The green cones are nanobubbles on a solid substrate. The bubbles have a height of about 10nm and a width of about 1000nm at the base. Source: phys.org.
Scientists agree that nanobubbles (also called 'bubstons' – an abbreviation of bubble, stabilised by ions) exist in solutions in two forms: 'surface' – on the solid surface of the container in which the solution is placed, and 'bulk' – dispersed in the volume of the solution. It has also recently been discovered that there is another type of nanobubble in solutions: micro- or nano-pancakes, up to several nanometres thick, however their structure has not yet been fully studied.
Bubstons are known to have a negative surface charge and can persist in a solution for up to several months. They do not have a shell and can exhibit biological activity in living systems. Due to these properties, bubstons are used in various fields, such as the semiconductor industry, wastewater treatment technology, chemical synthesis, etc. Bubstons can also be used to adsorb macromolecules and self-assemble materials with a specific curvature.
Three types of nanoscopic gas formations. Surface nanobubbles are found at the solid-liquid interface and have curvature radii of 100-1000nm, with a height of 5-20nm and a width of 50-100nm. Bulk nanobubbles are detected in solutions with curvature radii of 50-100nm. Micropancakes are located at the solid-liquid interface, and have a height of 1-2nm with typical width ranging from a few hundred nanometres to micrometres.
Bulk bubstons (also termed 'ultrafine'), are formed in a solution as a result of mechanical or ultrasonic exposure, or by passing the solution through a porous membrane under pressure. Because the range of applications for such bubbles is rather wide, an affordable method of generating them in solutions is needed. A large number of bubston devices are being created. For this purpose, in one of their works, Keiji Yasuda et al suggest using a compact ultrasonic generator (see fig. 2). The authors noted the following advantages of using such a device to create nanobubbles in a solution: compactness, ease of use, and short formation time. As this generator makes it possible to choose the ultrasound's frequency and power, one can control the concentration of nanobubbles in the system. The diameter of such bubbles will be in the range of 90-100nm.
Ultrasound generator for producing nanobubbles.
According to the results of the study, surface nanobubbles are formed on the surface of the substrate introduced into the solution. In his work, Jiachen Wei studied the dependence of bubbles' formation dynamics and stability on the surface composition of such a substrate. As part of this study, several types of substrates were considered: hydrophilic and hydrophobic (homogeneous), as well as compounds of these substrates (heterogeneous). In the study of homogeneous substrates, it was found that a large number of nanobubbles are generated without attaching to the surface. When using a heterogeneous substrate, nanobubbles are fixed and increase the contact area with a solid surface as their concentration in the system increases. Solution microvolume and isolation from external influences were important conditions for the experiments. If the solution is 'opened' (in other words, if there is a transition from microvolumes to macrovolumes and contact with the environment is restored), two conditions must be fulfilled simultaneously in order to ensure the stability of the nanobubbles: their attachment to the surface and an excess concentration in the solution.
Thus, thanks to the discoveries of recent years, nanobubbles are no longer merely a theoretical concept. We now know that they are an integral part of various natural systems, including biological systems. The study of bubston properties is becoming a field of utmost importance in both basic and applied research. Today, the challenge for scientists is to develop approaches to life cycle and sustainability analysis of such bubbles, and also to find opportunities to influence their structure and properties.