Surfactant adsorption kinetics in microfluidics.
Emulsions are metastable dispersions. Their lifetimes are directly related to the dynamics of surfactants. We design a microfluidic method to measure the kinetics of adsorption of surfactants to the droplet interface, a key process involved in foaming, emulsification, and droplet coarsening. The method is based on the pH decay in the droplet as a direct measurement of the adsorption of a carboxylic acid surfactant to the interface. From the kinetic measurement of the bulk equilibration of the pH, we fully determine the adsorption process of the surfactant. The small droplet size and the convection during the droplet flow ensure that the transport of surfactant through the bulk is not limiting the kinetics of adsorption. To validate our measurements, we show that the adsorption process determines the timescale required to stabilize droplets against coalescence, and we show that the interface should be covered at more than [Formula: see text] to prevent coalescence. We therefore quantitatively link the process of adsorption/desorption, the stabilization of emulsions, and the kinetics of solute partitioning-here through ion exchange-unraveling the timescales governing these processes. Our method can be further generalized to other surfactants, including nonionic surfactants, by making use of fluorophore-surfactant interactions.
Soft Microsystems, Centre de Recherche Paul Pascal, Unité Propre de Recherche 8641, CNRS, University of Bordeaux, 33600 Pessac, France; Droplets, Membranes and Interfaces, Max Planck Institute for Dynamics and Self-Organization, 37077 Goettingen, Germany.
Birte
B
Riechers
Soft Microsystems, Centre de Recherche Paul Pascal, Unité Propre de Recherche 8641, CNRS, University of Bordeaux, 33600 Pessac, France; Droplets, Membranes and Interfaces, Max Planck Institute for Dynamics and Self-Organization, 37077 Goettingen, Germany.
Florine
F
Maes
Department of Biochemistry, Ludwig Maximilians University, 81377 Munich, Germany; Gene Center, Ludwig Maximilians University, 81377 Munich, Germany; Department for NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, 37077 Goettingen, Germany.
Elias
E
Akoury
Droplets, Membranes and Interfaces, Max Planck Institute for Dynamics and Self-Organization, 37077 Goettingen, Germany; Laboratoire de Physique Statistique, École Normale Supérieure, Université de Recherche Paris Sciences et Lettres, 75005 Paris, France; Laboratoire de Physique Statistique, Université Paris Diderot Sorbonne Paris-Cité, 75005 Paris, France; Laboratoire de Physique Statistique, Sorbonne Universités, Pierre and Marie Curie University, University of Paris 06, 75005 Paris, France; Laboratoire de Physique Statistique, CNRS, 75005 Paris, France.
Benoît
B
Semin
Droplets, Membranes and Interfaces, Max Planck Institute for Dynamics and Self-Organization, 37077 Goettingen, Germany.
Philipp
P
Gruner
Soft Microsystems, Centre de Recherche Paul Pascal, Unité Propre de Recherche 8641, CNRS, University of Bordeaux, 33600 Pessac, France; Droplets, Membranes and Interfaces, Max Planck Institute for Dynamics and Self-Organization, 37077 Goettingen, Germany; jean-christophe.baret@u-bordeaux.fr.
Jean-Christophe
JC
Baret
droplet; emulsion; interfaces; microfluidics; surfactant
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