Effects of Salinity on Surface Lifetime of Large Individual Bubbles

Abstract

Abstract:The influence of salinity on the characteristics of individual bubbles (2-4 mm in diameter) in fresh and saline water (up to 40 practical salinity units) was investigated. Bubbles were produced by forcing air through capillary tubes. Aqueous solutions in distilled and filtered tap waters with minimized presence of organic additives were used. Salinity, surface tension, and water temperature were monitored. Parameters measured were the bubble surface lifetime, diameter, and rise velocity. The surface lifetime varies widely (in the range of 0.4-35 s) depending on the salinity concentration and the purity of the solutions. Variations with salinity of size and rise velocity of large individual bubbles are discussed. Interpretation of the results in terms of anti-foaming (negative adsorption), as well as the Marangoni and the Gibbs effects, is helpful in understanding the results.

Effects of Salinity on Bubble Cloud Characteristics

“…This study of the effects of salinity on bubble clouds beneath breaking waves complements our study of salinity influence on large individual bubbles comprising whitecaps floating on water surface [10]. Each study provides insights on a different pathway through which salinity could affect oceanic whitecap longevity (persistence) [11].…”

“…The horizontal and vertical scales were 0.296 mm pxl −1 and 0.698 mm pxl −1 . With these scales, the smallest bubbles within bubble clouds that we resolved have projected-area equivalent diameter [10] of ≈ 0.5 mm. Our system is thus suitable to study the large-size end of the bubble size spectrum formed during active wave breaking (Section 2).…”

“…Generally, bubbles are easily created in aqueous solutions when γ is lower than its value in pure water γ 0 , i.e., γ < γ 0 . The surface tension of an aqueous solution can become lower when water temperature increases or simple organics or surfactants are added; Appendices B and C in [10] give details and schematics of such variations of γ. At a fixed water temperature (i.e., isothermal conditions), adding inorganic salts [e.g., sodium chloride (NaCl)] to water may either increase or decrease γ depending on the purity of the water.…”

“…Because lower γ values (or, equivalently, positive film pressure ∆γ) facilitate bubble formation, the added salt now acts as a foaming agent. Moreover, positive ∆γ enhances the stabilization of bubbles against bursting (details in Appendix C of [10]). The antifoaming role of the salts therefore changes to that of foaming agents in contaminated aqueous solutions because salts start to contribute to both bubble formation and stabilization.…”

“…Each study provides insights on a different pathway through which salinity could affect oceanic whitecap longevity (persistence) [11]. Specifically, in [10], we addressed…”