Study On The Optical Tweezers Manipulation And Combustion Characteristics Of Single Carbon-based Particles

The manipulation and combustion characteristics of a single fuel particle are the basis for evaluating the fuel and interpreting the combustion characteristics of the particle group.In this paper,the laser optical tweezers are used to control and ignite the particles,and to further reveal the combustion mechanism of particulate fuel by measuring manipulation,ignition and combustion process parameters of individual particle.The optical tweezers are important tool to manipulate microparticles,and which has two different forms of forces to control particles: one is to utilize laser radiation pressure,and the other is to utilize the photophoretic force.The optical tweezers manipulation and ignition devices are systematically designed and self-built.The experiments were carried out for trapping polystyrene spheres in the deionized water and for manipulating strong absorption airborne graphite particles by using the experimental device.Furthermore,the ignition and combustion characteristics of the single graphite particles were measured and analyzed to reveal the reaction control mechanism.The research mainly focuses on the following contents and draw some conclusions.(1)In the design process of the optical tweezers manipulation and ignition device,the problems of matching the laser spot with the microscope objective,matching the objective with the illumination source,coupling the beam were solved.Simulation of shaping beam and adjusting trap position by ZEMAX software shows that the trap position is approximately linear with the distance of the adjustment lens.(2)The optical tweezers control parameters based on radiation pressure were investigated by capturing polystyrene(PS)microparticles.The results show that the minimum power of trapping 5.0μm PS in deionized water is 6.7m W.The maximum optical force at 10.4m W is2.37×10-12 N,and the optical trap stiffness is 4.74×10-7N/m.The distance between the stable and unstable trap points from the center of optical tweezers are 7.038μm and 7.467μm,respectively.The movement velocity and acceleration of PS manipulated by optical tweezers is variable.The movement tendency of the PS is similar at different power,and the higher the power,the higher the movement speed and acceleration become.(3)The optical control parameters based on photophoretic force were investigated by manipulating graphite microparticles.The results show that the minimum laser power for manipulating graphiteparticles is 7.3m W.The graphite particles have various motion modes by photophoretic force,including the variable acceleration curve motion and the rotational motion with variable angular acceleration around the fixed point.Manipulation power will directly affect its speed of movement.(4)The laser ignition and levitation combustion characteristics of single graphite particles are studied by igniting a single graphite particle,in which high-speed camera was used to record the process with 90000 fps.The results show that the ignition delay time of graphite particles is tens of μs and the ignition energy is as low as μJ.The ignition energy increases with the increase of the initial particle size,which has a square relationship with a correlation coefficient of 0.99.The graphite particles first underwent diffusion combustion,and the flame area increased linearly with the burning time.When the flame brightness reached the maximum,the flame area began to shrink linearly with the burning time.The flame propagation rate was slightly faster than the shrinkage rate,and the particles were gradually pushed away from the laser spot during the flame shrinkage process and then extinguished.In an air flow with natural convection,the oxidation reaction of a single graphite particle is mainly controlled by oxygen diffusion.