Molecular Simulation And Theoretical Study On The Properties Of Gaseous Nanostructures Immersed In Liquids

Over recent decades there emerged numerous gaseous nanostructures,such as interfacial(surface)nanobubbles,bulk nanbubbles and gas micropancakes,observed experimentally in liquid.Some of them show extraordinary stability,and are proven to have significant potential applications in many fields,including flotation,slippage of fluid,targeted drug delivery,promoting growth of plants and animals.Although there have extensive experimental studies on the gaseous nanostructures,a number of controversies still exist.For example,the actual behaviors of interfacial nanobubbles are still largely unknown due to the limits of experimental mesurement techniques.Similarly,the stability mechanisms for bulk nanobubbles and micropancakes are still a matter of fierce debate.Thus,in this thesis,we devote to investigate both the thermodynamic and dynamical properties of those gaseous nanostructures via computer simulations and theoretical analysis.Our research contents are as follows:1.The interaction between AFM tips and interfacial nanobubbles.We employed constraint lattice density functional theory(CLDFT)to investigate the interaction between AFM tips and interfacial nanobubbles systematically,and the effects of the wetting properties and the shape of AFM tips were discussed,respectively.In general,our obtained force curve is qualitatively in a good agreement with the experimental results.We also determined the responses of nanobubbles to the approaching/departing tips of various hydrophilicity.Under the effect of hydrophilic tips,surface nanobubble show elastic response,while interacting with hydrophobic tips,the viscous behaviors are observed for the nanobubbles.This indicates that the model discussed here catches the main intergradient of tip-nanobubble interactions for interfacial nanobubbles and may therefore provide important insight into how to design minimally invasive AFM experiments.2.Surface nanobubbles under undersaturated liquid.The effect of the degree of the gas supersaturation in the liquid on the stability of surface nanobubbles was investigated via constraint lattice density functional theory(CLDFT),and we explored the possibility of the existence of surface nanobubbles under undersaturated liquid.We demonstrated surprisingly that there exists a new type of surface nanobubble having a gas-liquid interface of negative curvature,in contrast to normal surface nanobubbles experimentally observed that always have a gas-liquid interface of negative curvature.The newly observed surface nanobubbles have some similar properties as traditional surface nanobubbles:they also needed contact line pinning to make them stable;the contact angle of the bubbles depends on the bubble size,but is independent on the properties of the substrates;the curvature radius of the bubbles depends on the chemical potential of the liquid,but is independent on the lateral radius.Finally,we provided some possible substrates where this hidden nanobubble can appear.3.Oscillation of nanodroplet formation/rupture transition in pinned surface nanobubbles with coupled fluctuation.The phase behavior of insoluble solute molecules inside pinned surface nanobubbles was investigated via molecular dynamics(MD)simulation.It is observed that with different number of solute molecules or external pressure,there are three phase states occurring inside surface nanobubbles:stable vapor state,stable nanodroplet-contained state,and two-state oscillation between droplet formation and rupture.At the same time we explored the relationship between the fluctuation of nanobubble interface and the oscillation of nanodroplet formation and rupture.We observed that the nanobubble breathing would be the key factor for the appearance of two-state oscillation between nanodroplet formation and rupture,which would in turn affect the nanobubble fluctuation.4.Coupling of gas supersaturation and vapor supersaturation on nucleation of the vapor/liquid transition.We considered the nucleation process occurring in pure liquid and in gas-containing solution,respectively,and the separated contributions of dissolved gas and superheating were analyzed via using molecular dynamics(MD)simulations and theories.Different from the nucleation in pure liquid,the dissovled gas would play a key role in the nucleation process under gas-containing solution.With increasing the concentration of gas dissolved,the supersaturation of the solvent or both of them,the nucleation barrier would decrease in different manners and the nucleation process became more easily to occur.Through MD simulations,it was observed that when the concentration of dissolved gas becomes sufficiently high,the gas molecules in the liquid would gather together and result in the spinnodal or low-barrier nucleation of bubble formation.In addition,within a given confining space and at the fixed concentration of gas molecules,a bulk nanobubble could be stabilized due to the finite size effect.However,at open environment,it becomes unstable,indicating that the bulk nanobubble need additional mechanism to maintain it stability.5.Stability mechanism of bulk nanobubbles.With thermodynamic and dynamic analysis,we attributed the stability of bulk nanobubbles to the dissolvable amphiphilic molecules(DAMs)abound on the gas-liquid interface of the bubbles:the adsorbed DAMs could decrease/increase the interface tension in response to bubble shrinking/growing,which provides a negative feedback mechanism to make the nanobubble stable in bulk liquid.Through dynamics diffusion theory and thermodynamics force equilibrium theory,we obtained the range of DAMs concentrations in which the bulk nanobubbles could be stable,and the obtained results are consistent with the experimental results.Furthermore,the size of the bulk nanobubbles is demonstrated to depend on the gas supersaturation and DAMs concentration.6.Stability mechanism of gas micropancakes.Our model calculations within the framework of CLDFT show that micropancakes surrounded by solvent on homogeneous surfaces cannot live stably.However,the collective interaction with neighboring posts on substrates,which was introduced to represent surface heterogeneity,induces the formation of large stable micropancakes.We demonstrated that the micropanke stability is enhanced by the taller and denser posts and the regular distribution of post height but is inhibited by the roughness that mismatches with the height of the gaseous domains.Our simulations also demonstrate that if the nanoscale roughness in the boundary region is sufficiently denser and the surface roughness inside the central area is relatively sparser,the boundary effect produces a strong pinning effect and interfacial nanobubbles form more easily.On the other hand,if the roughness inside strengthens while the boundary effect becomes weakened,micropancakes form instead.