Otic Clint John Cortes, Yonemura Shigeru
Department of Finemechanics, Graduate School of Engineering, Tohoku University, 6-6 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Miyagi, Japan.
Department of Mechanical Engineering, College of Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Aichi, Japan.
Micromachines (Basel). 2022 May 31;13(6):871. doi: 10.3390/mi13060871.
In micro/nano-scale systems where the characteristic length is in the order of or less than the mean free path for gas molecules, an object placed close to a heated substrate with a surface microstructure receives a propulsive force. In addition to the induced forces on the boundaries, thermally driven flows can also be induced in such conditions. As the force exerted on the object is caused by momentum brought by gas molecules impinging on and reflected at the surface of the object, reproducing molecular gas flows around the object is required to investigate the force on it. Using the direct simulation Monte Carlo (DSMC) method to resolve the flow, we found that by modifying the conventional ratchet-shaped microstructure into different configurations, a stronger propulsive force can be achieved. Specifically, the tip angle of the microstructure is an important parameter in optimizing the induced force. The increase in the propulsive force induced by the different microstructures was also found to depend on the Knudsen number, i.e., the ratio of the mean free path to the characteristic length and the temperature difference between the heated microstructure and the colder object. Furthermore, we explained how this force is formed and why this force is enhanced by the decreasing tip angle, considering the momentum brought onto the bottom surface of the object by incident molecules.
在特征长度为气体分子平均自由程量级或小于该量级的微纳尺度系统中，放置在具有表面微观结构的加热基板附近的物体受到推进力。除了边界上的感应力外，在这种情况下还会诱导产生热驱动流。由于作用在物体上的力是由撞击物体表面并反射的气体分子带来的动量引起的，因此需要重现物体周围的分子气流来研究作用在物体上的力。通过使用直接模拟蒙特卡洛（DSMC）方法求解流动，我们发现将传统的棘轮形微观结构修改为不同的构型，可以获得更强的推进力。具体而言，微观结构的顶角是优化感应力的一个重要参数。还发现不同微观结构诱导的推进力的增加取决于克努森数，即平均自由程与特征长度的比值以及加热的微观结构与较冷物体之间的温差。此外，考虑到入射分子带到物体底面的动量，我们解释了这种力是如何形成的，以及为什么顶角减小会增强这种力。
