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Presencenanobubbles, especially bulk nanobubbles, remain a mystery mainly because of their stability and lifetime, properties that have been studied by many research groups around the world. At the same time, a new generation of methods has been strongly developed, and their potential use has so far been extended to high-value applications. Some of them relate to drinking water,waste water treatment, surface cleaning, biomedicine, engineering, medical imaging and "food" applications such as fisheries and agriculture. Although most methods of nanobubble formation are well detailed and well known, there is still a significant backlog in the detailed explanation of their formation, specific physicochemical properties and stability mechanisms. In this short review, methods of nanobubble production, their main properties and potentially important applications are briefly presented. During the past four years of research work, the focus has been on results and discoveries.
The intriguing topic of gaseous nanobubbles has attracted a lot of attention thanks to its brilliant, unique, unexpected and somewhat "mysterious" properties. However, many questions remain unanswered for clustered nanobubbles, mainly related to the mechanisms of their formation and stability.
From 1950, when Epstein−Plesset  proposed a theory to predict the lifetime of individual bubbles as a function of bubble radius and saturation, until 2000, only a few papers were published in the field of nanobubbles. This fact changed a thousand years later, and published papers related to nanobubbles increased exponentially. For illustration, it should be mentioned that the number of published papers on nanobubble research was 5 in 2000, 61 in 2010 and 236 in 2020, according to the Scopus database.
Although we use the term "nanobubbles" in the context of both surface nanobubbles (SNB) and bulk nanobubbles (BNB), there are significant differences between the two. Surface nanobubbles are described as air-filled pockets/pockets on the surface in the form of spherical caps. The height of these spheres ranges from 10 to 100 nanometers, and the diameter of their contact wire varies between 50 and 500 nanometers. On the other hand, collective nanobubbles are described as spherical cavities filled with gas, the bubbles are obtained in the liquid phase, and their diameter is less than 1000 nm. Another interesting submerged gaseous region is the micro-pancake, which is typically 1-2 nm high but extends laterally for hundreds of nanometers or even micrometers . However, we must note that recently the scientific community has mentioned and discussed the approximation that micropancakes are not gaseous but originate from polymeric contaminants (i.e. PDMS) [3,4]. The above three forms are shown schematically in Figure 1.
The current short review article is organized as follows: In the introduction to the current section, the physicochemical properties of nanobubbles (NB) and information on mechanistic approaches to their stability are briefly presented. In the second part, an overview of the method of NB production is given with an emphasis on hydrodynamic cavitation. Finally, the last section summarizes some important topics for potential NB applications, and finally, we draw the conclusions of the paper.
With the increase in the number of related patents for the production and application of nanobubbles, the Fine Bubble Industry Association (FBIA)  has added new members, and countless potential applications of nanobubbles have been recorded in the literature, it is necessary to further study the "phenomenon" of nanobubble growth. Among the many necessary steps in studying the production of NBs and their potential uses, a key question arises regarding the existence of NBs: "Does NB really exist?'. The first evidence for the existence of NB was the Tyndall effect observed using a laser beam . Furthermore, the size of the nanobubbles can be estimated by more advanced scientific tools, such as high-resolution optical microscopy, confocal laser scanning microscopy, dynamic light scattering and cryoelectron microscopy, cryoelectron microscopy (SEM and/or TEM) . ] .
Today, NB is at the forefront of research due to its unique properties. Some of the most important properties are listed and discussed below. Both SNB and BNB have a long retention time and excellent stability, do not agglomerate easily and do not dissolve. In the case of massive formation of nanobubbles, the gas is more soluble in water and has a very high zeta potential (G-potential) values have been reported in these systems . Nanobubbles can also serve as nucleation sites for crystal growth [9,10], and water-nanobubble interfaces can be loaded with surfactants , while being characterized as low-buoyancy systems. Based on this phenomenon, it is also attributed to the long-term stability of the BNB (see Figure 1).
