Dynamically reconfigurable complex emulsions via tunable interfacial tensions

Abstract

Emulsification is a powerful, well-known technique for mixing and dispersing immiscible components within a continuous liquid phase. Consequently, emulsions are central components of medicine, food and performance materials. Complex emulsions, including Janus droplets (that is, droplets with faces of differing chemistries) and multiple emulsions, are of increasing importance in pharmaceuticals and medical diagnostics, in the fabrication of microparticles and capsules for food, in chemical separations, in cosmetics, and in dynamic optics. Because complex emulsion properties and functions are related to the droplet geometry and composition, the development of rapid, simple fabrication approaches allowing precise control over the droplets' physical and chemical characteristics is critical. Significant advances in the fabrication of complex emulsions have been made using a number of procedures, ranging from large-scale, less precise techniques that give compositional heterogeneity using high-shear mixers and membranes, to small-volume but more precise microfluidic methods. However, such approaches have yet to create droplet morphologies that can be controllably altered after emulsification. Reconfigurable complex liquids potentially have great utility as dynamically tunable materials. Here we describe an approach to the one-step fabrication of three- and four-phase complex emulsions with highly controllable and reconfigurable morphologies. The fabrication makes use of the temperature-sensitive miscibility of hydrocarbon, silicone and fluorocarbon liquids, and is applied to both the microfluidic and the scalable batch production of complex droplets. We demonstrate that droplet geometries can be alternated between encapsulated and Janus configurations by varying the interfacial tensions using hydrocarbon and fluorinated surfactants including stimuli-responsive and cleavable surfactants. This yields a generalizable strategy for the fabrication of multiphase emulsions with controllably reconfigurable morphologies and the potential to create a wide range of responsive materials.

Extended Data Figure 1. Dynamic interfacial tension data was used to estimate the equilibrium interfacial tensions for the hexane–water and perfluorohexane–water interfaces

a, Dynamic interfacial tension data (in blue) was obtained from the pendant-drop method; the representative data shown here was measured for the hexane–water interface at f_SDS = 0.9 (such that the aqueous solution contained 0.1% SDS and 0.1% Zonyl in a 9:1 ratio). The data was fitted to an empirical model (in red) to estimate the equilibrium value of the interfacial tension γ_eqb = γ(t → ∞). Such fitting was performed for all measured interfacial tensions and the fitted parameter results are tabulated in Extended Data Table 1. b, The estimated equilibrium interfacial tension values were used to plot the hexane–water (squares) and perfluorohexane–water (circles) interfacial tensions as a function of the fraction of 0.1% SDS, f_SDS, where the other fraction is 0.1% Zonyl. See discussion in Methods section for more details.

Extended Data Figure 2. The geometry of a Janus droplet can be used to estimate the interfacial tension between hydrocarbon and fluorocarbon internal phases

a, Sketch of a Janus droplet consisting of hydrocarbon (grey) and fluorocarbon (white) phases within an aqueous outer phase. The radii of curvature of the H–W (R_H), F–W (R_F) and F–H (R_FH) interfaces are related to their respective interfacial tensions through the Young–Laplace equation. The diameter of the circle of contact between the two phases (dashed line) is denoted as D. b, The Janus droplet is composed of three spherical caps, and the volume, V_cap, of each constituent spherical cap is a function of the radius of curvature of the spherical surface and the base diameter D. Here we show the cap at the intersection of the hydrocarbon and fluorocarbon phases in which V_cap is a function of R_FH and D. c, An exemplary image of a hexane–perfluorohexane Janus droplet obtained at f_SDS = 0.6, which was used to estimate R_FH and, in turn, γ_FH. d, The droplet pictured in c is subjected to edge detection to determine the H–W and F–W interfaces. e, The resulting edges are fitted to circles (red lines with green centres). The diameter of the circle of contact is then computed (green line). Given the ratio of the volumes of the two phases, we then determined the radius of curvature, R_FH, of the hexane–perfluorohexane interface, which was subsequently used to estimate γ_FH. See discussion in the Methods section for more details and Extended Data Table 2 for estimated values of γ_FH.

Extended Data Figure 3. Synthesis scheme and ¹H-NMR of fluorinated crosslinker

a, Reaction scheme for the synthesis of the fluorinated crosslinker. b, ¹H-NMR spectrum of the fluorinated crosslinker.

Extended Data Figure 4. ¹³C-NMR and ¹⁹F-NMR of the fluorinated crosslinker

a, ¹³C-NMR spectrum of the fluorinated crosslinker. b, ¹⁹F-NMR of the fluorinated crosslinker.

Figure 1. Temperature-controlled phase separation of hydrocarbon and fluorocarbon liquids can be used to create complex emulsions

a, Complex emulsion fabrication. b, Above T_c, hexane and perfluorohexane are miscible and emulsified in aqueous 0.1% Zonyl (top left). Below T_c, hexane and perfluorohexane phases separate to create a hexane-in-perfluorohexane-in-water (H/F/W) double emulsion (bottom right). Hexane is dyed red. Scale bar, 200 μm. c, Emulsions of uniform composition made by bulk emulsification (such as shaking). Scale bar, 100 μm. d, Lateral confocal cross-section of H/F/W double-emulsion droplets. Hydrocarbon-soluble Nile Red dye (green) selectively extracts into hexane. Rhodamine B dyes the aqueous phase (red). Scale bar, 100 μm. Monodisperse droplets in b and d were made using a microcapillary device.

