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The dynamics of collapsing bubbles close to a flat solid is investigated with respect to its potential for removal of surface attached particles. Individual bubbles are created by nanosecond Nd:YAG laser pulses focused into water close to glass plates contaminated with melamine resin micro-particles. The bubble dynamics is analysed by means of synchronous high-speed recordings. Due to the close solid boundary, the bubble collapses with the well-known liquid jet phenomenon. Subsequent microscopic inspection of the substrates reveals circular areas clean of particles after a single bubble generation and collapse event. The detailed bubble dynamics, as well as the cleaned area size, is characterised by the non-dimensional bubble stand-off γ=d/Rmax, with d: laser focus distance to the solid boundary, and Rmax: maximum bubble radius before collapse. We observe a maximum of clean area at γ≈0.7, a roughly linear decay of the cleaned circle radius for increasing γ, and no cleaning for γ>3.5. As the main mechanism for particle removal, rapid flows at the boundary are identified. Three different cleaning regimes are discussed in relation to γ: (I) For large stand-off, 1.8<γ<3.5, bubble collapse induced vortex flows touch down onto the substrate and remove particles without significant contact of the gas phase. (II) For small distances, γ<1.1, the bubble is in direct contact with the solid. Fast liquid flows at the substrate are driven by the jet impact with its subsequent radial spreading, and by the liquid following the motion of the collapsing and rebounding bubble wall. Both flows remove particles. Their relative timing, which depends sensitively on the exact γ, appears to determine the extension of the area with forces large enough to cause particle detachment. (III) At intermediate stand-off, 1.1<γ<1.8, only the second bubble collapse touches the substrate, but acts with cleaning mechanisms similar to an effective small γ collapse: particles are removed by the jet flow and the flow induced by the bubble wall oscillation. Furthermore, the observations reveal that the extent of direct bubble gas phase contact to the solid is partially smaller than the cleaned area, and it is concluded that three-phase contact line motion is not a major cause of particle removal. Finally, we find a relation of cleaning area vs. stand-off γ that deviates from literature data on surface erosion. This indicates that different effects are responsible for particle removal and for substrate damage. It is suggested that a trade-off of cleaning potential and damage risk for sensible surfaces might be achieved by optimising γ.

A laboratory filtration plant for drinking water treatment is constructed to study the conditions for purely mechanical in situ cleaning of fouled polymeric membranes by the application of ultrasound. The filtration is done by suction of water with defined constant contamination through a membrane module, a stack of five pairs of flat-sheet ultrafiltration membranes. The short cleaning cycle to remove the cake layer from the membranes includes backwashing, the application of ultrasound and air flushing. A special geometry for sound irradiation of the membranes parallel to their surfaces is chosen. Two frequencies, 35kHz and 130kHz, and different driving powers are tested for their cleaning effectiveness. No cleaning is found for 35kHz, whereas good cleaning results are obtained for 130kHz, with an optimum cleaning effectiveness at moderate driving powers. Acoustic and optic measurements in space and time as well as analytical considerations and numerical calculations reveal the reasons and confirm the experimental results. The sound field is measured in high resolution and bubble structures are high-speed imaged on their nucleation sites as well as during their cleaning work at the membrane surface. The microscopic inspection of the membrane surface after cleaning shows distinct cleaning types in the cake layer that are related to specific bubble behaviour on the membrane. The membrane integrity and permeate quality are checked on-line by particle counting and turbidity measurement of the permeate. No signs of membrane damage or irreversible membrane degradation in permeability are detected and an excellent water permeate quality is retained.

Collapsing cavitation bubbles produce intense microscopic flows. Here, in an aqueous environment, we seed single laser-induced bubbles (diameter about one millimeter) in proximity to a solid surface, in a regime that has not been well explored before in order to generate a “needle jet.” The needle jet propagates at supersonic speed through the gas phase toward the solid. It reaches average velocities of more than 850 ms−1 and thus is an order of magnitude faster than the regular jets that have frequently been observed in cavitation bubbles. The dynamics leading to the needle jet formation are studied with high speed imaging at five million frames per second with femtosecond illumination. This highly repeatable, localized flow phenomenon may be exploited for injection purposes or material processing, and it is expected to generate significantly larger water hammer pressures and may also play a role in cavitation erosion and peening.

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The time evolution of the liquid-film thickness of a single cavitation bubble in water collapsing onto a solid surface is measured. To this end, total internal reflection (TIR) shadowmetry is developed, a technique based on TIR and the imaging of shadows of an optical structure on a polished glass surface. The measurements are performed at frame rates up to 480 kHz. Simultaneous high-speed imaging of the bubble shape at up to 89 kHz allows relating the evolution of the film thickness to the bubble dynamics. With a typical maximum bubble radius of 410 µm, we varied the nondimensional stand-off distance γ from 0.47 to 1.07. We find that during the first collapse phase, the bubble does not come in direct contact with the solid surface. Instead, when the bubble collapses, the jet impacts on a liquid film that always resides between the bubble and solid. At jet impact, it is 5–40 µm thick, depending on γ. Also, during rebound, at any given point in time, most or all of the then overall toroidal bubble is not in contact with the solid surface.