The inception of a cavitation bubble in a liquid by focusing a short and intense laser pulse near its free surface develops not only an upwards directed jet, but a second jet of opposite direction into the bulk liquid. When the laser is focused a few microns below the surface, the rapid deposition of energy produces a splash, whose later sealing gives origin to two particularly elongated opposing jets. Interestingly, the evolution of the downward jet flowing into the liquid pool has many similarities to that observed in free water entry experiments, e.g. the creation of a slender and stable cavity in the liquid. The downward jet can reach speeds of up to $40$ m s $^{-1}$ and travels distances of more than 15 times the maximum radius of the laser induced cavity before losing momentum. The longer lifetime of this so-called ‘bullet’ jet as compared with conventional cavitation based jets, the alignment of the jet perpendicular to the free surface and the possibility of scaling the phenomenon opens up potential applications when generated on small droplets or in shallow liquids. In this work, the underlying mechanisms behind the formation of the bullet jets are initially investigated by performing a set of experiments designed to address specific questions about the phenomenon under study. Those were followed by numerical simulations used to give a quantitative and detailed explanation to the experimental observations.

Two pairs of converging and superimposing shock and Rayleigh waves are generated on a glass substrate by focusing laser pulses on two concentric rings in a bullseye configuration (67 and 96  μm radii). We experimentally study the threshold for the substrate damage as a function of the number of repetitions and the delay (0–20 ns). The bullseye focusing experiments are compared to a single focusing ring. Additionally, fluid–structure interaction simulations using a volume-of-fluid framework are utilized to estimate the stresses. The lowest number of repetitions to attain surface damage is found for constructive superposition of the Rayleigh waves, i.e., here for a delay of 10 ns. The observed damage is consistent with the simulations where the largest positive stresses ([Formula: see text] GPa) are achieved for bullseye focusing with [Formula: see text] ns followed by [Formula: see text] ns, which corresponds to a simultaneous shock wave focusing. In all these cases, the positive stresses are followed (a few nanoseconds later) by the negative stresses that can reach [Formula: see text] GPa.

We study the interaction between a laser-induced cavitation bubble and a submillimetre-sized water droplet submerged in silicone oil. High-speed imaging reveals the pathways through which droplet fragmentation occurs and three distinct regimes of bubble–droplet interaction are identified: deformation, external emulsification and internal emulsification. We have observed that during the bubble collapse, the droplet elongates towards the bubble, which acts as a flow sink pulling on the droplet. For silicone oils with higher viscosity, the droplet jets into the cavitation bubble and forms a satellite water droplet in the continuous oil phase. In contrast, for lower-viscosity oils, the droplet encapsulates the collapsing bubble as it jets inside and undergoes multiple cycles of expansion and collapse. These internal bubble collapses create tiny oil droplets inside the parent water droplet. The kinematic viscosity of the silicone oil, maximum bubble diameter and centre-to-centre distance between the bubble and the droplet are varied. The regimes are separated in a parameter space set up by the non-dimensional distance and a cavitation Reynolds number.

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A cavitation bubble expanding and collapsing near a rigid boundary develops a directed jet flow towards the boundary. In the case of a perforated plate, some of the jet flow passes through the plate and thus the bubble acts as a pump transporting liquid from one side of the plate to the opposite side. The transport is rather complex, is time dependent and varies with the geometric parameters of the bubble and the connecting channel. Therefore, we first model the transport of liquid through a perforated rigid plate for a large range of parameters and then compare some regimes with experiments using single laser-induced bubbles. The simulations are based on a Volume-of-Fluid solver in OpenFOAM and account for surface tension, compressibility and viscosity. The resulting flux and generated velocity in the channel obtained in the simulations are discussed with regards to the dependence of the channel geometry, liquid viscosity and stand-off distance of the bubble to the plate. In general, high flow rates are achieved for long cylindrical channels that have a similar width as the jet produced by the collapsing bubble. At low stand-off distances combined with thick plates, an annular inflow creates a fast and thin jet, also called needle jet, which is approximately a magnitude faster and significantly thinner than the usually encountered microjet. In contrast, for thin plates and small stand-off distances, liquid is pumped in the opposite direction via a reverse jet.