Characteristics of the interfacial region provide researchers with useful information about engineering processes and the forces that act on these surfaces are of a particular interest . The ability to study such interactions has been made possible thanks to the development and creation of surface force technology, first introduced in 1961 , followed by the atomic force microscope (AFM) in 1986 .
The original DLVO-theory is typically used to quantify the interactions between two surfaces, however despite its popularity, it does not provide an explanation as to how two hydrophobic surfaces can come into contact with each other in an aqueous solution.
It is believed that there are additional non-DLVO forces that are also active and have different properties, thereby increasing attractive interactions . Where these ‘hydrophobic forces’ are coming from is not known, and there is much debate over whether they should be classed as solvation forces or a type of far-reaching electrostatic or a Van der Waals interaction.
Presently, it is largely agreed upon in the community that ‘hydrophobic forces’ are made up of two parts: a close-ranged true hydrophobic and a far-reaching force, facilitated by capillary interactions.
The idea of tiny bridging bubbles being the underlying cause of far-reaching and big interactions between hydrophobic surfaces was first announced in 1994 by Parker et al. . His research group witnessed steps in force-distance graphs and perceived them as coalescing nanobubbles.
A considerable amount of time passed before a separate protocol was produced that could obtain consistent results and the generation of nanobubbles on surfaces, by Lou et al.  and Ishida et al.  in 2000.
Following this, a wave of experiments were carried out on producing nanobubbles , attempting to improve their stability , adjust their size and geometry [10-13], as well as to make better detection methods , and consider what impact they could have on engineering processes, as well as other applications.
For the use in the majority of experiments, nanobubbles are produced by what is known as the solvent exchange method: this involves a liquid (typically ethanol) which has high gas solubility and is exchanged for a liquid with less solubility. This results in an oversaturation of local gas and the nucleation of the bubbles .
An alternative method founded on a similar principle of oversaturating with gas is as follows: The researcher heats up the sample liquid forming a temperature gradient in the immediate surrounding area, which creates bubbles on the surface .
There are still disagreements over the existence of nanobubbles, since theoretically they should dissolve in microseconds  but have been shown to be present even after hours and days have elapsed . One theory, presented by Brenner and Lohse  is a model that pins the stable nature of the bubbles on a dynamic equilibrium between the flow of gas molecules in and out of the nanobubble, which had work carried out by Yasui et al. coming to similar conclusions .
A thick layer of gas sitting upon the surface of the bubble produces sufficient gas for the influx in the bubble. It is now thought that the prolonged stability of nanobubbles is a result of both supersaturation and contact line pinning [22, 23] which complements the dynamic equilibrium model.
Zhang et al. have observed a reduction in the size of the nanobubble contact angle in replace of lateral size . The bubble remains fixed on the three phase contact line, the Laplace pressure reduces to zero and no sizeable inner pressure or concentration gradient between the nanobubble and the surrounding environment happens. Fixation occurs as a result of chemical and morphological inhomogeneities, even on surfaces that are not dirty or rough.
Figure 1. 3D topographical scan (isotropic axes) of a hydrophobic rough surface covered with small cap-shaped nanobubbles generated via gas oversaturation (left), geometrical parameters (middle), and scheme of stability theory (right).
Phase contrast imaging can be used to differentiate between contaminations and nanobubbles. Previously, the majority of nanobubble research was carried out using atomic force microscopy (AFM) due to its powerful properties in analyzing microscopic surfaces. In theory, AFM can be used to scan the surface for the detection of bubbles directly, by changing between different scanning modes [25, 26] as well as indirectly by force spectroscopy .
For example, contact mode is inappropriate because of the lateral force it applies to the nanobubbles, which causes them to coalesce or disconnect from the surface. However, this mode has shown that they have a soft cap-shaped dome .
Typically, intermittent/Tapping® mode is most suitable for identifying bubbles, this is because of the minimal lateral forces as a result of cantilever oscillation. It is important that the amplitude ratios (set point amplitude to free amplitude) are kept greater than 0.7, or else there is a high chance of distorting the bubbles [28, 29] affecting the result. It is common practice to run phase contrast imaging alongside this technique in order to improve the differentiation of the bubble from the surface.
