Cmputer Simulation Studies On The Dewetting And Pinning Of Nanodroplet And The Stability Of Nanobubble

There have already been many beautifull descriptions of kinds of interfacial phenomena in the macroscopic world, but differing from the macrosystem, there are many interesting phenomena generated in microsystems because of the size effects, such as the dynamic dewetting of nanodroplet, the pinning effect of the surface acting upon the droplet, the long time existing nanobubble and so on. These phenomena and problems have brought up not only some questions against some classical theories but also discussions about the legality of some classical theories in microscopic world and is rather directly related to the development and applications of industrial products.Three different theories, i.e. the kinetic lattice density functional theory (KLDFT), constrained lattice density functional theory (CLDFT) and molecular dynamics (MD) are employed to study the dynamic dewetting of nanodroplets, the pinning effect of nanodroplets upon a surface and the stability of nanobubbles. Compared with the wetting and dewetting phenomenon of droplet formed with especially polymer in macrosystem, there are less detailed descriptions of, especially, the dewetting of nanoscale thin film formed with simple fluid in microsystem reported. What does the dewetting of simple fluid thin film turns to be? How does a piece of liquid film minimize its surface energy to become fully dewetted? How and what initiate the breakup of film? How do the solid surface and the size affect film dewetting? Polymer is often used in experiments to study the dewetting of nonvolatile liquid in large temporal and spatial scale. But it is nearly impossible for the volatile liquid to be used to study the dynamic dewetting phenomenon of nanoscale thin film. Fortunately, as the computer science and relivent theories becoming more and more powerful, a fully detailed story about the dynamic dewetting process of volatile nanodropts can be now possible to be describled using KLDFT, as we did in this work; The pinning effect is a very common phenomenon in daily life, while there are rare reports about the mechanism and quantative description of such phenomenon in nanoscale. What does the nanoscale triple phase line looks like? Is the nanoscale triple phase line really be fixed during the pinning process? Where does the so-called pinning force come from? How can the pinning effects on the boundaries with chemical heterogeneity and geographic heterogeneity be unified? How can we describle appropriately the pinning force? CLDFT is then employed to study the pinning effect of nanoscale thin film upon solid surface in order to answer those questions; As the counterpart of nanodroplet, nanobubble has nowadays arousen general interest due to its unbelievable long survival time. How can a nanobubble be kept stable for so long time in such a scale? Although several hypothesizes have been proposed to explain that. The question remains unclear. Motivated by the substantial pinning effect from surfaces to nanobubbles, the commonly accepted MD method is now employed to study directly the stability of nanobubble in order to confirm the importance of pinning effect. Our work is summarized as follows:1. the dewetting of nanoscale thin film formed with simple fluidThe dynamical dewetting process is describled using KLDFT. The dewetting mechanism is discussed according to the flow field distribution. The wave motion is found in nearly every one film and thus used to expain the inevitable breakup of films during dewetting. The effects of properties of solid surfaces and dimensions of liquid film to the dewetting are also systematically discussed.2. the microscopic mechanism for the pinning effectThe CLDFT is employed to draw the nanoscale picture of the pinning phenomenon. The triple phase line seems to be more like a "triple phase area" because of the small size effect. The triple phase line is found to keep moving during the so-called pinning process and thus the pinning of three phases line is proved to be actually the "soft pinning" of the "triple phase area". The soft pinning phenomenon has larger temporal and spatial scope. As proposed by Gennes, the pinning effect is caused by different local contact angles between the surface and droplet! A concept, namely the "local surface tension" is proposed here in order to extract the nature of pinning effect. The pinning effects on chemical and geographic heterogenouse boundaries are unified together. Many much complex descriptions about the additional "pinning force" can then be avoided and the pinning effect of the surface upon the triple phase area can then be describled instead by one assistant force and its expression is also extracted.3. nanobubble pinned on surfaces:evidence from molecular dynamics simulationDirect results extracted from MD simulations and theoretical analyses from multiple points of view are used to carefully explain the substantial importance of the pinning effects to the stability of nanobubble. The influence of solid surface and system environment upon the stability of nanobubble is discussed in detail. The solid surface is found to be a very important role in the nanobubble system. The active effects of supersaturation and hydrophobic surface upon the stability of nanobubble are proved successfully. The phase diagram of nanobubble system in a large phase space is drawn and the shapes of all the sub-phase regions and the paths along which the phase point walks from one to another sub-phase area are describled exactly. The possible stable nanobubble on hydrophilic surface is predicted. The effects of a second fluid constitutent to the nanobubble system are also discussed.