Production of Nanodrops Using Interfacial Electrokinetic Polarization at a Flow-Focused Microfluidic Constriction

Nanoemulsions are metastable dispersions of nanoscale droplets of one liquid suspended in a different immiscible liquid. Because they offer enhanced material properties and versatility, there is growing interest in their use in a wide variety of applications including the food and beverage, pharmaceutical, energy and cosmetic industries. Conventional high pressure liquid homogenizers and microfluidizers can produce nanoemulsions using high pressure to provide large mechanical stresses to break large droplets down into smaller sized nanoscale dispersions. One major problem with these methods is that they are expensive and require extremely large pressures – up to 20,000 psi – to create the required fluid shear stresses to form nanosized droplets. Alternatively, low pressure approaches such as self-emulsification require high surfactant concentrations, which can be undesirable or toxic in pharmaceutical or food-based applications. In this work we present a new electrokinetic method for producing nanoscale droplets. Unlike previous electro-spray emulsification techniques that promote droplet break-up using electrical shear stresses tangential to the flow direction, our approach combines classic microfluidic flow-focusing with downstream orthogonalelectrical stresses to produce droplets. In this way, the hydrodynamic and electrical influences are effectively decoupled and can be manipulated independently to optimize droplet emulsification. We show that nanosized droplets can beproduced by forcing a thin hydrodynamic jet of discontinuous fluid through a thin flow constriction. Within the constriction, we use locally patterned 3D carbon black electrodes to deliver a direct current (DC) electric field perpendicular to the direction of flow. The field induces an interfacial electrical stress at the discontinuous-continuous fluid phase interface, where the combined tangential-hydrodynamic and orthogonal-electrical stress forces the thin jet to break into small nanoscale drops.Using this method, we present data illustrating the ability to break thin liquid jets into sub-micron droplets and highlight important parameters such as elongational shear stress, electric field strength, surfactant concentration, and device geometry, that influence droplet diameter and show how our approach provides a novel low-cost scalable method for producing nanoscale emulsions at low pressure.