Nonmechanical micro-pumps can be divided into categories that include magnetohydrodynamic pumps 31, electroosmotic pumps 32, and Knudsen pumps (KP) 33, 34. Mechanical micro-pumps can be classified into various types including piezoelectric pumps 24, 25, electrostatic pumps 26, 27, thermo pneumatic pumps 28, shape-memory alloy (SMA) pumps 29, and electromagnetic pumps 30 based on the type of actuating force. Their working mediums are not limited to liquids but can also include gases or solids in the form of nanometer-sized particles. In contrast, fluid flow in nonmechanical micro-pumps is induced by nonmechanical forms of energy such as electric energy, thermal energy, chemical energy, and magnetic energy. Their life spans, sensitivity, and stability are not good due to the moving parts.

Mechanical micro-pumps use the mechanical energy of their moving parts to drive fluid flow, and their working mediums are basically liquids. There are mechanical micro-pumps and nonmechanical micro-pumps, classified according to whether they have moving parts. Micro-pumps exist with various actuating principles and morphologies. The micro-pump, which is a driven control engine for micro fluids, is the key to the development of microfluidic devices, and is a major area of research. Research has been conducted on various microfluidic devices, such as micro fuel cells 2, 3, 4, lab-on-a-chip systems 5, micro-gas chromatographs 6, 7, 8, 9, 10, micro mass spectrometers 11, 12, 13, micro spectrometers 14, 15, reformers 16, 17, 18, and micro air vehicles 19, 20, 21, 22, 23. They have since been widely used in many fields due to their large quantity production, high integration, easy extension, and interdisciplinary applications. Micro electro mechanical systems (MEMS) were first introduced in the 1990s 1. Finally, the current problems of KP are discussed, and some suggestions are provided for future research and applications. Then, the three aspects of configurations, performance, and applications of KPs in the current state of research are reviewed and analyzed.

In this paper, KPs are first defined and classified according to the flow mechanisms of the thermally induced flows. However, there seems to be no comprehensive review on KPs at present. In many circumstances, the simulation and experimental results have good agreement. In order to obtain efficient flow fields in KPs, it is necessary to adopt modern computational methods for simulation and analysis. With the development of Micro/Nano Electro Mechanical Systems (MEMS/NEMS), extensive studies have been conducted on KPs, and the applications of KPs have widened. It has the advantages of having no moving parts, simple structure, easy construction and extension, a wide range of energy sources, and low energy consumption. The Knudsen pump (KP) is a kind of micro-pump that can form thermally induced flows induced by temperature fields in rarefied gas environments.

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