By sweeping binary droplets with N2 gas

the Marangoni convection driven by desorption of light component into gas phase is observed with the help of laser shadowgraph optical method.The results show that Marangoni convection arises as small-scale vortex and large-scale symmetrical circular flow.The small-scale vortex structure appears in the ethanol-water droplet at the very beginning

and then vortex structure develops and merges to form larger vortex flow.At the interface of acetone-water binary droplet

symmetrical circular flow appears at first

and then vortex flow arises in the circular flow.For the quiescent single binary droplet composed of ethanol-water and acetone-water separately

the mass transfer coefficient is measured to compare with theoretical prediction while N2 gas purges the gas-liquid interface. Experimental results show that the large-scale symmetrical circular inside the droplet can promote mass transfer process much better than the small-scale vortex near the interface of droplet.