Confined flow of concenrated emulsions in microfluidic systems
Keywords: Aeronautics and Astronautics.

Emulsion droplets are metastable systems, in which one liquid is dispersed in another immiscible liquid. The flow of emulsion underlies many industrial applications ranging from oil recovery, drug delivery, food processing, to cosmetic production. Because of its rich and complex rheological responses, the study of emulsion, among others including colloids, foams, and granular media, has been central to soft matter physics and relevant material designs. Recent advances in high-throughput droplet microfluidic applications also rely on the flow of emulsion, where individual droplets act as separate biochemical reactors. Here, the ability to manipulate the drops in the form of a concentrated emulsion — an emulsion with a high disperse-phase volume fraction φ > 80% — is important to ensure uniformity in the flow profile and the associated incubation duration of the drops. As the number of drops that must be used increases for applications such as the directed evolution of enzymes which has a large library of mutants (> 1e7) to screen from, it is necessary to handle the drops in a concentrated manner that can remove the need to handle the large volumes of continuous phase for the same number of drops. This leads to a reduction in the cost and time in processing these drops. Understanding the flow of concentrated emulsion is thus key for practical applications in droplet microfluidics for the design of effective flow control and on-chip droplet manipulations. Previous work on concentrated emulsions have focused primarily on their bulk rheological properties. Meanwhile, the dynamics of a single droplet has been extensively studied and relatively well understood. The dynamics of individual droplets within a flowing concentrated emulsion, however, remains less explored. Understanding how individual droplets behave within a concentrated emulsion is necessary as each droplet acts as an individual micro-reactor and can encapsulate different assay samples or reagents. Motivated by this, my thesis dissertation aims to examine the fate of individual droplets within a concentrated emulsion flowing in a microfluidic system. In particular, my dissertation focuses on the flow features of concentrated emulsion droplets in two distinct strain-rate regimes, under which many practical emulsion-based applications are operated. The first regime (Capillary number Ca~1e-4) is a low strain-rate regime, in which the interfacial effect dominates over viscous effect. The second regime is a high strain-rate regime (Ca> 1e-2), in which viscous effect dominates over interfacial effect, and the emulsion starts to flow laminarly and can exhibit unstable phenomenon such as droplet breakup. The results presented in this dissertation is significant not only in understanding the flow of emulsions at the individual droplet level but also in guiding the design of flow control elements in microfluidic devices. The first part of this dissertation focuses on the low strain-rate regime. This part covers four sub-topics. (1) An ordered flow of a two-dimensional concentrated emulsion in a tapered microchannel. Under appropriate conditions, the cascade of successive rearrangement events shows a surprising periodicity both in space and time, similar to the gliding motion of crystal dislocations along slip planes. (2) A micro-PIV study on the internal flow inside droplets within a concentrated emulsion flowing in a straight microchannel. The results indicate the internal flow patterns inside droplets are dependent on confinement as well as the location of the droplets in microchannel. (3) The effects of droplet rearrangements on internal flow inside droplets within a concentrated emulsion. When droplets rearrange, vortical flow structures are induced inside droplets, which is a unique nature of concentrated emulsions and cannot be predicted from studies in diluted emulsions or single drops. (4) The timescale and spatial distribution of local plastic events in a concentrated emulsion. Upon an increase in applied strain rate, the duration of a droplet rearrangement event consists of distinct regimes, which can be associated with the emulsion transitioning from a solid-like to a liquid-like state and the loss of order in the flow of concentrated emulsion. The second part of this dissertation focuses on the high strain-rate regime, in which a stochastic droplet breakup behavior in concentrated emulsions is examined. This part includes two sub-topics. (1) Critical parameters for describing droplet breakups in concentrated emulsions. The droplet breakups in concentrated emulsions display a universal behavior that is described by the product of capillary number, viscosity ratio, and confinement. (2) Effects of nanoparticles on suppressing droplet breakups in concentrated emulsions. By switching from a surfactant-stabilized interface to an amphiphilic nanoparticles-stabilized interface, undesired droplet breakups in concentrated emulsions can be effectively reduced. Given the same tolerance of the droplet breakup fraction, the throughput rate can be increased up to 3 folds.