A backflipping motion of a bubble impacting tilted surfaces*

Bubbly flows are ubiquitous in both industrial and natural settings. A recent study [1] showed that forces arising from a bubble impacting a surface in a moderate Reynolds number range can remove active microorganisms or passive beads on the surface. In this work, we experimentally investigate a bubble impacting a tilted glass surface for a bubble diameter range of 1.1< d<1.4. Interestingly, we observe a backflipping-like behavior where the bubble moves backward against the surface slope after its first impact before it starts moving forward. To characterize the backflipping behavior, we find a characteristic backflipping distance for different bubble sizes and surface inclination angles. We find that the backflipping effect is stronger for larger bubbles as the impact velocity is larger. In addition, we find that for each bubble size there is a critical angle for the maximum backflipping effect which varies in the range of 5-10 degrees for bubbles of the current size range. Our PIV measurements indicate that the backflipping strength is directly correlated with the average vorticity around the bubble surface and the resultant lift force. We incorporate the PIV measurements into a simplified force balance model that only considers lift and buoyancy forces, which captures the main essence of the backflipping behavior. In addition, we perform numerical simulations to also consider the thin film pressure during the first impact and compare the numerical backflipping distance with the experiments and the simplified model.

[1] E. Esmaili, P. Shukla, J. Eifert, S. Jung, “Bubble impact on a tilted wall: Removing bacteria using bubbles” Phys. Rev. Fluids, 4, 043603 (2019)