What is it about?

Through computer simulations, it has been shown that a cold object placed close to a heated substrate with a surface microstructure such as a ratchet can self-propel. This is due to thermally induced force exerted tangentially on the surface of the object. In the previous study, it was pointed out that this force appears only when the gap distance between the object and the surface structure of the substrate is on the order of submicron under the atmospheric conditions. But, its mechanism had not been known. In this paper, we constructed the theory of this force focusing on the fact that the momentum can be brought only by molecules incident on and reflected on the surface of the object. The forces expected by using our theory together with viscous and thermal stresses agree very well with those obtained in numerical experiments. From this agreement, it was found that this force is generated mainly by high-momentum molecules incoming to the surface of the object from the hot heated substrate without experiencing intermolecular collisions.

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Why is it important?

This force can induce a relative sliding motion between two surfaces. If we apply this phenomenon to a coaxial pair of outer hollow cylinder and inner solid cylinder by fabricating a surface microstructure, the relative rotation of cylinders can be obtained. Thus, using this phenomenon, thermal energy can be converted into work in micro/nanoscale space without complicated mechanical system such as heat engines. In order to exploit this phenomenon efficiently, the understanding of the mechanism in the generation of the force is essential. This paper has clarified it clearly.


I hope that this work helps researchers and engineers to develop a novel and efficient system to exploit this thermally induced force.

Prof. Shigeru Yonemura
Chubu University

I believe that the theoretical approach presented in this study has a potential to be used in other related thermally induced phenomenon.

Clint John Otic
Tohoku University

Read the Original

This page is a summary of: Mechanism of tangential Knudsen force at different Knudsen numbers, Physics of Fluids, July 2022, American Institute of Physics,
DOI: 10.1063/5.0096324.
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