前进接触线薄液膜结构与运移机制

Structure and migration mechanism of thin liquid film in vicinity of advancing contact line

气-液-固三相接触线薄液膜微观结构认识不清使得动态接触角预测问题始终存在争议.本文通过大规模分子动力学模拟研究液滴动态润湿过程,获得了完全浸润液滴前进接触线区域的前驱液膜结构、部分浸润液滴前进接触线区域的纳米级凸起结构.首次在模拟中获得了微观接触角与宏观接触角随时间的动态演变规律.分子轨迹追踪揭示了接触线薄液膜区域流体在固体表面摩擦作用下,随速度增大从滑动为主向滚动为主的模式转变,进而在底层出现气体裹挟导致滑移、发生溅射的机制.本研究为进一步构建普适的动态接触角预测模型提供了关键物理依据.

Thin liquid film near the gas-liquid-solid three phase contact line is the core area of oil and gas production,phase-change heat transfer,and material synthesis systems.Although there are many experimental studies on fluid dynamics in the contact line region,the prediction of contact angle is still difficult,and the bottleneck lies in the special structure of thin liquid film in the contact line region.Because the microstructure of thin liquid film is not well understood,the prediction of dynamic contact angle is always controversial.At present,the main controversial points focus on whether the microscopic contact angle changes with speed,and whether the microscopic contact angle is the same as the macroscopic contact angle.Therefore,it is crucial to monitor the dynamic process of the microscopic contact angle in the thin liquid film region of the contact line.In this work,the wetting system of 50 nm liquid droplets on different solid surfaces is constructed by molecular dynamics simulation,and the structure and migration mechanism of thin liquid film are studied.The structure of the precursor liquid film in the completely wetting droplet advancing contact line region and the nanoscale convex structure in the partially wetting droplet advancing contact line region are obtained.The precursor liquid film is 2–3 molecular layers in thickness,leading the droplet to move forward.However,there is no precursor liquid film in a partially wetting system,and the convex nano-bending larger than 10 nm is formed in the wetting process,resulting in the microscopic contact angle.By comparing the difference between the absolute smooth surface dynamic wetting process and the actual solid surface dynamic wetting process,the dynamic evolution law of the micro contact angle and the macro contact angle with time are obtained for the first time in the simulation.The liquid molecules in the contact line region are tracked and statistically analyzed by means of particle tracer.It is revealed that the liquid molecules in thin liquid film change from sliding mode to rolling mode with speed increasing under the action of solid surface friction,and then the air entrainment at the bottom of the contact line leads to slip and sputtering.The research results are expected to provide theoretical guidance for the following three directions:1) improving heat transfer efficiency of micro and nano device based on wettability control;2) improving the imbibition displacement efficiency of shale oil micro-nano matrix based on wettability regulation;3) constructing universal contact angle prediction model.