| Summary: | 博士 === 國立成功大學 === 化學工程學系 === 89 === The motion driven by tangential gradients of surface tension, which are caused by temperature gradients, is called thermocapillary flow. In recent years, the researches and applications of high technology products have been extended to the fields of microgravity conditions in outer space and the fields of micro electro mechanical systems with the progress in science and technology. Within these fields, thermocapillary flow always plays an important role. However, the contaminants such as oil, dust, surfactant, and other impurities can adsorb at interfaces and change the interfacial properties, resulting in significant influence on thermocapillary flow.
The objective of this work is to study the effects of surface active solutes on thermocapillary flow at air/liquid interfaces. The thermocapillary flow was generated by a point heat source at the interface. With different fluids (pure water or ethylene glycol) and concentrations of surface active materials (sodium dodecyl sulfate (SDS) or n-heptanol) within the controlled temperature ranges, the fluids of various temperature coefficients of surface tension (β) resulted. The β value can be constant or not, and can vary from negative to positive. The surface velocity and temperature distributions of fluids with various β values were measured and compared with that obtained for pure fluids. The effects of surface elasticity and β variation were then considered elucidating the influence of surface active solutes on the thermocapillary flow at air/liquid interfaces.
The experimental results revealed that the thermocapillary flow for pure liquids with constant β values can be expressed by a dimensionless equation. When a small amount of SDS was added in pure water or ethylene glycol, the thermocapillary flow was significantly retarded. The inhibition was caused by the surface elasticity effect. When the elasticity number (E) of SDS exceeded its critical value (EC), the thermocapillary flow was totally inhibited. Depending on the temperature range, the surface tensions of aqueous n-heptanol solutions may decrease or increase with increasing temperature. Since the values of aqueous n-heptanol solutions varied with temperature and concentration, the effect of β variation within controlled temperature ranges can be evaluated by the magnitude of thermocapillary stress (Δσthermo). The experimental results demonstrated that the trends of the variations of average surface temperature and velocity were consistent with that of Δσthermo change, which implies that the thermocapillary flow of aqueous n-heptanol solutions was dominated by the βvariation. The inhibition ability of n-heptanol on surface flow was less than that of SDS. In addition to a smaller value of elasticity number, the less significant inhibitory effect of n-heptanol on surface flow as compared with that of SDS was due to the fast adsorption behavior of n-heptanol at the interface, resulting in fast restoration from a non-equilibrium interface. This may explain the insignificant influence of surface elasticity effect in aqueous n-heptanol solutions. On the contrary, since the adsorption/desorption of SDS were slow, the thermocapillary flow was apparently retarded by the surface elasticity effect.
Although the induced mechanism of thermocapillary flow is somewhat similar to that of diffusocapillary flow, the mechanisms for SDS inhibition on thermocapillary and diffusocapillary flows were quite different. While the surfactant-induced retardation of the thermocapillary flow was strongly related to the interfacial elasticity property of surfactant solutions, that of the diffusocapillary was mainly due to the reduction in the driving force which triggers the diffusocapillary flow.
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