Mass transfer enhancement produced by laser induced cavitation

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 not, vert, similar500–1000 ?m). 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 ?m 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

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

Electrodeposition of copper in the presence of an acoustically excited gas bubble

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

An electrochemical and high-speed imaging study of micropore decontamination by acoustic bubble entrapment

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 ?m × 400 ?m (aspect ratio 0.8), 125 ?m × 350 ?m (aspect ratio 2.8) and 50 ?m × 200 ?m (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)6(3-)). 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 ?m 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 study of surface processes under electrochemical control in the presence of inertial cavitation

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

A study investigating the sonoelectrochemical degradation of an organic compound employing Fenton's reagent

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)

Particle induced surface erosion – Tumbling and direct impact; a high-speed electrochemical, acoustic and visual study

A technique that monitors the impedance of a 250 µm diameter aluminium electrode as a function of time with a 2 µs resolution as sand particles (‘slurry erosion’) are impinged on the solid/liquid interface is reported using a submerged jet. The detection of individual particles as they approach an electrode, before any erosion/corrosion was registered, is demonstrated. This study shows that at least two types of erosion mechanisms are possible; direct or ‘primary’ impact and tumbling or ‘scrape’ events. The primary impact events are correlated to the acoustic emission from the environment which is shown to be significant for these events, whereas scrape events appear to produce far weaker acoustic emission signatures under the conditions employed. The velocities of the particles are reported and are of the order of 6–8 m s−1 at the jet mouth. However, high-speed imaging of the particles as they strike the substrate indicates a significant deceleration prior to impact and an order of magnitude reduction in kinetic energy compared to that as it exits the jet

Tailoring lipid crystal networks with high-intensity ultrasound

This chapter describes recent research in the area of lipid sonocrystallization and introduces how this technology can be used to tailor physical and functional properties of lipid crystalline networks. It describes the effects that sonication has on lipid crystallization and its use to tailor lipid crystalline network, and explains some fundamental principles and the physical events associated with high-intensity acoustic waves. Controlling and tailoring lipid crystallization is important in many food products to achieve high-quality products. Research performed over the past decade has shown that high-intensity ultrasound (HIU) with a frequency of approximately 20kHz can be used as a tool to tailor lipid crystal networks by modifying crystallization behavior of edible lipids. The chapter shows that HIU can be used to induce crystallization and to increase crystallization rate. These changes in crystallization behavior generate a harder and more elastic crystalline network characterized by small crystals and a sharp melting profile.</p