The Effect of Surface Interactions on the Coalescence of Water Droplets in Fuel

Coalescence is a heavily utilized treatment for phase separation of emulsions, especially in the industrial application of filtration. In fuel filtration, the water must be filtered out to prevent the corrosion of engine parts due to microbial growth. The coalescing filters used often contain fibers, to which the droplets first adhere before coalescing with incoming droplets to form larger drops. The contact with both fibers and other droplets certainly affects the coalescence process, but the extent of the impact remains relatively undercharacterized. In this work, the effect of surface interactions on droplet coalescence is studied by using microfluidic platforms. The film drainage times for water droplets suspended in fuel are measured in a contact device that studies the coalescence of droplets in contact with poly(dimethylsiloxane) (PDMS) traps and are compared to those found using a contactless microfluidic Stokes trap device that studies the coalescence of free droplets in a hydrodynamic cross-slot. The fuel contains mono-olein, a common molecule in biodiesel that acts as a surface-active surfactant at a fuel-water interface. A dimensionless film drainage time is compared at different capillary numbers (Ca), for the contact and contactless cases. It is found that for both cases the dimensionless drainage time is decreasing as a function of Ca. In addition, the coalescence events for droplets with multiple contacts, either with other droplets or with the PDMS droplet traps, had a lower dimensionless drainage time than that between two free droplets brought together in the Stokes trap. This work aims to serve as a foundation to understand coalescence for essential industrial applications such as firefighting foams, oil remediation, or pharmaceutical preparation.