Microfluidic Approaches for the Study of Emulsions: Transport of Solutes
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Microfluidic Approaches for the Study of Emulsions: Transport of Solutes Philipp Gruner1, Yousr Skhiri2,3, Benoit Semin1*, Quentin Brosseau1, Andrew D. Griffiths3, Valérie Taly2 and Jean-Christophe Baret1 1 Max Planck Institute for Dynamics and Self-organization, Am Fassberg 17, Goettingen, Germany. 2 Université Paris Descartes, 45 Rue des Saints-Pères, Paris, France. 3 ISIS-CNRS-Université de Strasbourg, 8 allée Gaspard Monge, Strasbourg, France. * Present address: ENS Paris, 45 Rue d'Ulm, France. ABSTRACT Molecular transport as an ageing process in emulsions is revisited using microfluidic droplet production, manipulation and analysis. We show how microfluidic systems provide extremely quantitative insights into the phenomenon. We designed microfluidic systems to address the specificity of molecular transport in fluorinated oils and showed the role of the surfactant solubilised in the oil phase on the time scale of the exchange and rationalize the effect of water soluble additives on the exchange rate. Finally, we also demonstrate that the droplet packing influences the exchange rate through the number of first neighbours. INTRODUCTION Droplet-based microfluidic systems are promising platforms for high-throughput screening applications [1,2]. The versatile encapsulation of various types of biomaterials, such as proteins, nucleic acids, single genes, or cells makes droplets perfectly suitable as a tool for miniaturized and automatized enzymatic assays, cell screening, cancer diagnostics, or drug screening platforms [3-5]. The success of the technology relies on the very high-throughput of each individual step of droplet manipulation, which reaches several kHz for droplet production, reinjection, splitting, or fusion [6]. The droplets are used as microcontainers behaving in principle as independent, miniaturized and closed reacting vessels that can be actuated on demand for the steps required in a typical biological assay [1]. In practice, droplets are produced in microchannels and stabilised against coalescence by surfactant molecules, resulting in an emulsion, i.e. a dispersion of one phase into another, stabilised by surfactant molecules [7]. From a fundamental perspective, emulsions are systems out of equilibrium as they only correspond to a local minimum of the free energy landscape and the final equilibrium state where the two phases are separated in two compartments of macroscopic size will eventually be reached. The typical time-scale for this process is linked to the ageing mechanism of the emulsion, involving floculation, coalescence and molecular transport between droplets [7]. The first relevant step for the use of droplet as microreactor is the stabilisation of the droplets by the surfactant, which is achieved by formulation and optimisation of surfactant concentration and additives. This side is well established and microfluidic systems have proven very efficient tools to study the underlying mechanisms in the stabilisation of emulsions by surfactants [8, 9].
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