Rarely, frequency modulation, or true contact mode is implemented. This involves the cantilever oscillating at its resonant frequency near to the sample containing nanobubbles with a small amplitude but never touching it. Advantageously, this mode has minimal distorting effects to the nanobubbles.
More recently, the PeakForce® mode is another popular mode option for the identification of nanobubbles. PeakForce® mode monitors the highest load force on the probe in real time by operating at frequencies less than the resonance. This is more beneficial than tapping mode, which struggles to quantify this force. The chosen imaging force influences the degree of bubble deformation, for example a greater force will result in more damage.
A soft and sharp cantilever is required to identify nanobubbles because of the soft material properties of the bubbles. Generally, cantilevers that have spring constants of between 0.2 and 4.8 N/m are chosen. When able to, adjustments in relation to the geometrical size of the nanobubbles should be made, unless using AFM mode, because otherwise the cantilever tip will affect the scanning results .
AFM has the advantage of being able to produce three-dimensional details, however there are some disadvantages that come with this technique. For example, it is common and easy to a deform the nanobubbles and you are not able to differentiate between gaseous molecules and contamination. Therefore it is crucial to carry out investigations under sterile conditions, for example ensuring glass syringes and cantilevers are clean .
To overcome this, there are a number of different observation techniques that can be implemented to detect nanobubbles, such as infrared spectroscopy , rapid cryofixation , interference-enhanced reflection microscopy , confocal microscopy or attenuated total internal reflection microscopy in combination with other spectroscopic methods .
Figure 3. Evaluation of nanobubbles on a rough hydrophobic surface via ImageJ by binarization (a certain threshold has to be chosen), geometrical size distributions of nanobubbles, and dependency of contact angle on height for different temperature gradients.
Figure 4. 3D and 2D topographical scan of a hydrophobic rough particle with nanobubbles (top left and right) with corresponding phase contrast image (bottom right) and line profile (bottom left).
The information found from investigations into nanobubbles helps to advance areas of engineering processes in agglomeration, filtration, or flotation.
Mineral flotation is an engineering technique which isolates hydrophobic from hydrophilic materials and contributes to understanding the attachment between nanobubble and valuable material, which is extremely useful. Typically, desired flotation quality is restricted to particles that are sized between 10 and 100 µm, when using minerals, or 50 to 600 µm, when using coal.
The recovery of finer particles can be made greater with the use of nanobubbles by making film rupture better [39, 40] and researchers have looked into their use for applications in chalcopyrite, coal, quartz, and scheelite. Not as much frother is needed due to their ability to stabilize the froth and so the required quantity of air is significantly less .
A high proportion of submicron bubbles can be successfully produced by ultrasound or hydrodynamic cavitation . Advancements have been happening in the industry of nanobubble-nanoparticle contact. In contrast to froth flotation, particle-bubble contact cannot be obtained by collision, however can be produced by the recently practiced nucleation of nanobubbles directly onto the nanoparticles itself .
Agglomeration is an additional engineering technique and finds itself also being heavily affected by the bubbles being present. Nanobubbles facilitate and promote agglomeration between hydrophobic particles because of the capillary interactions between them, an event that also occurs for fine as well as rough particles .
Figure 5. Agglomerate size (left) and particle-particle interaction dependency on wetting properties. The formation of nanobubbles leads to larger adhesive forces and therefore larger agglomerates.
The field of nanobubbles and their real-life applications has an exciting and optimistic future. There are many more promising areas that nanobubbles can be applied to which have not been discussed here such as wastewater treatment , micro- and nanofluidics  or electrolysis . The research into nanobubbles has interesting opportunities ahead and will contribute greatly to science.
Original authors: Lisa Ditscherlein, Scientific Researcher (PhD) at MVTAT, TU Bergakademie Freiberg, Mechanical Process Engineering and Material Processing, Specialization Particle Technology.
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