Experiments on the effect of wall distances for bubble collapse characteristics

At present, the energy released by bubble collapse can be used for the surface treatment of workpieces and can also be used to degrade pollutants. However, the mechanism of action between bubble collapse and a nearby wall has yet to be accurately explained. In order to grasp the relationship between the wall distance and collapse characteristics, the methods of spark discharge, high-speed photography, and pressure acquisition are used to study bubble shapes and the dynamic variation during pressure release with different wall distances in this paper. The results show that with the increasing of the distance L, the shape of a bubble changes from oblate to spherical and its collapse time shortens. The proportion of the total collapse time consumed by the slow collapse stage shows an increasing trend. In an experiment with two contractions, the time proportion of the slow collapse stage of the first contraction is much larger than that of the second contraction. The existence of the wall delays the collapse of the bubble. As the distance L increases, the bubble goes from undergoing one collapse to two collapses and then to one collapse again. The proportion of the duration of the slow collapse stage of the first contraction decreases rapidly, and the proportion of the slow collapse stage of the second contraction increases slowly, but the time proportion of second contraction decreases. When the distance L increases from 4.5 mm to 11 mm, the pressure received by the wall gradually decreases 28.19 MPa to 18.01 MPa. With an increase in the distance S from 0 to 8 mm, the maximum pressure received by the wall gradually decreases from 19.77 MPa to 9.37 MPa. The relationship found between the slow collapse stage (t_a), the second contraction (t_b), and the distance (L) can provide guidance for the effective application of the energy released by bubble collapse.