Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA.
Phys Rev E. 2019 Feb;99(2-1):022802. doi: 10.1103/PhysRevE.99.022802.
We investigate how the aspect ratio of micropillar or microwell arrays patterned on a surface affects the rolling and slipping motion of spheres under flooded conditions at low Reynolds numbers. We study arrays of rigid microstructures with aspect ratios varying over two orders of magnitude for surface coverages ranging from 0.04 to 0.96. We investigate how the surface features (dimensions, surface coverage, and geometry) individually impact the motion of the sphere. We find that increasing microstructure height results in higher rotational velocities on all studied surfaces. We then model the motion of the spheres using two physical parameters: an effective separation and a coefficient of friction between the sphere and the incline. We find that a simple superposition of resistance functions, previously shown to accurately predict the motion of spheres for different surface coverages and geometries, indeed shows good agreement with experimental outcomes for all microstructure heights studied. We also perform separate sliding friction measurements via a force microscope to measure the coefficient of friction between the sphere and incline, under identically flooded conditions. A comparison of the sliding friction measurements at different Hersey numbers suggests that the effect of the microstructure on the coefficient of friction becomes more important as the Hersey number increases.
我们研究了表面微柱或微槽阵列的纵横比如何影响低雷诺数下淹没条件下球体的滚动和滑动运动。我们研究了具有两个数量级纵横比的刚性微结构阵列，其表面覆盖率范围为 0.04 至 0.96。我们研究了表面特征（尺寸、表面覆盖率和几何形状）如何单独影响球体的运动。我们发现，增加微结构的高度会导致所有研究表面上的旋转速度更高。然后，我们使用两个物理参数来模拟球体的运动：有效分离和球体与斜面之间的摩擦系数。我们发现，先前被证明可准确预测不同表面覆盖率和几何形状下球体运动的阻力函数的简单叠加，确实与所有研究的微结构高度的实验结果吻合良好。我们还通过力显微镜进行了单独的滑动摩擦测量，以测量球体和斜面之间的摩擦系数，在相同的淹没条件下进行。在不同的赫西数下的滑动摩擦测量的比较表明，随着赫西数的增加，微结构对摩擦系数的影响变得更加重要。
