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various scattered and irregular energy sources in the natural environment. [10] Although this type of energy is small, it is distributed extremely widely. If utilized reasonably, it will certainly make a significant contribution to human energy strategy.Coupled with contact electrification and electrostatic induction, Prof. Wang's group first invented the triboelectric nanogenerator (TENG) in 2012. [11] Presently, as the mainstream technology for energy harvesting, TENG has shown great application prospects in the fields of micro-nano energy, self-powered system, and blue energy due to its environmental friendliness, high efficiency, and accessibility. [12][13][14][15] After decades of research, the theory and output of TENG based on solid-solid interface has been deeply explored and improved. [16][17][18] However, in addition to the mechanical energy-electricity conversion between the solidsolid interface, the triboelectricity at solid-liquid interface can also be seen everywhere in the environment, such as in the falling raindrops. [19] Recently, the mechanism of solid-liquid electrification has been explored, and theories such as the electric-double-layer model and Wang's hybrid layer model have also been proposed to expand its applications. [20,21] However, these works of research are mainly concentrated on the solid-liquid electrificationThe working principle of the triboelectric nanogenerator (TENG), contact electrification and electrostatic induction, has been used to harvest raindrop energy in recent years. However, the existing research is mainly concentrated on solid-liquid electrification, and adopts traditional electrostatic induction (TEI) for output. As a result, the efficiency of droplet electricity generators (DEGs) is severely constrained. Therefore, previous studies deem that the DEG output is limited by interfacial effects. This study reveals that this view is inappropriate and, in reality, the output strategy is the key bottleneck restricting the DEG performance. Here, a switch effect based on an electricdouble-layer capacitor (EDLC) is introduced, and an equivalent circuit model is established to understand its working mechanism. Without pre-charging, a single droplet can generate high voltage over 100 V and the output is directly improved by two-orders of magnitude compared with TEI, which is precisely utilizing the interfacial effect. This work provides insightful perspective and lays solid foundation for DEG applications in large scale.

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SU-8 is a negative, epoxy type, near-UV photoresist. This resist has been specifically developed for ultrathick, high-aspect-ratio MEMS-type applications using standard lithography equipment. However, in practice, SU-8 has shown to be very sensitive to process parameter variation. The orthogonal array was used in our experiments in order to improve the lithography quality and analyze the interaction among the parameters. The analyses show that the interaction between the exposure dose and post exposure bake has played an important role in adhesion between SU-8 resist and the substrate. The proposed process conditions are given. The output structure has straight sidewall profile, fine line and good space resolution. The aspect ratio is larger than 20. Moreover, several metallic films are used as substrates. The Ti film with oxidation treatment was found to have the strongest adhesion to the resist. The result will help to open possibilities for low-cost LIGA-type process for MEMS applications.

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a b s t r a c tEffect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid was investigated experimentally. Three types of surfactants including Sodium Dodecyl Sulfate (SDS), Cetyltrimethyl Ammonium Bromide (CTAB) and Sorbitan Monooleate (Span-80) were used in the experiments. The refrigerant-based nanofluid was formed from Cu nanoparticles and refrigerant R113. The test surface is horizontal with the average roughness of 1.6 lm. Test conditions include a saturation pressure of 101.3 kPa, heat fluxes from 10 to 80 kW m À2 , surfactant concentrations from 0 to 5000 ppm (parts per million by weight), and nanoparticle concentrations from 0 to 1.0 wt.%. The experimental results indicate that the presence of surfactant enhances the nucleate pool boiling heat transfer of refrigerant-based nanofluid on most conditions, but deteriorates the nucleate pool boiling heat transfer at high surfactant concentrations. The ratio of nucleate pool boiling heat transfer coefficient of refrigerant-based nanofluid with surfactant to that without surfactant (defined as surfactant enhancement ratio, SER) are in the ranges of 1.12-1.67, 0.94-1.39, and 0.85-1.29 for SDS, CTAB and Span-80, respectively, and the values of SER are in the order of SDS > CTAB > Span-80, which is opposite to the order of surfactant density values. The SER increases with the increase of surfactant concentration and then decreases, presenting the maximum values at 2000, 500 and 1000 ppm for SDS, CTAB and Span-80, respectively. At a fixed surfactant concentration, the SER increases with the decrease of nanoparticle concentration. A nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant is proposed, and it agrees with 92% of the experimental data within a deviation of ±25%.