Gas content strongly affects cavitation dynamics; however, most studies rely solely on dissolved gas measurements, overlooking the influence of undissolved bubbles. This study investigates their role by introducing air bubbles (<200 µm) into water samples with identical dissolved gas levels and analyzing cavitation at both inception and developed phases using high-speed imaging and hydrophone measurements. The results show that the presence of pre-existing bubbles alters cavitation dynamics at the inception and developed phases. Under low dissolved gas and in the absence of air bubbles, cavitation can initiate from a single nucleus, and the developed phase exhibits transient vaporous cavitation with the highest acoustic intensity. In contrast, pre-existing bubbles promote the formation of conical-like bubble structures early in the inception phase and affect their dynamics in the developed phase, reducing the acoustic pressure and attenuating the noise spectrum. These effects are reversible upon bubble removal and independent of dissolved gas concentration, demonstrating that dissolved gas alone cannot represent gas-related influences on cavitation. Characterizing not only dissolved but also undissolved gas content is therefore essential for cavitation studies and applications.

The treatment of dye-contaminated wastewater remains a significant challenge for environmental management due to the high stability and persistence of many azo dyes. The purpose of this study was to develop and evaluate a novel hybrid rotor-stator hydrodynamic cavitation non-thermal plasma device designed to enhance the decolorization of Reactive Red 120. The configuration integrates atmospheric-pressure plasma directly into a cavitating flow field, attempting to increase gas-liquid interfacial contact and promote mass transfer of reactive species into the liquid. The device performance was investigated by assessing the effects of various parameters on H2O2 production, pH, conductivity, and energy consumption. The results show generation of reactive species at all tested parameters, with higher H2O2 production in deionized compared to tap water. Rotational speed, discharge voltage and air flow rate all influenced H2O2 production and the highest concentration achieved was 9.9 mg/L in 15 min. Decolorization was evaluated at three initial concentrations and followed apparent pseudo first-order kinetics, with higher degradation rates observed at lower initial dye concentrations (0.0273, 0.0234 and 0.0168/min at 25, 50 and 100 mg/L, respectively). The study demonstrates that coupling of hydrodynamic cavitation with non-thermal plasma produces synergistic effects and has potential to treat complex wastewaters.

Nanocellulose dispersions and hydrogels represent a promising class of sustainable soft materials with dynamic responsiveness to external stimuli. This study employs visualization, statistical analysis, and particle image velocimetry to investigate the dynamic behavior of cationic (CCNF) and TEMPO-oxidized (TOCNF) cellulose nanofibril dispersions under sonication, focusing on hydrogel network formation at varying concentrations (0.5-2.0 wt% for CCNF and 1.0-2.0 wt% for TOCNF). CCNF dispersions rapidly formed physically crosslinked layers even at low concentrations due to attractive interactions via quaternary ammonium groups, resulting in persistent hydrogel networks. In contrast, TOCNF required concentrations over 1.3 wt% to overcome electrostatic repulsion between carboxylate groups and formed weaker, transient gels. Image sequence analysis revealed that crosslinked layer thickness and lifetime increased with concentration for both nanocellulose types, with 2.0 wt% samples exhibiting robust, resilient hydrogel structures throughout sonication. While CCNF showed resistance to ultrasonic disruption, TOCNF networks degraded rapidly due to weaker intermolecular interactions. Results highlight how nanofibril surface chemistry and concentration govern the interplay between ultrasound-induced network formation and its destabilization. The study provides mechanistic insights into sonication-driven gelation and establishes a methodological framework for designing nanocellulose-based materials with tailored real-time structural responsiveness, bridging nanostructural dynamics and macroscopic behavior for advanced soft material applications.

