59th Annual Report on Research 2014

Rheology of foams and concentrated emulsions impacts many industries, including enhanced oil recovery. The use of foams or emulsions is particularly attractive for mobility control. Our research objectives are to 1) identify conditions for the stability of surfactant- and nanoparticle-laded emulsions flowing through constrictions with size equal to or smaller than droplet diameter. 2) Determine the degree of reversibility and dispersion of the emulsion’s microstructure, and if initially irreversible structures self-organize into reversible patterns after cycles of oscillations.

RESULTS
    1. Stability of surfactant-laden emulsions flowing through narrow constrictions

We generated monodisperse droplets using a flow-focusing nozzle. The drops were collected and reinjected as a concentrated emulsion (φ~92%) into microfluidic channels having different constriction geometries. We developed a custom Matlab script (Figure 1) to track droplet positions, measure droplet deformations, and identify droplet splitting of a large number of drops (N>4000). We observed that, unlike single drops, break-up in a concentrated emulsion was stochastic in nature, and depended on instantaneous interactions among the drops. The ability to examine a large number of drops allowed us to define a probability of droplet break-up and to identify the critical flow conditions beyond which the drops became unstable and started to split. Figure 2a shows that the break-up probability increased with increasing entrance angle and applied flow rate. Figure 2b shows that the break-up probability increased with increasing droplet size. Figure 2c shows the probability of droplet break-up as a heat map. Below some critical values of flow rate and droplet deformation, break-up probability was zero. This probability increased with increasing flow rate and increasing droplet deformation.

   
Figure 1: (a) snap shot of the flow of concentrated emulsion through a narrow constriction. (b) The corresponding MATLAB code used to track the drops.  

 
Figure 2: (a) The fraction of drops that split as a function of flow rate at three different constriction entrance angles. (b) The fraction of drops that split as a function of flow rate at three different droplet volumes. (c) Heat map showing the probability of droplet splitting as a function of maximum deformation of a drop and the constriction velocity (or capillary number). White regions indicate that there are no drops observed in our experiments having that particular set of maximum deformations and constriction velocities. For all data in this figure, the droplet volume was 40 pL and the entrance angle was 30°.

2. Stability of Nano-particles laden droplets

We have synthesized silica nanoparticles as effective emulsifiers to replace surfactants (Figure 3). The surface chemistry of the particles was modified to render the particles amphiphilic, that is, partially wettable by water and partially wettable by oil. We showed that these particles can stabilize monodisperse picoliter aqueous drops against coalescence (Figure 4). Work is in progress to compare the stability drops stabilized by nanoparticles against those stabilized by surfactants when the drops are injected into a constriction.

Figure 3. SEM image of drops stabilized by nanoparticles after excess particles from the continuous phase were washed off and after the fluids evaporated.

Figure 4: Nanoparticles can stabilize droplets against coalescence. Size of drop ~ 50 mm.

3. Degree of reversibility and dispersion of the emulsion’s microstructure:

Here we explored the transition from reversible to chaotic behavior in an oscillatory shear flow of water-in-oil emulsions. The emulsion was injected through a microchannel and was forced to rearrange due to the presence of a constriction in the channel. We defined a drop to be reversible if it returned to a position within 5% of its starting position after one oscillation cycle. We found that the emulsion exhibit behaviors that vary from complete reversibility to complete irreversibility depending on the volume fraction and strain rate. The reversibility phenomena was reproducible even when the drops undergo many rearrangement events over distances of >150 droplet diameters. Work is in progress to identify the mechanism of reversibility.

IMPACT OF RESEARCH

The study on droplet stability is useful for understanding the behavior of concentrated emulsions in applications such as mobility control in enhanced oil recovery, and for extrapolating critical parameters such as injection rates to ensure the stability of the fluids going through small pore throats. Understanding hydrodynamic reversibility and discovering any self-organization of the fluid structure allows one to elucidate the origin of hysteresis and history dependence of the microstructure. One could potentially prime the foams or emulsions prior to actual use to fine-tune their bulk properties. The research has resulted in publication in a peer-reviewed journal Soft Matter, and two conference presentations in American Physical Society Division of Fluid Dynamics Annual meeting 2013. The work has enabled research in my lab on two-phase flow and supported a postdoc who is interested in a career in two-phase flow and interfacial phenomena.