Mixing of laser-induced plumes colliding in a background gas

We collided silicon and germanium laser-induced plumes in helium background gas to clarify the behavior of the plumes after the collision. The expansion of the silicon and germanium species in the mixed plume after the collision was observed separately by spectroscopic measurements and the degree of mixing was evaluated by the experimental results. When the pressure of the background gas was 2000 Pa, the plumes moved backward after the collision with the counter-propagating shock wave, and no mixing of the plumes was observed. The effect of the counter-propagating shock wave was discussed by comparing with the results of numerical calculations based on the compressible Euler equations. At 300 Pa, the plumes concentrated at about 1 mm around the central region just after the collision and almost 100% mixing occurred in this region. The concentration and mixing of the plumes in the central region is a key to forming well-mixed plume. Stagnation and partial mixing of the plumes were observed at intermediate pressures. By decreasing the background helium gas pressure from 2000 to 300 Pa, the degree of mixing increased from 0 to about 100%. The results are compared with those in argon background gas to discuss effects of mean free path. The effects of counter-propagating shock wave, mean free path and the onset time of the collision on the backward movement and mixing of the plumes are discussed.