溶解润湿动力学的物理力学

Dynamics of dissolutive wetting: Physical mechanics investigations

溶解润湿涉及润湿、扩散及对流多个复杂的物理过程.本文采用物理力学方法研究了溶解润湿的动态过程,分析了溶质粒子进入液体内部对于液滴铺展、溶剂结构以及液滴内部流场造成的影响,结合理论得到了溶解润湿过程中的标度律.研究结果表明在液滴铺展前期界面张力驱动占主导,标度关系满足Tanner律.随着溶解的进行,溶质粒子进入液滴内部一方面造成溶剂分子结构的变化,另一方面浓度梯度的存在加快了液滴的铺展速度并改变了液滴内部的流场,且液滴内部流场中有无对流对于液滴铺展标度律会产生显著的影响.因此,溶解润湿对于液滴的内部结构、流场及铺展过程都会产生重要的影响,其研究对于理解、控制和优化涉及溶解润湿的实际问题具有十分重要的意义.

Dissolutive wetting, i.e. droplet wets and simultaneously dissolves its solid substrates, is of great significance in both academic research and practical applications. The diffusion interface at the moving contact line is one of the answers to the Huh-Scriven paradox. Besides, dissolutive wetting is the bottleneck problem in many practical fields, such as metal alloy processes, shale gas exploitation, drug release, etc. Furthermore, dissolutive wetting involves complex physical processes, i.e. wetting, diffusion and convection. The coupling transport of mass and momentum as well as the internal convection in the droplet increases the difficulty in studying dissolutive wetting. Even though previous works have been done on metal/metal, metal/ceramic systems, there has been no experimental observation on the flow details in droplets and changes of the solid-liquid interface due to the opacity of the materials in dissolutive wetting. Thus, the physical mechanism of dissolutive wetting is still far from being well understood. Based on the problems mentioned above, in this paper, dynamics of dissolutive wetting of droplets on solid surfaces is investigated by physical mechanics. From a new perspective, we carry out the molecular dynamic simulations of glucose and water dissolution pairs, we match the pa- rameters that dominate the dissolutive wetting in our simulations with real experimental parameters. By this method, the simulation systems are simplified and the glucose molecules can be regarded as Lennard Jones particles. Experiments are also designed to show the in situ real time measurement of the dissolutive wetting. In order to obtain the flow details in the droplet, we select three pairs of high transparent materials. Beyond that, we built a dimensionless number space that divides the dissolutive wetting into two types. For small Pelect number, the convection effect is ignored. But when Pelect number is greater than one, the convection effect that leads to the uniform of the solute distribution must be considered. Morever, we obtain that the droplet follows different scaling laws in different dissolutive wetting stages. In the early stage, the surface tension dominates and the scaling law satisfies the Tanner law. Whereas scaling law in dissolutive wet- ting changes while diffusion and convection effect are more obvious with the dissolution of solutes. Compared with the non-convective process, the internal convection increases the energy dissipation within the dissolution pairs in our ex- periments and inhibits the spreading of the droplets to a certain extent. We also find that the coupling effect of dissolution and wetting process will have important influences on the molecular structure of the droplet. Overall, we establish mul- tiscale dissolutive wetting model and reveal the scaling laws of droplet spreading in dissolutive wetting by physical me- chanics. Our findings may help to further understand the mechanisms of dissolutive wetting and provide theoretical guidance of practical applications that involve dissolutive wetting.