A spherical bubble becomes unstable when it is subject to forces such as gravity, or when it is in proximity of surfaces such as walls or free-surfaces. The presence of such surfaces alters the surrounding pressure field, causing the bubble to lose its original sphericity, fold on itself and collapse. As the bubble collapses, some interesting and noteworthy flow phenomena take place. The detailed high-speed visualisations in the following video reveal some of these complex dynamics for a bubble collapsing close to a free surface.
In this case, it is the proximity of the free surface that causes the violent collapse. Once the laser-generated bubble grows to its maximum size, the collapse stage starts as the bubble begins to lose its sphericity. The upper part of the bubble accelerates downwards and becomes pierced by a liquid microjet. The jet itself tends to be difficult to observe visually, but its location can be identified by the protrusion it leaves when pushing part of the bubbles gaseous contents downwards.
When the jet impacts on the opposite bubble wall, a set of shock waves are emitted. In this case, the jet is so wide that it touches the interface not at a single point but in a ring, thus making the shock wave emission mechanism even more complex. Following jet impact, the bubble is separated into two parts, as the vapour cavity swept along by the jet becomes detached from the main toroidal bubble. Upon collapse of the main bubble, a stronger second set of shock waves is emitted which is reflected by the free surface causing excitation of nearby small bubbles.
More details here: https://doi.org/10.1063/1.4931098