SURFACTANT EFFECTS ON THE MOTION OF A DROPLET IN THERMOCAPILLARY MIGRATION

A nearly density matched system (water/n-butyl benzoate) was used to s tudy the effects of surfactants (sodium dodecyl sulfate and Triton X-1 00) on the motion of droplet in thermocapillary migration under earth gravity. The thermal coefficient of interfacial tension over a tempera ture range of 20-50 degrees C was determined for water/n-butyl benzoat e by a method which is suitable for both equal and unequal density liq uids, and consequently the theoretical prediction for the motion of wa ter droplet in n-butyl benzoate in the presence of a temperature gradi ent can then be made. Experimental results of thermocapillary migratio n for the clean interface were found to be 19-30% less efficient than the theoretical predictions. This is attributed to the thermal convect ion 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 depr essed by the deliberately introduced water soluble surfactants. Seven hundred parts per million SDS and 1600 ppm Triton X-100 were both show n to inhibit the thermocapillary effect completely. No theoretical mod el, however, is available in the literature for describing this effect of soluble surfactants. (C) 1997 Elsevier Science Ltd.