Nanosecond resolution photography system for laser-induced cavitation based on PIV dual-head laser and industrial camera

The detailed study of the initial and collapse processes of the laser-induced cavitation requires nanosecond resolution (both nanoseconds exposure and nanoseconds interframe time) of the photography measurement system. The high-speed video cameras are difficult to achieve nanoseconds interval time. The framing and streak cameras are able to reach the nanosecond resolution, but their complex technology and expensive prices make them far from being commercially available. The present study builds a nanosecond resolution photography system based on PIV dual-head laser and conventional industrial camera. The exposure time of the photography system is controlled by the laser pulse width, which is 5 ns. The two heads of the PIV laser are operated independently thus the smallest time interval between two laser pulses can be set to less than 10 ns. A double-pulse per-exposure imaging technique is used to record the information from two laser pulses on single frame on a low-speed industrial camera. The nanosecond resolution photography system was applied to the laser-induced cavitation experiments to verify the reliability of the measurement results. The measurement of the shock wave velocity demonstrates the ability of the system to capture ultrafast phenomena, which reduces from 3611 m/s to approximately 1483 m/s within 400 ns. The experimental results also reveal the asymmetric evolution of laser-induced cavitation bubbles. The major axis of the ellipsoidal bubble has twice reversals along the laser propagation and perpendicular direction from the laser-induced breakdown to the first collapse.     

Effect of ultrasound and chlorine dioxide on Salmonella Typhimurium and Escherichia coli inactivation in poultry chiller tank water

This study evaluated the application of ultrasound alone or combined with chlorine dioxide (ClO₂) for Salmonella Typhimurium and Escherichia coli inactivation in poultry processing chiller tank water. A Full Factorial Design (FFD) 2² was conducted for each microorganism to evaluate the effect of ultrasound exposure time (x₁: 1 to 9 min; fixed: 37 kHz; 330 W; 25 °C) using a bath, and ClO₂ concentration (x₂: 1 to 17 mg L⁻¹) on microorganism count expressed in log CFU mL⁻¹ in distilled water. Variable x₂ had a negative effect on Salmonella Typhimurium (-5.09) and Escherichia coli (-2.00) count, improving the inactivation; while a x₁ increase present no inactivation improvement, explaining the use of x₁ lower level (1 min) and x₂ higher level (17 mg L⁻¹). The best condition for microorganism inactivation based on FFD was evaluated in chiller tank water (with organic matter) at 25, 16, and 4 °C; x₁ was kept (1 min), however x₂ was adjusted to obtain the same residual free chlorine (2.38 mg L⁻¹) considering the ClO₂ consumption by organic matter, achieving the value of 30 mg L⁻¹. An inactivation of 49% and 31% were observed for Salmonella Typhimurium and Escherichia coli. When ultrasound was replaced by a simple agitation in the presence of ClO₂, there was no inactivation for both microorganisms. Moreover, at poultry carcass pre-chilling (16 °C) and chilling (4 °C) conditions, the synergism of ultrasound combined with ClO₂ was more pronounced, with microorganisms’ reductions up to 100%.     

Unsupported gold nanocones as sonocatalytic agents with enhanced catalytic properties

Gold catalysts have attracted attention for enabling sustainable chemical processes under ambient conditions. This reactivity is attributed to the small size of the catalysts (<5 nm); however, their size also creates difficulty when removing from product streams and often require rare-metal additives to enhance reaction rate kinetics, thereby limiting the environmental benefits of these catalysts. Comparatively, submicron gold catalysts are easier to separate but are much less reactive under ambient conditions. In this study, we synthesized submicron gas-stabilising gold nanocones (gs-AuNCs) that are acoustically responsive to afford greater reaction rates than other conventional gold catalysts. We explore the catalytic performance of acoustically responsive gs-AuNCs exposed to focussed ultrasound at 5.0 MPa peak negative pressure and 1.1 MHz center frequency. Cavitation nucleated from gs-AuNCs significantly increased the sonocatalytic degradation of water pollutants without the need for co-catalysts. The ability to amplify catalysis with ultrasound by tailoring the morphology of the catalyst to control cavitation opens new paths for future designs of sonocatalysts that may enable a sustainable chemical approach needed for a broad range of industrial processes.     

