Why an object can self-propel when placed close to a heated microstructure

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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Photo by Joshua Hoehne on Unsplash

Photo by Joshua Hoehne on Unsplash

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.

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