Abstract
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.
Highlights
Amphiphilic molecules are ubiquitous in our daily lives, widely represented in natural systems[1,2] and intensively used in the pharmaceutical, cosmetic, food and petroleum industries, among others.[3,4] Surfactant molecules are used for example as detergents and dispersing materials, coating and foaming agents, emulsi ers and biocides, spanning a wide range of functions.[5,6,7]More than two hundred years a er the rst observations reported by Benjamin Franklin of the damping of waves on a lake by fatty acids,[8] understanding the dynamics of surfactant lms remains a challenge, both theoretically and experimentally
Understanding the dynamics of the surfactant requires an accurate description of the bulk molecular self-assembly, of the transport towards the interface and of the bulk–interface equilibrium.[12,13,14,15]
The accuracy of the measurement value of kdes is limited by the large error bars due to the determination of the time-scale and the crude approximations of the model, we provide here the rst estimate of the rate constants for the PEGPFPE surfactant using our micro uidic chip, and a new on-chip tensiometry method to quantitatively analyse surfactant dynamics at the liquid–liquid interface
Summary
Amphiphilic molecules are ubiquitous in our daily lives, widely represented in natural systems[1,2] and intensively used in the pharmaceutical, cosmetic, food and petroleum industries, among others.[3,4] Surfactant molecules are used for example as detergents and dispersing materials, coating and foaming agents, emulsi ers and biocides, spanning a wide range of functions.[5,6,7]. More than two hundred years a er the rst observations reported by Benjamin Franklin of the damping of waves on a lake by fatty acids,[8] understanding the dynamics of surfactant lms remains a challenge, both theoretically and experimentally. We address here the dynamics of surfactant adsorption in the context of micro uidic emulsi cation
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