Unlike bulk nanobubbles, surface nanobubbles can be easily detected, mainly using atomic force microscopy (AFM) techniques. Also, the assumptions about how it remains stable over a period of time are well explained, which has not yet happened in the case of clustered nanobubbles. According to many published works, the stability of most of the observed SNBs can be calculated with high precision by pinning the contact wire  and supersaturation theory . The explanation of how SNBs are strongly anchored to surfaces is based on the suggestion that SNBs can remain remarkably stable in gas-supersaturated environments if their contact lines are anchored by geometric or chemical substrate inhomogeneities. The so-called "pinning of the contact wire" has been confirmed theoretically and experimentally [14,15]. However, An et al questioned the approximation of the assumption that “contact line pinning” causes SNBs to be tightly attached to surfaces. , who found that nanobubbles on the surface of fluorinated silicon wafers (PFOTS) were highly mobile under mechanical disturbances imposed by an AFM cantilever.
On the other hand, BNBs are evenly distributed in the solution and do not like to stick to surfaces. Furthermore, the small size, low number density, and sometimes homogeneity with high degrees of dispersant make it difficult to detect and distinguish BNB from other dispersed nanostructures such as amphiphiles and/or contaminants. Thus, bulk NBs are small, spherical, gaseous voids that move into a liquid solution, and since their pressure inside the bubble is inversely proportional to their diameter, the nanobubbles should have a very high internal pressure (this will be discussed later).Based on the fact that the gas inside the nanobubble cannot be kept in equilibrium with its surroundings. Therefore, due to the phenomenon of "Ostwald ripening" , bulky nanobubbles should immediately dissolve within a few microseconds in favor of larger bubbles. In contrast, βNBs last for weeks, and in some cases even months, which is beyond doubt [6,18].
Although in the past decade, BNBs have been reported as carriers with a very low carrying capacity, mostly compared to microbubbles and ultrafine bubbles, they are not often mentioned anymore, and many studies support the approach that BNBs have a high gaseous character. Carrying capacity ]. One of the characteristics of nano bubbles is their high surface energy . Very small fluctuations in concentration and/or temperature are sufficient to induce large surface energy changes in nanobubbles. Due to fluctuations in nanoparticle activity among different acclaimed catalysts, nanobubbles with high surface energy can be generalized as homogeneously distributed two-phase catalysts or as surfactant-mediated catalytic systems .
According to the Young-Laplace equation , the internal pressure of the bubble is described as:WherePInterioris the internal gas pressure,Pexternalis the liquid pressure outside the bubble,Cis the surface tension of the liquid, iRis the bubble diameter.
This means that for a hollow bubble with a diameter of about 1000 nm, the internal pressure is about 3.9 bar at 25°C, and this pressure increases as the size of the bubble becomes smaller. The stability of the large number of nanobubbles is attributed to gas supersaturation and high surface charge. The presence of a negative charge is described by a large magnitudeG- The potential, which leads to repulsion between and within the bubbles, thus providing resistance to the merging of bubbles and ensuring their colloidal stability . However, it is not clear how to stabilize a single charged NB under high Laplace pressure .
Based on the unique physicochemical properties of NBs, their specific characteristics, such as lifetime, interface composition (gas-liquid, liquid-liquid, gas-solid and liquid-solid), controlled gas transport rate, controlled directionality of ultrasound migration and/or optical plasmonic fields have received much attention from the scientific community [24,21].
The classical method of finding bubbles refers to the creation of microbubbles with a diameter of micrometers mainly by cavitation. Such microbubbles have been reported to be used in environmental protection, i.e. to use oily water cleaning functions, such as gas and liquid dissolution and flotation, or/and to stimulate the growth of aquatic animals and plants and to use their growth promotion effect. Methods of improving such microbubble functions
Although nanobubbles have been proposed for many important applications so far, the technology is still stuck in a laboratory setting. However, the likelihood of nanobubbles moving from the laboratory to the market is high enough to be a promising scenario for commercial use in the next decade, mainly in horticulture. Among them, the potential application of NB is accelerating in many fields, including mining industry, medical application, wastewater
It is clear that nanobubbles have attracted a lot of attention from the scientific community in the last two decades. This is due to their exceptional properties that make NB candidates for numerous applications, from engineering and agriculture to pharmaceuticals and medicine. There are two main questions about NB, the first is: 'How can we produce stable aggregated nanobubbles? ’ and secondly: ‘What are their potential applications? " is partially answered in this short review article. About their first promising results
Statement of competing interests
The authors declare that they have no known financial interests or personal relationships that could influence the work reported in this article.
