Nanometer titanium pigment is widely used to degrade organic pollutants, such as an organic pollutant. It is also used as an UV protective agent and it’s a part of the fields of new energy technology, such as photochemical catalysis and photo electricity. One thing the public doesn’t know is that its large scale of production comes from a flame-synthesized method. In fact, the fine particles generated from coal, oil, gas and other fossil fuels all come from its burning process, which can also apply to the formation of submicron materials in interstellar dust. All of the phenomena above instill a common science problem in the nature—the mechanism that gaseous plasma transferred itself to condensed particle phase. Therefore the research on online laser diagnosis in multiphase systems has been under the scrutiny of different scientists for a long time.

Recently, Tsinghua’s doctoral student Ren Yihua from the Department of Thermal Engineering made a breakthrough in this field. In conjunction with Professor Stephen D. Tse from Rutgers State University of New Jersey, and Professor Marshall B. Long from Yale University, Ren Yihua found that under a low- level laser circumstance, the nano-oxide particles would produce nanoplasmas, on this basis, a new technology called Phase-selective Laser-induced Breakdown Spectroscopy was developed. Their research findings titled Absorption-Ablation-Excitation Mechanism of Laser-Cluster Interactions in a Nanoaerosol System was published on Physical Review Letters, 2015, 104, 023115. Ren Yihua is the main author of the article and his advisor Professor Li Shuiqing is the corresponding author. Professor Yao Qiang from the Department of Thermal Engineering, Professor Pu Yikang from the department of Engineering Physics, and Professor Wang Hai from Stanford University have provided essential assistances to the completion of the article.

The article featured a physical diagram of “absorption-ablation-excitation” mechanism by measuring the time evolution of TiO2 nanoparticles and atomic spectrum signal. Later they found out that the energy absorption mechanism comes from conduction-band electrons instead of the heating effect, which was formally reported to be the origin. Non-dimensionalizing the Fokker-Planck equation, is an analogy with theories of mass transfer and burning conduct assessment of the electron energy distribution during the ablation process and classic flow mentioned in thermal engineering courses. Strouhal SlE, Peclet PeE, and Damkoumlhler DaE numbers were also defined to characterize the interactional physics mechanism between low- level laser and nanoparticles. This research lays a foundation for developing and using the technology of Phase-selective Laser-induced Breakdown Spectroscop.

In recent years, the Department of Thermal Engineering at Tsinghua University have conducted a series of research on online laser diagnosis where extra fine particles are produced in high temperature flame fields, which contributed to many articles, such as Combustion and flame, 2013; Appl. Phys. Lett., 2014; Proc. Combust. Inst., 2015. International cooperation has also launched that their research projects have won financial aids from The National Natural Science Fund and The National Basic Research Program.