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.