Study on Nanobubble-on-Pancake Objects Forming at Polystyrene/Water Interface

Toward a Comprehensive Understanding of the Anomalously Small Contact Angle of Surface Nanobubbles

Experimental studies have demonstrated that the gas phase contact angle (CA) of a surface nanobubble (SNB) is much smaller than that of a macroscopic gas bubble. This reduced CA plays a crucial role in prolonging the lifetime of SNBs by lowering the bubble pressure and preventing gas molecules from dissolving in the surrounding liquids. Despite extensive efforts to explain the anomalously small CA, a consensus about the underlying reasons is yet to be reached. In this study, we conducted experimental investigations to explore the influence of gas molecules adsorbed at the solid-liquid interface on the CA of SNBs created through the solvent exchange (SE) method and temperature difference (TD). Interestingly, no significant change is observed in the CA of SNBs on highly oriented pyrolytic graphite (HOPG) surfaces. Even for nanobubbles on micro/nano pancakes, the CA only exhibited a slight reduction compared to SNBs on bare HOPG surfaces. These findings suggest that gas adsorption at the immersed solid surface may not be the primary factor contributing to the small CA of the SNBs. Furthermore, the CA of SNBs formed on polystyrene (PS) and octadecyltrichlorosilane (OTS) substrates was also investigated, and a considerable increase in CA was observed. In addition, the effects of other factors including impurity, electric double layer (EDL) line tension, and pinning force upon the CA of SNBs were discussed, and a comprehensive model about multiple factors affecting the CA of SNBs was proposed, which is helpful for understanding the abnormally small CA and the stability of SNBs.

Manipulating Trapped Nanobubbles Moving and Coalescing with Surface Nanobubbles

Trapped nanobubbles are observed nucleating at nanopits on a pitted substrate, while surface nanobubbles are usually formed on the smooth solid surface in water. In this work, trapped nanobubbles and surface nanobubbles were captured by a tapping-mode atomic force microscope (AFM) on a nanopitted substrate based on the temperature difference method. A single trapped nanobubble was manipulated to change into a surface nanobubble, then to change into the trapped nanobubble again. At the same time, surface nanobubbles can be moved to merge into a trapped nanobubble. Our results show that the scan load and the size of the scan area were the main factors that significantly affect the mobility of surface/trapped nanobubbles. The coalescence and mutual transformation of the two kinds of nanobubbles indicate that trapped nanobubbles and surface nanobubbles have the same chemical nature, which also provides vital experimental proof of the existence of nanobubbles in the course of contact line depinning. Our results are of great significance for understanding nanobubble stability and providing guidelines in some industrial applications.

Interfacial Micropancakes: Gas or Contaminations?

Micropancake, a flat domain with micrometer-scale lateral size and a few nanometer thickness, is usually accompanied by the generation of interfacial nanobubbles at the liquid/solid surfaces. Unlike the nanobubbles, micropancakes are difficult to be produced efficiently, impeding further investigations of their mysterious properties. Very recently, An et al. even argued that the previously observed micropancakes were most likely the contaminate, not the gas layers. Herein, to reveal the nature of micropancakes with solid evidence, we presented the in situ characterization of micropancakes at a highly oriented pyrolytic graphite (HOPG) surface produced by the ethanol-water exchange or gas-supersaturated water. By washing with deeply degassed water (DW), the dissolution of those micropancakes was clearly observed, indicating that they may very well be composed of gas. In addition, the analysis of the force measurements showed the intrinsic differences between those gaseous micropancakes and the insoluble organic films. The data and results supported the interpretation that the real existence of gas micropancakes at liquid/solid surfaces.

