The effect of contamination on the bubble cluster formation in swarm of spherical bubbles rising along an inclined flat wall

The motion of mono-dispersed spherical bubbles rising along an inclined flat wall is investigated experimentally. An inclination angle of the flat wall is changed to control the bubble Reynolds number set to be 100. As an experimental condition, the boundary condition on the bubble surface (free-slip and no-slip conditions) is controlled, and its effect on the motion of the bubbles has been analyzed. MgSO4 solution and Triton X-100 solution are used to achieve free-slip and no-slip bubble surfaces, respectively. Bubble coalescences are almost inhibited by using these additives. The bubbles tend to be horizontally arranged in lines and such arrangements pile up to be remarkable bubble clusters. The radial pair distribution function and the conditional average of the relative velocity of two bubbles are calculated to evaluate the spatial distribution and bubble-bubble interaction quantitatively. The results show that the tandem configuration of bubble pair next to each other frequently appears due to the increment in the rising velocity of the trailing bubble; a horizontally-oriented (side-by-side) configuration is stable compared to a vertically-oriented (tandem) one, which leads to a horizontally arranged bubbles in a single line. A similar tendency for the bubble clustering is confirmed for both free-slip bubbles and no-slip bubbles. However, there are some quantitative differences such as the faster approaching velocity of the trailing bubble in wider wake region of the leading bubble for no-slip bubbles, etc. The bubble cluster velocity as a velocity of highly accumulated bubbles is also defined and evaluated.