Dynamics of air bubbles in removing proteins or bacteria on a tilted surface

In this study, we conduct experiments to test the cleaning efficacy of bubbles at different inclination angles of a surface. We use two different surfaces coatings of either a protein solution or a bacterial biofilm. Our experimental results indicate that bubbles exhibit the best cleaning efficacy at the surface angle of θ=20^o for polydisperse bubbles in the range of 0.3-2 mm and with an average radius of 0.6 mm in radius. To gain a better understanding of the underlying mechanism, we perform numerical analysis of a single air bubble impacting surfaces with different angles. Our numerical and theoretical results show that the shear stress, that is proportional to the sliding speed but inversely proportional to the film thickness, result in the maximum shear force occurring at θ≈22^o ≈π/8 in good agreement with the experiments.