Micro and nanobubbles have achieved great importance in recent years due to their wide applications in many fields of science and technology. Various applications of these bubbles have been predicted and established for commercial purposes. In the current review article, we discuss the existence and applications of micro and nanobubbles in many diverse fields like cleaning and defouling, medical therapy, marine engineering, degradation of toxic compounds and many more. Looking at the wide applications of these new technology bubblers one can predict that the future prospects of micro and nanobubbles are immense and yet more to be explored.
Keywords: Microbubbles, nanobubbles, application
Introduction
The microbubbles (MBs) and nanobubbles (NBs) are a new class of nano species which came into account a few years back. Initially the existence of NBs as stable and spontaneously forming entity was considered as a topic of certain disputes due to number of thermodynamic objections. For example, the total free energy of the system was considered to increase upon the formation of NBs unless the surface is extremely rough, moreover due to very high Laplace pressure inside the NBs they were assumed to dissolve and disappear quickly[1-2]. In spite of initial skepticism, the conformity for the existence[3] and special properties of these bubbles is now growing. The formation and characteristics of NBs is now extensively becoming a subject of research in present. MBs and NBs are tiny bubbles with diameter 10 to 50 µm and <200 nm respectively. Due to the propensity of MBs to
shrink and subsequently to collapse under water makes them a suitable candidate for new technical applications. Figure 1 shows the key difference between macro, micro and nanobubbles. MBs gradually decrease in size due to the dissolution of interior gases by the surrounding water and then vanish ultimately below the surface of water[4] whereas a macrobubbles bursts at the water surface; however in contrast the NBs remains as such for months and do not burst out at once.
Figure 1: Schematic diagram showing macro, micro and nanobubbles.
Subsistence of bubbles
The existence of NBs at the liquid-solid interface has been predicted by numerous experiments employing atomic force microscopy [5-11], and other techniques [12-13] as well. In contrast, some experimental results obtained using neutron reflectivity[14], electrochemical quartz crystal microbalance [15], and ellipsometry[16] neglected the existence of NBs. However after an agitation from equilibrium, the formation of these NBs was found to occur immediately, under certain conditions like gas supersaturation, change in temperature, mixing of solvents, or by chemical reaction, [10, 17-19] Interpretation from experiments revealed that these nanoscopic species resemble spherical caps with heights and diameters of the order of 10 nm and 100 nm respectively and hence so-called surface NBs, i.e., nanoscale gas bubbles sited at the liquid-solid interface. This claim was supported by the fact that NBs can be fused by the tip of an AFM to form a larger bubble[7]. Initially it was predicted from calculations that NBs being very small in size might have high surface tension[20] and thus the gas must be 'pressed out' of the NBs within microseconds. However it is now clear that under right conditions such bubbles can both form freely and remain stable for long periods of time. This stability is related to a lower than expected interfacial curvature, due to an anomalously high contact angle.[10, 21] Some results for the formation of NBs in aqueous solutions of small organic molecules like tetrahydrofuran, ethanol, urea, or α-cyclodextrin which cause scattering of light has also been stated.[22-23] It was then concluded that the formation of stable NBs was responsible for the scattering of light and the formation of such stable NBs in the aqueous solution of small organic molecule was a universal phenomenon. However, in contrast to the reports for the formation of NBs in aqueous solutions of organic molecules, Habich et al.,[17] predicted the results which were inconsistent with the presence of gas NBs in bulk solution. The experiments conducted by Habich et al., revealed that the scattering of light in aqueous solutions of organic molecules was not due to the gaseous NBs rather it was due to the water-insoluble impurities originated from the organic solvents or from the plastic labware that coalesce into nanoparticles when the solvent quality was changed.
Kaneo, C. & Masayoshi, T.[24] produced oxygen NBs which were capable of being present in an aqueous solution for a long time, by applying physical irritation to oxygen-containing MBs contained in an aqueous solution, thereby abruptly reducing the bubble diameter of the MBs. The formation of micro and NBs in bulk solutions has also been reported. Such methods involve the transfer of gas from bulk phase to the NBs by making use of flow through a porous membrane[25], electrochemical reaction[26-27], or ultrasound[28-29]. Bunkin et al.[30], reported the spontaneous generation of bubbles and the stabilization of bubbles through the adsorption of ions (bubstons). It was supposed that due to the high stability of such bubbles, they can prove to be highly efficient as ultrasound contrast agents.[28-29, 31] Besides the generation of oxygen and ozone NBs, nitrogen, methane and argon NBs with an average radius of 50 nm, as measured by scanning microscopy and having an average lifetime of more than two weeks were also created successfully under atmospheric conditions.[32] It was found that hard hydrogen bonds at the surfaces of the NBs were responsible for reducing the diffusivity of gases through interfacial films. Morphology of air NBs trapped at hydrophobic nanopatterned surfaces has been recently studied by Checco et al.[33] The microscopic information thus obtained correlated very well with macroscopic surface wetting behavior too.
