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

Abstract

Electrocoalescence is a powerful technique for separating water from hydrocarbons. The present study is a detailed experimental investigation of the influence of surfactant (Span 80) on electrowetting-induced surface electrocoalescence of micron-scale water droplets in hexadecane. 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 dimensionless parameters i) modified electric capillary number (Ca_e^* ), ii) frequency (τ), and iii) surfactant concentration (C^* ). The effectiveness of electrocoalescence is quantified using the parameter (δ/α), where δ is the droplet density per unit area and α is the fraction of surface not covered by droplets.

Strong coalescence (no surfactant) corresponds to δ/α < 10 droplets/mm², with best-case δ/α = 1.6 droplets/mm², with no droplets < 20 μm diameter and electrocoalesced droplets as large as 750 μm. With surfactant, electrocoalescence weakens; the parameter space for strong electrocoalescence progressively reduces with concentration. Nonetheless, electrocoalescence at all concentrations results in substantial radius enhancement (after/before electrocoalescence); measured ratio ranged from 3.1 - 6.3 in the parameter space of Ca_e^*: 3.3 - 4.9, and τ ≤ 1.25 x 10^-2. Surface electrocoalescence therefore promotes significant aggregation of water droplets over a wide range of interfacial energies. This study also characterizes satellite droplet ejection (SDE), which adversely affects electrocoalescence due to the ejection of 2 - 10 μm radii droplets. SDE scales with voltage, frequency and concentration, and inversely with electrode spacing. Overall, we identify the recommended parameter space for surface electrocoalescence for separations and microfluidic applications.