(535g) Microgel-Covered Drops: Effects on Mass Transfer and Fluid Dynamics

Microgels are micron-sized, cross-linked polymer particles with surface active properties. They adsorb and spread at interfaces. [1, 2] The extent of spreading is related to their properties such as size and degree of cross-linking [3]. Due to their soft and porous character, microgels clearly behave differently from rigid particles at the interface. [1] However, their most outstanding property is their switchable structure change. This structure change leads to a change in properties, especially their interfacial properties [4, 5]. Their surface active, stabilizing properties combined with their switchability make them promising candidates to intensify liquid-liquid systems such as extraction processes. In an extraction column, microgels will stabilize the drops while passing the active part of the column to suppress coalescence. At the top of the column, where coalescence is required for phase separation, the microgel properties will be switched by a temperature shift to enable coalescence. The feasibility of the process concept has been demonstrated before.

In this contribution, we present the impact of cross-linked poly(N-isopropylacrylamide) (PNIPAM) microgels on mass transport through a liquid-liquid interface. With regard to application in disperse liquid-liquid systems, the investigation is based on rising droplets. Therefore, two aspects need to be considered: The mass transport resistance of the microgel layer and the fluid dynamics of the microgel-covered drop resulting from the interfacial conditions. The effect of microgels on both phenomena is determined experimentally and compared to existing model approaches.

The mass transfer resistance of the microgel layer is determined at a flat interface. Taking the cross-linked character of the microgels into account, we quantify the impact of the mass transfer agent size and molecular weight, respectively. Furthermore, the mass transport resistance is related to microgel properties such as cross-linker content.

In addition, the interfacial conditions of microgel-covered drops in a continuous flow field are investigated by their fluid dynamic behavior. The sedimentation velocity of drops as a function of their diameter can be correlated to the interfacial mobility. We determined these effects as a function of the cross-linker content and the spreading of the microgels at the interface, respectively. [6] At drops with a mobile interface, momentum transfer at the interface induces an internal circulation. This circulation leads to an enhanced mass transfer as the dispersed phase is well mixed, which overcomes diffusion limitation. [7] Therefore, the second part of this contribution focuses on the impact of the microgels on mass transport by their effect on the fluid dynamics of drops.

References

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