This article investigates the development and evaluation of a prototype hydrodynamic cavitation generator, driven by the electromagnetically induced linear motion of a hollow piston. This so-called magnetic generator of hydrodynamic cavitation (MGHC) is designed for applications requiring small liquid volumes, with its simplicity being a key advantage. It does not require additional pumps or seals and provides a high level of sample protection against contamination. The electromagnetic drive is designed to generate a strong magnetic field, allowing for rapid acceleration of the piston and periodic generation of pressure pulsations associated with the collapse of cavitation clouds throughout the control volume of the device.In the study, the electrical parameters of the electromagnetic drive, such as voltage, current, and frequency, were adjusted to modify the piston’s motion characteristics, thereby influencing the intensity of cavitation, as reflected in the generation of intense pressure oscillations. Simultaneous pressure measurements and visualisation of cavitation structures show that the piston motion induces intense hydrodynamic cavitation along its entire stroke path, particularly during the collapse phases of cavitation clouds. The results offer qualitative and quantitative analysis of the measured variable interdependence and provide a foundation for parametric studies across different spatial and temporal scales aimed at optimising the device in the future.

HYPOTHESIS:Cavitation-driven emulsification is governed by bubble collapse and microjet formation at the liquid-liquid interface, which in turn is correlated with the system's hydrophilic-lipophilic balance (HLB). Modifying the HLB value through the use of surfactants affects interfacial properties, one of which is the static interfacial tension between liquids. This might change emulsification pathways and consequently the type of formed emulsion. EXPERIMENTS:The HLB value of an oil-water system was systematically tailored using surfactants Tween 80 and Span 80, which were separately dissolved in demineralized water and silicone oil, respectively. The resulting systems were first characterized in terms of viscosity, surface tension, and static oil-water interfacial tension. Single laser-induced cavitation bubbles were then formed at various distances from the liquid-liquid interface, and their collapse dynamics were characterized using high-speed visualization. Bubble dynamics through bubble centroid displacement were correlated with the HLB value of the respective system. FINDINGS:In systems with higher HLB values (≥ 12.9), bubbles always collapsed away from the oil-water interface, following classical density-driven jetting behaviour. Intermediate values (between 10.7 and 6.4) produced bubble collapse both towards and away from the interface. At the lowest HLB value of 4.3, bubbles collapsed towards the oil-water interface, a reversal of the expected jetting direction. These indicate different emulsification pathways, which are consistent with typical HLB ranges for surfactant applications. This demonstrates that in cavitation-driven emulsification, the system's HLB value affects directly the collapse dynamics of cavitation bubbles, and consequently, the type of emulsion they produce.

Ultrasonic emulsification is a widely used technique for generating fine dispersions of immiscible liquids, yet its underlying mechanisms remain only partially understood, particularly under reduced-gravity conditions. In the absence of gravity, buoyancy-driven phase separation vanishes, fundamentally altering the organization of liquid-liquid interfaces and their interaction with cavitation structures. This study investigates the dynamics of ultrasonic emulsification in microgravity using high-speed visualization during parabolic flight experiments, with systematic variation of oil-water volume ratios. The results reveal that microgravity leads to capillarity-dominated interface configurations, where the position and stability of the oil-water interface become less predictable compared to normal gravity conditions. As a consequence, sustained interaction between the cavitation zone and the interface is more difficult to achieve. Since such interaction was previously identified as the key mechanism driving droplet breakup, its reduction leads to slower emulsification rates and lower final emulsion homogeneity. Image analysis shows that emulsification under normal gravity proceeds faster and produces more homogeneous dispersions, while microgravity conditions lead to delayed evolution and lower overall homogeneity. The findings demonstrate that ultrasonic emulsification in microgravity is governed by an interplay between interface topology and cavitation dynamics and highlight the importance of controlled interface positioning for enabling efficient multiphase fluid processing in space environments.

In this study, laboratory-scale Pinned Disc Rotary Generator of Hydrodynamic Cavitation was used to treat waste-activated sludge with a Total Solids concentration of 0.7 %. Five different rotor-stator arrangements were tested, focusing on waste-activated sludge physicochemical and rheological parameters of industrial relevance: general chemical analysis, rheometry, dewaterability, interfacial tension, UV-Vis and FTIR spectroscopy. Radical formation in all five arrangements was confirmed using salicylic acid dosimetry before sample testing. Three of the arrangements generated twice the radical concentration of the other two and achieved a disintegration degree three times higher (17 % compared to 5 %). Capillary Suction Time tests demonstrated a 14-fold reduction in filterability across all arrangements, accompanied by an increase in interfacial tension exceeding 10 %. Statistically significant changes in the UV-Vis spectra indicated alterations in dissolved organic matter humification, aromaticity, and molecular size of colorimetric dissolved organic matter, DNA, and RNA. FTIR analysis revealed characteristic peaks at 1537 cm(-1) and 1648 cm(-1), signifying microbial cell wall damage. Rheological analysis showed a reduction in apparent viscosity within the low shear stress zone (tau < 5 Pa) and a shift in the yield stress point to lower shear stresses (tau < 0.14 Pa compared to tau = 0.17 Pa for the untreated samples). Pearson's correlation test revealed strong, statistically significant correlations between cell wall damage (as identified by FTIR) and hydrodynamic conditions in the reactor, while the correlation with radical formation was not statistically significant. This suggests that hydrodynamic forces were the primary drivers of cell wall damage, with potential synergetic effects from radicals.

