Molecular dynamics investigation of nanobubble formation and their role in thermophysical and thermochemical behaviour of polar and nonpolar liquids

Abstract

The rapid increase in atmospheric greenhouse gas concentrations, particularly CO₂ emissions from anthropogenic activities, poses a significant global challenge requiring urgent mitigation. Key strategies include improving energy efficiency through advanced thermal management and heat-transfer technologies, developing engineered working fluids, and deploying next-generation fuels alongside carbon capture and storage (CCS) technologies. Growing attention is being directed toward nanoparticle incorporation and nanoscale mechanisms to advance these strategies by enhancing fluid and fuel properties, accelerating hydrate formation, and improving overall process efficiency. Nanobubbles (NBs), defined as gaseous cavities smaller than 1 μm, exhibit exceptional stability, high surface-to-volume ratios, and the capacity for free radical generation, highlighting their potential in energy, power, and environmental applications. A fundamental understanding of NB nucleation and behaviour is essential to elucidate their effects on the thermophysical properties of host liquids and on key processes such as CO₂ hydrate formation and combustion, which are difficult to capture experimentally. Classical and reactive molecular simulation techniques provide a rigorous approach for probing the influence of NBs on intrinsic material behaviour, process kinetics, and reaction pathways at the molecular scale. Integrating data-driven approaches with molecular dynamics (MD) simulations enables systematic characterization of NB dynamic behaviour and properties. This study investigates the possible formation and effects of NBs generated by nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), and hydrogen (H₂) on the thermophysical properties of four host liquids, including dodecane, isooctane, water, and methanol, examines solid–liquid–gas interactions in graphene-enhanced water and methanol nanofluids (NFs), explores the phase transition and thermal behaviour of NB-enhanced dodecane and its combustion processes, and evaluates their potential impact on CO₂ hydrate formation using classical and reactive MD simulations. Furthermore, the DBSCA (density-based spatial clustering of applications with noise) algorithm was utilised to analyse the formed NB clusters, assessing their size, density, and motion over time in water and methanol as polar and alcoholic host liquids. The findings reveal that the formation of NBs reduces the density of polar and alcoholic liquids due to the establishment of a nanolayer. The presence of NBs and dispersed graphene increases the viscosity of water-based NFs but decreases viscosity in graphene-methanol NFs containing two-atom gases. Under optimal thermodynamic conditions, NBs markedly enhance hydrate formation by providing hydrophobic nucleation sites, generating local concentration gradients, and accelerating clathrate kinetics, with hydrogen and nitrogen NBs further promoting growth through rapid core-to-solution diffusion. Low temperatures suppress molecular mobility and hinder these effects. At elevated pressures, NBs exhibit a dual role, with a pressure-dependent shift from NB-induced interfacial ordering to bulk-phase interactions. In organic host liquids, NB formation and coalescence are governed by molecular interactions between the host liquid and the dispersed gas, as demonstrated by oxygen forming NB in dodecane but not in isooctane. The presence of NBs, particularly hydrogen NB, depresses the liquid-to-solid transition temperature of dodecane and enhances both thermal conductivity and specific heat capacity, with nitrogen NB yielding the highest thermal conductivity increase of approximately 14%. Nitrogen and hydrogen NBs also exhibit excellent stability over a wide temperature range, making them promising candidates for high-temperature applications. Furthermore, nitrogen NB reduce the activation energy to 52.89 kcal/mol in samples with a density of 0.17 g/mL by promoting the formation of intermediates and radicals, thereby accelerating dodecane consumption, whereas oxygen NB increase the activation energy to 66.93 kcal/mol under the same conditions.