CONVECTION GENERATED BY A GRADIENT OF INTERFACIAL-TENSION IN A PHASE-SEPARATED BINARY-LIQUID MIXTURE WITH A MISCIBILITY GAP

Results of measurements of the migration velocity of droplets in a pha se separated butanedinitrile/water mixture under the influence of a te mperature gradient are reported. The droplet phase and the suspending phase coexist. The driving force of this motion is a gradient of the l iquid/liquid interfacial tension generated by a temperature gradient ( thermocapillary motion, Marangoni convection). The experiments are car ried out in the vicinity of the temperature at which the coexisting ph ases have the same density (isopycnic temperature T-ip). Under that co ndition the influence of the force of gravity on the thermocapillary m otion of the droplets can be neglected. The results show that the meas ured migration velocity is larger by factor bf about 1.4 than theoreti cally expected if only the temperature dependence of the interfacial t ension is taken into account. This discrepancy is assumed to be caused by a contribution of a gradient of the interfacial tension originated from a concentration gradient along the interface of the droplets. Th e concentration gradient is the result of a temperature dependent loca l mass transport of the components of the mixture across the liquid/li quid to maintain the temperature dependent diffusive equilibrium betwe en the two phases.