Cavitation clusters and streamers are characterised in lipid materials (specifically sunflower oil) and compared to water systems. The lipid systems, which are important in food processing, are studied with high-speed camera imaging, laser scattering and pressure measurements. In these oils, clusters formed at an aged (roughened) tip of the sound source (a piston like emitter, PLE) are shown to collapse with varied periodicity in relation to the drive amplitude employed. A distinct streamer (an area of increased flow emanating from the cavitation cluster) is seen in the lipid media which is collimated directly away from the tip of the PLE source whereas in water the cavitation plume is visually less distinct. The velocity of bubbles in the lipid streamer near the cluster on the order of 10 m s−1. Local heating effects, within the streamer, are detected using a dual thermocouple measurement at extended distances. Viscosity, temperature and the outgassing within the oils are suggested to play a key role in the streamer formation in these systems.

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.

An investigation of surface cleaning using a swarm of gas bubbles within an acoustically activated stream is presented. Electrolysis of water at Pt microwires (100 μm diameter) to produce both hydrogen and oxygen bubbles is shown to enhance the extent of ultrasonic surface cleaning in a free flowing water stream containing an electrolyte (0.1 M Na2SO4) and low surfactant concentration (2 mM SDS). The surfactant was employed to allow control of the average size of the bubble population within the swarm. The electrochemical bubble swarm (EBS) is shown to perturb acoustic transmission through the stream. To optimise the cleaning process both the ultrasonic field and the electrochemical current are pulsed and synchronized but with different duty cycles. Cleaning action is demonstrated on structured surfaces (porcine skin and finger mimics) loaded with fluorescent particles. This action is shown to be significantly enhanced compared to that found with an inherent bubble population produced by the flow and acoustic regime alone under the same conditions.

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.

Electrochemical and high-speed imaging techniques are used to study the abilities of ultrasonically-activated bubbles to clean out micropores. Cylindrical pores with dimensions (diameter × depth) of 500 μm × 400 μm (aspect ratio 0.8), 125 μm × 350 μm (aspect ratio 2.8) and 50 μm × 200 μm (aspect ratio 4.0) are fabricated in glass substrates. Each pore is contaminated by filling it with an electrochemically inactive blocking organic material (thickened methyl salicylate) before the substrate is placed in a solution containing an electroactive species (Fe(CN)63−). An electrode is fabricated at the base of each pore and the Faradaic current is used to monitor the decontamination as a function of time. For the largest pore, decontamination driven by ultrasound (generated by a horn type transducer) and bulk fluid flow are compared. It is shown that ultrasound is much more effective than flow alone, and that bulk fluid flow at the rates used cannot decontaminate the pore completely, but that ultrasound can. In the case of the 125 μm pore, high-speed imaging is used to elucidate the cleaning mechanisms involved in ultrasonic decontamination and reveals that acoustic bubble entrapment is a key feature. The smallest pore is used to explore the limits of decontamination and it is found that ultrasound is still effective at this size under the conditions employed.

The flow patterns generated by a pulsating jet used to study hydrodynamic modulated voltammetry (HMV) are investigated. It is shown that the pronounced edge effect reported previously is the result of the generation of a vortex ring from the pulsating jet. This vortex behaviour of the pulsating jet system is imaged using a number of visualisation techniques. These include a dye system and an electrochemically generated bubble stream. In each case a toroidal vortex ring was observed. Image analysis revealed that the velocity of this motion was of the order of 250 mm s−1 with a corresponding Reynolds number of the order of 1200. This motion, in conjunction with the electrode structure, is used to explain the strong ‘ring and halo’ features detected by electrochemical mapping of the system reported previously.Highlights► High-speed images of the flow generated from a pulsating jet used to study HMV are shown. ► Dye solutions or bubble tracers show the flow under different conditions. ► A repetitive vortex is studied as it moves through the fluid and onto an electrode surface. ► This vortex, and its characteristics, produces the spatial mass transfer pattern observed.

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.

