Electrokinetic flow, which can generate electrical voltage (i.e., streaming potential) through the flow of an electrolyte over a charged surface, may be used for energy transduction. However, the yielded low streaming potential of electrokinetic flow on flat surface limited its practical applications. Though it was proposed that superhydrophobic surfaces could be used to promote fluid slip to enhance the generated streaming potential, there is a significant ambiguity of the charge property of liquid-air interface, e.g. whether liquid-air interface is charged, and its capability in promoting the electricity generation. Here, I will first present that enhanced streaming potential can be obtained through the flow of salt water on liquid-filled surfaces that are infiltrated with a lower dielectric constant liquid, such as oil, to harness both of the electrolyte slip and associated surface charges. The highest figure of merit, in terms of the voltage generated per unit applied pressure, of 0.043 mV/Pa is obtained through the use of the liquid-filled surfaces. In comparison with flat and air-filled superhydrophobic surfaces, the figure of merit associated with the liquid-filled surface increases by a factor of 1.4. The hydrodynamic slip length is related to the obtained streaming potential which offers insight into flow characteristics, as modulated by the liquid interfaces in the liquid filled surfaces. I will show the tunability of electrokinetic flow and streaming potential by changing the geometry of liquid-filled surface and the properties of filled liquid including density, viscosity, surface tension, as well as the dielectric constant. We have developed an empirical relationship between the generated streaming potential and filled liquid properties through dimensionless analysis and experiment verification. It is also shown that the magnitude of the streaming potential can be significantly enhanced two orders of magnitude from ∼0.02 V to as much as ∼1.6 V through optimization of filled liquid viscosity. The analytical relationship between streaming potential and filled liquid viscosity was derived and verified through corresponding experimental results. We also investigated the electroosmotic mobility related tensorial effects on the streaming potential through experiments and theoretical analysis. A significant modulation of the streaming potential - as high as 100%, due to transverse pressure gradients caused by tensorial effects, is demonstrated. These results lay the basis for innovative surface charge engineering methodology for the study of electrokinetic phenomena at the microscale, with possible application in new electrical power sources, and yield insights into understanding geometrical effects in electrolyte flows with implications to the establishment of local electric fields, energy generation, and biological separations.&nbsp;&nbsp;&nbsp;

Enhanced Voltage Generation Through Electrolyte Flow Over Liquid-Filled Surfaces

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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