CAREER: Measuring the Effect of Nano-Scale Charge Nonuniformity on Colloidal Forces

目前对胶体力的理解一般假定粒子上的电荷是均匀分布的。不均匀的电荷分布将显著改变粒间作用力。研究的目标是：在微米级的单个胶体颗粒上测量纳米尺度的电荷不均匀；胶体颗粒上的电荷不均匀将与测量到的粒子之间的法向和切向胶体力有关。这项研究的部分原因是来自PI的最新数据显示，两个不接触的粒子之间作用的与时间相关的切向胶体力可能是由于电荷分布不均匀造成的。电荷不均匀性对悬浮体稳定性和流变学的建模有重要影响。通过结合微电泳法、电动理论和蒙特卡罗模拟方法测量单个颗粒上的电荷不均匀性。颗粒上的电荷不均匀将与颗粒间的作用力有关。利用最近发展起来的“差示电泳法向力”技术，可以测量两个粒子之间的法向力(沿中心线作用的力)和它们之间的切向力(沿粒子相邻表面横向作用的力)，分辨率为亚皮牛顿。差示电泳法可以测量间距小于100纳米的两个布朗粒子之间的粒间力，范德华力在这一点上很重要。

The current understanding of colloidal forces generally assumes that the charges on particles are uniformly distributed. A nonuniform charge distribution will significantly alter interparticle forces. The research goals are: the nanoscale charge nonuniformity will be measured on individual colloidal particles of micron size; and the charge nonuniformity on colloidal particles will be related to the measured normal and tangential colloidal forces between the particles. This research was prompted in part by recent data from the PI that shows time-dependent tangential colloidal forces acting between two non-touching particles that could result from a nonuniform charge distribution. Charge nonuniformity could have a major impact on the modeling of bulk suspension stability and rheology.The charge nonuniformity on individual particles will be measured by combining microelectrophoresis experiments, electrokinetic theory, and Monte Carlo simulations. The charge nonuniformity on particles will be related to the forces between particles. Using the recently-developed technique of "differential electrophoresis", the normal forces can be measured between two particles (those forces acting along the line of centers) and the tangential forces between them (those forces acting laterally along adjacent surfaces of the particles) with sub-piconewton resolution. Differential electrophoresis can measure interparticle forces between two Brownian particles at gaps less than 100 nanometers, where van der Waals forces are important.