Dynamic blocking of dispersed flows in microchannels

Alfir Akhmetov, Artur Rakhimov, Azat Valiev, Alexander Vasilyev

Objective:

Experimental studies of peculiarities of emulsion hydrodynamics in microchannels

Emulsion flows are accompanied by physicochemical transformations as well as by coalescence and fragmentation of droplets. Moreover, emulsion motion is accompanied by redistribution of volume fraction of the dispersed phase and restructuring of micelles in the dispersion phase, by forth and back transfer of surfactant molecules between the bulk of the flow and the droplet surface and migration of surfactant molecules along the droplet surface, and, finally, by an ambiguous interaction of droplets at a reduced spacing between them. Such a variety of transformations and interactions complicates the hydrodynamic behavior of the system enormously. In microchannel flows the presence of walls leads to a stronger manifestation of these processes and the appearance of the effects associated with the self-organization of complex systems. Hence, the hydrodynamics of emulsions in microchannels has important peculiarities.

The dependence of volume the emulsion passing through the microchannel and the emulsion flow rate in the state of blocking in a microchannel on the time
The dynamic blocking effect

The dynamic blocking effect occurs in different types of flows in microchannels. The effect is that for the emulsion flow through a microchannel at a constant pressure drop, over time is slowed by several orders of magnitude (3-4 orders). This effect obtains its name due to the fact that despite the observed visually stopping the flow in the scale of the model, a much slower microflow is detected on the microscale. It should be noted that in all cases the manifestation of the effect of dynamic blocking the sizes of the droplets of the emulsion are much smaller than the characteristic cross-section the microchannel. Moreover, the observed effect can be treated as manifestation of instability of emulsion flow in microchannels, when the emulsion flow rate at constant pressure drop slows down over time by several orders of magnitude, while the decrease of pressure can lead to the increase of speed and intermittent pressure release can result in a partial recovery of the flow rate.

The use of visualization techniques together with measurements of hydrodynamic flow parameters enables detection of structural changes in the flows and effects leading to emulsion flow instabilities. By comparing of the video information obtained at different magnifications it is possible to identify factors affecting the change of flow parameters and its structure under constant ambient conditions.

Although the most significant transformation in the flow in microchannels occurs at high concentrations of the droplets, it is important to begin the study with low concentrated emulsions. This should provide understanding of the interaction of droplets with the flow and characterization of the influence of the unstable behavior of droplets on their deformation, orientation, fragmentation, coalescence and redistribution across the flow.

The studies are carried out using various types of microfluidic devices produced by traditional methods (axisymmetric capillaries, capillaries with narrowings, Healy-Shaw cells) and using soft lithography techniques.

Prospects:

A detailed experimental study of the dynamic blocking effect of emulsions will enable development of new technologies for enhanced oil recovery