EPF thanks the support of the project MIS 5002567 which isInitiative for the strategic development of the research and technology sector', financed from the Operational Plan'Competitiveness, entrepreneurship and innovation’ (NSRF 2014-2020), co-financed by Greece and the European Union (European Fund for Regional Development).
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Surface enrichment with ions leads to stabilization of massive nanobubbles
Nucleation mechanism and steady state of electrochemically generated nanobubbles
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Aggregate nanobubbles: formation and investigation of their formation/stabilization mechanisms
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Identifying surface-attached nanobubbles
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Wetting of nanophases: nanobubbles, nanodroplets and microplates on hydrophobic surfaces
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Stability of bubbles in liquid-gas solutions
Journal of Physical Chemistry
Overview of surface and bulk nanobubbles
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Pinning and gas supersaturation imply stable nanobubbles with a single surface
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Microbubble and nanobubble technology as CO
2-EOR i CO 2New horizons in geological storage technologies: a review
carbon monoxide2Capture, use and storage technology is one of the most effective ways to reduce CO2 emissions2.Although this technology is relatively mature, the main problem limiting large-scale application is the safety of geological storage. The advantages of microbubbles and nanobubbles are widely recognized due to their size and stability. Microbubbles and nanobubbles are tiny gas-liquid dispersion systems with unique physical properties. Important features of microbubbles and nanobubbles include their excellent stability, high internal pressure, extremely large surface-to-volume ratio, and high gas dissolution rate, which have wide prospects for application in various fields. This review discusses the research conducted in the field of microbubbles and nanobubbles, with a special emphasis on CO2-EOR i CO2Geological storage. The classification and composition of microbubbles and nanobubbles is briefly presented. The method of preparation of micro-nano bubbles and the main factors influencing their properties are studied in detail. Characterization parameters of state-of-the-art measurement and analysis techniques for microbubble and nanobubble technology are summarized. In addition, the main application of CO2-EOR i CO2Geological storage is discussed. Based on this review, various potential areas and gaps in MNB research in CO2– EOR i CO2A geological repository has been identified for further exploration.
In situ transmission electron microscopy reveals quasi/non-equilibrium states in nanobubble growth trajectories
2023, Today's Nano
In recent years, nanobubbles have attracted much attention due to their potential applications in various fields such as engineering, environment, biology and medicine. In addition, different types of nanobubbles are used in different fields due to their different properties. However, understanding the growth dynamics of interfacial and aggregated nanobubbles at the nanoscale to meet the demands of various applications remains a challenge. Using liquid-cell transmission electron microscopy, we reveal in real time the quasi-equilibrium growth trajectories of electron beam-induced nanobubbles in aqueous solution. In particular, we successfully distinguished interfacial nanobubbles from aggregated nanobubbles by combining growth kinetics analysis and the Fresnel fringe method. In addition, the methods described above revealed nonequilibrium growth of some nanobubbles from the solution to the interface. These insights into the growth kinetics of nanobubbles at the nanoscale can help in the better design of tailored nanobubbles with specific kinetic behavior for different applications.
The role of surfactants in the modulation of surface charge and surface tension in the stabilization of aggregated nanobubbles
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Since the classical Epstein-Plesset theory predicted that bubbles cannot spontaneously maintain a stable thermodynamic equilibrium, the unusual stability of a large number of nanobubbles has attracted much attention from academia and industry. The dual nature of ionic surfactants, which simultaneously act as charge carriers and surface tension reducers at the gas-liquid interface, enables them to play a unique role in the stabilization of clustered nanobubbles. In this work, the stability of a large number of nanobubbles in solutions of a wide range of concentrations of anionic, cationic and nonionic surfactants is investigated. Experimental results show that different types of molecules of surfactants participate in different ways in the nucleation and stabilization of collective nanobubbles. We show that the accumulation of net charges carried by the surfactant ions at the interface is primarily responsible for the stabilization of the nanobubbles and not for the reduction of the surface tension. Through theoretical calculations, we additionally quantified the effect of coupling of interfacial tension and surface charge on the steady state equilibrium of clustered nanobubbles. The results show that interfacial tension and gas saturation together determine the upper limit of the bubble size that can exist stably. Our study identifies avenues for investigating the mechanisms underlying the stability of aggregated nanobubbles.
