Comparing electrowettability and surfactants as tools for wettability enhancement on a hydrophobic surface

Reduced-Order Model for Surfactant-Laden Electrified Sessile Droplets

A comprehensive understanding of the physics of electrowetting of a surfactant-laden droplet is important for applications in rapid healthcare diagnostics. A majority of biological samples examined during point-of-care (POC) diagnostics are biofluids with dissolved surfactants, such as the respiratory droplets containing protein (mucin) and surfactant molecules like dipalmitoylphosphatidylcholine. The presence of these surfactant molecules is anticipated to have a significant impact on the performance of electrowetting-based POC diagnostic devices. A reduced-order model is developed using the weighted residual integral boundary layer theory for the electrowetting of a surfactant-laden sessile droplet in a parallel plate electrode configuration. Thin film evolution equations are obtained for the fluid-fluid interface, the surfactant concentration, the depth-integrated flow rate, and the interfacial charge density. We show that the presence of surfactants opposes and decreases the strength of the electrohydrodynamic flow due to Marangoni stress-driven convection. The droplet then responds to an AC field with a suppressed amplitude of oscillation and the same mean deformation as that under DC forcing. Thus, low-frequency AC forcing with a suitable surfactant can plausibly be employed as a viable alternative to more energy-intensive high-frequency AC forcing.

Adaptive liquid lens with controllable light intensity

An adaptive liquid lens with controllable light intensity is demonstrated, which can modulate both light intensity and beam spot size. The proposed lens consists of a dyed water solution, a transparent oil, and a transparent water solution. The dyed water solution is used to adjust light intensity distribution by varying the liquid-liquid (L-L) interface. The other two liquids are transparent and designed to control the spot size. In this way, two problems can be solved: the inhomogeneous attenuation of light can be achieved through the dyed layer, and a larger optical power tuning range can be achieved through the two L-L interfaces. Our proposed lens can be used for homogenization effects in laser illumination. In the experiment, an optical power tuning range from - 44.03 m^-1 ∼ + 39.42 m^-1 and an ∼ 89.84% homogenization level are achieved. Our proposed lens may also ease the vignetting problem in imaging systems.

Experimental study and predicted model analysis of nanofluid wetting behavior under high voltage

To explore the wetting behavior of nanofluid under high voltage, a contact angle measurement system under electric field is designed and set up. The effects of mass concentration, the type of nanoparticles and the temperature of dielectric layer are considered. The experimental results manifest that the contact angle reduction rate of SiO₂-water nanofluid is gradually increased with the increase of nanofluid concentrations from 0 to 0.05 wt%. While, it is decreased when the concentration is varied from 0.05 to 0.25 wt%. On the other hand, the contact angle reduction rate of Al₂O₃-water nanofluid is generally greater than SiO₂-water nanofluid with the same volume concentration. In addition, the reduction rate of the contact angle of the SiO₂-water nanofluid would be gradually increased with the increase of the surface temperature of the dielectric layer. Moreover, the experimental values are greatly deviated from the results calculated by Young-Lippmann equation and its modified form of nanofluid. Hence, the present study proposes a dimensionless surface tension correct factor to obtain the modified equation which is based on the Young-Lippmann equation. The influence of electric charge, electric field force, drag force and Brownian force on nanoparticles under high voltage are considered in the modified equation. The results show that the modified equation can predict the trend of the nanofluid contact angle under higher voltage.

Foaming properties, wettability alteration and interfacial tension reduction by saponin extracted from soapnut (Sapindus Mukorossi) at room and reservoir conditions

In this study, a natural surfactant, saponin was isolated from soapnut (Sapindus Mukorossi). The extracted surfactant was characterized by Fourier-transform infrared spectroscopy (FTIR) analysis. The effectiveness of the isolated surfactant as EOR agent was evaluated from foam generation/stabilization properties, wettability alteration of the rock surfaces, as well as oil-water interfacial tension (IFT) reduction characteristics. The performance of the extracted saponin was compared with that of a commercial saponin and sodium dodecyl sulfate (SDS). The foaming properties of the saponin with carbon dioxide (CO2) was characterized using Teclis Foamscan instrument at room condition and 60 °C. The IFT and contact angles at room conditions and reservoir conditions were measured using KRŰSS Drop Shape Analyzer (DSA 25 and DSA 100) via pendant drop and sessile drop techniques respectively. The foamability of the saponin-stabilized foam was good at ambient condition and 60 °C. Moreover, the time taken for almost 100% liquid drainage was higher in saponin-stabilized foam than the SDS-stabilized foam. The optimum concentration for attaining maximum foam stability decreased from 0.4 wt% at room temperature to 0.1 wt% at 60 °C. Signifying that the quantity of the surfactant to be used in foam generation could reduce at high temperature. The isolated saponin exhibited relatively good interfacial activities individually and in synergistic interaction with silicon dioxide (SiO2) nanoparticles at reservoir conditions. Precisely, at 8 MPa and 80 °C, the crude-oil water IFT was reduced from 23.24 mN/m to 1.59 mN/m (about 93.2%) by 0.2 wt% saponin concentration. The IFT was further reduced to 0.87 mN/m (about 96.3%) by a mixed system of 0.5 wt% saponin and 0.05 wt% SiO2 nanoparticles concentration. Increasing IFT with increasing temperature were observed at very high temperature due to phase separation resulting from clouding phenomenon. However, the clouding temperature increased with 0.1 wt% saponin concentration, and in presence of SiO2 nanoparticles (0.05 wt% and 0.1 wt%). The study suggests that the extracted saponin could be considered as supplementary alternative to conventional EOR surfactants.