Complex droplet interfaces at the microscale: Surfactant and hydrodynamic effects in the separation of water-in-oil emulsions

Abstract

Complex, surfactant-stabilized emulsions are relevant to various technological applications, such as the removal of dispersed water from diesel fuel in engines. Due to chemical stabilization of micrometer-sized dispersed droplets by surfactant molecules, emulsions can be challenging to separate, especially because surfactant transport to the interface is enhanced by the small droplet size and large interfacial curvature. The main goal of this work is to measure fundamental emulsion properties affecting their stability, such as dynamic interfacial tension and interfacial rheological properties, on the microscale, and relate these properties to droplet dynamics and coalescence behavior in water-in-fuel emulsions. First, dynamic interfacial tension (IFT) of water-in-diesel fuel systems containing surface-active additives such as monoolein and poly(isobutyl) succinimide (PIBSI), relevant to fuel filtration, is measured using a microfluidic tensiometer with contraction-expansion geometries. Microfluidic dynamic IFT measurements are compared with pendant drop tensiometry measurements employing millimeter-sized droplets. It is found that the dynamic interfacial tension decreases on orders of magnitude faster timescales on the microscale due to enhanced diffusive flux to curved microscale interfaces. This result has implications for fuel-water separation testing in the filtration industry. Next, a microfluidic hydrodynamic ‘Stokes’ trap is used to trap droplets in a cross-slot geometry. A four-channel hydrodynamic trap is applied towards studying drop shape relaxation as well as binary droplet coalescence of water droplets in mineral oils, stabilized by SPAN 80. It is found that the film drainage time for coalescence increases with droplet radius and surfactant concentration, while it decreases with incoming drop velocity. Critical conditions for flocculation and rebound of droplets are identified in terms of the capillary number. Finally, interfacial dilatational rheological properties of water-in-diesel fuel systems are measured using a capillary pressure microtensiometer. PIBSI and monoolein are added to the diesel fuel, and the dependence of the dilatational modulus on oscillation frequency and surfactant concentration is investigated. The dilatational modulus is found to increase with oscillation frequency and decrease with surfactant concentration. PIBSI-laden interfaces have higher modulus than monoolein-laden interfaces. Collectively, these experiments enhance our understanding of the intricate relationship between surfactant transport on the microscale, and droplet coalescence leading to emulsion separation.