Theoretical And Molecular Simulation Studies On Micro-and Nano-bubbles And Droplets

Fluids(including liquids and gases)that are ubiquitous in nature and life are commonly inhomogeneous microscopically,containing micro-and nano-particles(immiscible micro-and nanobubbles and droplets,etc.).The existence of these particles will greatly change the properties of macroscopic fluids,such as tensile strength,cavitation pressure and interface properties.These characteristics promote the industrial applications of micro-and nano-bubbles and droplets.However,due to the limitations of current experimental methods,we still lack of micro-cognition and theoretical explanation of the state and properties of these tiny particles.Therefore,in this thesis,the molecular dynamics(MD)simulation method and thermodynamic theory are applied to study the stability mechanism of micro/nano bubbles and droplets and the effect surfactants on the interfaces of these tiny particles.The main contents are as follows:1.First,combining molecular dynamics simulation and thermodynamic theoretical analysis,two mechanisms for the stability of bulk nanobubbles are proposed: i)surface enrichment of ions leads to stability of bulk nanobubbles.The electric field by ion enrichment promotes the thermodynamic metastability state of nanobubbles.The excess surface charges mechanically generate a size-dependent force,which balances the Laplace pressure and acts as a restoring force when a nanobubble is thermodynamically perturbed away from its equilibrium state;ii)Secondly,it is found that the adsorbed surfactant-like molecules,with their amphiphilic character,will affect the dissolution of the existing bulk nanobubbles under low gas supersaturation environments.Depending on the concentration of the dissolved gas and the molecular structure of surfactants,two fates of bulk nanobubbles whose interfaces are saturated by surfactants are found: either remaining stable or being completely dissolved.Two basic conditions for stable nanobubbles at low gas saturation are identified: vanishing surface tension due to bubble dissolution and positive spontaneous curvature of the surfactant monolayer.2.Secondly,thermodynamic theory and molecular dynamics simulation are used to study the effect of interface properties on the molecular state in the bubble.The van der Waals fluid model with surface tension is used to calculate the size fluctuation of the droplet for various confinement conditions,NVT,NPT,and NBT(in which the system is confined in a nano-bubble immersed in a host liquid).The results show that in the NBT system,the size flexibility along with space confinement induces a wealth of properties that are not found in NVT and NPT.It exhibits richer phase behaviors.The NBT system shows not only the oscillatory fluctuation between the two stable states but also a large fluctuation in the total volume and the pressure.In addition,molecular dynamics simulation reveals that if the interface effect is not enough to affect the entire nanobubble,the internal pressure of stable nanobubbles is independent of bubble radius and surface tension(via adding of surfactant molecules).Below the critical size,the macroscopic thermodynamics breaks down,and there does not exist a clear-cutting separation of bubble interface and bubble core.3.By combining molecular dynamics simulation and thermodynamic theoretical analysis,we study the effect of surfactant on the state of dissolved gas in solution and the properties of liquid film.Thermodynamic theoretical analysis reveals that an interesting reversible dissolution-nanobubbles-dissolution transition for gas molecules in ethanol/water mixtures appears as the fraction of ethanol varies.Besides,we show with molecular dynamics simulations that there exists another kind of capillary pressure.Our simulation demonstrates the existence of such a pressure difference for the gas-liquid interface covered by compressed surfactant monolayer,even though the interface is flat.We further demonstrate with theoretical analysis that for this kind of capillary pressure,it is the difference of interfacial curvature and spontaneous curvature of the monolayer that induces the pressures difference.Similarly,this kind of capillary pressure also show a size effect,i.e.,the capillary pressure decreases with the increase of the monolayer size,as a consequence of the size-dependence of the effective bending rigidity of the monolayer.4.Finally,Lattice Boltzmann simulation and thermodynamic theory are used to study the reverse Oswald ripening mediated by scaffolds.The results show that through pinning three phase contact line of sessile droplets,heterogeneous substrates or solid nanoparticles can behave as a scaffold to suppress Oswald ripening,to regulate droplet morphology and to enhance droplet stability.Different from the conventional methods by functionalizing nanoemulsions with adequate ripening inhibitors,an alternative strategy to stabilize nanoemulsions by inhibiting Ostwald ripening was proposed.