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.

In a rarefied gas with a non-uniform temperature field, one phenomenon that arises is the tangential Knudsen force. Various researches have investigated the tangential Knudsen force but have been limited to specific cases. In this study, we investigated the mechanism of the thermally induced tangential Knudsen force, using theoretical analysis under fully diffusive conditions and for a range of Knudsen numbers. Specifically, we formulated a theoretical expression to describe the tangential Knudsen stress by considering the two kinds of -momentum fluxes transferred on a surface of interest. One is brought by molecules directly coming from the other surface without experiencing intermolecular collisions and the other is brought by molecules coming from the bulk region after experiencing intermolecular collisions there. As reference, we used a channel where the lower surface is a hot ratchet structure and the upper surface is a flat cold object. The tangential Knudsen force on the object obtained by the theoretical analysis was compared with the results from our previous work where we performed numerical experiments by the direct simulation Monte Carlo (DSMC) method. Based on the comparison, it is found that the tangential Knudsen force is caused by three mechanisms. First is the contribution of impinging molecules coming from the other surface with different temperature. Second is the contribution of viscous effect of thermally driven flows. While the third is the contribution of thermal stress, which is noticeable in small Knudsen numbers.

In this paper, we propose that thermally induced Knudsen forces in a rarefied gas can be exploited to achieve a tweezer-like mechanism that can be used to trap and grasp a micro-object without physical contact. Using the direct simulation Monte Carlo (DSMC) method, we showed that the proposed mechanism is achieved when a heated thin plate, mounted perpendicularly on a flat substrate, is placed close to a colder object; in this case, a beam. This mechanism is mainly due to the pressure differences induced by the thermal edge flows at the corners of the beam and the thermal edge flow at the tip of the thin plate. Specifically, the pressure on the top surface of the beam is smaller than that on its bottom surface when the thin plate is above the beam, while the pressure on the right side of the beam is smaller than that on its left side when the thin plate is located near the right side of the beam. These differences in pressure generate a force, which attracts the beam to the plate horizontally and vertically. Furthermore, this phenomenon is enhanced when the height of the beam is shorter, such that the horizontal and vertical net forces, which attract the beam to the plate, become stronger. The mechanism proposed here was also found to depend significantly on the height of the beam, the temperature difference between the thin plate and the beam, and the Knudsen number.

An evaporating object placed on an asymmetric surface that is heated above its Leidenfrost temperature, will not only levitate but will also be self-propelled in a well-defined direction and speed 1. In contrast to continuum flow, in a slightly rarefied gas

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