Bubble Growth From Submerged Orifice Plates: Part 1—Decoupling Interfacial Tension and Surface Wettability Effects

Abstract Liquid–gas interfacial tension (σ), substrate wettability (characterized in terms of contact angle (θ)), and upstream gas chamber volume strongly affect the departure size and the growth dynamics of a bubble emanating from an orifice plate submerged in a quiescent liquid pool. These are experimentally investigated in two companion papers. Companion part 2 examines the extent to which upstream gas chamber volume influences the growth dynamics and how that influence is modulated by liquid viscosity and surface tension. While in part 1 (this article), to decouple and understand the effects of σ and θ, bubble growth dynamics in aqueous surfactant solutions and binary mixtures of ethanol–water are studied. Seven different substrates with contact angles for water ranging from 43 deg to 128 deg are used. High-speed videography and precision digital image processing were utilized to experimentally analyze bubble growth. Aqueous solutions of sodium dodecyl sulfate (SDS) in concentrations of 1 critical micellar concentration (CMC) and ½ CMC (equilibrium σ of 38.8 and 53.0 mN/m, respectively) along with ethanol–water binary mixtures of similar σ, but different contact angle θ, are used. Spreading of the bubble base, or surface de-wetting, on a hydrophobic Teflon substrate is observed for aqueous ½ CMC SDS solution and its ethanol–water mixture counterpart. However, the differences in the extent of spreading of bubble base led to significant differences in the bubble departure diameters for the two solutions. In contrast, bubbles formed in 1 CMC SDS solution and ethanol–water mixture of nearly the same equilibrium σ had similar bubble sizes at departure, with the bubble base remaining attached to the orifice from incipience to departure. Additional experiments using ethanol–water mixtures with surface tension varying between 35.5 and 72.5 mN/m on substrates with different wettability revealed that there is a critical liquid–gas interfacial tension (σ∼46mN/m) below which no de-wetting is observed on both hydrophobic and hydrophilic surfaces, and the bubble departure size is independent of surface wettability. For σ greater than the critical surface tension, bubble base spreads on substrates with contact angles above a critical value giving rise to larger bubble sizes at departure. Using aqueous solutions of Triton-X 305 at six different concentrations, the value of the critical contact angle was obtained as (θ ∼ 64 deg) below which no de-wetting of the bubble base is observed. A σ − θ regime map is provided that demarcates the capillary-like behavior (θ ≤ 64 deg or σ≤46mN/m) from the region where surface de-wetting occurs (θ > 64 deg and σ>46mN/m).