The Control Mechanism Of Liquid-liquid Interface In Micro-extraction Process

Liquid-liquid microextraction has been applied to many fields such as environmental monitoring, analytical chemistry and chemical industry. The mechanism of interface control has been studied in this dissertation. For the liquid-liquid viscosity-stratified flow considering the effects of apparent viscosity and slip boundary, three models, called break interface model, single film model and double film model, are proposed to simulate the structure of liquid-liquid interface. The results show that the stability of the interface can be controlled by adjusting the wall material, viscosity ratio, interface location and so on. The interface instability caused by Marangoni effect is firstly studied. The outcome lays a theoretical basis for improving the inter-phase mass transfer. Main results are as following:(1)The apparent viscosity modification is controlled by the parameters of n andξ, which depends on the wall material and fluid properties. With the increase of n andξ, the critical Reynolds number decreases. The apparent viscosity is disadvantage to the stabile interface. On the other hand, the effect of boundary slip stabilizes flow and has advantage to control the interface. When the combined effects of apparent viscosity modification and boundary slip are considered, the effect of slip boundary on the flow stability is greatly reduced.(2)When the viscosity ratio of the upper fluid to lower fluid is less than 0.2, the critical Reynolds number is increased with the decrease of the viscosity ratio, and the stability of interface is enhanced. The effect of the upper boundary slip plays a major role to the increase of the critical Reynolds number. When viscosity ratio is greater than 0.2, the critical Reynolds number is also increased with the increase of viscosity ratio. The effect of the upper boundary slip on the critical Reynolds number is reducing and that of the lower boundary slip is increasing. Finally, the effect of the lower boundary slip plays a major role.(3)The single film model and double film model are proposed to study the limited miscible stratified flow. When the center of mixed film approaches to the critical layer, the critical Reynolds number decreases sharply, and it leads to an easily instability at the interface. When the film center is located at the lower half channel, and the viscosity ratio is equal to 2, the effect of upper boundary slip plays a major role on the growth of the critical Reynolds number. When the film center moves towards to the wall and exceeds the critical layer, the effect of lower boundary slip is almost no effect on the growth of the critical Reynolds number. However, when the film center is located at the upper half channel, the effect of lower boundary slip plays a major role on the increase of the critical Reynolds number.(4)In the single film model, only when the film center closes to the critical layer (generally between 0.65-0.7), the effect of film thickness on the critical Reynolds number is obvious. The smaller the film thickness is, the bigger the critical Reynolds number is. In the double film model, there is a certain discontinuity of viscosity at the interface. Regardless of the film is at any location, the effect of film thickness is an obvious on the stability of interface.(5)For the case of mass transfer from liquid 1 to liquid 2( 1→2), if the viscosity and solute diffusivity ratio of the upper fluid to lower fluid is small than one, the instability of interface occurs to enhance mass transfer between phases. However, the above conclusion is correct only when the Reynolds number is small. Because when the Reynolds number exceeds a certain value, the effect of Marangoni is becoming weak, and finally the unstable region disappears.