Study On Low-voltage Electrowetting Behavior In Surfactant Solutions

Electrowetting is a phenomenon of change in surface wettability caused by an applied electric field.In recent years,electrowetting has been widely used in microfluidic control,pharmaceutical detection,crude oil extraction,and chemical cleaning due to its flexibility in regulation,and has received extensive attention from researchers.It has been shown that the wettability could be effectively regulated by adding surfactants or modifying the surface to reduce the driving voltage of electrowetting and increase the range of wettability variation.Existing studies have been mostly based on“Sessile Drop”method to study the wetting behavior of droplets in the gas-phase environments,while in practical applications such as heat exchangers and Lab-on-Chip,captive bubble/oil-droplet are the common forms.Herein,the gas-liquid and oil-liquid interface in the immersed state can significantly affect the system performance(heat transfer,flow friction resistance,etc.).Therefore,the present study aims to investigate the electrowetting behavior of captive bubble/oil droplets in surfactant solutions under a low-voltage electric field.In the present study,an experimental system for electrowetting visualization is built to investigate the low-voltage electrowetting behaviors of bubble/oil droplets in different ionic surfactant solutions.In addition,the relationship between the surfactant concentration and the electrowetting characteristics is analyzed.The experimental results showed that the driving voltage can be significantly reduced by the surfactant(SDS,DTAB)solution compared with deionized water.At 0.1 CMC,the captive bubble presents contact angle reduction,and bubble slip under the action of a low-voltage electric field(0?-6 V).In addition,the captive bubbles exhibit an inverse Cassie-Baxter state,and their gas-solid area fractions f_sbdecrease significantly after the electrowetting response,prompting the occurrence of bubble slip.By comparing the electrowetting phenomena in SDS or DTAB solutions at different concentrations,it is found that the increase of surfactant concentration could reduce the voltage required for bubble slip,but would decrease the range of contact angle variation.Moreover,the experimental results show that the DTAB solution with 0.05?0.10 CMC can present better electrowetting characteristics.Only a voltage of 3 V is required to achieve bubble slip and underwater superoleophobic characteristics of the silver surface(contact angle less than 30°).This finding can be of practical significance for applications such as regulating bubble nucleation and improving the degreasing efficiency of the electrode surface.Finally,the morphological evolution of captive bubble/oil droplet is quantitatively analyzed,and the influence of electrostatic force on the deformation of oil droplet is explored according to the electrowetting behavior of oil droplets in surfactant solutions with different polarities.The results show that the contact angle and contact diameter of the captive bubble/oil-droplet gradually decrease after the voltage is applied and present a"slow-fast-slow"variation pattern.Besides,the oil droplet in the solution exhibits different morphologies due to the electrostatic force in the negative electric field.In the SDS solution,the oil droplet remains nearly spherical due to the electrostatic repulsion and the buoyancy force in the opposite direction.On the contrary,in the DTAB solution,the oil droplets are squeezed by both electrostatic suction and buoyancy forces,resulting in a semi-ellipsoidal shape near the electrode.