Self-limiting droplet fusion in ionic emulsions.

We make an oil-in-water emulsion, which is initially stabilized using a first ionic surfactant, and mix it with a solution of a second ionic surfactant having the opposite charge, thereby inducing massively parallel droplet fusion. A transient disruption of the screened-charge repulsive barrier between interacting droplets, caused by the second ionic surfactant, arises from significant yet temporary charge neutralization of the first ionic surfactant on the surfaces of the oil droplets while mixing occurs. Interestingly, if a moderate molar excess of one surfactant exists, then the resulting emulsion re-stabilizes after limited droplet fusion. By adjusting the droplet volume fraction, concentrations of first and second surfactants, and volumes of the emulsion and the solution of the second surfactant, we control the degree of droplet coalescence and achieve a self-limiting droplet fusion process. Using optical microscopy, we observe that flat, thin, crystalline films can form between the two oil compartments after fusion of two or more immiscible microscale droplets. However, no such crystalline films are seen on the highly curved oil-oil interfaces inside nanoscale droplets that are composed of two or more immiscible oils and have been fused in the same manner, as revealed by cryogenic transmission electron microscopy.

Cerberus nanoemulsions produced by multidroplet flow-induced fusion.

Through extreme flow-induced fusion and rupturing of microscale droplets within a mixture of two or more oil-in-water emulsions, each having a different type of mutually immiscible oil, we create complex oil-in-water nanoemulsions composed of multicomponent compartmentalized nanodroplets. The extreme flow temporarily overcomes the repulsive barrier between oil droplets, arising from stabilizing surfactant molecules on the droplet interfaces, thereby causing multidroplet fusion as well as droplet fission down to the nanoscale. After the droplets leave the vicinity of extreme flow, they remain stable against subsequent coarsening and coalescence. Using this highly parallel, top-down, nonequilibrium synthetic approach, we create bulk quantities of engulfed-linear Cerberus oil-in-water nanoemulsions. Each Cerberus nanodroplet contains three different immiscible oils that form complex-shaped internal compartments, as revealed by cryogenic transmission electron microscopy (cryo-TEM). Within a given Cerberus nanodroplet, depending upon the interfacial tensions and relative volume fractions of the different oils, the internal oil-oil interfaces can be significantly deformed. Such multicomponent compartmentalized oil nanodroplets have the capacity of holding different types of oil-soluble cargo molecules, including fluorinated drug molecules, which have a wide variety of functional capacities and the potential for local synergistic effects. Their size range is small enough to permit a wide variety of pharmaceutical applications. As such, Cerberus nanoemulsions open up possibilities for simultaneously delivering several different types of oil-soluble drug molecules, each of which is readily soluble in at least one of the different types of immiscible oils, to the same cell or tissue.

Nanoinclusions in cryogenically quenched nanoemulsions.

Nanodroplets containing mixtures of silicone oil and squalene are dispersed in a simple aqueous surfactant solution, quenched in liquid ethane, and examined using cryogenic transmission electron microscopy (CTEM). Depending on the phase of ice that forms around the nanodroplets and on the composition of the oil mixture, nanoinclusions can be observed inside oil nanodroplets, independent of surfactant type. Our observations suggest that these nanoinclusions arise from nucleation of vapor cavities as the water freezes and expands while the oil remains liquid during the quench.

Advanced nanoemulsions.

Recent advances in the growing field of nanoemulsions are opening up new applications in many areas such as pharmaceuticals, foods, and cosmetics. Moreover, highly controlled nanoemulsions can also serve as excellent model systems for investigating basic scientific questions about soft matter. Here, we highlight some of the most recent developments in nanoemulsions, focusing on methods of formation, surface modification, material properties, and characterization. These developments provide insight into the substantial advantages that nanoemulsions can offer over their microscale emulsion counterparts.

Optically probing nanoemulsion compositions.

Many types of colloids, including nanoemulsions, which contain sub-100 nm droplets, are dispersed in molecular and micellar solutions, especially surfactant solutions that confer stability. Since it would be desirable to measure the droplet volume fraction ϕ and surfactant concentration C of a nanoemulsion non-destructively, and since the droplet and surfactant structures are significantly smaller than the shortest wavelengths of visible light, optical refractometry could provide a simple and potentially useful approach. By diluting a silicone oil-in-water nanoemulsion having an unknown ϕ and C with pure water, measuring its refractive index n(ϕ,C) using an Abbé refractometer, and fitting the result using a prediction for n that treats the nanoemulsion as an effective medium, we show that ϕ and C can be deduced accurately over a relatively wide range of compositions. Moreover, we generalize this approach to other types of nanoemulsions in which a molecular constituent partitions in varying degrees between the dispersed and the continuous phases.

Rotational Fourier tracking of diffusing polygons.

We use optical microscopy to measure the rotational Brownian motion of polygonal platelets that are dispersed in a liquid and confined by depletion attractions near a wall. The depletion attraction inhibits out-of-plane translational and rotational Brownian fluctuations, thereby facilitating in-plane imaging and video analysis. By taking fast Fourier transforms (FFTs) of the images and analyzing the angular position of rays in the FFTs, we determine an isolated particle's rotational trajectory, independent of its position. The measured in-plane rotational diffusion coefficients are significantly smaller than estimates for the bulk; this difference is likely due to the close proximity of the particles to the wall arising from the depletion attraction.

Shear-induced disruption of dense nanoemulsion gels.

The structural evolution and rheology of dense nanoemulsion gels, which have been formed by creating strong attractions between slippery nanodroplets, are explored as a function of steady shear rate using rheological small-angle neutron scattering (rheo-SANS). For applied stresses above the yield stress of the gel, the network yields, fracturing into aggregates that break and reform as they tumble and interact in the shear flow. The average aggregate size decreases with increasing shear rate; meanwhile, droplet rearrangements within the clusters, allowed by the slippery nature of the attractive interaction, increase the local density within the aggregates. At the highest shear rates, all clusters disaggregate completely into individual droplets.