Unraveling the effects of gas species and surface wettability on the morphology of interfacial nanobubbles

The morphology of interfacial nanobubbles (INBs) is a crucial but controversial topic in nanobubble research. We carried out atomistic molecular dynamics (MD) simulations to comprehensively study the morphology of INBs controlled by several determinant factors, including gas species, surface wettability, and bubble size. The simulations show that H₂, O₂ and N₂ can all form stable INBs, with the contact angles (CAs, on the liquid side) following the order CA(H₂) < CA(N₂) < CA(O₂), while CO₂ prefers to form a gas film (pancake) structure on the substrate. The CA of INBs demonstrates a linear relation with the strength of interfacial interaction; however, a limited bubble CA of ∼25° is found on superhydrophilic surfaces. The high gas density and high internal pressure of the INBs are further confirmed, accompanied by strong interfacial gas enrichment (IGE) behavior. The morphology study of differently sized INBs shows that the internal density of the gas is drastically decreased with the bubble size at the initial stage of bubble nucleation, while the CA remains almost constant. Based on the simulation results, a modified Young's equation is presented for describing the extraordinary morphology of INBs.

Gas-stabilizing nanoparticles for ultrasound imaging and therapy of cancer

The use of ultrasound in the clinic has been long established for cancer detection and image-guided tissue biopsies. In addition, ultrasound-based methods have been widely explored to develop more effective cancer therapies such as localized drug delivery, sonodynamic therapy, and focused ultrasound surgery. Stabilized fluorocarbon microbubbles have been in use as contrast agents for ultrasound imaging in the clinic for several decades. It is also known that microbubble cavitation could generate thermal, mechanical, and chemical effects in the tissue to improve ultrasound-based therapies. However, the large size, poor stability, and short-term cavitation activity of microbubbles limit their applications in cancer imaging and therapy. This review will focus on an alternative type of ultrasound responsive material; gas-stabilizing nanoparticles, which can address the limitations of microbubbles with their nanoscale size, robustness, and high cavitation activity. This review will be of interest to researchers who wish to explore new agents to develop improved methods for molecular ultrasound imaging and therapy of cancer.

Recent advances for understanding the role of nanobubbles in particles flotation

Traditional froth flotation is the primary method for the separation and upgrading of fine mineral particles. However, it is still difficult for micro-fine and low-quality minerals to effectively separate. It is generally believed that bubble miniaturization is of great significance to improve flotation efficiency. Due to their unique physical and chemical properties, the application of nanobubbles (NBs) in ore flotation and other fields has been widely investigated as an important means to solve the problems of fine particle separation. Therefore, a fundamental understanding of the effect of NBs on flotation is a prerequisite to adapt it for the treatment of fine and low-quality minerals for separation. In this paper, recent advances in the field of nanobubble (NB) formation, preparation and stability are reviewed. In particular, we highlight the latest progress in the role of NBs on particles flotation and focus in particular on the particle-particle and particle-bubble interaction. A discussion of the current knowledge gap and future directions is provided.

Probing the "Gas Tunnel" between Neighboring Nanobubbles

Surface nanobubbles are the main gaseous domains forming at solid-liquid interfaces, and their abnormally long lifetime (stability) is still an open question. A hypothesis "gas tunnel" was presented in a recent simulation study [ACS Nano 2018, 12 (3), 2603-2609], which was thought to connect two neighboring nanobubbles and make the nanobubbles remain stable. Herein, we aim to experimentally investigate the existence of gas tunnel and its role in governing nanobubble dynamics. By using an atomic force microscope, mutual effects between different gaseous domains including nanobubbles, nanopancakes, and nanobubble-pancake composite on a PS substrate undergoing violent tip perturbation and their effects on the undisturbed neighbors were investigated. The pancake between two nanobubbles can behave as a visible gas tunnel under the tip-bubble interaction. Based on statistical analysis of volume change in the different gas domains, the concept of a generalized gas tunnel is presented and experimentally verified. Nanobubbles are surrounded by a water depletion layer which will act as a channel along solid/liquid surfaces for adjacent nanobubbles to communicate with each other. Moreover, the change in contact angle of nanobubbles with the concentration of local gas oversaturation was studied, and the equilibrium contact angle of nanobubbles is further verified experimentally.

