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.

In the absence of sufficient cleaning of medical instruments, contamination and infection can result in serious consequences for the health sector and remains a significant unmet challenge. In this paper we describe a novel cleaning system reliant on cavitation action created in a free flowing fluid stream where ultrasonic transmission to a surface, through the stream, is achieved using careful design and control of the device architecture, sound field and the materials employed. Cleaning was achieved with purified water at room temperature, moderate fluid flow rates and without the need for chemical additives or the high power consumption associated with conventional strategies. This study illustrates the potential in harnessing an ultrasonically activated stream to remove biological contamination including brain tissue from surgical stainless steel substrates, S. epidermidis biofilms from glass, and fat/soft tissue matter from bone structures with considerable basic and clinical applications.

This paper reports on noninertial cavitation that occurs beyond the zone close to the horn tip to which the inertial cavitation is confined. The noninertial cavitation is characterized by collating the data from a range of measurements of bubbles trapped on a solid surface in this noninertial zone. Specifically, the electrochemical measurement of mass transfer to an electrode is compared with high-speed video of the bubble oscillation. This gas bubble is shown to be a "noninertial" event by electrochemical surface erosion measurements and "ring-down" experiments showing the activity and motion of the bubble as the sound excitation was terminated. These measurements enable characterization of the complex environment produced below an operating ultrasonic horn outside of the region where inertial collapse can be detected. The extent to which solid boundaries in the liquid cause the frequencies and shapes of oscillatory modes on the bubble wall to differ from their free field values is discussed.

Thin films of (Ba x Sr 1−x ) 1+y Ti 1−y and Zr, Gd codoped (Ba x Sr 1−x ) 1+y Ti 1−y were deposited on platinized sapphire substrates at 640 °C under constant flux of atomic oxygen or a mixture of atomic oxygen and nitrogen to synthesize perovskites of (Ba x Sr 1−x ) 1+y Ti 1−y O 3−δ (also referred to as BSTO), (Ba x Sr 1−x ) 1+y Ti 1−y O 3−z N z (also referred to as BSTON), and Zr, Gd codoped (Ba x Sr 1−x ) 1+y Ti 1−y O 3−z N z . Structural characterization was done via XRD and XPS, and electrical characterization was done via LCR (dielectric properties and tunability under DC bias) and P−E measurements. Although the levels of nitrogen incorporated within the perovskite structure appear to be very low as determined by XPS analysis, definite improvements in dielectric properties and tunability have been achieved by synthesis of the BSTON oxynitride films. For a composition of Ba 0.8 Sr 0.2 TiO 3−z N z an improvement by a factor of 1.72 for tunability and 1.44 for relative permittivity has been observed between the oxide and the oxynitride. The oxynitride achieved a tunability ratio of 6.78 to 1 (close to 7:1) under an applied electrical field of 34 kV/mm. The high throughput approach allowed us to highlight a compositional shift for the material with the best dielectric properties when comparing the oxide with the oxynitride thin films. Effects of codoping the perovskite structure with Zr and Gd have also been investigated and although the tunability and dielectric constant of the thin films were not improved, some improvements in dielectric losses were observed, along with a superparaelectric state as observed by P−E hysteresis measurements.

A cylindrical ultrasonic reactor was driven at eight discrete frequencies in the range 20-150 kHz. Imaging of multibubble sonoluminescence (MBSL) within this cell showed discrete modes of activity throughout this frequency range. This modal activity was compared to the pressure distribution through the cell and also to the erosion/corrosion activity. The erosion/corrosion was detected using an electrochemical method employing a passivated aluminum electrode (250 µm diameter). Each erosion/corrosion event was counted over a fixed time period (specifically 30 s) and used to map this phenomenon throughout a region of the cell. A strong spatial correlation was shown between the MBSL imaging, the acoustic pressure, and the erosion mapping at relatively low ultrasonic frequencies (here <50 kHz). However, at higher frequencies, although MBSL activity and relatively high acoustic amplitudes were detected, the rate of the erosion/corrosion activity of the system decreased. High-speed imaging (>100000 fps) of a bubble cloud near the electrode surface showed a region of bubble activity, the dynamics of which were correlated to the erosion/corrosion transients produced. These observations contribute to the growing body of knowledge which will allow the development of ultrasonic cleaning systems optimized for particular scenarios.