The electrochemistry of Pt nanostructured electrodes is investigated using hydrodynamic modulated voltammetry (HMV). Here a liquid crystal templating process is used to produce platinum-modified electrodes with a range of surface areas (roughness factor 42.4−280.8). The electroreduction of molecular oxygen at these nanostructured platinum surfaces is used to demonstrate the ability of HMV to discriminate between faradaic and nonfaradaic electrode reactions. The HMV approach shows that the reduction of molecular oxygen experiences considerable signal loss within the high pseudocapacitive region of the voltammetry. Evidence for the contribution of the double layer to transient mass transfer events is presented. In addition, a model circuit and appropriate theoretical analysis are used to illustrate the transient responses of a time variant faradaic component. This in conjunction with the experimental evidence shows that, far from being a passive component in this system, the double layer can contribute to HMV faradaic reactions under certain conditions.

An electrochemical technique that can detect inertial cavitation within an ultrasonic reactor is reported. The technique relies on the erosion and repassivation of an oxide covered electrode (specifically aluminum). The sensitivity of the technique (<46 fg per erosion event) is significantly greater than normal weight loss measurements. A novel opto-isolation system is discussed which enables the electrochemical measurements to be undertaken within an earthed metallic container. Events detected in this manner are reported and compared to the noise in the absence of appropriate isolation. This system is combined with a multichannel analyzer to map the erosion/corrosion activity within an operating ultrasonic bath.

The electrochemistry of nanostructured electrodes is investigated using hydrodynamic modulated voltammetry (HMV). Here a liquid crystal templating process is used to produce a platinum modified electrode with a relatively high surface area (Roughness factor, Rf = 42.4). The electroreduction of molecular oxygen at a nanostructured platinum surface is used to demonstrate the ability of HMV to discriminate between Faradaic and non-Faradaic electrode reactions. The HMV approach shows that the reduction of molecular oxygen shows considerable hysteresis correlating with the formation and stripping of oxide species at the platinum surface. Without the HMV analysis it is difficult to discern the same detail under the conditions employed. In addition the detection limit of the apparatus is explored and shown, under ideal conditions, to be of the order of 45 nmol dm−3 employing [Fe(CN)6]4− as a test species.

A new system for the generation of hydrodynamic modulated voltammetry (HMV) is presented. This system consists of an oscillating jet produced through the mechanical vibration of a large diaphragm. The structure of the cell is such that a relatively small vibration is transferred to a large fluid flow at the jet outlet. Positioning of an electrode (Pt, 0.5 mm or 25 μm diameter) over the exit of this jet enables the detection of the modulated flow of liquid. While this flow creates modest mass transfer rates (time averaged ∼0.015 cm s−1) it can also be used to create a HMV system where a ‘lock-in’ approach is adopted to investigate the redox chemistry in question. This is demonstrated for the Fe(CN)63-/4- redox system. Here ‘lock-in’ to the modulated hydrodynamic signal is achieved through the deployment of bespoke software. The apparatus and procedure is shown to produce a simple and efficient way to obtain the desired signal. In addition the spatial variation of the HMV signal, phase correction and time averaged current with respect to the jet orifice is presented.

Copper has been electrodeposited in the presence of an acoustically excited gas bubble (Ar bubbles with radii ∼1.5 mm held below a copper plate). Under the conditions employed, an acoustic pressure amplitude of 69.5 Pa is sufficient to excite multiple surface wave modes on the bubble wall. This is observed using high-speed imaging. This oscillation generates significant micromixing, which brings fresh electrolyte to the electrode surface leading to an enhanced deposition current. Scanning electron microscopy reveals radial streaming patterns in the resulting copper deposit. Experiments carried out using a lower acoustic pressure amplitude of 50.5 Pa (such that only the Faraday wave is excited) exhibit a lesser degree of streaming and mass transfer enhancement. No significant spatially averaged current enhancement is seen if the bubble is only undergoing breathing mode oscillation.

A microelectrode is used to measure the mass transfer perturbation and characteristics during the growth and subsequent collapse of a single bubble (which, following its initial expansion, achieved a maximum radius, Rm, of ∼500–1000 μm). This mass transfer enhancement was associated with the forced convection, driven by bubble motion, as the result of a single cavitation event generated by a laser pulse beneath a 25 μm diameter Au microelectrode. Evidence for bubble growth and rebound is gained from the electrochemical and acoustic measurements. This is supported with high-speed video footage of the events generated. A threshold for the formation of large cavitation bubbles in electrolyte solutions is suggested.

