Oscillations, Waves, and Interactions - GWDG

150 W. Lauterborn et al.
Figure 10. Dynamics of a laser-produced spherical bubble in water of reduced surface
tension in the neighbourhood of a plane solid boundary observed at 300000 frames per
second. Size of the frames is 6.7 mm × 2.7 mm.
where Rmax ist the maximum radius of the bubble and s the distance from the wall
at that instant, has turned out to be a good measure of perturbation of the bubble
collapse by a neighbouring wall.
The development of a jet by involution of the part of the bubble opposite to the
solid wall, the formation of a toroidal bubble or bubble vortex ring, and the rebound
of the torus bubble with the formation of a ‘counterjet’ is exemplified by Fig. 11. The
photographic series is taken at one million frames per second for better resolving the
dynamics and at an angle of 45 ◦ from above the wall with front illumination. The
normalized distance to the wall has the quite large value of γ =2.6. The jet is broad
and after penetrating the opposite bubble wall turns the bubble into a short living
torus bubble or vortex ring, the life time being less than a microsecond. Also within
less than a microsecond the torus bubble has rebounded and is expanding whereby a
‘counterjet’ develops sticking out upwards perpendicular to the torus ring and with
obviously little physical connection to the expanding torus bubble. It is produced
by the tension part of the torus shock wave developing upon collapse of the torus as
secondary cavitation [23]. It consists of many tiny bubbles that nucleate in the strong
negative pressure region along the perpendicular to the torus ring by confluence of the
shock/tension waves emanating from the torus ring. Figure 12 shows the evolution
of this secondary cavitation appearing on top of the bubble (torus) photographed in
side view for a bubble of Rmax = 1.5 mm and γ = 1.4.
When femtosecond laser pulses are used for bubble generation the high intensity
in the laser beam alters the properties of the medium in which it propagates, in
particular its index of refraction. In water this leads to self-focussing of the beam and
self-guiding or channelling. This effect may be used to produce elongated bubbles for