近壁亚微米尺度下纳米粒子布朗运动实验研究

Brownian motion of nanoparticles has been well described by classical theory like Stokes-Einstein equation, however, this theory is suitable for rigid particles with smooth surface in unlimited flow field. In micro/nanoflows, the Brownian motion of nanoparitcles are usually confined, influenced by fluid flow near wall, electrostatic field of solid surface and particle surface properties, especially from recent observation, experimental data of vertical diffusion coefficient Dz are not consistent with the Stokes-Einstein equation prediction with the near wall hydrodynamic correction. With previous support of NSFC, we have built the TIRFM system which can be used to measure the flow field within 500nm to wall. This application will continue to utilize this system to study confined Brownian motion. We focus on measuring the vertical diffusion coefficient Dz of nanoparticles, studying the influences of wall and particle surfaces on Brownian motion， and analyzing theoretically the diffusion very close to wall based on fokke-plank equation. This research will not only contribute to understand the mechanism of particle confined diffusion, but also help develop the nanoparticle probe in microfluidics.

粒子布朗扩散运动虽已有经典的Stokes-Einstein公式描述，但该理论针对无界流场中光滑刚性粒子。在微纳米流动中，作布朗运动的纳米粒子往往处在受限空间内，受到近壁流场、壁面静电场及粒子表面特性的影响，特别是最近实验发现纳米粒子垂直扩散系数的测量值与经过近壁水动力学修正的S-E公式不符。本课题组在前期基金资助下，已经建立了全内反射荧光显微镜系统，可观测到近壁500nm以内的流场。本申请目的是继续在受限条件下研究粒子布朗运动，着重测量荧光纳米粒子垂向扩散系数Dz，研究壁面和粒子表面特性对纳米粒子布朗运动的影响，并根据福克-普朗克方程对近壁扩散运动进行理论分析。此研究有助于阐明粒子受限扩散的机理，而且有利于开发纳米粒子探针在微流控中的应用。

在微纳米流动中，纳米粒子往往处在近壁受限空间内，其扩散运动会展现出不同于经典Stokes-Einstein理论的特征。在本基金资助下，我们改进了全内反射荧光显微测量系统，可观测到近壁z < 500 nm以内的流场并追踪荧光纳米粒子的运动垂向分层精度达到20 nm。在此基础上，我们开展壁面受限条件下纳米粒子布朗运动研究，着重在不同浓度盐溶液中测量不同粒径纳米粒子的水平和垂向扩散系数Dxy及Dz。实验测量结果给出了近壁区纳米粒子扩散系数受粒径、盐浓度、表面属性等影响的变化规律，并表征了其受粒子粒径及壁面静电作用影响。进一步从郎之万(Langevin)运动方程出发，引入静电作用对近壁扩散运动给出了对经典水动力学Brenner-Goldmann理论的修正，理论结果和实验测量结果符合较好。此研究有助于阐明粒子受限扩散的机理，而且有利于开发纳米粒子探针在微流控中的应用。