Fundamentals and applications of nanobubbles: an overview
2023, Chemical Research and Design
Nanobubble technology is a new solution for solving climate change, environmental challenges, reducing costs and energy in industrial processes, optimizing therapeutic and diagnostic techniques and other applications. Although the production and development of nanobubbles is a recently developed field, there are many reports and studies on their characteristics and promising applications in various fields. This paper aims to provide a summary of the latest (as of 2017) scientific findings on the potential of nanobubbles as a multifunctional and sustainable technology. Applications in the environment, agriculture, medicine/biomedicine and others are reviewed, and the most indicative applications in each domain are listed in detail. Special attention is paid to the implementation of water and wastewater treatment.
The effect of aggregated nanobubbles on ultrafiltration membrane performance: physicochemical, rheological and microstructural properties of the obtained dispersions of skimmed milk concentrate
2023, Journal of Food Engineering
The purpose of this research was to evaluate the effect of bulk nanobubble (BNB) incorporation during ultrafiltration (UF). Laboratory and pilot ultrafiltration experiments were conducted to evaluate the effect of BNB incorporation on the ultrafiltration process by evaluating permeate flux, membrane microstructure, fouling resistance, and energy consumption. In addition, control dispersions and dispersions of skimmed milk concentrate (SMC) with incorporated BNB were characterized in terms of physicochemical properties, rheology and microstructure. Running the UF with the installation of BNB enabled a better performance of the membrane as evidenced by a better permeate flow. The permeation flux of the control SMC is 9.27 and 6.89 kg/h∙m2After adding BNB, permeate flow increased significantly (P<0.05) to 14.57 and 9.59 kg/h∙m2For laboratory and pilot UF tests. The incorporation of BNB also resulted in a significant decrease in apparent viscosity (P<0.05) and showed a significant change in the microstructure of the skimmed milk concentrate dispersion obtained. In conclusion, the incorporation of BNB helps to improve the performance of the UF membrane, so this study shows that more efficient UF treatment can be achieved by incorporating BNB.
Nanobubble Ozonation for Water Regeneration at Different Sites in India: A Holistic and Sustainable Approach
2022, Engineering results
In this study, four different pond sites were analyzed at different time periods to demonstrate the effect of nanobubble ozonation (NBO). NBO is an environmentally friendly new technology that uses only air, ozone and almost no chemicals. All case studies were conducted in real time during different seasons and locations, demonstrating the importance of NBO for treating water bodies with techniques that are completely natural, environmentally sustainable, cost effective and less time consuming. Three case studies were studiedin place(Locations 1, 2, 4) and the previous one- On the spot(location-3), the effectiveness of NBO in treating pond water was confirmed. The NBO-treated water was found to be within standard limits, odors were removed, and the water was purified enough for the animals to drink. Test results showed that ozone nanobubbles (NBO) achieved 85-99% reduction in total soluble solids (TSS), 80-90% reduction in biochemical oxygen consumption (BOD) and 55% reduction in chemical oxygen consumption (COD) in wastewater treatment The % COD of locations 3 and 4 was reduced by 82%. Improved dissolved oxygen suitable for organisms is achieved. Therefore, this paper highlights the effectiveness of NBO treatment on water regeneration and ecological restoration to achieve the sustainable goals of clean water and environmental sustainability. DO levels in all ponds increased significantly after NBO treatment, and measurements showed a value of 14.5 mg/L even after 50 hours. The Nanobubble gas dissolving system is not only capable of raising dissolved oxygen to supersaturation levels, but also keeps it stable for over 14-15 hours. Various reasons for this consistency are that the size of the nanobubbles is about <0.5 μm and that the NBs do not follow buoyancy and therefore move by Brownian motion in water. NBG systems are the future of rejuvenating and maintaining water quality in our lakes and ponds.