Visualisation and les simulation of cavitation cloud formation and collapse in an axisymmetric geometry

Visualisation and Large Eddy Simulations (LES) of cavitation inside the apparatus previously developed by Franc (2011) for surface erosion acceleration tests and material response monitoring are presented. The experimental flow configuration is a steady-state closed loop flow circuit where pressurised water, flowing through a cylindrical feed nozzle, is forced to turn 90° and then, move radially between two flat plates towards the exit of the device. High speed images show that cavitation is forming at the round exit of the feed nozzle. The cavitation cloud then grows in the radial direction until it reaches a maximum distance where it collapses. Due to the complexity of the flow field, direct observation of the flow structures was not possible, however vortex shedding is inferred from relevant simulations performed for the same conditions. Despite the axisymmetric geometry utilized, instantaneous pictures of cavitation indicate variations in the circumferential direction. Image post-processing has been used to characterize in more detail the phenomenon. In particular, the mean cavitation appearance and the cavity length have been estimated, showing good correlation with the erosion zone. This also coincides with the locations of the maximum values of the standard deviation of cavitation presence. The dominant frequency of the ‘large-scale’ cavitation clouds has been estimated through FFT. Cloud collapse frequencies vary almost linearly between 200 and 2000 Hz as function of the cavitation number and the downstream pressure. It seems that the increase of the Reynolds number leads to a reduction of the collapse frequency; it is believed that this effect is due to the agglomeration of vortex cavities, which causes a decrease of the apparent frequency. The results presented here can be utilized for validation of relevant cavitation erosion models which are currently under development.     

On the possibility of shock-induced cavitation in submerged cylindrical shell systems

A circular cylindrical shell loaded by one or two fluids and responding to an external shock wave is analyzed in the context of the possible inception of shock-induced cavitation. Several scenarios of fluid contact are considered including a submerged evacuated shell and a submerged fluid-filled shell for three different combinations of the parameters of the internal and external fluids. A semi-analytical shell-shock interaction model is employed in order to predict the regions of the fluids where cavitation is likely to occur, and the respective cavitation development is hypothesized about. The most interesting and practically important finding is that when fluid is present both inside and outside the shell, there exist conditions when cavitation is expected to occur in both the internal and external fluid, resulting in a particularly complex and violent structural re-loading occurring upon the collapse of the respective cavitation regions. The inception of cavitation in the internal fluid alone and in the external fluid alone is also possible. The findings are summarized in a manner that is suitable for use at the pre-design stage as a guide for preliminary assessment of the possibility of shock-induced cavitation in fluid-interacting industrial systems.     

Degradation of methylparaben by sonocatalysis using a Co–Fe magnetic carbon xerogel

The degradation of methylparaben (MP) through 20 kHz ultrasound coupled with a bimetallic Co-Fe carbon xerogel (CX/CoFe) was investigated in this work. Experiments were performed at actual power densities of 25 and 52 W/L, catalyst loadings of 12.5 and 25 mg/L, MP concentrations between 1 and 4.2 mg/L and initial pH values between 3 and 10 in ultrapure water (UPW). Matrix effects were studied in bottled water (BW) and secondary treated wastewater (WW), as well as in UPW spiked with bicarbonate, chloride or humic acid. The pseudo–first order kinetics of MP degradation increase with power and catalyst loading and decrease with MP concentration and matrix complexity; moreover, the reaction is also favored at near–neutral conditions and in the presence of dissolved oxygen. The contribution of the catalyst is synergistic to the sonochemical degradation of MP and the extent of synergy is quantified to be >45%. This effect was ascribed to the ability of CX/CoFe to catalyze the dissociation of hydrogen peroxide, formed through water sonolysis, to hydroxyl radicals. Experiments in UPW spiked with an excess of tert-butanol (radical scavenger), sodium dodecyl sulfate or sodium acetate (surfactants) led to substantially decreased rates (i.e. by about 8 times), thus implying that the liquid bulk and the gas-liquid interface are major reaction sites. The stability of CX/CoFe was shown by performing reusability cycles employing magnetic separation of the catalyst after the treatment stage. It was found that the CX/CoFe catalyst can be reused in up to four successive cycles without noteworthy variation of the overall performance of the sonocatalytic process.     

