Influence of surfactant on electrowetting-induced surface electrocoalescence of water droplets in hydrocarbon media

Electrocoalescence is a powerful technique for manipulating water-hydrocarbon media. This study is a detailed experimental investigation of the influence of surfactant on electrowetting-induced surface electrocoalescence of micron-scale water droplets in hexadecane. Surface electrocoalescence is studied over a vast parameter space comprising surfactant concentration, voltage, frequency and electrode geometry. We develop phase maps for various electrocoalescence possibilities and identify the parameter space for significant coalescence using three dimensionless parameters: i) modified electric capillary number (Ca-e*), ii) frequency (tau), and iii) surfactant concentration (C*). Electrocoalescence effectiveness is quantified using the parameter (delta/alpha): delta is the droplet density/area and alpha is the fraction of surface not covered by droplets. Strong coalescence (no surfactant) corresponds to delta/alpha < 10 droplets/mm(2), with best-case delta/alpha = 1.6 droplets/mm(2), with no droplets < 20 mn diameter and electrocoalesced droplets as large as 750 mu m. With surfactant, electrocoalescence weakens; parameter space for strong electrocoalescence progressively reduces with concentration. Nonetheless, electrocoalescence at all concentrations results in substantial radius enhancement (after/before electrocoalescence); measured ratio ranges from 3.1 to 6.3 in the parameter space of Ca-e*: 3.3-4.9, and tau <= 1.25*10(-2). Surface electrocoalescence therefore promotes significant aggregation of water droplets over a wide range of interfacial energies. This study also characterizes droplet generation (via satellite droplet ejection (SDE)) of 2-10 mu m radii droplets. SDE scales with voltage, frequency and concentration, and inversely with electrode spacing. Overall, we show that water droplets can be coalesced or generated using the same microfluidic device, and identify the parametric space to enable surface-electrocoalescence-based fluidic separation and droplet generation.

Automated summary

This study investigates the influence of surfactant on the electrowetting-induced surface electrocoalescence of micron-scale water droplets. The research shows that with the presence of surfactant, electrocoalescence weakens, but there is still a substantial increase in droplet radius. The study also characterizes the droplet generation process and identifies the parameter space for surface-electrocoalescence-based fluidic separation and droplet generation.