Adhesion and friction during physical contact of solid components in microelectromechanical systems (MEMS) often lead to device failure. Translational stages that are fabricated with traditional silicon MEMS typically face these tribological concerns. This work addresses these concerns by developing a MEMS vertical translation, or focusing, stage that uses electrowetting-on-dielectric (EWOD) as the actuating mechanism. EWOD has the potential to eliminate solid-solid contact by actuating through deformation of liquid droplets placed between the stage and base to achieve stage displacement. Our EWOD stage is capable of linear spatial manipulation with resolution of 10 um over a maximum range of 130 um and angular deflection of approximately +/-1 degrees, comparable to piezoelectric actuators. We also developed a model that suggests a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capability to operate the stage without solid-solid contact offers potential improvements for applications in micro-optics, actuators, and other MEMS devices.
EWOD can also be used for fundamental fluid mechanics research. For example, we studied the conversion of surface energy into kinetic energy that arises as an initially-deformed droplet is ejected from a surface. In this case, electrowetting provides a means of preparing a droplet on a substrate for lift-off. When a voltage is applied between a water droplet and a dielectric-coated electrode, the wettability of the substrate increases in a controlled way, leading to the spreading of the droplet. Once the voltage is released, the droplet recoils due to a sudden excess in surface energy, and droplet detachment may follow. The process of drop detachment and lift-off, prevalent in both biology and micro-engineering, has to date been considered primarily in terms of qualitative scaling arguments for idealized superhydrophobic substrates. We considered the eletrically-induced ejection of droplets from substrates of finite wettability and analyzed the process quantitatively. We compared experiments to numerical simulations and analyzed how the energy conversion efficiency is affected by the applied voltage and the intrinsic contact angle of the droplet on the substrate. Our results indicate that the finite wettability of the substrate significantly affects the detachment dynamics; from this conclusion, we provide new rationale for the previously reported large critical radius for drop ejection from micro-textured substrates.
D.J. Preston, A. Anders, B. Barabadi, E. Tio, Y. Zhu, D.A. Dai, E.N. Wang
A. Cavalli, D.J. Preston, E. Tio, D.W. Martin, N. Miljkovic, E.N. Wang, F. Blanchette, J.W.M. Bush
Heat pipes, which are incresingly being used to cool chips in phones and laptops, rely on wicking materials to return fluid from the condenser back to the evaporator. Fluid is pulled through the wick by capillary pressure generated by small menisci, but it is resisted by viscous losses when flowing through small pores, so an optimal wick would maximize the capillary pressure while minimizing the viscous resistance. We developed highly porous copper nanowire aerosponges produced by direct assembly of copper nanowires as a potential high-performance wick. This method offers not only great simplicity, but also excellent properties such as extremely low density, high electrical conductivity, and remarkable mechanical properties. Furthermore, these Cu aerosponges exhibit excellent wicking behavior as compared to state-of-the-art methods like sintered copper wicks, suggesting their potential for heat-exchange applications.
S.M. Jung, D.J. Preston, H.Y. Jung, Z. Deng, E.N. Wang, J. Kong
Water vapor condensation is commonly observed in nature and routinely used as an effective means of transferring heat, with dropwise condensation on nonwetting surfaces exhibiting heat transfer improvement compared to filmwise condensation on wetting surfaces. However, state-of-the-art techniques to promote dropwise condensation rely on functional hydrophobic coatings that either have challenges with chemical stability or are so thick that any potential heat transfer improvement is negated due to the added thermal resistance of the coating. In this work, we show the effectiveness of ultrathin scalable chemical vapor deposited (CVD) graphene coatings to promote dropwise condensation while offering robust chemical stability and maintaining low thermal resistance. Heat transfer enhancements of 4x were demonstrated compared to filmwise condensation, and the robustness of these CVD coatings was superior to typical hydrophobic monolayer coatings. Our results indicate that graphene is a promising surface coating to promote dropwise condensation of water in industrial conditions with the potential for scalable application via CVD.