In some circumstances, the erosive effects of inertial (transient) cavitation have been usefully employed in the acceleration of chemical processes that are dependent on surface reactions. However, in other situations the erosion of materials can be detrimental. For example, problematic erosion/corrosion phenomena have been well documented. It will be demonstrated here that the employment of inertial cavitation can be beneficial to the study of surface processes and indeed has a number of advantages. These include rapid erosion and the removal of small quantities of the surface. To highlight these effects, high-temporal resolution of the re-oxidation transients produced from a passivated microelectrode placed within a cavitation cloud will be reported. These will be compared to the multi bubble sonoluminescence (MBSL) output of the cell.

This paper describes the approach to bubble related phenomena using a novel `acoustoelectrochemical' technique designed to investigate the physical and chemical effects of the acoustically induced motion of the bubble wall. In particular it describes the behaviour of a suspended gas bubble irradiated with sound of an appropriate frequency and pressure to induce bubble wall oscillation.The first electrochemical measurement of the growth of a bubble through rectified diffusion is demonstrated. The technique employed relies on the sensitivity of a scanning electrochemical microscope (SECM) deployed close to the gas/liquid interface of a bubble. The growth rate of the bubble (<0.1 μm s−1) is reported. It will be also demonstrated that gas exchange across the phase boundary at the bubble wall, can be successfully probed when the bubble is stationary.

A novel dual microelectrode system has been developed to study the effects of cavitation at the solid/liquid interface. By sealing lead and platinum microelectrodes in close proximity, the mass transfer and surface effects from the same inertial cavitation event have been recorded simultaneously for the first time. A number of advantages of the system have been outlined. In addition supporting evidence for an erosion/corrosion mechanism on the lead electrode is reported.

The characterisation of a small sonochemical reactor has been performed using electrochemical, luminescent and photographic techniques. The electrochemical experiments have employed a novel flow system to determine the formation of sonochemical products (in this case hydrogen peroxide) in semi-real time with high sensitivity. The rate of production of hydrogen peroxide is reported as a function of driving pressure amplitude. The degradation of an organic molecule, specifically the organic dye amaranth, within the sonochemical cell is also reported.

The interaction of a tethered bubble with sound is demonstrated using novel electrochemical characterisation technology. A 25 μm diameter microelectrode, positioned close to the gas/liquid interface is used to monitor the motion of the bubble wall as a function of time in the presence and absence of sonic irradiation. Evidence for ‘breathing’ mode oscillation of the bubble and its effect on mass transfer to the microelectrode is presented.

A variety of reactions, which are known to be enhanced or driven by sonochemical effects, have been studied and their absolute rate measured as a function of the ultrasonic frequency employed within a cylindrical reactor. The rate is shown to be highly dependent on the ultrasonic frequency employed in the range of 20 kHz to 160 kHz. The frequency dependence of the system is the net result of the frequency dependencies of the transducer, the reverberant sound field, the cavitation dynamics and the chemistry. Rate variation of the reactions studied is correlated to light emission (sonoluminescence) as a function of the acoustic driving frequency with a resolution down to 1 kHz. The results are discussed with reference to the acoustic characteristics (particularly the modal nature) of the cell employed. The results are compared to the spatial peak acoustic pressure amplitude within the cell and broadband audio emission. Chemical activity could be predicted by sonoluminescence activity, which correlated with the more spatially complex sound field produced at higher frequencies. The most important finding is that characterisation of the sound field is vital in sonochemical experiments: a <3% change in the driving frequency was found to change the chemical activity by 3 orders of magnitude, because of the tuning effect of the modal sound field.

The degradation of an organic dye molecule (specifically meldola blue, MDB) is reported under the influence of power ultrasound in combination with electrochemically-generated hydrogen peroxide. A novel flow system is employed to measure the degradation as a function of time while minimising the disturbance to the acoustics of the sonoelectrochemical reactor employed. The effect of adding Fe2+ to the rate of dye degradation is measured and demonstrated to be significant. Under optimum conditions the rate constant for dye degradation was found to reach a maximum value of (23.7±0.35)×10−3 min−1 assuming pseudo-first order kinetics. The rate constant for the complete destruction of MDB, determined by chemical oxygen demand, was found to be significantly slower at (10.2±2.6)×10−3 min−1.

Electrochemical evidence of H˙ produced by cavitation as the result of ultrasonic irradiation of an aqueous solution is presented.

