Controlling molecular transport in minimal emulsions

Abstract

Emulsions are metastable dispersions in which molecular transport is a major mechanism driving the system towards its state of minimal energy. Determining the underlying mechanisms of molecular transport between droplets is challenging due to the complexity of a typical emulsion system. Here we introduce the concept of 'minimal emulsions', which are controlled emulsions produced using microfluidic tools, simplifying an emulsion down to its minimal set of relevant parameters. We use these minimal emulsions to unravel the fundamentals of transport of small organic molecules in water-in-fluorinated-oil emulsions, a system of great interest for biotechnological applications. Our results are of practical relevance to guarantee a sustainable compartmentalization of compounds in droplet microreactors and to design new strategies for the dynamic control of droplet compositions.

Figure 1. Microfluidics for the control of minimal emulsions.

(a) Sketch of the trapping channel based on rails and anchors. (b) The geometry of the channels determines the spacing distance between droplets (h_c=20 μm, h_r=15 μm, w_r=20 μm, w_t=80 μm, w_c=100 μm, l varies between 36 and 6 μm). (c) Time sequence (100 ms interval) of droplets buffering each other from one local minimum in surface energy to the next (scale bar, 100 μm). (d) Sketch of the membrane model used to describe the transport process in the array based on the transport through a single membrane (see Supplementary Fig. 2 and Supplementary Note 2).

Figure 2. Molecular transport for two fluorophores.

Fluorescein (a–c) is exchanged at a time scale of order several days in a water-in-fluorinated oil emulsion stabilized by a PFPE–PEG–PFPE surfactant (in HFE-7500). At production, the bright droplets contain Fluorescein at 100 μM in PBS water and the dark droplet PBS only. In contrast, Rhodamine 6G (d–f) is exchanged over minutes (scale bar, 100 μm). At production, the bright droplets contain Rhodamine 6G at 100 μM in millipore water and the dark droplet millipore water only. Resorufin is an intermediate case with a time scale of exchange of the order of 1 h (ref. 33).

Figure 3. Fabrication and control of minimal emulsions.

(a) Sketch of the device for droplet production and storage as alternating row (see Supplementary Fig. 1 and Supplementary Note 1). (b) On one single chip, several packing distance are tested under the same condition. (c) White-light micrograph of an immobilized row of droplets (scale bar, 100 μm) and time evolution of one row of droplets (0, 1.5, 3, 6, 12 and 24 h after immobilization, for a surfactant concentration C=1 wt% and a distance d_c=4 μm.) in fluorescence. (d) Relaxation to equilibrium of the average fluorophore concentrations of initially filled (filled circles) and empty droplets (empty circles) (e) The concentration difference between the two droplet populations decays exponentially. (f) Variation of the kinetics as a function of surfactant (surf) concentration (for d_c=15 μm). (g) Relaxation dynamics of five one-dimensional droplet microarrays for edge-to-edge distance d_c−2r between the droplets of 30 μm (black), 15 μm (red), 10 μm (blue), 4 μm (pink) and 1 μm (olive) (for C=1 wt%).

Figure 4. Oil Permeability P.

(a) The permeability is a function of the centre-to-centre distance between the droplets d_c normalized by the droplet diameter 2r. Experimental data are fitted using equation (1) (see Supplementary Fig. 2 and Supplementary Note 2) to obtain the diffusion coefficient of the dye in the oil-surfactant mixture using the values of K determined independently (b). We obtain D=7.6 × 10⁻⁹ cm² s⁻¹ (5% surfactant), D=7.8 × 10⁻⁹ cm² s⁻¹ (2% surfactant), D=8.2 × 10⁻⁹ cm² s⁻¹ (1% surfactant). The value indicates that transport is mediated by surfactant aggregates affect the partitioning coefficient of the organic molecule between both the phases (error bars are standard deviation over three experiments); conc, concentration.

Figure 5. Flow-induced targeted delivery.

(a) A droplet trapped in a microchannel is exposed to an oil doped with fluorescent dye (see inset). (b) The partitioning to the droplet leads to an increase of fluorescence (scale bar, 40 μm). The rate of molecule uptake by the droplet is given by the delivery rate from the oil (Q_oil=0.2 μl min⁻¹). The principle for a controlled uptake of multiple dyes is shown in c. (c) Droplets of 100 μm in diameter are trapped in a microfluidic chamber. We selectively inject fluorescent dyes (fluorescein and resorufin) from the oil and surfactant mixture into the droplet. The dye injected depends on the position of the droplet in the chamber. (d) Experimental realization: fluorescent micrograph of the droplets showing the dye uptake occuring at a time scale of the order of 1 min. Here Q_oil=0.2 μl min⁻¹.

Figure 6. Emulsion-based targeted delivery.

(a) White-light picture of an immobilized row of droplets and time sequence of fluorescence signals (0, 0.5, 1, 1.5, 2, 2.5, 3 h after immobilization, scale bar, 100 μm). (b) Relaxation of the concentration difference towards the opposite composition. A complete inversion of the concentration is obtained according to an exponential relaxation for droplets initially containing 100 μM resorufin sodium salt and 10 mg ml⁻¹ sodium chloride (filled circles, see inset) and millipore water droplets (empty circles, see inset). (c) Dependency of permeability P on the partitioning coefficient K: the transport rate is driven by the largest partitioning coefficient in the system. Inset: dependency of partitioning coefficient K on the salt concentration in the aqueous phase c_NaCl (error bars are standard deviation over three experiments).

Figure 7. The possible structures of the surfactant supramolecular assemblies.

(a) The swolen micelle structure with an aqueous core, (b) the vesicle structure where the surfactant is either extended or forms bilayers. The vesicle structure is compatible with our experimental results. With the micelle structure, the exchange rate of water should depend on surfactant concentration and all dyes should have the same exchange rates (see Supplementary Fig. 4 and Supplementary Note 4).

Figure 8. Surfactant-mediated extraction of solutes.

(a–d) Fluorescence images of microfluidic droplet production and resulting emulsions in the presence of 0.5% PFPE–PEG–PFPE surfactant and additionally low (a,b<1%) or high (c,d=30%) mass fractions of carboxylic acid fluorosurfactants (scale bar, 300 μm). The aqueous and fluorous phase flow rate are 1 and 4 μl min⁻¹, respectively. (e–h) Macroscopic partitioning experiments. (e) Absorption spectra of the fluorous phase after equilibration shown for various concentrations of the additive. The inset shows the absorption at 500 nm. (f) Corresponding images of the partitioning experiments. Aqueous solutions of rhodamine 6G (initial concentration: 100 μM) are exposed to the fluorous phase with additive concentrations from 1 to 9 as: 0, 3.75, 7.51, 18.8, 37.5, 93.8, 188, 375 and 751 μM. Solution 10 contains only the PFPE–PEG–PFPE surfactant at a concentration of 877 μM (0.5 wt%). (g) Absorption spectra of the fluorous phase for various mole fractions of fluorophore and surfactant. Inset: absorbance at 500 nm as a function of the mole fraction of the additive (Job plot). (h) Images of the corresponding partitioning experiments. In samples 11–19, the fraction of the additive increases as: 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, 1.0.