Cavitation is a phase change phenomenon that generates highly energized bubbles due to low local pressures. The collapse of these bubbles releases this energy to the surrounding area in different forms upon the pressure recovery. Free radical production, which is considered as chemical effect of the bubble collapse, plays a major role in many applications, from wastewater treatment to material exfoliation. Although some studies underscore the importance of chemical effects for acoustic cavitation (AC), their investigations in hydrodynamic cavitation (HC) are challenging due to the difficulty in controlling cavitating flows. One of the approaches that could shed light on this challenging aspect is to shrink the reactor scale to micro-scale size ("HC on a chip"). In this regard, we investigated the chemical effects of HC using Salicylic Acid (SA) dosimetry in three different micro-scale designs (long diaphragm, micro-orifice, and micro-venturi configurations) and compared the results to those of a macro-scale HC reactor. High-speed visualization revealed important links between flow patterns and the formation of hydroxyl radicals (center dot OH), which contributed to the SA products. This study thus focused on comparing the effectiveness of the three micro-scale reactors in terms of center dot OH formation. According to the results, the "HC on a chip" concept demonstrated significantly higher efficiency in generating SA products compared to the macro-scale HC reactor. For instance, the micro-scale HC reactors achieved an SA concentration of approximately 0.6 mu g/mL in just 5 cycles, while the macro-scale HC reactor required 164 cycles to reach a similar concentration (0.45 mu g/mL). This substantial reduction in the number of cycles highlights the potential of micro-scale HC reactors for efficient and rapid generation of SA products.

In this work, we experimentally and numerically investigate cavitation bubble dynamics in a thin liquid layer surrounded by gas. We focus on configurations featuring strongly confined bubbles at dimensionless bubble-free surface stand-off distances D* below unity. Additionally, we impose the condition of null Kelvin impulse, subjecting a bubble to the oppositely equal influence of two opposing free surfaces, resulting in the formation of two convergent water jets. We observe a diverse spectrum of jetting phenomena, including broad jets, mushroom-capped jets, and cylindrical jets. These jets become progressively thinner and faster with lower D* values, reaching radii as small as 3% of the maximal bubble radius and speeds up to 150 m/s. Numerical results reveal a linear relationship between the jet impact velocity and the local curvature at the bubble region proximal to the free surface. This suggests that the magnitude of bubble deformation during its growth phase is the primary factor influencing the observed fivefold increase in the jet impact velocity in the parameter space considered. Our findings show that bubble collapse intensity is progressively dampened with increased confinement of its environment. As D* decreases beyond a critical value, the liquid layer separating the bubble and ambient air thins, leading to the onset of interfacial shape instabilities, its breakdown, and bubble atomization. Furthermore, we compare bubbles at zero Kelvin impulse to corresponding anisotropic scenarios with a single free surface, revealing that the dynamics of axial jets until the time of impact is primarily influenced by the proximal free surface. The impact of convergent axial jets at null Kelvin impulse results in local pressure transients up to 100 MPa and triggers the formation of a fast and thin annular outflow in the form of a liquid sheet, affected by the Rayleigh–Plateau and flapping shape instability.

