We consider the dynamics of equilibration of the chemical potential of a fluorophore in a monodisperse emulsion containing droplets with two initially different concentrations of the fluorophore. Although the exchange mechanism involves a single timescale at the droplet (microscopic) level, the organisation of the droplets determines the exchange dynamics at the population (macroscopic) level. The micelle concentration in the continuous phase and the chemistry of the fluorophore control the microscopic exchange rate while the disorder of the initial condition determines the power-law of the long timescale, recovered in a minimal analytical model. We also show here that an additive in the droplet such as Bovine Serum Albumin (BSA) acts on the microscopic exchange rate and slows down the exchange process by increasing the solubility of the fluorophore in the dispersed phase rather than by creating a viscoelastic layer at the droplet interface.

We designed, developed and characterized a microfluidic method for the measurement of surfactant adsorption kinetics via interfacial tensiometry on a microfluidic chip. The principle of the measurement is based on the deformability of droplets as a response to hydrodynamic forcing through a series of microfluidic expansions. We focus our analysis on one perfluoro surfactant molecule of practical interest for droplet-based microfluidic applications. We show that although the adsorption kinetics is much faster than the kinetics of the corresponding pendant drop experiment, our droplet-based microfluidic system has a sufficient time resolution to obtain quantitative measurement at the sub-second time-scale on nanoliter droplet volumes, leading to both a gain by a factor of ∼10 in time resolution and a downscaling of the measurement volumes by a factor of ∼1000 compared to standard techniques. Our approach provides new insight into the adsorption of surfactant molecules at liquid-liquid interfaces in a confined environment, relevant to emulsification, encapsulation and foaming, and the ability to measure adsorption and desorption rate constants.

Tip-streaming generates micron-and submicron-sized droplets when a thin thread pulled from the pointy end of a drop disintegrates. Here, we report streaming from the equator of a drop placed in a uniform electric field. The instability generates concentric fluid rings encircling the drop, which break up to form an array of microdroplets in the equatorial plane. We show that the streaming results from an interfacial instability at the stagnation line of the electrohydrodynamic flow, which creates a sharp rim. The flow draws from the rim a thin sheet which destabilizes and sheds fluid cylinders. This streaming phenomenon provides a new route for generating monodisperse microemulsions.A highly conducting drop in a uniform electric field elongates into a prolate ellipsoid whose poles in strong fields deform into conical tips (Taylor cones) emitting jets of charged tiny droplets [1][2][3][4][5]. This so called electrohydrodynamic (EHD) streaming or cone-jetting occurs in many natural phenomena (e.g., drops in thunderclouds) and technological applications (printing, electrospraying, electrospinning) [4,6].The streaming is related to a generic interfacial instability due to a convergent flow [7], see Figure 1.a. The interface is compressed and a local perturbation at the stagnation point (e.g., drop tips) gets drawn by the flow. If the viscous stresses overcome the interfacial tension, the perturbation grows into a fluid filament. This is the tip-streaming phenomenon commonly observed in the microfluidic co-flow geometry [8][9][10]. If instead of a point, the flow is converging to a stagnation line, then a thin sheet can be entrained [11]. By analogy with the cone-jet geometry resulting from the destabilization of a stagnation point, it is expected that the instability of a stagnation line would give rise to an edge-sheet structure. In this Letter, we report for the first time streaming resulting from a stagnation line instability.Experimentally, we exploit the electrohydrodynamic flow about a neutral drop placed in a uniform electric field [12,13]. By varying the fluid conductivities, we are able create flow converging either at the drop poles (Figure 1.b) to generate cone-jet, or at the equator ( Figure 1.c) to generate an edge-sheet. The latter case is the focus of this work. The electrohydrodynamic flow is driven by electric shear stresses due to induced surface charges [12,13]. For a drop in a uniform electric field the resulting flow is axisymmetrically aligned with the applied field. For a spherical drop with radius a placed in DC electric field E = Eẑ, the surface velocity is [12]where λ = µ in /µ ex is the viscosity ratio between the drop and suspending fluids and θ is the angle with the applied field direction. The direction of the surface flow depends on the difference of conductivity, σ, and permittivity, ε, of the drop and suspending fluids R = σ in /σ ex and S = ε in /ε ex . For highly conducting drops, R/S > 1, the surface flow is from the equator to the poles. Accordingly, the poles become stagnation p...

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