Cavitation onset caused by a dynamic pressure wave in liquid pipelines

When liquids flow in the pipelines, the onset of cavitation can be characterized by a variant of the Euler number known as the cavitation number (CN), which is based on the velocity and denoted by C in this paper. Conventionally, cavitation is considered to be induced when C ~ 1. However, experimental observations and several pipe bursts indicate that the CN may incorrectly predict the onset of cavitation. For example, when leakage occurs in the pipeline or a valve in the pipeline is opened, the resultant pressure loss generates a dynamic pressure wave with a small amplitude, which may lead to bubble formation, even though C ~ 1 is not satisfied. Hence, this paper proposes another CN based on the amplitude of the generated dynamic pressure wave, rather than the velocity, for ascertaining the onset of cavitation. The validity of the proposed CN was verified through experiments and a case study. The results indicated that the proposed CN can be effectively used for cavitation prediction induced by pressure fluctuations and for investigating phenomena such as pressure fluctuation, leakage, and corrosion in liquid pipelines, tanks, and pressure vessels, as well as the safety design of liquefied natural gas tanks and tankers.     

Controlled positioning of microbubbles and induced cavitation using a dual-frequency transducer and microfiber adhesion techniques

We report a study on two methods that enable spatial control and induced cavitation on targeted microbubbles (MBs). Cavitation is known to be present in many situations throughout nature. This phenomena has been proven to have the energy to erode alloys, like steel, in propellers and turbines. It is recently theorized that cavitation occurs inside the skull during a traumatic-brain injury (TBI) situation. Controlled cavitation methods could help better understand TBIs and explain how neurons respond at moments of trauma. Both of our approaches involve an ultrasonic transducer and bio-compatible Polycaprolactone (PCL) microfibers. These methods are reproducible as well as affordable, providing more control and efficiency compared to previous techniques found in literature. We specifically model three-dimensional spatial control of individual MBs using a 1.6 MHz transducer. Using a 100 kHz transducer, we also illustrate induced cavitation on an individual MB that is adhered to the surface of a PCL microfiber. The goal of future studies will involve characterization of neuronal response to cavitation and seek to unmask its linkage with TBIs.     

A computational simulation study on the acoustic pressure generated by a dental endosonic file: Effects of intensity,file shape and volume

One of the uses of ultrasound in dentistry is in the field of endodontics (i.e. root canal treatment) in order to enhance cleaning efficiency during the treatment. The acoustic pressures generated by the oscillation of files in narrow channels has been calculated using the COMSOL simulation package. Acoustic pressures in excess of the cavitation threshold can be generated and higher values were found in narrower channels. This parallels experimental observations of sonochemiluminescence. The effect of varying the channel width and length and the dimensions and shape of the file are reported. As well as explaining experimental observations, the work provides a basis for the further development and optimisation of the design of endosonic files.     

Quantitative analysis of oxidative DNA damage induced by high-voltage pulsed discharge with cavitation

Pulsed discharge is used for sterilization and disinfection, but the details of the molecular mechanisms remain largely unknown. Since pulsed discharge generates reactive oxygen species (ROS), we analyzed the oxidative DNA damages after pulsed discharge treatment to consider the involvement of ROS in the damaging process. We applied pulsed discharge with cavitation to plasmid DNA molecules and estimated the yields of the damages by agarose gel electrophoresis. The treated DNA contained various oxidative DNA damages, including single and double strand breaks and base lesions. The yields of the damages increased in response to the energy used for pulsed discharge. We also measured the yield of 8-hydroxyguanine (8-OH-G), one of the major oxidative base lesions, in the treated plasmid DNA by mass spectrometry quantitatively and found that the yield of the oxidative base lesion corresponded to the increment of the applied energy. In addition, we observed the involvement of mutM gene, which is responsible for repair of 8-OH-G, in the increased sensitivity of Escherichia coli to pulsed discharge. Therefore, ROS seem to mediate the sterilization ability of pulsed discharge.