Shave Molly K, Balciunaite Aiste, Xu Zhou, Santore Maria M
Department of Polymer Science and Engineering , University of Massachusetts at Amherst , 120 Governors Drive , Amherst , Massachusetts 01003 , United States.
Department of Physics , University of Massachusetts at Amherst , 666 North Pleasant Street , Amherst , Massachusetts 01003 , United States.
Langmuir. 2019 Oct 8;35(40):13070-13077. doi: 10.1021/acs.langmuir.9b01871. Epub 2019 Sep 24.
We compare the electrostatically driven capture of flowing rod-shaped and spherical silica particles from dilute solutions onto a flow chamber wall that carries the opposite electrostatic charge from the particles. Particle accumulation and orientation are measured in time at a fixed region on the wall of a shear flow chamber. Rod-shaped particle aspect ratios are 2.5-3.2 and particle lengths are 1.3 and 2.67 μm for two samples, while sphere diameters were 0.72, 0.96, and 2.0 μm for three samples. At a moderate wall shear rate of 22 s, the particle accumulation for both rods and spheres is well described by diffusion-limited kinetics, demonstrating the limiting effect of particle diffusion in the near-wall boundary layer for electrostatically driven capture in this particle shape and size range. The significance of this finding is demonstrated in a calculation that shows that for delivery applications, nearly the same (within 10%) particle volume or mass is delivered to a surface at the diffusion-limited rate by rods and spheres. Therefore, in the absence of other motivating factors, the expense of developing rod-shaped microscale delivery packages to enhance capture from flow in the diffusion-limited simple shear regime is unwarranted. It is also interesting that the captured orientations of the larger rods, 2.6 μm in average length, were highly varied and insensitive to flow: a substantial fraction of rods were trapped in standing and slightly leaning orientations, touching the surface by their ends. Additionally, for particles that were substantially tipped over, there was only modest orientation in the flow direction. Taken together, these findings suggest that on the time scale of near-surface particle rotations, adhesion events are fast, trapping particles in orientations that do not necessarily maximize their favored adhesive contact or reduce hydrodynamic drag.
我们比较了从稀溶液中通过静电驱动捕获流动的棒状和球形二氧化硅颗粒,并将其捕获到与颗粒携带相反静电荷的流动腔室壁上的过程。在剪切流动腔室壁上的固定区域,实时测量颗粒的聚集和取向。两个样品的棒状颗粒长径比为2.5 - 3.2,颗粒长度分别为1.3和2.67μm,而三个样品的球形颗粒直径分别为0.72、0.96和2.0μm。在22 s的中等壁面剪切速率下,棒状颗粒和球形颗粒的聚集情况都可以用扩散限制动力学很好地描述,这表明在该颗粒形状和尺寸范围内,颗粒扩散在近壁边界层中对静电驱动捕获具有限制作用。这一发现的重要性在一项计算中得到了证明,该计算表明,对于输送应用,棒状颗粒和球形颗粒以扩散限制速率输送到表面的颗粒体积或质量几乎相同(在10%以内)。因此,在没有其他驱动因素的情况下,开发棒状微尺度输送包以增强在扩散限制简单剪切状态下从流动中捕获颗粒的成本是不必要的。同样有趣的是,平均长度为2.6μm的较大棒状颗粒的捕获取向变化很大,并且对流动不敏感:相当一部分棒状颗粒以直立和略微倾斜的取向被捕获,通过其端部接触表面。此外,对于大幅倾倒的颗粒,在流动方向上只有适度的取向。综上所述,这些发现表明,在近表面颗粒旋转的时间尺度上,粘附事件很快,将颗粒捕获在不一定能使其最有利的粘附接触最大化或降低流体动力阻力的取向中。