Traditional methods for algae removal in drinking water treatment, such as coagulation and sedimentation, face challenges due to the negative charge on algae cells’ surfaces, resulting in ineffective removal. Ultrasonic cavitation has shown promise in enhancing coagulation performance by disrupting extracellular polymer structures and improving cyanobacteria removal through various mechanisms like shear force and free radical reactions. However, the short lifespan and limited mass transfer distance of free radicals in conventional ultrasonic treatment lead to high energy consumption, limiting widespread application. To overcome these limitations and enhance energy efficiency, advanced carbon-based materials were developed and tested. Nitrogen-doped functional groups on nanodiamond surfaces were found to boost sonosensitivity by increasing the production of reactive oxygen species at the sonosensitizer-water interface. Utilizing low-power ultrasound (0.12 W/mL) in combination with N-ND treatment for 5 min, removal rates of Microcystis aeruginosa cells in water exceeded 90 %, with enhanced removal of algal organic matters and microcystins in water. Visualization through confocal microscopy highlighted the role of positively charged nitrogen-doped nanodiamonds in aggregating algae cells. The synergy between cell capturing and catalysis of N-ND indicates that efficient mass transfer of free radicals from the sonosensitizer’s surface to the microalgae’s surface is critical for promoting cyanobacteria floc formation. This study underscores the potential of employing a low-intensity ultrasound and N-ND system in effectively improving algae removal in water treatment processes.

The persistence and toxicity of hazardous pollutants present in wastewater effluents require the development of efficient and sustainable treatment methods to protect water resources. In this study, the efficacy and efficiency of a novel combination of two advanced oxidation processes - sub-atmospheric-pressure plasma and hydrodynamic cavitation - were systematically tested for the removal of valsartan (VAL), sulfamethoxazole, trimethoprim, naproxen, diclofenac (DF), tramadol, propyphenazone, carbamazepine, 17(3-estradiol (E2) and bisphenol A (BPA). The results show that both sample temperature and plasma power play a role and the highest removal, from 29-99 %, was achieved at 25 degrees C and 53 W of plasma power. E2, BPA, DF, and VAL were removed to the highest degree. These results are particularly important in the case of E2 and BPA, whose harmful environmental effects may start to occur already at sub-ng/L to mu g/L levels. The differences in the removals obtained depend strongly on the physicochemical properties, and the compounds with the highest logKOW were removed to the highest extent. The energy yield, in terms of plasma power, was between 1 and 26 mg/kWh under optimal experimental conditions. Our results show that the novel plasma-cavitation treatment shows potential that could prove valuable for upcoming regulatory requirements.

Droplet deposition with material-jetting methods such as thermoplastic 3D printing (T3DP) depends greatly on the rheological properties of the feedstocks. This study investigated the effect of particle interactions and the degree of weak flocculation on the shear thinning behaviour, the yield stress and the storage/loss moduli of paraffin-wax-based feedstocks containing 40 vol.% of zirconia (3Y-TZP) micron-sized powder. Steric stabilization of the feedstocks was provided by varying the ratios of the surfactants with different chain lengths, i.e., stearic acid (2.4nm) and Solsperse 3000® (10nm), which in turn affected the dynamics of the droplet formation and the quality of the layers when jetting non-Newtonian, thermoplastic ceramic feedstocks. The results of the study extend the guidelines for the processing of printable feedstocks used in T3DP additive manufacturing.

Sewage sludge (SS) is rich in plant nutrients, including P, N, and organic C, but often contains toxic metals (TMs), which hinders its potential use in agriculture. The efficiency of removal of TMs by washing with ethylenediamine tetraacetate (EDTA), in combination with hydrodynamic cavitation (HC) and the usability of washed sewage sludge as fertilizer were investigated. The environmental risk was assessed. During 8 wash batches an average 35, 68, 47 and 45 % of Pb, Zn, Cd and Cu, respectively, as well as 22 and 5 % Mn and Fe were removed from the SS. The process solutions and EDTA were recycled at a pH gradient of 12.5 -2, which was achieved by adding quicklime (CaO) and then acidification by H 2 SO 4 , so that no wastewater was produced, only solid waste (ReSoil (R) method). The quality of the recycled process solutions (they remained unsaturated with salts) and the efficiency of the washing process were maintained across all batches. On average, 46 % of the EDTA was lost during the process and was replenished. The initial leachability of EDTA-mobilized Pb, Zn, Cu, Cr and Fe remaining in the washed SS increased 6-, 17-, 3-, 11- and 11 -fold, respectively, but not to hazardous levels except for Zn. After washing, P and K remained in the SS, plant -available P increased 3.3 -fold, while total N and C were reduced by 20.28 and 2.44 %, respectively. Washed SS was used as fertilizer in the pot experiment. The yield of Brassica juncea did not improve, the uptake of TMs by the plants and the leaching of TMs from the soil were minimal. Our study highlighted the drawbacks and potential feasibility of the new SS washing method.

