Till Nowak

Study on the bubble transport mechanism in an acoustic standing wave field.

The use of bubbles in applications such as surface chemistry, drug delivery, and ultrasonic cleaning etc. has been enormously popular in the past two decades. It has been recognized that acoustically-driven bubbles can be used to disturb the flow field near a boundary in order to accelerate physical or chemical reactions on the surface. The interactions between bubbles and a surface have been studied experimentally and analytically. However, most of the investigations focused on violently oscillating bubbles (also known as cavitation bubble), less attention has been given to understand the interactions between moderately oscillating bubbles and a boundary. Moreover, cavitation bubbles were normally generated in situ by a high intensity laser beam, little experimental work has been carried out to study the translational trajectory of a moderately oscillating bubble in an acoustic field and subsequent interactions with the surface. This paper describes the design of an ultrasonic test cell and explores the mechanism of bubble manipulation within the test cell. The test cell consists of a transducer, a liquid medium and a glass backing plate. The acoustic field within the multi-layered stack was designed in such a way that it was effectively one dimensional. This was then successfully simulated by a one dimensional network model. The model can accurately predict the impedance of the test cell as well as the mode shape (distribution of particle velocity and stress/pressure field) within the whole assembly. The mode shape of the stack was designed so that bubbles can be pushed from their injection point onto a backing glass plate. Bubble radial oscillation was simulated by a modified Keller-Miksis equation and bubble translational motion was derived from an equation obtained by applying Newtons second law to a bubble in a liquid medium. Results indicated that the bubble trajectory depends on the acoustic pressure amplitude and initial bubble size: an increase of pressure amplitude or a decrease of bubble size forces bubbles larger than their resonant size to arrive at the target plate at lower heights, while the trajectories of smaller bubbles are less influenced by these factors. The test cell is also suitable for testing the effects of drag force on the bubble motion and for studying the bubble behavior near a surface.

Acoustic streaming and bubble translation at a cavitating ultrasonic horn

Acoustic cavitation at a 20 kHz ultrasonic horn is investigated by means of high-speed imaging and particle image velocimetry. In one experimental set-up, bubble dynamics is visualized synchronously with the acoustic streaming liquid flow to reveal their connection. By switching an elevated static pressure, cavitation can be turned off and on for otherwise identical conditions. If cavitation is present, an average increase of liquid streaming velocities by a factor of 30 is found as compared to the non-cavitating case, and high flow velocities are well confined to the bubbly regions. Further results show that individual bubble trajectories do not always coincide with the liquid flow direction, but can even run in opposite direction. This is highlighted in a second set-up where the periodic back-and-forth translation of a single bubble near the horn tip in phosphoric acid is analyzed. It is concluded that translation of larger cavitation bubbles is mainly determined by acoustic forces, even in the presence...

Characterization of a cavitation bubble structure at 230 kHz : bubble population, sonoluminescence, and cleaning potential

Introduction Applications of acoustic cavitation gain in importance and become more widespread recently. While its utilization in traditional fields persists and is subject to optimization, new types and areas of application emerge as well. In many cases, however, it is realized that not all physical processes involved are well enough understood yet. In particular the link between process parameters like acoustic field geometry, frequency or intensity, and the observed or desired effects, might not be sufficiently clear. This is sometimes true even qualitatively, and then a quantitative analysis is naturally out of the scope anyway. An important aspect of this link in acoustic cavitation is the formation of bubble structures: The applied sound field generates certain bubble distributions in space and time with specific bubble size populations, which in turn mediate the microscopic effects via their oscillation and/or collapse properties. A systematic characterization and comprehension of different bubble structures has started only recently [1], and here we want to give a further contribution to advance the knowledge with respect to the process chain in acoustic cavitation. A distinct bubble structure at 230 kHz has been observed and investigated by means of high-speed recordings, sonoluminescence measurements, and cleaning tests. We speculate that the observed phenomena are universal for a class of acoustic field geometries over a broader frequency range.

Cleaning of semiconductor substrates by controlled cavitation

The continuing downscaling of device geometries in the semiconductor industry is driving the requirements for both process and contamination control. Historically, the physical and the chemical processes required for contamination control were evolutionarily scaled with device geometry. However, todays tailored wet‐chemical cleaning approaches must strive to meet stringent requirements to assure a minimal material loss and no damage to extremely fragile structures. While chemical solutions exist for the control of molecular‐organic and metallic ion contamination, the physico‐chemical solutions for the removal of nanosized particulate contamination to critical diameters below 20 nm are still undetermined. Therefore, the potential and the limitations of megasonic cleaning, which is mainly based on cavitation, are carefully balanced and a detailed understanding of the ongoing physical mechanisms is necessary to maintain a stable window of operation. The relevant active mechanisms present in such a cavitatio...

Investigation of bubble dynamics and sonoluminescence in megasonic fields

Cavitation bubble motion and bubble structures in water are investigated for standing wave fields in the megasonic range by high‐speed imaging. Larger degassing bubbles and small bubbles with high translation speeds can be resolved. Groups of bubbles arrange in lines or arrays, as reported earlier by Miller [Miller, JASA 62, 1977]. Additional, sonoluminescence is measured in overall long‐term and phase‐resolved (gated) long‐term exposures. Several distinct luminescing islands can be detected. The findings seem to be strongly related to the standing wave nature of the pressure field in our setup. Conclusions on bubble distributions and for cleaning applications are drawn.