles (38 deg ≤ θ ≤ 128 deg) and varying air flow rates (1–300 ml/min) were made using high speed videography and image processing. In the static or constant-volume regime, below a critical contact angle θ_c, the bubble base remains attached to the orifice, and the equivalent departure diameter D_b is independent of θ. On the other hand, above the critical contact angle, the bubble base spreads on the surface resulting in larger D_b. For θ > θ_c, D_b is strongly dependent on θ and increases with it. Using minimum energy method, it is shown that the wettability effects can be scaled and correlated by a modified capillary length, defined as a function of the Laplace length and contact angle. The proposed correlation provides predictions of D_b that agree with experimental data of this study as well as those available in the literature to within ±15%. Moreover, for a hydrophobic surface when D₀ > twice the modified capillary length, the bubble grows inside the orifice; for a hydrophilic surface, this scales with twice the capillary length, and effect of θ is not seen.