Alexander Lippert

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.

Collective bubble dynamics near a surface in a weak acoustic standing wave field

The transport of bubbles to a neighboring surface is very important in surface chemistry, bioengineering, and ultrasonic cleaning, etc. This paper proposes a multi-bubble transport method by using an acoustic standing wave field and establishes a model that explains the multi-bubble translation by expressing the balance between Bjerknes forces and hydrodynamic forces on a bubble in a liquid medium. Results indicated that the influence of primary Bjerknes force, secondary Bjerknes force, and buoyancy force on the bubble translation depends on the position of the target bubble in the acoustic field. Moreover, it was found that increasing the size of a bubble or pressure amplitude can accelerate the bubble motion and enhance the bubble-bubble interaction. The secondary Bjerknes force between two bubbles can switch from an attractive one when they oscillate in phase to a repulsive one when the bubble oscillations are out of phase. These findings provide an insight into the multi-bubble translation near a surface and can be applied to future bubble motion control studies, especially in drug delivery, sonoporation, and ultrasonic cleaning.

Behaviour of a Well-Designed Megasonic Cleaning System

In this paper, we will demonstrate the performance of a new generation of megasonic systems. It has been improved beyond the state-of-the-art through a thorough, space resolved study and correlation of cavitation events coupled with multi-bubble-sonoluminescence (MBSL) [1], particle removal efficiency (PRE) and device damage creation. Minimizing “killer particles” becomes extremely critical as feature sizes of semiconductor devices continue to shrink. Wet processing has been fulfilling these stringent requirements to control particulate contamination in the sub-micron size range through the introduction of megasonic systems. By doing this, the chemical activity is supported through additional mechanical forces. However, recent reports are giving significant hints that current megasonic systems are creating too much device damage and paying a price for highparticle removal efficiency [2].

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.

Study of non-spherical bubble oscillations near a surface in a weak acoustic standing wave field.

The interaction of acoustically driven bubbles with a wall is important in many applications of ultrasound and cavitation, as the close boundary can severely alter the bubble dynamics. In this paper, the non-spherical surface oscillations of bubbles near a surface in a weak acoustic standing wave field are investigated experimentally and numerically. The translation, the volume, and surface mode oscillations of bubbles near a flat glass surface were observed by a high speed camera in a standing wave cell at 46.8 kHz. The model approach is based on a modified Keller-Miksis equation coupled to surface mode amplitude equations in the first order, and to the translation equations. Modifications are introduced due to the adjacent wall. It was found that a bubbles oscillation mode can change in the presence of the wall, as compared to the bubble in the bulk liquid. In particular, the wall shifts the instability pressure thresholds to smaller driving frequencies for fixed bubble equilibrium radii, or to smaller equilibrium radii for fixed excitation frequency. This can destabilize otherwise spherical bubbles, or stabilize bubbles undergoing surface oscillations in the bulk. The bubble dynamics observed in experiment demonstrated the same trend as the theoretical results.

Single Bubble Cleaning and Vortex Flow

Ultrasonic cleaning is a well proven technique in many industrial, laboratory and even household applications. It is known that cavitation bubbles can induce fast microscale flows and thus are responsible for cleaning and even corrosion [1,2]. Nevertheless there are numerous effects that can have a potential role in cleaning processes, as the behavior of an acoustic bubble is very complex: radial oscillations, surface oscillations, leading sometimes to the disintegration of a bubble, collapses, rebounds and subsequently shockwaves, liquid jets and vortex flows can be observed. But as bubbles in sound fields typically appear in a random fashion and in complicated interactions, it is very hard to identify the processes and their effects with respect to cleaning. To isolate the various ongoing processes and to study them in detail, single cavitation bubbles and their interaction with a surface are examined in this work. The single bubbles are of sizes around 500 μm in radius and are produced by a pulsed laser that is focused into water, which allows creating bubbles of a repeatable size at a defined position.

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.