纳米流体动态湿润的格子-Boltzmann模拟

Dynamic wetting by nanofluids: Lattice Boltzmann method study

中文关键词: 纳米流体,湿润动力学,微观耗散,自组装,分离压

英文关键词: Nanofluids, dynamic  wetting, microscopic  dissipation, self-assembly, disjoining  pressure

中文摘要:

      纳米流体动态湿润特性与纳米颗粒的微观运动密切相关。由于缺乏纳米尺度的实验观测技术及相关理论描述,纳米流体动态湿润的研究极具挑战,相关机理仍未明晰。本文采用格子-Boltzmann方法研究纳米颗粒在纳米尺度下(10_-9 m)的微观运动及颗粒沉积所导致的基液流体表面张力、流变性改变及结构分离压力对宏观动态湿润(10_-3 m)的影响机制。结果表明纳米颗粒对基液的表面张力的改性影响纳米流体平衡湿润特性,决定纳米流体是完全浸润还是部分浸润。而纳米颗粒对基液流体流变性的改变影响纳米流体动态湿润过程的铺展指数。纳米颗粒在液滴底部的沉积对动态湿润过程影响较小,而在接触线区域的沉积显著改变纳米流体的动态湿润行为。本研究尝试从跨尺度的角度阐释纳米颗粒微观运动对宏观动态湿润行为的影响,探索从微观层面调控纳米流体动态湿润的新方法。

英文摘要:

      Nanofluids have been reported to exhibit attractive and tunable dynamic wetting behaviors due to the complex nanoparticle kinetics. Studies of the dynamic wetting by nanofluids are of great challenge because the wetting behavior crosses several length and time scales. The mechanism of dynamic wetting by nanofluids is still unclear due to limitations of nanoscale experimental techniques and fundamental theories. A lattice Boltzmann method with some simple but effective treatments with consideration of nanofluid surface tension and rheology modification, as well as the nanoparticle sedimentations, was used to investigate the effects of nanoparticle kinetics at the nanoscale (10-9 m) on the dynamic wetting behaviors occurring at the macroscopic scale (10-3 m). The individual effect analysis shows that the surface tension modification is related to the equilibrium wettability of nanofluids (complete wetting or partial wetting), while the spreading exponents of nanofluid droplets depend on the rheology modification. The additional hydrophobic nanoparticles, which function like surfactants, facilitate the wettability and result in a complete wetting behavior. Only considering the rheological modification cannot predict the wettability of nanofluids at equilibrium stages. However, without considering the rheology modification, the spreading exponents of nanofluid droplets are underestimated. The shear-thinning non-Newtonian behavior can further enhance the dynamic wetting of nanofluids. The nanoparticle deposition at the bottom of the droplet has few effects on the dynamic wetting for both the complete wetting and partial wetting. However, the nanoparticle deposition in the vicinity of contact line strongly facilitates the contact line motion, especially for the partial wetting droplet at the late spreading stage when the contact angle is small, which can be explained by the additional structural disjoining pressure due to the nanoparticle self-assembly. The study provides multiscale understanding and guidelines to tune the nanofluid dynamic wetting behaviors.