Surfactant effects on the motion of a droplet in thermocapillary migration

Abstract

A nearly density matched system (water/ n-butyl benzoate) was used to study the effects of surfactants (sodium dodecyl sulfate and Triton X-100) on the motion of droplet in thermocapillary migration under earth gravity. The thermal coefficient of interfacial tension over a temperature range of 20–50°C was determined for water/ n-butyl benzoate by a method which is suitable for both equal and unequal density liquids, and consequently the theoretical prediction for the motion of water droplet in n-butyl benzoate in the presence of a temperature gradient can then be made. Experimental results of thermocapillary migration for the clean interface were found to be 19–30% less efficient than the theoretical predictions. This is attributed to the thermal convection effect and/or the trace surface active impurities effect which are neglected in the theory. On the other hand, the experimental results reveal that thermocapillary migration of droplet is significantly depressed by the deliberately introduced water soluble surfactants. Seven hundred parts per million SDS and 1600 ppm Triton X-100 were both shown to inhibit the thermocapillary effect completely. No theoretical model, however, is available in the literature for describing this effect of soluble surfactants.