The paper investigates the oil–water emulsification process inside a micro-venturi channel. More specifically, the possible influence of Kelvin-Helmholtz instability on the emulsification process. High-speed visualizations were conducted inside a square venturi constriction with throat dimensions of 450 µm by 450 µm, both under visible light and X-Rays. We show that cavity shedding caused by the instability results in the formation of several cavity vortices. Their rotation causes the deformation of the oil stream into a distinct wave-like shape, combined with fragmentation into larger drops due to cavitation bubble collapse. Later on, the cavity collapse further disperses the larger drops into a finer emulsion. Thus, it turns out that the Kelvin-Helmholtz instability is similarly characteristic for hydrodynamic cavitation emulsification inside a microchannel as is the Rayleigh-Taylor instability for acoustically driven emulsion formation.

Cavitation erosion is one of the most severe problems encountered in hydraulic turbomachinery. When testing the materials, the engineers usually rely on standardized procedures. The most common one being the vibratory ASTM G-32 test, which offers two possibilities of performing the test - the direct, where the specimen is attached to the ultrasonic device and the indirect, where the specimen is stationary and exposed to the ultrasonic horn, positioned just 0.5 mm from it. The erosion rates from the two are significantly different and a question may be asked if they are at all comparable and further on are they comparable to the "real-life" hydrodynamic cavitation which occurs in turbomachinery. In this study we performed erosion tests on a stationary specimen where the gap between the specimen and the horn was varied from 0.3 to 4 mm. In addition, we used high speed visualization to observe the cavitation in the gap. We observed that the cavitation erosion rate strongly depends on the gap. From visualization we see that the cavitation dynamics significantly changes in a small gap, leading to a large, but 2-dimensional cavitation bubbles which collapse very slowly, compared to the small spherical ones in a larger gap. We investigated the probability of shock wave occurrence and derived a very simple model, which gives accurate qualitative predictions of experimental data. Finally, the study puts into question the validity of ASTM G32 test - the most common approach used in engineering today.

Aiming at modeling the cavitation bubble cluster, we propose a novel nonlinear dynamic cavitation model (NDCM) considering the second derivative term in Rayleigh-Plesset equation through strict mathematical derivation.There are two improvements of the new model: i) the empirical coefficients are eliminated by introduction of the nonuniform potential functions of ψ v and ψ c for growth and collapse processes respectively, and ii) only two model parameters are required, which both base on physical quantities-the Blake critical radius R b and the average maximum growth radius R m .The corresponding cavitation solver was developed by using OpenFOAM in which we implemented the modified momentum interpolation (MMI) method to ensure that the calculated results are independent of time step size.Three validation cases, namely numerical bubble cluster collapse, ultrasonic horn experiment, and hydrodynamic cavitation around slender body are employed.The results indicate that ψ v and ψ c can reveal the nonlinear characteristics for cavity accurately, and R b and R m can reflect the relevance between cavitation model and actual physical quantities.Moreover, it is discussed the potentiality of NDCM that is generally applied on the cavitating flow possessing with dispersed bubbly cloud.

Water scarcity, one of the most pressing challenges we face today, has developed for many reasons, including the increasing number of waterborne pollutants that affect the safety of the water environment. Waterborne human, animal and plant viruses represent huge health, environmental, and financial burden and thus it is important to efficiently inactivate them. Therefore, the main objective of this study was to construct a unique device combining plasma with supercavitation and to evaluate its efficiency for water decontamination with the emphasis on inactivation of viruses. High inactivation (>5 log10 PFU/mL) of bacteriophage MS2, a human enteric virus surrogate, was achieved after treatment of 0.43 L of recirculating water for up to 4 min. The key factors in the inactivation were short-lived reactive plasma species that damaged viral RNA. Water treated with plasma for a short time required for successful virus inactivation did not cause cytotoxic effects in the in vitro HepG2 cell model system or adverse effects on potato plant physiology. Therefore, the combined plasma-supercavitation device represents an environmentally-friendly technology that could provide contamination-free and safe water.

