Abstract
Knudsen force generated by thermally driven gas flow in a microscale structure has been used for gas detection and has shown immeasurable potential in the field of microelectromechanical system (MEMS) gas sensors due to its novel sensing characteristics. In this article, the performances of three kinds of Knudsen force gas sensors with improved isosceles triangular shuttle arm structures were studied. In the first design, the top side and right side lengths were equal; in the second, the top side and bottom side lengths were equal; and for the third, the bottom side and right side lengths were equal. A detailed investigation including gas flow, thermal characteristics, Knudsen force, and coupling effects between the shuttle-heater pairs was conducted using the direct simulation Monte Carlo (DSMC) method and the main mechanisms for gas flow presented were almost the same in this work. However, the second design returned the highest Knudsen force performance. The value increased by 42.9% (P = 387 Pa) compared to the Knudsen force of the original square shuttle arm. The results also demonstrate that the coupling effects become weak toward the right with an increase in the number of shuttle-heater pairs.
Highlights
IntroductionIn rarefied (low-pressure) gases without any initial pressure gradient, thermal stress is produced due to the inhomogeneous temperature
In rarefied gases without any initial pressure gradient, thermal stress is produced due to the inhomogeneous temperature
Gas temperature gradient and thermally driven flow along the shuttle arm surfaces are illustrated as Figure 3. (Nonlinear) thermal stress flow is induced by stress due to the non-uniform temperature distribution [44,48], three circulation flows or vortices were formed
Summary
In rarefied (low-pressure) gases without any initial pressure gradient, thermal stress is produced due to the inhomogeneous temperature. A gas flow is induced and exerts a force (Knudsen force or radiometric force) on the immersed structures. The effect first appeared in the famous Crookes radiometer developed by Sir William Crookes in the 1870s [1]. Ketsdever et al [2] conducted a broad literature review for Knudsen force from the 19th to the 21st century. Note that the Knudsen force appears in the environment with a Knudsen number (Kn) higher than 0.1, which significantly limits its macroscopic application. Passian et al [3,4] first demonstrated that the micro-cantilever focused by laser illumination can be deflected by the Knudsen force, fully displaying the application potential of Knudsen force at the micro and nano-scale
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