Mechanism of the Decrease in Surface Tension by Bulk Nanobubbles (Ultrafine Bubbles)

Free gas micro-/nano-bubble water: a novel dispersion system to prepare ultrasound imaging vehicles

OBJECTIVES

Free gas micro-/nano-bubbles (MNBs) in water have demonstrated significant potential in various industrial applications, including water treatment, enhanced transport processes, and disinfection. However, the feasibility of utilizing MNBs water as a dispersed system for preparing ultrasound imaging vehicles is seldom explored. This study aims to investigate the potential of MNBs water for this purpose.

METHODS

Initially, MNBs water containing sulfur hexafluoride (SF6) was prepared and characterized. Subsequently, the potential of SF6 MNBs water to form lipid-shelled bubbles for ultrasound imaging was evaluated. This involved the incubation of lyophilized phospholipids with SF6 MNBs water.

RESULTS

The study confirmed the presence of SF6 MNBs in water. Through the incubation process, it was possible to obtain lipid-shelled bubbles with a nano-sized and narrow size distribution. These bubbles exhibited comparable echogenicity to those produced by conventional mechanical agitation methods during the initial 5 min of in vitro observation.

CONCLUSIONS

SF6 MNBs water represents a novel dispersion medium for generating nano-sized lipid-shelled bubbles. This approach offers a promising new method for extravascular ultrasound imaging and drug delivery, potentially expanding the applications of MNBs in medical imaging and therapeutic delivery systems.

The Existence and Stability Mechanism of Bulk Nanobubbles: A Review

Since they were shown to be a potential phenomenon through experimentation, bulk nanobubbles (BNBs) have been a long-standing controversy. The controversy mainly originates from the fact that their stability cannot be well explained by the established theories. Although nanobubbles have been applied in many fields, the controversial stability issue has been a hanging "cloud" looming over the nanobubble research. This review focuses on why the stability of nanobubbles cannot be depicted by the current theories from thermodynamics and dynamics perspectives. Moreover, a number of current models pertaining to bulk nanobubble stability are compiled. It is anticipated that this review will give readers a better grasp of the current state of bulk nanobubble research and provide some insight for further studies in this area.

Impact of Water Purity and Oxygen Content in Gas Phase on Effectiveness of Surface Cleaning with Microbubbles

Cleaning of surfaces without complex cleaning agents is an important subject, especially in food, pharmaceutical, and biomedical applications. The subject of microbubble and nanobubble cleaning is considered one of the most promising ways to intensify this process. In this work, we check whether and how the purity of water used for microbubble generation, as well as the gas used, affects the effectiveness of cleaning stainless-steel surfaces. Surfaces contaminated with Pluronic L-121 solution were cleaned by water of three purities: ultrapure water (<0.05 μS/cm), water after reversed osmosis (~6.0 μS/cm), and tap water (~0.8 mS/cm). Similarly, three different gases were supplied to the generation setup for microbubble generation: air, oxygen, and nitrogen. Stainless steel plates were immersed in water during microbubble generation and cleaned for a given time. FTIR (Fourier Transform Infrared Spectroscopy) and contact angle analysis were employed for the analysis of surfaces. The results of cleaning were repeatable between plates and showed different cleaning effects depending on both the purity of water (concentration of ions) and gas composition. We have proposed different mechanisms that are dominant with respect to specific combinations of ion concentration and oxygen content in gas, which are directly connected to the microbubble stability and reactivity of gas.

Nanobubble-enabled foam fractionation to remove algogenic odorous micropollutants

Due to climate change and environmental pollution, natural lakes and reservoir water suffer increasingly serious algal blooms and associated water quality problems due to the presence of algal or algogenic organic matter (AOM) such as algal odour and toxins. Effective removal of these micropollutants, especially in the event of algal blooms, is critical to aesthetic values of water bodies, drinking water security and human health. The study investigated the removal efficiency of two common odorous compounds, trans-1,10-dimethyl-trans-9-decalol (geosmin) and 2-Methylisoborneol (2-MIB), using foam fractionation enabled by air nanobubbles with addition of two common cationic and anionic surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), to enhance foaming ability and stability. The results showed that the cationic surfactant (i.e., CTAB), a low pH, and high ionic strength significantly promoted the removal of geosmin and 2-MIB. For example, the removal tests using the synthetic water determined that the conditions of pH = 7, [CTAB] = 20 mg·L-1 and IS = 10 mM as NaCl resulted in both the highest geosmin removal rate of 91.81% and highest 2-MIB removal rate of 85.0%. The removal of two odorous compounds in real lake water was evaluated, which yielded removal rates of 83.2% for geosmin and 48.1% for 2-MIB, highlighting the minor inhibition from water matrixes on the removal performances. Compared to microbubbles, nanobubbles enabled greater surface areas of foam and higher removal efficiencies. The study provided new insights into the use of foam fractionation with air nanobubbles to enhance the removal of odorous compounds from impaired water and mitigate the negative environmental and health impacts of harmful algal blooms (HABs).

Interfacial Thermal Fluctuations Stabilize Bulk Nanobubbles

Consensus on bulk nanobubble stability remains elusive, despite accepted indirect evidence for longevity. We develop a nanobubble evolution model by incorporating thermal capillary wave theory that reveals that dense nanobubbles generated by acoustic cavitation tend to shrink and intensify interfacial thermal fluctuations; this significantly reduces surface tension to neutralize enhanced Laplace pressure, and secures their stabilization at a finite size. A stability criterion emerges: thermal fluctuation intensity scales superlinearly with curvature: sqrt[⟨h^{2}⟩]∝(1/R)^{n}, n>1. The model prolongs the time frame for nanobubble contraction to 2 orders of magnitude beyond classical theory estimates, and captures the equilibrium radius (90-215 nm) within the experimental range.

Understanding the Role of Surface Charge on Nanobubble Capillary Bridging during Particle-Particle Interaction

The interactions between particles due to long-range hydrophobic forces have been extensively investigated. The hydrophobic force is likely a capillary force that arises from the formation of capillary bridges due to the merging of nanobubbles. In this study, we aim to investigate the impact of the nanobubble surface charge on the capillary bridge and, subsequently, the interaction between particles. The surface charge of the nanobubbles was altered in the presence of various surfactants (cationic, anionic, and nonionic) and salts (mono-, di-, and trivalent). The particle-particle interaction was quantified by measuring the aggregate size of the hydrophobized glass particles. Both experimental and theoretical findings confirm that the interaction between particles was enhanced when the surface potential of the nanobubble was around the neutral regime. This is probably because, when the surface potential was close to neutral, the interaction between two surface-deposited nanobubbles dominated over electrostatic repulsion, which was more conducive to the formation of the nanobubble capillary bridge. The estimation of the constrained Gibbs potential also showed the capillary bridge to be more stable when surface charge density along the bridge gas-liquid interface was minimal.