Cavitation is a potentially useful phenomenon accompanied by extreme conditions, which is one of the reasons for its increased use in a variety of applications, such as surface cleaning, enhanced chemistry, and water treatment. Yet, we are still not able to answer many fundamental questions related to efficacy and effectiveness of cavitation treatment, such as: "Can single bubbles destroy contaminants?" and "What precisely is the mechanism behind bubble's cleaning power?". For these reasons, the present paper addresses cavitation as a tool for eradication and removal of wall-bound bacteria at a fundamental level of a single microbubble and a bacterial cell. We present a method to study bubble-bacteria interaction on a nano- to microscale resolution in both space and time. The method allows for accurate and fast positioning of a single microbubble above the individual wall-bound bacterial cell with optical tweezers and triggering of a violent microscale cavitation event, which either results in mechanical removal or destruction of the bacterial cell. Results on E. coli bacteria show that only cells in the immediate vicinity of the microbubble are affected, and that a very high likelihood of cell detachment and cell death exists for cells located directly under the center of a bubble. Further details behind near-wall microbubble dynamics are revealed by numerical simulations, which demonstrate that a water jet resulting from a near-wall bubble implosion is the primary mechanism of wall-bound cell damage. The results suggest that peak hydrodynamic forces as high as 0.8 μN and 1.2 μN are required to achieve consistent E. coli bacterial cell detachment or death with high frequency mechanical perturbations on a nano- to microsecond time scale. Understanding of the cavitation phenomenon at a fundamental level of a single bubble will enable further optimization of novel water treatment and surface cleaning technologies to provide more efficient and chemical-free processes.

The collapse of microbubbles near a fiber is an example often encountered in water treatment situations and cavitation fibrillation processes. However, due to the broken symmetry conditions, this process has not been studied in detail experimentally or numerically, making it difficult to precisely measure or simulate the rapid bubble evolution during collapse. In this work, we present a novel experimental method, allowing for precisely repeatable cavitation events observation, combined with numerical simulations offering insight into pressure and velocity fields distribution developments in time. Both experimental and numerical works focused on small distances between the bubble and the fiber, where the physical interaction between subjects is the strongest. Four different bubble offsets were considered within the scope of this work, and very good agreement of numerical simulations with experiments was found in all cases. Two modes of bubble collapse were identified, leading to mushroom-shaped bubbles at positions closest to the fiber and a pear-shaped bubble at the farthest position. It is noteworthy that in all four cases, a planar jet formation toward the fiber was observed. The formed jet initially assumes an elongated shape, whereas its stability depends on the mode of bubble collapse. Numerical analysis of the planar jet as the defining feature of the collapse defined lower bounds for the actual values of peak jet velocities, ranging between 250 and 330 m/s, and the resulting impact pressures, which range from 100to 500 MPa.

Anaerobic digestion in wastewater treatment plants converts its unwanted end product - waste activated sludge into biogas. Even if the process is well established, pre-treatment of the sludge can further improve its efficiency. In this study, four treatment regimes for increasing methane production through prior sludge disintegration were investigated using lab-scale cavitation generator and real sludge samples. Three different cavitating (attached cavitation regime, developed cloud shedding cavitation regime and cavitation in a wake regime) and one non-cavitating regime at elevated static pressure were studied in detail for their effectiveness on physical and chemical properties of sludge samples. Volume-weighted mean diameter D[4,3] of sludge's particles decreased by up to 92%, specific surface area increased by up to 611%, while viscosity (at a shear rate of 3.0 s- 1) increased by up to 39% in the non-cavitating and decreased by up to 24% in all three cavitating regimes. Chemical changes were more pronounced in cavitating regimes, where released soluble chemical oxygen demand (sCOD) and in-crease of dissolved organic matter (DOM) compounds by up to 175% and 122% were achieved, respectively. Methane production increased in all four cases, with the highest increase of 70% corresponding to 312 mL CH4 g-1 COD. However, this treatment was not particularly efficient in terms of energy consumption. The best energy balance was found for the regime with a biochemical methane potencial increase of 43%.