Figure 2. Reconfiguration of droplet morphology is dynamic and results from changes in the balance between interfacial tensions

a, Sketch of the effect of interfacial tensions on the configuration of a complex droplet. In (1), γ_F > γ_H + γ_FH, favouring encapsulation of phase F within phase H. In (3), γ_H > γ_F + γ_FH and phase H is encapsulated within phase F. At intermediate values of γ_F and γ_H, a Janus droplet with geometry typified by (2) is formed. γ_F, γ_H and γ_FH can be reconfigured into a Neumann triangle solvable for θ_H and θ_F. b, Hexane–perfluorohexane droplets reconfigure in response to variation in the concentration of Zonyl as it diffuses through 0.1% SDS from right (high Zonyl concentration) to left (low Zonyl concentration). Scale bar, 100 μm. c, Configurational stability diagram for the hexane–perfluorohexane–water system showing γ_F – γ_H as a function of the fraction of 0.1% SDS, f_SDS, where the other fraction is 0.1% Zonyl. The green band denotes the region |γ_F – γ_H| < γ_FH = 1.07 ± 0.1 mN m⁻¹, obtained from geometrical analysis of Janus droplets. The red dashed lines correspond to γ_FH = 0.4 mN m⁻¹ as predicted for a temperature of 10 °C (ref. 23), the approximate temperature at which the droplets in d were imaged. Filled and unfilled triangles indicate conditions under which Janus droplets and double emulsions were observed, respectively. Labels I–VII correspond to the droplets in d. d, Optical micrographs of hexane–perfluorohexane droplets in solutions of 0.1% Zonyl and 0.1% SDS in varying ratios as plotted in c. Hexane is dyed and appears grey. Scale bars, 50 μm.

Figure 3. Emulsions reconfigure in response to light and pH

a, Sketch of how the variations in γ_F and γ_H induced by alterations in the effectiveness of a hydrocarbon surfactant translate into differences in drop morphology on a phase-stability diagram. Grey represents hexane and white represents perfluorohexane. b, Chemical structure of the light-responsive surfactant which reversibly isomerizes under ultraviolet (UV) and blue light between the more effective trans form of the surfactant (left) and the less effective cis form (right). Aligned beneath are optical micrographs of hexane–perfluorohexane emulsions that are tuned to undergo specific morphological transitions in response to light. Hexane is dyed red, and the aqueous phase consists of Zonyl and the light-responsive surfactant pictured. Top: droplets undergo complete inversion. Middle: F/H/W double-emulsion drops transition to Janus droplets. Most droplets are viewed from the top, but one is lying on its side allowing a view of the droplet profile. Bottom: Janus droplets transition to an H/F/W double emulsion. Scale bar, 100 μm. c, A pH-responsive surfactant, N-dodecylpropane-1,3-diamine, is used in combination with Zonyl to create pH-responsive droplets. Acid diffuses through the solution from right (high concentration) to left (low concentration), reducing the pH below pK_a = 4.7 and so generating a less effective surfactant and inducing inversion of the hexane–perfluorohexane emulsions. Scale bar, 100 μm. d, Emulsions stabilized by a combination of Zonyl and acid-cleavable surfactant, sodium 2,2-bis(hexyloxy)propyl sulphate, undergo morphological changes as the cleavable surfactant is degraded over time at pH = 3. Scale bar, 50 μm.

Figure 4. Magnetic complex emulsions, complex emulsions as templates and four-phase emulsions

a, Janus droplets of ethyl nonafluorobutyl ether and dichlorobenzene containing magnetite nanoparticles in the dichlorobenzene phase are oriented with a magnet. Scale bar, 200 μm. b, Scanning electron micrograph of hemispherical particles made from photopolymerized Janus droplets containing hexanediol diacrylate and methoxyperfluorobutane. Scale bar, 100 μm. c, Top: scanning electron micrograph of a Janus particle with hydrocarbon and fluorinated polymeric hemispheres. Bottom: the energy-dispersive X-ray spectral map reveals the fluorinated hemisphere. Scale bar, 50 μm. d, Four-phase emulsions reconfigure in response to changes in interfacial tension. Drops contain hydrocarbon oil (H, mineral oil with octadecane), silicone oil (Si) and fluorinated oil (F, ethyl nonafluorobutyl ether) emulsified in water (W). The silicone phase is enriched with a fraction of the two other phases. Left: 1% Zonyl generates an H/Si/F/W triple emulsion. Right: 1% SDS generates an F/Si/H/W triple emulsion with a thin outer shell. Phases were identified with fluorescent dyes. Nile Red (green) preferentially dyes the hydrocarbon oil, and fluorous-tagged coumarin dye preferentially dyes the fluorous phase (blue). Scale bars, 50 μm.