Ping Cheng

Frequency-dependent resonance and asymmetric droplet oscillation under ac electrowetting on coplanar electrodes

Sessile droplet oscillations in electrowetting on dielectric with a coplanar-electrode configuration are studied experimentally under the actuation of ac voltage with different frequencies. It was found that the experimental resonance frequencies and the number of lobes at different resonance modes agree reasonably well with a previous linear analysis. Oscillations of contact width and droplet height are in-phase at resonance modes P2n+2 while out-of-phase at P2n with n = 2, 4, 6,.... At certain critical frequencies, the droplet oscillations are very weak and switch from in-phase (out-of-phase) to out-of-phase (in-phase). For the oscillations after resonance frequency but before critical frequency, at low frequency, the large amplitude oscillation of the contact line deforms it from a circle to having lobes; the number of lobes increases with the frequency and their position alternates in the azimuthal direction, through periodical droplet spreading and receding. For the oscillations after critical frequency but before resonance frequency, the droplet oscillation demonstrates droplet waggling with an obvious contact line at normal and abnormal stops, due to the contact line pinning at low frequency, or the transportation of lobes on the droplet surface from one end to the other at high frequency. These asymmetric oscillations will produce a more chaotic fluid flow inside the droplets than symmetric oscillations and could be used purposely to enhance mixing in droplet-based micro-fluidics.

Characterization on the performance of a fractal-shaped microchannel network for microelectronic cooling

Previous theoretical and analytical studies have shown that microchannel heat sinks with a fractal-shaped network have many advantages over traditional parallel microchannels with respect to thermal resistance, temperature uniformity and pressure drop. However, to the best knowledge of the authors, no experimental investigations on fractal-shaped microchannel network heat sinks have been conducted so far to verify their performance. In this paper, we designed and fabricated a silicon-based microchannel heat sink with a single-layered fractal-shaped microchannel network using MEMS technology, and experimentally studied its pressure drop and thermal resistance characteristics under different mass flow rate and heat flux conditions. Numerical simulations are performed to predict the heat sink performance under the same experimental conditions. It is found that the experimentally measured pressure drop in the heat sink has a nonlinear relationship with the mass flow rate, which agrees very well with the numerical simulation result. It is also found that the experimentally measured thermal resistance is also in reasonably good agreement with the numerical simulation, and therefore indirectly verifies the conclusion of previous numerical simulations that the performance of the fractal-shaped microchannel network is better than that of traditional parallel microchannels.

Promoting Effect of Layered Titanium Phosphate on the Electrochemical and Photovoltaic Performance of Dye-Sensitized Solar Cells

We reported a composite electrolyte prepared by incorporating layered α-titanium phosphate (α-TiP) into an iodide-based electrolyte using 1-ethyl-3-methylimidazolium tetrafluoroborate(EmimBF4) ionic liquid as solvent. The obtained composite electrolyte exhibited excellent electrochemical and photovoltaic properties compared to pure ionic liquid electrolyte. Both the diffusion coefficient of triiodide (I3−) in the electrolyte and the charge-transfer reaction at the electrode/electrolyte interface were improved markedly. The mechanism for the enhanced electrochemical properties of the composite electrolyte was discussed. The highest conversion efficiency of dye-sensitized solar cell (DSSC) was obtained for the composite electrolyte containing 1wt% α-TiP, with an improvement of 58% in the conversion efficiency than the blank one, which offered a broad prospect for the fabrication of stable DSSCs with a high conversion efficiency.