46510-G5
Application of an Electric Field to Control Wetting of Thin Fluid Films
Overview:
The ACS-PRF grant (type G) has helped support two separate projects over the last year. They both involved the investigation of solid-liquid interfaces in the presence of an electric field. The focus of the first project is to quantify proton transfer at a mineral/solution interface. The goal of the second project is to develop tools allowing us to investigate the limits of electrowetting on dielectric (EWOD) for nanoscale drops. The ACS-PRF-supported work has led to additional funding from the National Science Foundation and 3M Corp., as well as long-term collaboration with other researchers. The Type G grant has also had a large impact on my own career via support of summer salary, which allowed me to focus on the research related to the project. Additionally, some of the research results have resulted in a submitted manuscript.
Summary of Activities:
Project 1) Proton transfer at mineral/solution interface.
The weathering of soil plays a major role in agricultural sustainability, removal of pollutants, and oil recovery. Here we study the mechanisms and kinetics for the interaction of muscovite mica with protons in the presence of an applied electric field. Muscovite mica is a clay mineral (one of the main soil component) and is a good candidate for the study of mineral/solution interactions. Muscovite mica has the added advantage that it can easily be cleaved into thin molecularly smooth sheet, which allows for quantitative studies. In addition, mica is the substrate of choice for a wide range of surface characterization techniques: the surface force apparatus (SFA), the electrochemical microscope, and the atomic force microscope. Therefore a better understanding of how muscovite mica interacts with solution provides surface characteristics that will help in the interpretation of surface force measurements. We have studied the reactivity of the mica surface under applied electric fields using electroanalytical techniques such as cyclic voltammetry and impedance spectroscopy. Using single crystal thin mica sheets we have shown that protons transfer from the bulk solution to the bulk of the mica. We have also shown that this process occurs at lower field strength compared to dehydroxylation that is observed in the absence of solution. We have shown that the capture and release of protons from the mica/solution interface is a function of pH and, using cyclic voltammetry, we have determined the rate constant for this proton transfer.
Project 2) Thin film electrowetting
Electrowetting is a means to control the solid-liquid interfacial energy via capacitive charging. Electrowetting is a fast, low-power option to move fluid in microfluidic devices. Moreover, wetting is a very important issue for secondary and enhanced oil recovery, especially at the current price of oil. By contributing directly to the field of wetting, this work aims to provide a better understanding of the effect of charge and electric fields at a solid/liquid interface, which could help design better recovery schemes. Moreover, there are current efforts to employ electric fields directly into oil reservoir to enhance recovery and the work proposed would help better understand this effect at the fundamental level. Traditional electrocapillary formalism (Young-Lippmann equation) describes well the phenomenon for thick fluid films (in the absence of contact angle hysteresis). However, there are serious limitations to the theory for very thin films (for example in the case of a geometrically confined film). The main challenges in the study of nanoscale electrowetting are to develop an experimental approach that would allow for the actuation and the determination of the contact angle of a nanofluidic drop. To that end, we have decided to extract contact angles from capillary force measurements. We use the SFA for both the capillary force measurement and to visualize the drop (since the SFA can image the interacting geometry and detect a local change in refractive index). Preliminary results show that we can control and actuate nanofluidic droplets, and we are currently working on comparing our results with theoretical predictions.