Applications of bubbles
The application of MB and NBs has been achieved in various fields of science and engineering. On one hand where NBs have been used in the design and as templates for micro devices[34] and in the manufacturing of nanostructures[35] respectively; on the other hand MB technology has shown great interest in industrial fields such as for hydrate formation due to its excellent gas-dissolving abilities and providing suitable conditions for hydrate nucleation[4]. Cavicchi, et al.[36] predicted that traces of NBs determine the nano-boiling. This experiment was believed to be the first confirmation that NBs can form on hydrophilic surfaces. This work has immediate inference for inkjet printers, in which a metal film is heated with a voltage pulse to create a bubble that is used to eject a droplet of ink through a nozzle.
Degradation mediated application of bubbles
Since few decades the extensive use of small gas bubbles have been done in both industrial and environmental separation processes for the treatment of potable water and wastewaters;[37-38] for the separation of particulate materials from the aqueous phase and for the remediation of volatile contaminants in the aqueous phase.[39-40] Finally micro and NBs were used by Yamasaki, et. al.[41] in a production device and a detoxification device as one example of the upstream treatment devices which perform specified treatment with the use of water. Bathing pool assembly having water full of ozone NBs for rehabilitation have also been developed to prevent germs from infecting a person while taking bath, especially for elder and children who have weak resistance against disease.[42] The ξ potential of MBs in aqueous solutions revealed that the bubbles were negatively charged under a wide range of pH conditions moreover OH- and H+ ions were found to play a vital role in explaining the charging mechanism of gas-water interface[43]. Ozone MBs were found to generate free hydroxyl radicals on their collapse under strong acidic conditions which decomposed phenol as well as polyvinyl alcohol[44], which is considered to be ozone resistant. Ozonation of a mixture of soluble organics like benzene, toluene, ethylbenzene and xylenes dissolved in aqueous solutions having salt concentrations ranging from 0 to 2 M have been investigated.[45] It was observed that the production of MBs mitigated the mass transfer limitation and increased the removal rate of soluble organics from simulated seawater. Later Takahashi at al. investigated the decomposition of phenol in aqueous solution with air MBs in the absence of a dynamic stimulus like UV irradiation and incident ultrasonic wave.[44] Their study shows that due to the free radical generation from the collapse of air MBs, the small amount of organic compounds could be removed.
The MB technology has shown promising results for the enhanced mass transfer and ozone oxidation of dye stuffs in waste water.[46] The production of OH• radicals using vacuum UV irradiations[47] has been reported for the mineralization of organic compounds in water and waste water systems to offer faster oxidation rates of organic compounds as compared to the conventional ozone system.[48] In addition to this, the use of vacuum UV is restricted because of the recombination of carbon-centered radicals during photo degradation which yields byproducts such as oligomers and polymers.[49] Recently the use of micro and NBs technique to improve the bulk limited vacuum UV process has been explored.[50] Under vacuum UV irradiations the degradation of Methyl Orange in the presence of oxygen MBs was found to be accelerated because of the enhanced mass transfer of oxygen and substrate within the vacuum UV reactor.[51] The rate of mineralization of sodium dodecylbenzenesulfonate with NBs having diameter of 720 nm was found to be much faster than that with MBs. The critical role of NBs for the degradation of surfactants and nonsurfactant solutions under vacuum UV irradiations was also investigated. The results showed better mineralization of surfactants as compared to nonsurfactant in the vacuum UV reactor.[50]
Cleaning and defouling actions
Application of NBs in the prevention and removal of protein adsorption has also been achieved. Protein adsorption on varied surfaces was found to be inhibited by NBs, thus protecting the surfaces from fouling.[52-53] In order to predict the effect of NBs on protein adsorption at the liquid/solid interface, bovine serum albumin adsorbed on mica was used as a model system. It was found that the NBs blocked bovine serum albumin adsorption and resulted in circular hollows on the bovine serum albumin films on mica.[53] The cleaning effect of nanobubbles on highly oriented pyrolytic graphite[52, 54] and gold surfaces,[55] was found to be highly effective as cleaning agents for the removal of proteins from these surfaces. Furthermore, NBs were readily produced electrochemically on both hydrophobic and hydrophilic surfaces, and their efficacy on the former was found to be superior.[55] Recently, similar defouling effect of NBs was also detected on stainless steel.[42] Hence it can be concluded that NBs may find their wide applications where materials come into contact with biological media, such as medical equipment, food, membrane cleaning, ship and filter regeneration. For the above industries the application of NBs may provide a promising path for a convenient, clean, cheap and environmentally friendly technique suitable for cleaning of conducting surfaces.
