(89c) Characterization Non-Newtonian Transport Character of Oil-in-Water Emulsions in Micro Tubes and Channels at High Content of Oil

The well-known non-Newtonian fluid behaviour of emulsions is caused by shear-forced deformation of droplets (disperse phase), by induction of phase internal convection and by shear-forced induced lateral movement of droplets. In many applications, a moderate non-Newtonian behaviour was observed, if emulsions with high volume ratios between disperse phase (coherent phase) and embedded phase were applied [1,2].

Here, the flow behavior of oil-in-water emulsions in micro channels and capillaries at low water volume fraction was studied. Silicon oil was applied as non-aqueous phase, sodium dodecylsulfate (SDS) was used as surfactant. The transport behavior was characterized by the flow rate-depending pressure drop and by the determination of local trajectories inside micro channels using micro particle image velocimetry (:-PIV).

The emulsions were generated immediately before the measurements by flow-through emulsification using a static chip micro mixer [3]. It was found, that the character of the transport behavior is changed considerably, if the volume fraction of the aqueous phase is shifted below about 30%. At lower volume fraction of water, the fluid resistance decrease much stronger with increasing flow rate than at higher water content (Fig. a). The :-PIV investigations showed, that this effect is due to a transition from a more laminar to a strong plug-like flow regime. The plug flow character is marked by a steep velocity gradient in the liquid near the channel walls (Fig. b). So, the transport behavior can be described by a significant slip-effect. In addition, the direct mechanical contact between the single oil droplets contributes to the change in the response behavior of the emulsion against the shear stress. This effect becomes particular important, if the critical volume ratio for dense stacked micro sphere is achieved in case of a homogenous droplet size. The exact value for critical volume ratio depends on the variation of droplet sizes inside emulsion and on the flow-induced droplet deformation.

The investigations show that the flow of emulsions of low volume fraction of coherent phase through micro channels and tubes is strongly determined by geometrical conditions of spatial self-organization of droplets of disperse liquid. Beside the volume ratio itself, the viscosity of both phases, the size distribution, the flow rate and the channel width are additionally effecting the transport behavior.

Acknowledgement

We thank T. Frank (LTF Ilmenau) for cooperation and discussion, F. Möller and S. Schneider (TU Ilmenau) for support and technical assistance. Financial support by the German Environmental Foundation (DBU) is gratefully acknowledged

References [1] J.W. VanEgmond, Current opinion in colloid and interface science 3 (1998), 385 [2] H. Löwen, M. Schmidt, Progr. Colloid Polym. Sci. 104 (1997), 81 [3] T. Kirner, J. Albert, M. Günther, G. Mayer, K. Reinhäckel, J.M. Köhler, Chem. Engin. J. 101 (2004), 65

Figure

Non-Newtonian transport behavior of oil-in-water emulsions at high volume fractions of oil: a) flow-rate dependence of apparent viscosity (pressure drop) for different volume ratios, b) examples of a velocity profile inside a micro channel measured by :-PIV