When bubbles expand and collapse close to boundaries, a shear flow is generated which is able to remove particles from the surface, thus locally cleaning it. Here the authors demonstrate experimentally with microparticle tracking velocimetry that the strongest forcing of particles occurs during a very brief time interval of the bubble oscillation period. During this interval a jet flow impacts and spreads radially along the surface, thus transporting the particles with it.

The device described here consists of a millimeter-size tube immersed in a liquid, closed at one end, and partially filled with gas.

No abstract

Lab-on-a-chip devices are in strong demand as versatile and robust pumping techniques. Here, we present a cavitation based technique, which is able to pump a volume of 4000 microm3 within 75 micros against an estimated pressure head of 3 bar. The single cavitation event is created by focusing a laser pulse in a conventional PDMS microfluidic chip close to the channel opening. High-speed photography at 1 million frames s(-1) resolves the flow in the supply channel, pump channel, and close to the cavity. The elasticity of the material affects the overall fluid flow. Continuous pumping at repetition rates of up to 5 Hz through 6 mm long square channels of 20 microm width is shown. A parameter study reveals the key-parameters for operation: the distance between the laser focus and the channel, the maximum bubble size, and the chamber geometry.

The wall shear stress from cavitation bubbles collapsing close to a rigid boundary is measured with a constant temperature anemometer. The bubble is created with focused laser light and its dynamics is observed with high-speed photography. By correlating the frames, a hydrophone signal, and the wall shear stress we find that the highest stresses are created after the impact of the jet, thus during its radial spreading on the surface. The maximum of the wall shear stress varies with the power of-2.75 as a function of the distance from the jet impact and in accordance with the prediction for a steady wall impinging jet. The highest amplitude of the signal of the wall shear stresses is found for bubbles oscillating close to the boundary and reaches more than 3 kPa. Additionally, it contains a slowly decaying weaker component which may be generated by an expanding vortex ring.

The collapse of a cavitation bubble near a rigid boundary induces a high-speed transient jet accelerating liquid onto the boundary. The shear flow produced by this event has many applications, examples are surface cleaning, cell membrane poration, and enhanced cooling. Yet the magnitude and spatio-temporal distribution of the wall shear stress are not well understood, neither experimentally nor by simulations. Here we solve the flow in the boundary layer using an axisymmetric compressible Volume of Fluid (VOF) solver from the OpenFOAM framework and discuss the resulting wall shear stress generated for a non-dimensional distance, γ = 1.0 (γ = h/R max , where h is the distance of the initial bubble centre to the boundary, R max the maximum spherical equivalent radius of the bubble). The calculation of the wall shear stress is found reliable once the flow region with constant shear rate in the boundary layer is determined. Very high wall shear stresses of 100 kPa are found during the early spreading of the jet followed by complex flows composed of annular stagnation rings and secondary vortices. Although the simulated bubble dynamics agrees very well with experiments we obtain only qualitative agreement with experiments due to inherent experimental challenges.

Cavitation bubble dynamics close to a rigid surface gives rise to a rapid and transient fluid flow. A single bubble is created with a laser pulse at different stand-off distances from the rigid surface, where the stand-off distance gamma is defined by gamma = h/R(max), with h being the initial distance and R(max) being the maximum bubble radius. When the surface is covered with adherent cells, molecular delivery and cell detachment after single cavitation activity are observed at different locations. We find a maximum of cell detachment at a normalized stand-off distance of gamma approximately 0.65. In contrast, the maximum of the molecular uptake is found when gamma approaches 0. The single cavitation event has only little effect on the viability of cells in the non-detached area. We find apoptosis of cells only very close to the area of detachment and, additionally, the metabolism of the non-detached cells shows no pronounced difference compared to control cells according to an MTS assay. Thus, although the cavitation event is responsible for the detachment of cells, only few of the remaining cells undergo a permanent change.

The growth and collapse of a vapour bubble inside a microtube is studied both experimentally and theoretically. The length of the bubble, and the velocity and acceleration of its interface, are obtained from a high-speed image recording (typically 1.25 × 105 fps) for various energy inputs and two tube diameters. To understand the underlying dynamics of the system, two theoretical models are compared with experiment. A model based on a discontinuous time dependence of the vapour pressure inside the bubble is at variance with the data. It proves necessary to account in greater detail for the time dependence of the vapour pressure. A new model is proposed for this purpose which includes heat transfer in addition to inertia and viscous friction. Both the data and the model show that the vapour pressure decreases with time continuously instead of abruptly. The length, velocity and acceleration from the numerical simulations are found to be in good agreement with experimental data. Both the experiments and simulations clearly indicate that thermal effects play an important role throughout the whole growth and collapse process.