Douglas G. Offin

Multiple observations of cavitation cluster dynamics close to an ultrasonic horn tip

Bubble dynamics in water close to the tip of an ultrasonic horn (∼23 kHz, 3 mm diameter) have been studied using electrochemistry, luminescence, acoustics, light scattering, and high-speed imaging. It is found that, under the conditions employed, a large bubble cluster (∼1.5 mm radius) exists at the tip of the horn. This cluster collapses periodically every three to four cycles of the fundamental frequency of the horn. Following the collapse of the cluster, a short-lived cloud of small bubbles (each tens of microns in diameter) was observed in the solution. Large amplitude pressure emissions are also recorded, which correlate temporally with the cluster collapse. Bursts of surface erosion (measured in real time using an electrochemical technique) and multibubble sonoluminescence emission both also occur at a subharmonic of the fundamental frequency of the horn and are temporally correlated with the bubble cluster collapse and the associated pressure wave emission.

Experimental and theoretical characterisation of sonochemical cells.

Cavitation theory is used to predict the acoustic pressure at the boundary of the inertial/non inertial threshold for a range of bubble sizes. The sound field generated from a commonly employed sonoelectrochemical cell is modelled. The model is tested with a calibrated hydrophone far from the transducer to avoid spatial averaging. This allows the model to provide the absolute pressure amplitude as a function of axial distance from the source. An electrochemical technique for detecting both inertial and non-inertial cavitation within the solution is employed. This technique uses a dual microelectrode to map the boundary between the regions where inertial cavitation occurs (associated with surface erosion), and where it does not. This zone occurs close to the transducer for the microelectrode employed (<1.5 mm). Further characterisation of the inertial cavitation zone is achieved by imaging of multibubble sonoluminescence (MBSL). The pressures at the boundary between inertial and non inertial cavitation that are determined from the electrochemical and imaging experiments are compared to a sound field model and cavitation theory. Qualitative arguments for the invasive nature of the electrode into the sound field are proposed. Evidence for cavity cluster collapse and shock wave emission is presented and discussed in relation to luminescence, the electrochemical experiments and cavitation theory.

Removal of Dental Biofilms with an Ultrasonically Activated Water Stream

Acidogenic bacteria within dental plaque biofilms are the causative agents of caries. Consequently, maintenance of a healthy oral environment with efficient biofilm removal strategies is important to limit caries, as well as halt progression to gingivitis and periodontitis. Recently, a novel cleaning device has been described using an ultrasonically activated stream (UAS) to generate a cavitation cloud of bubbles in a freely flowing water stream that has demonstrated the capacity to be effective at biofilm removal. In this study, UAS was evaluated for its ability to remove biofilms of the cariogenic pathogen Streptococcus mutans UA159, as well as Actinomyces naeslundii ATCC 12104 and Streptococcus oralis ATCC 9811, grown on machine-etched glass slides to generate a reproducible complex surface and artificial teeth from a typodont training model. Biofilm removal was assessed both visually and microscopically using high-speed videography, confocal scanning laser microscopy (CSLM), and scanning electron microscopy (SEM). Analysis by CSLM demonstrated a statistically significant 99.9% removal of S. mutans biofilms exposed to the UAS for 10 s, relative to both untreated control biofilms and biofilms exposed to the water stream alone without ultrasonic activation (P < 0.05). The water stream alone showed no statistically significant difference in removal compared with the untreated control (P = 0.24). High-speed videography demonstrated a rapid rate (151 mm2 in 1 s) of biofilm removal. The UAS was also highly effective at S. mutans, A. naeslundii, and S. oralis biofilm removal from machine-etched glass and S. mutans from typodont surfaces with complex topography. Consequently, UAS technology represents a potentially effective method for biofilm removal and improved oral hygiene.

An activated fluid stream – new techniques for cold water cleaning

Electrochemical, acoustic and imaging techniques are used to characterise surface cleaning with particular emphasis on the understanding of the key phenomena relevant to surface cleaning. A range of novel techniques designed to enhance and monitor the effective cleaning of a solid/liquid interface is presented. Among the techniques presented, mass transfer of material to a sensor embedded in a surface is demonstrated to be useful in the further exploration of ultrasonic cleaning of high aspect ratio micropores. In addition the effect of micropore size on the cleaning efficacy is demonstrated. The design and performance of a new cleaning system reliant on the activation of bubbles within a free flowing stream is presented. This device utilised acoustic activation of bubbles within the stream and at a variety of substrates. Finally, a controlled bubble swarm is generated in the stream using electrolysis, and its effect on both acoustic output and cleaning performance are compared to the case when no bubbles are added. This will demonstrate the active role that the electrochemically generated bubble swarm can have in extending the spatial zone over which cleaning is achieved.

Development and first results of a new mesoporous microelectrode DO-sensor

A new dissolved oxygen (DO) sensor for marine application was developed and first tests in the field were carried out. The sensor uses a mesoporous microelectrode as the sensing element that has an inherently fast response and an increased resistivity to bio-fouling due to the properties of the electrode material, mesoporous platinum. Introduction of a cleaning cycle, where the potential applied to the electrode is altered, allows the elimination of material build up at the electrode surface as well as the reconditioning of the electrode-surface, thus minimising drift and the effect of bio-fouling. Mesoporous microdisc electrodes ranging between 10 and 50 /spl mu/m diameter have been tested. The sensor has been adapted to operate on a CTD system. Results of two test cruises are presented, showing that although the general sensor performance is good further work has to be done to exploit the inherently fast response of the microelectrodes.