Efficient mass transfer enhancements as the result of acoustically oscillated gas bubbles are detected using a microelectrode positioned at variable distances from the gas/liquid interface.

The electrochemical reduction of oxygen is studied using electrochemically deposited mesoporous platinum microelectrodes, which exhibit efficient mass transfer of material to the electrode surface and accelerated reduction kinetics when compared to polished microelectrodes.

A new method to detect the uncompensated resistance, the capacitance and the Faradaic current at an electrode exposed to ultrasonic cavitation is presented. The method enables these parameters to be resolved with a 2 microsecond resolution and relies on the detection of the impedance of an electrode recorded as a function of time with a suitable AC excitation signal (here 500 kHz). Data obtained from an aluminium electrode, held under potentiostatic control, is used to illustrate the technique with particular relevance to the effects of cavitation bubbles generated by ultrasound. Analysis of the data recorded shows that the cavitation bubbles form close to the surface of the electrode and collapse, causing damage to the passive film formed at the aluminium surface. The capacitance, uncompensated resistance and Faradaic signals are used to explore the dynamic processes and show expansion and collapse of bubbles prior to erosion/corrosion. The close proximity of the bubbles to the surface is deduced from the reductions in capacitance and increases in resistance prior to bubble collapse, which is then shown to trigger the onset of a Faradaic signal, thus confirming the erosion/corrosion mechanism previously assumed.

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.

Electrochemical and high-speed imaging techniques are used to study the abilities of ultrasonically-activated bubbles to clean out micropores. Cylindrical pores with dimensions (diameter × depth) of 500 μm × 400 μm (aspect ratio 0.8), 125 μm × 350 μm (aspect ratio 2.8) and 50 μm × 200 μm (aspect ratio 4.0) are fabricated in glass substrates. Each pore is contaminated by filling it with an electrochemically inactive blocking organic material (thickened methyl salicylate) before the substrate is placed in a solution containing an electroactive species (Fe(CN)63−). An electrode is fabricated at the base of each pore and the Faradaic current is used to monitor the decontamination as a function of time. For the largest pore, decontamination driven by ultrasound (generated by a horn type transducer) and bulk fluid flow are compared. It is shown that ultrasound is much more effective than flow alone, and that bulk fluid flow at the rates used cannot decontaminate the pore completely, but that ultrasound can. In the case of the 125 μm pore, high-speed imaging is used to elucidate the cleaning mechanisms involved in ultrasonic decontamination and reveals that acoustic bubble entrapment is a key feature. The smallest pore is used to explore the limits of decontamination and it is found that ultrasound is still effective at this size under the conditions employed.

An investigation of surface cleaning using a swarm of gas bubbles within an acoustically activated stream is presented. Electrolysis of water at Pt microwires (100 μm diameter) to produce both hydrogen and oxygen bubbles is shown to enhance the extent of ultrasonic surface cleaning in a free flowing water stream containing an electrolyte (0.1 M Na2SO4) and low surfactant concentration (2 mM SDS). The surfactant was employed to allow control of the average size of the bubble population within the swarm. The electrochemical bubble swarm (EBS) is shown to perturb acoustic transmission through the stream. To optimise the cleaning process both the ultrasonic field and the electrochemical current are pulsed and synchronized but with different duty cycles. Cleaning action is demonstrated on structured surfaces (porcine skin and finger mimics) loaded with fluorescent particles. This action is shown to be significantly enhanced compared to that found with an inherent bubble population produced by the flow and acoustic regime alone under the same conditions.

Cavitation clusters and streamers are characterised in lipid materials (specifically sunflower oil) and compared to water systems. The lipid systems, which are important in food processing, are studied with high-speed camera imaging, laser scattering and pressure measurements. In these oils, clusters formed at an aged (roughened) tip of the sound source (a piston like emitter, PLE) are shown to collapse with varied periodicity in relation to the drive amplitude employed. A distinct streamer (an area of increased flow emanating from the cavitation cluster) is seen in the lipid media which is collimated directly away from the tip of the PLE source whereas in water the cavitation plume is visually less distinct. The velocity of bubbles in the lipid streamer near the cluster on the order of 10 m s−1. Local heating effects, within the streamer, are detected using a dual thermocouple measurement at extended distances. Viscosity, temperature and the outgassing within the oils are suggested to play a key role in the streamer formation in these systems.