Small: a new mechanism of micro engine driven by photoinduced bubble growth

our cars are driven by engines which are closely related to human life. Like the heart driving the blood flow of the whole body as the source of power micro engine has great potential. Optical drive artificial intelligence robot drug delivery to the diseased parts of the human body enhancement of multi fluid mixing chemical reaction optical fluid devices (optical control pump valve etc.) regulation of bubble dynamics of solar energy system are all the application scenarios of micro engine.

all things grow by the sun light has the advantages of non-contact non pollution no electromagnetic interference to the surrounding environment. However the development of optical drive engine is not easy. When light irradiates the surface the force produced by the momentum change of "light particles" is called light pressure which is too small to be applied. Some scientists have carried out experiments on driving the liquid-liquid interface by light pressure which shows that light pressure can only drive surface deformation with surface tension less than 10-7 n / m which is 9 orders of magnitude smaller than that of water.

in order to solve the problem that the traditional light pressure is too small Professor Xu Jinliang's team of North China Electric Power University proposed the concept of indirect light pressure theoretically. When light irradiates nanofluids due to the plasmon resonance effect of nanomaterials the absorption of light energy is enhanced bubbles are generated. When the bubble size is larger than the wavelength of light wave the light propagation on the bubble surface follows the principle of geometric optics resulting in refraction reflection the bubbles are divided into sunny side sunny side. The evaporation of liquid on the sunny side is faster than that on the sunny side. The expansion of bubbles causes momentum exchange between gas liquid the bubble surface is forced toward the center. In the direction perpendicular to the light propagation the evaporation momentum force counteracts each other the resultant force is 0; along the direction of the light propagation the bubble non-uniform expansion produces a net resultant force. The medium of momentum exchange is gas-liquid fluid not "light particles" so the force of light induced bubble growth is called indirect light pressure. The traditional light pressure has a linear relationship with the light intensity while the indirect light pressure is directly proportional to the second power of the light intensity which indicates that the indirect light pressure has a good sensitivity to the change of light intensity provides convenience for its application.

papers published by Professor Xu Jinliang's team in small（ DOI:10.1002/smll.202001548 ）The theoretical expression of indirect light pressure is given three new dimensionless criteria are proposed to characterize the importance of indirect light pressure relative to other forces. When the bubble diameter is in a certain range the indirect light pressure is greater than other forces on the bubble. When pulsed laser irradiates nanofluids vertically downward it has been observed that bubbles always stay under the water surface never overflow from the water surface which breaks the limit that bubbles must overflow from the water surface in the open system. This discovery promotes the birth of photoinduced bubble engine. In the pulse laser on stage the indirect light pressure provides an acceleration of 3 to 4 orders of magnitude greater than the gravity acceleration which makes the bubble rush to the depth of the water surface quickly. In the laser off stage the bubble floats due to the instantaneous cancellation of the indirect light pressure. Pulse laser maintains the reciprocating motion of bubbles which is similar to the reciprocating motion of piston in the cylinder. Photoinduced bubble engine opens up a wide space for the application of photofluid.

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