Wall-Bounded Dynamics of a Single Surfactant-Contaminated Bubble in Linear Shear Flow

This paper systematically investigates the dynamic characteristics of a single surfactant-contaminated bubble in the near-wall region of a linear shear flow. The experiment is conducted on a vertical water tunnel platform using dual-angle high-speed shadow imaging technology. The surfactant selected is sodium dodecyl sulfate (SDS, Concentration CSDS=۰-۵۰ ppm). The ascent trajectory, instantaneous velocity, average velocity, aspect ratio, bubble wavelength and quasi-steady lift and drag coefficients of bubbles near the wall is investigated, with different equivalent diameters deq=۲.۴۹-۳.۵۴ mm, bubble Reynolds numbers ۴۷۰-۸۳۰, and dimensionless wall distances S*=۱.۴۱-۲.۰۲. The results indicate that the surfactant triggers a rapid transition from a mobile to a rigidified interface. Compared to the pure water condition (vertical velocity ~۳۰۰ mm/s), the introduction of ۱۰ ppm SDS immediately drops the average rising velocity to approximately ۲۰۰ mm/s. Quantitatively, as the concentration increases from ۱۰ to ۵۰ ppm, the net lateral migration distance significantly diminishes from ۵.۶۸ mm to ۰.۷۲ mm, demonstrating robust trajectory stabilization. The quasi-steady lift coefficient (CL) exhibits a drastic monotonic decline (e.g., dropping by over ۶۰% for large bubbles) with increasing contamination and loses its sensitivity to the dimensionless wall distance (S*) in the highly contaminated regime. Conversely, the quasi-steady drag coefficient (CD) universally increases near the wall (low S*) due to geometric streamline compression. Based on these experimental datasets, empirical correlations utilizing the Marangoni and Eötvös numbers are developed to predict CL and CD, demonstrating high predictive accuracies with R۲ values of ۰.۹۱۳ and ۰.۸۵۳, respectively. These findings provide a practical predictive model and reveal the coupling mechanism between wall confinement, shear effects, and Marangoni stresses.

کلیدواژه ها:

Linear shear flow field ، Wall effect ، Surfactant ، Bubble dynamics ، Marangoni effect