Droplet Migration in Ionic Surfactant Gradients

N. Shi, A. Mashat, T. Squires, A.I. Abdel-Fattah
Exploratioin and Petroleum Engineering Center-Advanced Research Center (EXPEC ARC), Saudi Aramco,
Saudi Arabia

Keywords: emulsions, ionic surfactants, dead-end channels, Diffusiophoresis, soluto-capillary flow


Particle migration under chemical concentration gradients offers an intriguing mechanism to understand and manipulate suspended particles. Among different chemicals, ionic surfactant is a unique group that not only ubiquitously exists in many areas of science and technology but also drives diffusiophoretic (DP) and soluto-capillary migration of fluid particles such as droplets. While often investigated individually, whether DP and solute-capillarity can simultaneously drive droplet migration has not been studied yet. Utilizing a facile microfluidic technique to fabricate micromodel environments, direct observation of droplet migration driven by ionic surfactant gradients is presented. We choose two ionic surfactants, sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) as the anionic and cationic surfactants, respectively. Transient gradients of these surfactants are established by their diffusion into dead-end channels, which suppresses convection and enables a wide range of surfactant concentration in the gradient. We find that DP and solute-capillarity are simultaneously involved so that the magnitude and direction of droplet migration depends on both concentration profiles and surfactant dynamics. In addition, droplet migration velocities become extremely slow when the surfactant concentration exceeds the critical micelle concentration value (CMC) in spite of bulk concentration gradients. These results provide a better understanding of droplet migration in ionic surfactant gradients. We envision such understanding could contribute to a conceptual picture for manipulating emulsions with engineered chemical environments of surfactants.