Understanding the Stabilization of a Bulk Nanobubble: A Molecular Dynamics Analysis

Abstract

Bulk nanobubbles (NBs) have received considerable attention because of their extensive potential applications, such as in ultrasound imaging and water management. Although multiple types of experimental evidence have supported the existence and stabilization of bulk NBs, the underlying mechanism remains unclear. This study numerically investigates the bulk NB stabilization with molecular dynamics (MD) methods: the all-atom (AA) MD simulation is used for NBs of several nanometers diameter; the coarse-grained (CG) MD simulation is for the NBs of about 100 nm. The NB properties are statistically obtained and analyzed, including the inner density, inner pressure, surface charge, interfacial hydrogen bond (HB), and gaseous diffusion. The results show that the gas inside an NB has ultrahigh density (tens of kilograms per cubic meter). A double-layer surface charge exists on the NB. The inner/outer layer is positively/negatively charged, and the electrostatic stress can counteract part of the surface tension. In addition, the interfacial HB is weakened by the interaction between gas and water molecules, causing less surface tension. The above features are beneficial to NB stabilization. The NB equilibrium radii solved by the interfacial mechanical equilibrium equation agree with the MD results, indicating that this equation can describe the force balance of an NB as small as several nanometers. Besides, supersaturation appears to be necessary for the NB thermodynamic equilibrium. Based on Henry’s law and the ideal gas law, the theoretical analysis suggests that the stability of the NB thermodynamic equilibrium is conditional: the number of gas molecules in NBs should be more than half that dissolved in liquid. This study unravels a stabilized bulk NB’s properties and discusses the NB equilibrium and stabilization mechanism, which will advance the understanding and application of bulk NBs.

On Some Aspects of Nanobubble-Containing Systems

“…With regard to the high inner-density model that the lifetime of UFBs is longer as the density inside a bubble is higher, experimental evidence is required [38]. As the existence of the boundary layer of a UFB has been confirmed experimentally [39], its role on stability of a UFB should be studied further [40,41]. Other models for the stability of a UFB are discussed in Refs.…”

Analysis of Gas Nanoclusters in Water Using All-Atom Molecular Dynamics

“…While direct experimental observations are lacking, molecular simulation is a great tool for gaining important insights into the interfacial properties of gas clusters and the stability of bulk nanobubbles. However, expensive computational overhead has heretofore hindered the analysis of nanobubbles with radii larger than tens of nanometers …”

“…During the last 20 years, the stability of interfacial nanobubbles (INB) has been attributed to surface pinning, gas oversaturation conditions, high gas epitaxial layering density, and nonuniform gas solubility in the water region. − Nonetheless, these explanations were derived from fluid–solid interactions, which are not necessarily sufficient to explain the existence of bulk nanobubbles. Indirect experimental observations have verified that the material inside BNBs is fluid; however, speculations abound as to whether the fluid is a gas or liquid. ,, Researchers have posited a number of theories to explain the stability of BNBs. ,− One hypothesis proposed the Tolman effect, wherein interfacial tension decreases proportionally to the size of the bubble. However, this size-dependent contribution remains insignificant unless the bubble size is less than 1 nm .…”

“…However, expensive computational overhead has heretofore hindered the analysis of nanobubbles with radii larger than tens of nanometers. 8 During the last 20 years, the stability of interfacial nanobubbles (INB) has been attributed to surface pinning, gas oversaturation conditions, high gas epitaxial layering density, and nonuniform gas solubility in the water region. interactions, which are not necessarily sufficient to explain the existence of bulk nanobubbles.…”

Molecular Approach for Understanding the Stability, Collision, and Coalescence of Bulk Nanobubbles

“…Weijs et al have shown that supersaturation of the surrounding medium stabilizes the NBs. Gao et al have demonstrated the presence of EDL of thickness ∼10 Å. The surface charges are present due to the orientation of water around the NBs.…”