The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. Here, we show that enhanced electrical potential differences (i.e., streaming potential) may 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 electrolyte slip and associated surface charge. A record-high figure of merit, in terms of the voltage generated per unit applied pressure, of 0.043 mV Pa−1 is obtained through the use of the liquid-filled surfaces. In comparison with air-filled surfaces, the figure of merit associated with the liquid-filled surface increases by a factor of 1.4. 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.

A significant enhancement in the streaming potential (V s ) was obtained in experiments considering the flow of electrolyte over liquid-filled surfaces (LFSs), where the grooves in patterned substrates are filled with electrolyte immiscible oils. Such LFSs yield larger V s (by a factor of 1.5) compared to superhydrophobic surfaces, with air-filled grooves, and offer tunability of electrokinetic flow. It is shown that the density, viscosity, conductivity, as well as the dielectric constant of the filling oil, in the LFS, determine V s . Relating a hydrodynamic slip length to the obtained V s offers insight into flow characteristics, as modulated by the liquid interfaces in the LFS.

The relative influence of the capillary, Marangoni, and hydrophobic forces in mediating the evaporation of water from carbon foam based porous media, in response to incident solar radiation, are investigated. It is indicated that inducing hydrophilic interactions on the surface, through nitric acid treatment of the foams, has a similar effect to reduced pore diameter and the ensuing capillary forces. The efficiency of water evaporation may be parameterized through the Capillary number (Ca), with a lower Ca being preferred. The proposed study is of much relevance to efficient solar energy utilization.

Textured surfaces, comprised of grooves filled with air, e.g., air-filled surfaces (AFS), or with liquid, e.g., liquidfilled surfaces (LFS), significantly influence fluid flows and the related electrokinetic streaming potential (V s ). Here, electroosmotic mobility related tensorial effects on the V s were experimentally investigated. A significant modulation of the V s , as high as 100%, due to transverse pressure gradients, was demonstrated. The study yields insights into understanding geometrical effects in electrolyte flows with implications to the establishment of local electric fields, energy generation, and biological separations.

A significant enhancement of solar irradiation induced evaporation of water, and ethanol-water mixtures, through the use of carbon foam based porous media, is demonstrated. A relationship between the consequent rate of mass loss, with respect to the equilibrium vapor pressure, dynamic viscosity, surface tension, and density, was developed to explain experimental observations. The evaporative heat loss was parametrized through two convective heat transfer coefficients-one related to the surface and another related to the vapor external to the surface. The work promotes a better understanding of thermal processes in binary liquid mixtures with applications ranging from phase separation to distillation and desalination.

A new plasma processing-based methodology for enhancing the streaming potential (V s ) that may be obtained in electrokinetic flows for a given pressure gradient over a silicon surface-based microchannel is indicated. The dependence of the V s on both the surface zeta potential and the electrolyte slip length was carefully determined through a series of experiments involving the variation of CF 4 -and Ar-based plasma parameters, incorporating pressure, exposure time, and power. It was determined through analytical estimates that, while the zeta potential is always increased, the slip length may be diminished under certain conditions. A record value of ∼0.1 mV/Pa was obtained using CF 4 plasma at 500 W, 10 mTorr, and 300 s of exposure. The implications of the work extend to the investigation of whether smooth surfaces may be effective for generating large V s 's for new modalities of electrical voltage sources in microfluidics-based applications.

A methodology for substantially increasing the magnitude of the electrokinetic streaming potential (Vs) from ~ 0.02 V to as large as ~ 1.6 V is proposed. This is done through deploying textured, liquid-filled surfaces (LFS), filled with low viscosity oils, for electrolyte flow. The charge density at the electrolyte-oil interface as well as the enhanced slip may be responsible for enhancement of the Vs. It was found, through experimental analysis as well as computational simulations, that the fluid slip length was inversely proportional to the filling oil viscosity, and influences the Vs. The study provides new perspectives related to complex electrolyte flow conditions as may be relevant for energy harvesting applications.

A significant difference in the wetting angles of water and oil was observed on patterned substrates, combining interstitial spaces along with hydrophobic solid surfaces, as a function of the orientation. The difference was ascribed to a modification of the liquid–interstice interfacial surface energy due to different degrees of penetration of the liquid. A roughness metric related to the extent to which the liquid infiltrates the interstice normalized by the geometrically determined area is proposed. This study has implications in modulating surface slip behavior and would be of importance in guiding liquid droplets.