Wettability & coalescence modulation of water droplets through surface engineering, surfactants and electrowetting

Fluidic separation of two or more immiscible fluids is a key process in several applications. While oil-water separation has been extensively studied, there remain significant avenues for further improvement in the effectiveness, energy consumption and speed of separation. This dissertation includes multiple fundamental studies on the influence of surface engineering (texture and chemistry), surfactants and electric fields towards enhancing separation by controlling wettability of droplets and droplet coalescence. The first task (Chapter 3) details a study of wettability of water (in oil) and oil (in water) on sub-millimeter/micro/nano textured surfaces fabricated on a variety of substrates (metals, polymers, elastomers). Importantly, all the fabrication processes employed involved non-cleanroom-based scalable techniques. Textured metal surfaces coated with Teflon AF were superhydrophobic (in oil) with very low roll-off angles (4°–7°). Uncoated textured metal surfaces were superoleophobic (in water) with roll-off angles of 3°–9°. Secondly, textured polymer and elastomer surfaces exhibited ultrahydrophobicity (in oil); however not all textured elastomers exhibited superoleophobicity (in water). Thirdly, droplet roll-off was not observed on any textured elastomer and polymer surface, despite very favorable contact angles, indicating that high contact angles do not always translate to superhydrophobicity/oleophobicity. Chapter 4 analyzes and quantifies the extent of wettability alteration of water droplets on a hydrophobic surface (in air) via the use of surfactants and electrowetting (EW). Nine surfactants were chosen from the categories of anionic, cationic and zwitterionic surfactants. EW further enhanced wettability of surfactant solutions, and further reduced the contact angle (CA) by as much as 35°. Interestingly, it was seen that the influence of EW in enabling CA reduction was reduced by the addition of surfactants at pre-CMC (critical micelle concentration) levels. Conversely, surfactants strengthened the influence of EW at higher concentrations. Finally, it was seen that at post CMC concentrations, the saturation contact angles were independent of surfactant concentrations. Chapter 5 analyses dielectrophoretic (DEP) control of a water droplet at the interface of two other immiscible liquids. An analytical model was developed which balances gravity, buoyancy, capillary, and dielectrophoretic forces to predict the change in the position of the droplet and the immersion angle. Experiments and analysis were conducted for Bond numbers ranging from 0.1 to 1.7, the latter being the critical size at which a droplet will ‘sink’ due to its weight. The predicted immersion angles and threshold voltage showed a good match with the measurements. Chapter 6 studies the influence of surfactant concentration, applied voltage, frequency and electrode geometry (spacing) on surface electrocoalescence for micron-scale water droplets in hydrocarbon media. Phase maps were developed for various electrocoalescence possibilities to identify the parameter space for significant coalescence using three dimensionless parameters: i) modified electric capillary number (Ca [superscript asterisk over subscript e], ii) frequency (τ), and iii) surfactant concentration (C*). Electrocoalescence effectiveness was quantified using the parameter (δ/α): δ is the droplet density/area and α is the fraction of surface not covered by droplets. Strong coalescence (no surfactant) corresponded 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 weakened; parameter space for strong electrocoalescence progressively reduced with concentration. Nonetheless, electrocoalescence at all concentrations resulted in substantial radius enhancement (after/before electrocoalescence); measured ratio ranged from 3.1-6.3 in the parameter space of Ca [superscript asterisk over subscript e]: 3.3-4.9, and τ ≤ 1.25 ∗ 10⁻². This study also characterized droplet generation (via satellite droplet ejection (SDE)) of 2-10 µm radii droplets. SDE was seen to scale with voltage, frequency and concentration, and inversely with electrode spacing. Overall, it was shown that water droplets can be coalesced or generated using the same microfluidic device; the parametric space to enable fluidic separation and droplet generation was identified. Chapter 7 models the microfluidic system discussed in Chapter 6 using machine learning (ML) algorithms, such as artificial neural network (ANN), eXtreme gradient boosting (XGBOOST) and polynomial regression. Features such as voltage, frequency, electrode spacing, concentration and initial droplet density normalized with uncovered area ratio (δᵢ/αᵢ), were utilized to predict nine targets: uncovered area ratio (α [subscript f]), final droplet density normalized with uncovered area (δ [subscript f]/α [subscript f]), and seven droplet density distribution (radius) bins ranging up to 500 µm. The ANN was the most accurate and consistent among the three ML models with R² of 0.89. The model accurately predicted the droplet distribution bins for three distinct test cases consisting of good coalescence, poor coalescence and satellite droplet ejection (droplet generation). SHAP (Shapley Additive exPlanations) dependence plots highlighted the parametric influence of the features for each output. Overall, this dissertation has led to significant contributions in the field of droplet coalescence and generation. This multidisciplinary work has involved experiments, analytical modeling, numerical simulations and statistical modeling. The results show that surface engineering, surfactants and EW, in conjunction, offer powerful approaches to enhance droplet wettability and coalescence. This research impacts applications in energy (oil-water separation, enhanced oil recovery), pharmaceutical (droplet emulsion generation) and infrastructure (municipal and industrial water treatment, oil spills) areas.