D.J. Preston, D.L. Mafra, N. Miljkovic, J. Kong, E.N. Wang
Natural surface contamination due to hydrocarbon adsorption, particularly on noble metals, has been explored as an alternative approach to realize stable dropwise condensing surfaces. While noble metals are prohibitively expensive, the recent discovery of robust rare earth oxide (REO) hydrophobicity has generated interest for dropwise condensation applications due to material costs approaching 1% of gold; however, the underlying mechanism of REO hydrophobicity remains under debate. Careful experiments and modeling were used to show that REO hydrophobicity occurs due to the same hydrocarbon adsorption mechanism seen previously on noble metals. To investigate adsorption dynamics, we studied holmia and ceria REOs, along with control samples of gold and silica, via X-Ray photoelectron spectroscopy (XPS) and dynamic time-resolved contact angle measurements. The contact angle and surface carbon percent started at approximately 0 on in-situ argon-plasma-cleaned samples and increased asymptotically over time after exposure to laboratory air, with the rare earth oxides displaying hydrophobic (>90 degrees) advancing contact angle behavior at long times (>4 days). The results indicate that REOs are in fact hydrophilic when clean and become hydrophobic due to hydrocarbon adsorption. Furthermore, this study provides insight into how REOs can be used to promote stable dropwise condensation, which is important for the development of enhanced phase change surfaces.
D.J. Preston, N. Miljkovic, J. Sack, J. Queeney, E.N. Wang
The development of superhydrophobic surfaces has been pursued to enhance condensation heat transfer, where the low droplet surface adhesion and small droplet departure sizes increase the condensation heat transfer coefficient. Specifically, when two or more small (10-100 um) droplets coalesce on a superhydrophobic surface, they can spontaneously jump away from the surface due to the reduced droplet-surface adhesion and release of excess surface energy, which has been shown to increase heat transfer by 30 - 40% compared to that observed during gravitational shedding of droplets. While this droplet jumping phenomenon has been studied on a range of surfaces, past work has neglected electrostatic interactions and assumed charge neutrality of the droplets.
Recently, we have shown that jumping droplets on a variety of superhydrophobic surfaces, including copper oxide, zinc oxide, and silicon nanopillars, gain a net positive charge that causes them to repel each other mid-flight. The charge is determined experimentally by observing droplet motion in a uniform electric field. The mechanism for the charge accumulation is associated with the formation of the electric double layer at the droplet-coating interface and subsequent charge separation during droplet jumping governed by the fast time scales of droplet coalescence. One application of this charging phenomenon is further enhancement of condensation heat transfer by preventing droplet reversal and return to the condenser surface due to the presence of vapor flow towards the surface, which increases the drag on the jumping droplets. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface that impede heat transfer until they can be either removed by jumping again or finally shedding via gravity. By characterizing individual droplet trajectories during condensation on superhydrophobic nanostructured copper oxide surfaces, this vapor flow entrainment is shown to dominate droplet motion for droplets smaller than R ~ 30 um at moderate heat fluxes (q" > 2 W/cm2). Subsequently, electric-field-enhanced (EFE) condensation is demonstrated, whereby an externally applied electric field prevents jumping droplet return due to the positive charge obtained by the droplets upon jumping. As a result, with scalable superhydrophobic CuO surfaces, a 50% higher overall condensation heat transfer coefficient is demonstrated compared to a jumping-droplet surface with no applied field for low supersaturations (<1.12).
Another application of charged jumping droplets is use of these droplets for electrostatic energy harvesting. Here, the charged droplets jump between superhydrophobic copper oxide and hydrophilic copper surfaces to create an electrostatic potential and generate power during formation of dew under atmospheric conditions. Power densities of ~0.06 nW/cm2 are demonstrated, which, in the near term, can be improved to ~1 uW/cm2. This work demonstrates a surface engineered platform that is low cost and scalable for atmospheric energy harvesting and electric power generation. These applications of charged jumping droplets offer new avenues for improving the performance of self-cleaning and anti-icing surfaces as well as thermal diodes, and may also provide a competitive mode of energy harvesting from temperature gradients.
D.J. Preston, N. Miljkovic, R. Enright, E.N. Wang
N. Miljkovic, D.J. Preston, R. Enright, E.N. Wang
N. Miljkovic, D.J. Preston, R. Enright, E.N. Wang
N. Miljkovic, D.J. Preston, R. Enright, E.N. Wang
B. Bhatia, D.J. Preston, D.M. Bierman, N. Miljkovic, A. Lenert, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, W.R. Chan, I. Celanovic, M. Soljacic, E.N. Wang
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