Therapeutic applications of bubbles
The use of MBs and NBs has widely been studied for treating tumors and cancers. Initially MBs were used as ultrasound contrast agents for blood flow imaging, however development of molecular ultrasound imaging that uses sub-MBs or nanoparticles that were appropriate for tissue penetration have poor echogenic sensitivity for ultrasound imaging instruments (typically 50 to 100 mm).[56] The need to use nanosized carriers for in vivo imaging and therapy has led to the development of laser-induced photoacoustic ultrasound.[57-58] Hence the generation of NBs that can be imaged by ultrasound and are derived from gas-generating polymeric nanoparticles have been studied. The proposed mechanism involves localization of echogenic gas-generating polymeric nanoparticles in a tumor and coalescence of generated NBs, followed by fusion of NBs into MBs.[59]
The sandblasting of blood clots using NBs is considered to be the best technique in future. Biotech company ImaRx Therapeutics, Tucson, Arizona, presented its NB stroke therapy, called sonolysis. According to this therapy the bubbles called MRX-815 which was perfluroropropane-filled lipid encapsulated bubbles when injected into the body gets accumulated at blood clots in the brain - the cause of stroke - and when an ultrasound was applied the bubbles pulsated and dissolved the clot.[60] In order to improve and combine the selectivity of diagnostic and therapeutic processes into one (theranostics) to the cellular level which may provide significant benefits in various research and disease systems; a novel method based on the gold nanoparticle-generated transient photo thermal vapor NBs referred to as plasmonic NBs have been developed.[61] After delivery and clusterization of the gold nanoparticles to the target cells the intracellular plasmonic NBs were optically created and controlled through the laser impact. Selective and fast damage to specific cells with bigger plasmonic NBs, and optical regulation of the damage through the damage-specific signals of the bubbles have been achieved to a significant level. In the similar fashion, the optically guided controlled release from plasmonic NBs has also been reported recently. Liposomes containing a molecular load and gold nanoparticles were exposed to short laser pulses in order to induce transient vapor bubbles called plasmonic NBs around the nanoparticles, to disrupt the liposome so as to eject its molecular contents.[62] Besides the use of NBs for the treatment of tumors and controlled release of drugs, these nanoscopic species also find their application in tissue preservation and hence are useful in the field of medicine and medical experiments.[63] In tissue transplant it is important that the tissue removed from a donor be transplanted right away to the donor, but it is not always transplanted instantaneously hence in order to get rid of this problem the use of oxygen NBs has proved to be the most effective solution.
Bubbles as effective carriers
At the end of World War II the research on oxygen carriers started in order to fulfill the demand of blood transfusions for injured soldiers.[64] Since then many studies thus focused on the hemoglobin loaded particles[65-70] as well as biodegradable polymers[67, 71-76] for use as oxygen carriers. Studies regarding in vivo administration of hemoglobin particles in mice demonstrated same oxygen-carrying capacity as that of native hemoglobin. The use of MBs for the same purpose has also been predicted. Porter et al., formulated dextrose albumin MBs for oxygen and nitrogen for use as contrast agents.[77] Application of oxygen enriched lipid-coated perfluorocarbon MBs for oxygen delivery in anemic rats as found to maintain the rat's survival at very low hematocrit levels.[29, 78] Research conducted by Cavalli et al., showed that oxygen-filled micro/NBs coated with chitosan exchanged oxygen with the surrounding hypoxic solution and the release of oxygen was found to be enhanced by sonicating the particles with ultrasound at a frequency of 45 kHz.[79] Oxygen filled NBs consisting of dextran coating were developed and bubble stabilization was improved by adding a poorly soluble gas perfluoropentane[80] and polyvinylpyrrolidone[81] to the shell. In view of many possible applications in the treating of chronic wounds and anaerobic infections, dextran NBs might be proposed for the delivery of oxygen, particularly in the case of targeted delivery of oxygen by means of ultrasound and MBs/NBs.
Bubble application in marine technology
Another interesting application of these small bubbles is in the field of marine engineering. Cargo ships that carry heavy loads of commercial commodities such as crude oil, ores and grains, play a vital role in worldwide transportation. Due to their huge size these ships suffer a large skin frictional drag and hence they move very slowly. In order to reduce this frictional force component the use of MBs which are injected into the boundary layer on a solid wall have been studied intensively since the revolutionary works by McCormick and Bhattacharyya.34, 35 Several experimental studies were carried out to understand the mechanism of skin friction reduction by MBs.36-41[82-84] Kato et al.42 studied the effect of bubble size by changing the main flow velocity. It was observed that with increase in the main flow velocity, the bubble size decreased, resulting in larger reduction in the skin friction rate. Kato et al.36 also changed the bubble size by changing the surface tension using water with 0.1% ethanol, and found it to be a more effective system. It was hence established that the diameter of the bubble plays a vital role in reducing the skin friction.
Conclusions
The commercialization of MBs and NBs is increasing at astonishing rates due to their excellent applications in many fields. For example, wide use of oxygen NBs has been predicted due to their extremely high bioactivity effect which makes it possible to raise freshwater fish and marine fish simultaneously in the same environment. Besides this, the applications of NBs also find their way in pharmaceuticals, food processing and others to a great extent. Food preservation without the use of preservatives and fish culture/livestock industries which do not depend on antibiotics and other drugs are also expected to become possible. In the field of medicine, prevention of contagious diseases and therapeutic effects on diseases area are also finding their way in large numbers. Hence it can be concluded that the use of MBs and NBs in developing new technology is still ahead to be explored. Moreover several other practical applications of MBs and NBs are yet to be discovered in near future.
