Library Expert opinion
Click to enlarge Click to watch video Click to follow link
Dilutions Digest #3
Dilutions Digest #3

In 2007, just 13 years ago, scientists from the University of Melbourne, Australia, were the first to prove the existence of nanobubbles (NB) – a new stable form of the gas phase of a substance. Nanobubbles are 'nanoscopic' gas bubbles with a diameter of less than 1 micron that occur in a liquid under exposure to a mechanical, physical, or chemical action. NBs possess a number of unique physicochemical properties, including an unexpectedly long lifespan (more than a day). Due to these properties, NBs can be used in manufacturing, medicine, agriculture, and wastewater treatment. This article is an overview of the latest research in this area over the past six months (from September 2019 to February 2020).

Dilutions Digest #3

Diagrammatical representation of the features of macro-, micro- and nanobubbles. Source

NBs are divided into two classes - bulk and surface. The former appears in the volume of the liquid and are in contact only with the liquid phase, while the latter are found at the solid-liquid interface. Bulk NBs are already used in ultrasound imaging technology, to improve seed germination, and to process and purify water and various surfaces. Nevertheless, a generally accepted theory explaining the existence and stability of bulk NBs does not currently exist. In October 2019, an article by scientists from Australia was published in the Journal of Colloid and Interface Science . In the article, the authors showed that the supersaturation of a solution with gas due to a chemical reaction does not lead to the formation of NBs. After analysing the nano-objects formed as a result of the reaction of ammonium chloride and sodium nitrite (in which nitrogen gas is formed), the scientists found, among other things, that their density is higher than that of the surrounding solvent. It is also worth noting that the fixed objects are not subject to compression and cannot, therefore, be bubbles filled with gas.

Scientists from the University of Birmingham (UK) conducted an extensive study to prove the existence and stability of bulk NBs. Their work, with the Shakespearean title “Bulk Nanobubbles or Not Nanobubbles: That is the Question” was published in the February issue of Langmuir. They investigated three different methods of generating NBs in water: mechanical (a rotating rotor-stator mechanism), ultrasonic (acoustic cavitation), and chemical (mixing water with ethanol). The scientists used many advanced analytical methods to study nano-objects formed in water as a result of these type of influences. Among other methods, they used infrared and Raman-scattering spectroscopy (to determine the chemical composition of samples), cryogenic scanning and transmission electron microscopy (to visualise the nano-objects), and gas chromatography and inductively coupled plasma mass spectrometry (to detect organic matter and inorganic impurities in the water containing NBs). The paper provides evidence that the nano-objects formed in pure water when exposed to the studied influences are indeed stable NBs filled with gas. The authors believe that their work should put an end to the debate over the existence of bulk NBs, allowing the scientific community to focus on studying fundamental problems in this field.

Scientists from Greece have studied the optimal conditions for bulk oxygen and air NBs to form in water, as well as their properties. For this purpose, they created a new energy-efficient NB generator, which can continuously create NBs mechanically by means of countercurrent hydrodynamic cavitation. Using electron paramagnetic resonance spectroscopy and dynamic light scattering, the scientists revealed that the properties of nanobubbles are dependent on the production time, the type of gas used, pH, and salinity. It was found that NBs obtained in this manner have similar sizes (190-680nm) and are extremely stable, their lifespan being up to 3 months! In this case, the optimal NB generation duration – at which uniform and stable bubbles form – is 30 minutes. What is more, the scientists give the results of an analysis of the interaction of water and gas in modelling the process of a diffusion layer forming between NBs and water. The authors believe that stable NBs form mainly due to hydrogen bonds. This study was published in the Journal of Colloid and Interface Science in March 2020, and has been available online since December 2019.

In September 2019, a paper by American scientists dedicated to studying the diffuse layer between NBs and liquid was published in the journal Langmuir. The work examines the distribution of ions around oxygen NBs in saline solutions of various concentrations. The diffuse double layer theory is applied to calculations. Since NBs in aqueous solutions have electrically charged boundaries, this affects the distribution of ions in the volume of the solution. Therefore, there is a high concentration of diffusely distributed oppositely charged ions near the surface of the bubble. Thus, a double diffuse electric layer is formed. Using experimental data, as well as data obtained by computer simulation, the scientists calculated the thickness of the double electric layer, the surface charge density on NBs, and their interaction energy. Ions around the surface of the bubble create a thin film which acts as a diffusion barrier, decreasing the rate of gas dissolution. This, in turn, leads to an increase in the particle's life span. However, NBs obtained in a NaCl solution with a lower concentration (0.001M compared with 0.1M) were found to be more stable. This is because with an increase in the concentration of the solution, the size of the NBs increases while the charge on their surface decreases and, consequently, the thickness of the double electric layer decreases.