Study on the Formation and Properties of Trapped Nanobubbles and Surface Nanobubbles by Spontaneous and Temperature Difference Methods

Trapped nanobubbles are gas domains trapped at nanopits on the solid-liquid interface. This is different from surface nanobubbles that usually form at the smooth surface. Herein, both trapped nanobubbles and surface nanobubbles formed on the nanopitted polystyrene film were studied by a spontaneous formation method and a temperature difference method. Trapped nanobubbles behave more flexibly than surface nanobubbles under different scanning loads. The nanopits under trapped nanobubbles appear after being subjected to large force scanning, and both trapped nanobubbles and surface nanobubbles can recover after reducing the scan load. The contact angles of the two kinds of nanobubbles were calculated and were found to be approximately constant. Configurations of trapped nanobubbles including under the pit mouth, protruding out but pinning at the pit mouth, and protruding out and extending around the pit mouth were experimentally observed. Gas oversaturation in the liquid after replacing the low-temperature water with high-temperature water was evaluated and was found to be a key factor for nanobubble formation and led to trapped nanobubbles protruding out and extending. Our study should be helpful in understanding the formation mechanism and properties of trapped nanobubbles and surface nanobubbles, and it will also be useful for further research on the control of nanobubble distribution.

Properties of Blisters Formed on Polymer Films and Differentiating them from Nanobubbles/Nanodrops

When studying surface nanobubbles on film-coated substrates, a class of bubble-like domains called blisters are probably forming at the solid-liquid interface together with nanobubbles. This may easily lead to a misunderstanding of the characteristics and applications of surface nanobubbles and thus continue to cause problems within the nanobubble community. Therefore, how to distinguish surface nanobubbles from blisters is a problem. Herein, the morphology and properties of blisters are investigated on both smooth and nanopitted polystyrene (PS) films in degassed water. The morphology and contact angle of blisters are similar to those of surface nanobubbles. However, blisters were observed to be punctured under the tip-blister interaction and be torn broken by an atomic force microscope tip during the process of scanning. At the same time, nanopits on the surface of blisters that formed on a pitted PS film can be seen clearly. These provide direct and visual evidence for distinguishing blisters from surface nanobubbles. In addition, surface nanobubbles and blisters on smooth and pitted PS films in air-equilibrated water are studied. No punctured surface nanobubble was observed, and the force curves obtained on surface nanobubbles and the change in height of blisters and surface nanobubbles under a large scanning force show that surface nanobubbles are much softer than blisters.

Revealing the Activity Distribution of a Single Nanocatalyst by Locating Single Nanobubbles with Super-Resolution Microscopy

It is challenging to uncover the catalytic activity at different locations of a single nanocatalyst for gas-generating reactions in real time. This research uses super-resolution microscopy to localize the center of single nanobubbles and reveal the local activity distribution at several to tens of nanometers accuracy. The distances between the centers of the nanobubbles and the center of the nanoplate usually distribute in a certain range from 0 to 500 nm, with the maximum population exhibiting at ∼200 nm. This research also shows that more nanobubbles appear near the tips of the Pd-Ag nanoplate compared with the edges, which indicates higher activity at the tips. In addition, the relationship between the location, lifetime, and turnover rate of the nanobubbles was also carefully studied. This work presents an effective, high-resolution method to localize the activity distribution of nanocatalysts during gas-generating reactions, such as photocatalytic water splitting, dehydrogenation, and electro-oxidation.

Study of the Relationship between Boundary Slip and Nanobubbles on a Smooth Hydrophobic Surface

Surface nanobubbles, which are nanoscopic or microscopic gaseous domains forming at the solid/liquid interface, have a strong impact on the interface by changing the two-phase contact to a three-phase contact. Therefore, they are believed to affect the boundary condition and liquid flow. However, there are still disputes in the theoretical studies as to whether the nanobubbles can increase the slip length effectively. Furthermore, there are still no direct experimental studies to support either side. Therefore, an intensive study on the effective slip length for flows over bare surfaces with nanobubbles is essential for establishing the relation between nanobubbles and slip length. Here, we study the effect of nanobubbles on the slippage experimentally and theoretically. Our experimental results reveal an increase from 8 to 512 nm in slip length by increasing the surface coverage of nanobubbles from 1.7 to 50.8% and by decreasing the contact angle of nanobubbles from 42.8 to 16.6°. This is in good agreement with theoretical results. Our results indicate that nanobubbles could always act as a lubricant and significantly increase the slip length. The surface coverage, height, and contact angle are key factors for nanobubbles to reduce wall friction.