Bulk liquid soluble surfactant induced retardation of the thermocapillary migration of a droplet

Thermocapillary migration of droplet under earth gravity was successfully d emonstrated by using a density matched system (water / n-butyl benzoate) in the application of the Plateau configuration. The thermal coefficient of i nterfacial tension over a temperature range of 20-50 degrees C was determin ed for water / n-butyl benzoate, and consequently the theoretical predictio n for the motion of water droplet in n-butyl benzoate in the presence of a vertical temperature gradient can then be made. Experimental results of the rmocapillary migration for the clean interface were found to be less effici ent than the theoretical ones. This is attributable to the thermal convecti on effect, trace surface active impurities effect and/or wall effect which are neglected in the theory. On the other hand, the experimental results re veal that thermocapillary migration of water droplet is significantly depre ssed by the deliberately introduced bulk liquid soluble surfactants. Severa l millimolars of Span 80 and Span 85 were shown to inhibit the thermocapill ary effect completely.