Dilutions Digest #3

A simplified model of a double (diffuse) electric layer around nanobubbles in an electrolyte solution. Source

Another study of bulk NBs was devoted to their collapse. Scientists from South Korea created stable NBs of hydrogen in water using their own technology, and then subjected the water to ultrasound to destroy them. It was confirmed that when NBs collapse, an almost instantaneous increase in temperature occurs. The scientists believe that this can lead to the thermal destruction of the compounds surrounding the NBs, and the formation of highly reactive radicals. This effect can be used, for example, to fight cancer cells in the human body, as well as in other high-tech areas. The study was published in the Journal of Nanoscience and Nanotechnology.

One example of the potential use of NBs in the future has been suggested by a group of Japanese scientists . It was shown that due to the oxidation process, water containing bulk oxygen NBs contributes to a decrease in the rate of soil depletion during rice cultivation.

A number of studies have also been devoted to the properties of surface NBs. In October 2019, a detailed review dedicated to the current state of this field of science was published in the journal Advances in Colloid and Interface Science. In the study, the authors systematise and compare the main theoretical, experimental, and modelled results of surface NB studies, and discuss the field's development prospects.

Surface NBs are studied at the interface between a liquid and various solid object: various types of electrodes, nanoparticles, and even nanopores. The features of NBs formed on the surface of gold nanoparticles in water are also of interest. Scientists from Sweden and China have dedicated their research to various aspects of this field. Swedish scientists have studied the processes of formation and destruction of NBs which are formed near gold nanoparticles when heated by a laser. Gold nanoparticles are isolated plasmon structures and are used as localised heating elements to generate NBs, which can be modulated at frequencies of up to several kilohertz, orders of magnitude faster than previously demonstrated. It was also shown that the size and lifespan of NBs depend on the size of the nanoparticles and heat energy. Thus, nanoparticles can be a means by which it is possible to perturb the system by precise optical excitation, as well as return it to its original state within milliseconds. The paper was published in November 2019 in the serious journal Nano Letters.

Scientists from China have used gold nanoparticles to study the properties of water around nanoparticles using a specially developed surface plasmon resonance microscopy method. Using the proposed method, it was shown that a low-density water layer called a 'Water depletion layer' approximately 0.6nm thick was present. The layer is homogeneous i.e. does not consist of numerous isolated NBs. This study was published in the journal Analytical Chemistry in September 2019.

In an article published in the same journal in February 2020, scientists from the University of Washington, USA, present images of single NBs of hydrogen and oxygen during the electrolysis of water. In order to study the NBs generated at the electrode-solution interface, a new type of transparent highly conductive catalysed electrode was developed from an indium tin oxide semiconductor material coated with a thin gold-palladium film. The team investigated the nucleation dynamics and the behaviour of oxygen and hydrogen NBs using total internal reflection fluorescence microscopy, considering the difference between NBs of different gases.

Surface NBs can also be obtained using nanopores. On this basis, Scientists from China studied hydrogen nanobubble nucleation dynamics during the chemical reaction of sodium borohydride with water. A thin silicon nitride-based membrane was made containing nanopores with a diameter of less than 10nm. Such pores can simultaneously act as a generator of NBs and as a supersensitive sensor to monitor the hydrogen nanobubble nucleation process.

Articles by Chinese scientists were published in the journal Langmuir in September and November 2019. The first work is devoted to the formation and properties of 'trapped' and surface NBs. 'Trapped' NBs are formed due to gas molecules entering cracks, scratches, or pits on surfaces, and then form as a result of the gas diffusing or expanding, whereas surface NBs form predominantly on smooth surfaces. In the study, NBs were obtained by two methods: spontaneous formation – by the interaction of water with a hydrophobic surface, and the temperature difference method – replacing hot water with cold. With atomic force microscopy, it was shown that these two types of NBs have a similar morphology, however, the 'trapped' NBs are more 'flexible'.

In the second study , atomic force microscopy was also used, but for a different purpose. Using this method, they tested whether surface NBs are stable due to the so-called 'gas tunnel' hypothesis, which had been calculated by molecular dynamics equations. According to this hypothesis, if the gas-solid interaction is sufficiently intense, a 'gas tunnel' can be formed in the volume of the solvent, allowing the transfer of gas molecules along a solid surface between two adjacent surface NBs. In this case, larger NBs will remain stable on account of gas from neighbouring smaller NBs because of pressure differences. Based on the results of a statistical analysis of volume changes in various gas domains (nanobubbles, 'nanopancakes', and nanobubble–pancake composites), the concept of a 'gas tunnel' was theorised and confirmed in experiments. The scientists discovered that 'nanopancakes' between NBs can behave like gas tunnels when the microscope probe interacts with NBs. In addition, the results of the study are evidence of surface NB stability.