Mo Jianyong, Simha Akarsh, Raizen Mark G
Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2015 Dec;92(6):062106. doi: 10.1103/PhysRevE.92.062106. Epub 2015 Dec 2.
Brownian motion of particles in confined fluids is important for many applications, yet the effects of the boundary over a wide range of time scales are still not well understood. We report high-bandwidth, comprehensive measurements of Brownian motion of an optically trapped micrometer-sized silica sphere in water near an approximately flat wall. At short distances we observe anisotropic Brownian motion with respect to the wall. We find that surface confinement not only occurs in the long time scale diffusive regime but also in the short time scale ballistic regime, and the velocity autocorrelation function of the Brownian particle decays faster than that of a particle in bulk fluid. Furthermore, at low frequencies the thermal force loses its color due to the reflected flow from the no-slip boundary. The power spectrum of the thermal force on the particle near a no-slip boundary becomes flat at low frequencies. This detailed understanding of boundary effects on Brownian motion opens a door to developing a 3D microscope using particles as remote sensors.
受限流体中粒子的布朗运动在许多应用中都很重要，但在很宽的时间尺度范围内边界的影响仍未得到很好的理解。我们报告了对一个光学捕获的微米级二氧化硅球体在靠近近似平坦壁面的水中的布朗运动进行的高带宽、全面测量。在短距离内，我们观察到相对于壁面的各向异性布朗运动。我们发现表面限制不仅发生在长时间尺度的扩散 regime，也发生在短时间尺度的弹道 regime，并且布朗粒子的速度自相关函数比体流体中的粒子衰减得更快。此外，在低频时，由于无滑移边界的反射流，热力失去了其颜色。靠近无滑移边界的粒子上热力的功率谱在低频时变得平坦。对布朗运动边界效应的这种详细理解为开发一种使用粒子作为远程传感器的三维显微镜打开了一扇门。
