Zhu Taishan, Ye Wenjing, Zhang Jun
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong.
Phys Rev E Stat Nonlin Soft Matter Phys. 2011 Nov;84(5 Pt 2):056316. doi: 10.1103/PhysRevE.84.056316. Epub 2011 Nov 18.
Knudsen force acting on a heated microbeam adjacent to a cold substrate in a rarefied gas is a mechanical force created by unbalanced thermal gradients. The measured force has its direction pointing towards the side with a lower thermal gradient and its magnitude vanishes in both continuum and free-molecule limits. In our previous study, negative Knudsen forces were discovered at the high Knudsen regime before diminishing in the free-molecule limit. Such a phenomenon was, however, neither observed in experiment [A. Passian et al., Phys. Rev. Lett. 90, 124503 (2003)], nor captured in the latest numerical study [J. Nabeth et al., Phys. Rev. E 83, 066306 (2011)]. In this paper, the existence of such a negative Knudsen force is further confirmed using both numerical simulation and theoretical analysis. The asymptotic order of the Knudsen force near the collisionless limit is analyzed and the analytical expression of its leading term is provided, from which approaches for the enhancement of negative Knudsen forces are proposed. The discovered phenomenon could find its applications in novel mechanisms for pressure sensing and actuation.
在稀薄气体中，作用于与冷基板相邻的加热微梁上的克努森力是由不平衡热梯度产生的机械力。测量到的力的方向指向热梯度较低的一侧，并且其大小在连续介质极限和自由分子极限中均消失。在我们之前的研究中，在自由分子极限中减小之前，在高克努森 regime 中发现了负克努森力。然而，这种现象在实验中 [A. Passian 等人，《物理评论快报》90, 124503 (2003)] 未被观察到，在最新的数值研究 [J. Nabeth 等人，《物理评论 E》83, 066306 (2011)] 中也未被捕捉到。在本文中，通过数值模拟和理论分析进一步证实了这种负克努森力的存在。分析了无碰撞极限附近克努森力的渐近阶，并给出了其主导项的解析表达式，由此提出了增强负克努森力的方法。所发现的现象可在压力传感和驱动的新型机制中找到应用。