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Gas dispersion parameters (air entrapment - yes).G, air surface velocity-Jg and bubble surface flow-Sb), the concentration of nanobubbles formed in the hydrodynamic cavitation tube was measured separately. The best results obtained at an air/liquid volume ratio of 30%; 49m nautical miles−1Air/liquid interfacial tension results in 16% air entrapment and a Jg of 0.87 cm−1, in the year 85 Sb−1and nanobubbles (230–280nm)The concentration is 6.4×108NBsmL−1.The lower the air/liquid interfacial tension, the higher the air retention rate; this occurs by promoting cavitation and the formation of micro- and nano-sized gas bubbles. The obtained data are discussed in terms of solutions, hydrodynamics and interfacial phenomena. The main mechanisms involved in the action of nanobubbles and their interaction with solids and larger bubbles are predicted. These discoveries are believed to be important because techniques and equipment are needed to produce nanobubbles at low cost and at high speeds. This work shows that the tested cavitation tubes have great potential due to the large size of the formed bubbles and the high flow achieved on the surface of the bubbles, especially due to the high concentration of generated nanobubbles. These gas bubbles are of great importance in the separation of pollutants in modern flotation of small minerals and wastewater treatment, and some examples are presented and discussed.
As bulky nanobubbles remain stable over a wide temperature range
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Clustered nanobubbles are nanoscale gas domains in aqueous solutions. Because of the common assumption that spherical bubbles cannot reach stable equilibrium, their surprising long-term stability remains controversial. In order to reveal the intrinsic stabilization mechanism, the thermodynamic behavior of nanobubbles in water was investigated in a wide temperature range.
Acoustic cavitation is used to create massive nano bubbles with a typical radius of 50-200 nm. Increasing the temperature significantly narrows the bubble size distribution and their average radius decreases to at least 50 nm at about 45°C. A slight increase was observed for higher temperatures. Heat-induced shrinkage is reversible: after cooling, they return to their original state.
The observations can be explained by a charge stabilization mechanism. A complex balance of competing interactions between water autoionization and surface ion mobility leads to this non-monotonic dependence. Thus, nanobubbles undergo charge loss at lower temperatures and charge conservation at higher temperatures, which corresponds to their contraction and slight expansion. Through theoretical calculations, we further quantified the equilibrium properties of nanobubbles and their zeta potentials under different initial conditions. The temperature-sensitive properties of aggregated nanobubbles represent an important step forward in the research and industrialization of their stability.
Aqueous dispersions of nanobubbles: formation, properties and characteristics
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Nanobubbles (NB) possess interesting and exotic properties, such as high stability, long lifetime, and large surface area per unit volume, leading to important applications in mining, metallurgy, and the environment. NBs are also of great interest for the study of interfacial phenomena involving long-range hydrophobic attraction, microfluidics, and adsorption on hydrophobic surfaces. However, there is little data on the efficient creation of concentrated aqueous dispersions of NB and their physicochemical and surface properties. In this work, air was dissolved in pH 7 water at different pressures, and depressurized using a needle valve to create 150–200 nm (mean diameter) NB and MBs microbubbles (about 70 μm). Micrographs of NB were taken only in the presence of methylene blue dye as a contrast agent. The main results show that high NB concentration (amount per unit volume) can be achieved by reducing saturation pressure and surface tension. Number of NBs at 2.5 bar of 1.0×108NB ml−1at 72.5 mNm−1do 1,6×109NB ml−1na 49mNm−1(100 mg l−1alpha-terpineol). The average diameter and concentration of NBs changed only slightly over 14 days, demonstrating the high stability of these high-concentration aqueous NB dispersions. Finally, after NBs were attached to the surface of pyrite particles (a rather hydrophobic mineral), NBs significantly increased the number of MBs, indicating increased particle hydrophobicity due to NB adhesion. The results are explained by surface phenomena, and it is believed that these tiny air bubbles dispersed in water in high concentrations will lead to cleaner and more sustainable mineral flotation.
Microbubble and nanobubble technology as a new horizon in water treatment technology: a review
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This review article compares research conducted in the field of microbubbles and nanobubbles, with a special emphasis on water treatment. Based on the interpretation of different researchers, basic definitions of bubble types and their size ranges have also been proposed. Characterization parameters of state-of-the-art measurement and analysis techniques for microbubble and nanobubble technology are summarized. Some main applications of these technologies in water treatment processes are presented and briefly discussed. Based on the review, various potential research areas and gaps in the application of air bubbles in water and wastewater treatment technologies have been identified for further study. This article has been prepared in such a way as to provide a step-by-step introduction to the topic with the aim of focusing on the application of microbubbles and nanobubbles in water purification technology.
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