The size of a functioning "system" rather than the individual components determines the success of many miniaturization efforts. While most of the existing micro fuel-cell research has been focusing on the fuel-cell stack, our approach has been to systematically eliminate all the ancillary components with the goal of miniaturizing the full system. In this paper, we present a miniature fuel-cell system that combines the self-pumping of fuel and self-generation of oxidant altogether in a box-shape device of a few centimeters. Since the fuel is pumped on demand inside the system without requiring any external assistance, the device is self-sufficient and portable. Furthermore, the oxygen is generated on demand inside the system without requiring the ambient air, so that the device can be stacked in multiple. Constructed simply as liquids in a solid container, this active fuel-cell system resembles a battery to the user.

We introduce a simple and compact fuel-cell architecture consisting of only solid channels and demonstrate its validity by developing a miniature direct formic acid fuel cell (DFAFC). The proposed architecture generates electric power while pumping the fuel and removing byproduct CO 2 without any discrete pump, gas separator, or membrane electrode assembly (MEA). The fuel pump and gas separator are embedded in the channel, as reported before, by directionally growing and venting CO 2 byproduct bubbles formed inside the reaction microchannels using virtual check valve and microporous hydrophobic venting membrane. The new architecture further eliminates the MEA, along with the issues associated with it, by flowing one stream of fuel and electrolyte mixture in a single channel consisting of both an anode and an air-breathing cathode. The reported system obtains a supply of oxygen directly from quiescent air through a gas-diffusion cathode rather than using an oxygen tank. By eliminating all the ancillary parts, the so-called packaging penalty, which has been hindering the miniaturization of fuel cells below the order of a centimeter, is avoided. This simple and self-standing fuel-cell unit produces 16.7 mW/cm 2, a performance comparable to the existing bulkier DFAFCs that use external pumps, pressurized oxygen or MEA. © 2012 IEEE.

Despite the fact that superhydrophobic surfaces possess useful and unique properties, their practical application has remained limited by durability issues. Among those, the wetting transition, whereby a surface gets impregnated by the liquid and permanently loses its superhydrophobicity, certainly constitutes the most limiting aspect under many realistic conditions. In this study, we revisit this so-called Cassie-to-Wenzel transition (CWT) under the broadly encountered situation of liquid drop impact. Using model hydrophobic micropillar surfaces of various geometrical characteristics and high speed imaging, we identify that CWT can occur through different mechanisms, and at different impact stages. At early impact stages, right after contact, CWT occurs through the well established dynamic pressure scenario of which we provide here a fully quantitative description. Comparing the critical wetting pressure of surfaces and the theoretical pressure distribution inside the liquid drop, we provide not only the CWT threshold but also the hardly reported wetted area which directly affects the surface spoiling. At a later stage, we report for the first time to our knowledge, a new CWT which occurs during the drop recoil toward bouncing. With the help of numerical simulations, we discuss the mechanism underlying this new transition and provide a simple model based on impulse conservation which successfully captures the transition threshold. By shedding light on the complex interaction between impacting water drops and surface structures, the present study will facilitate designing superhydrophobic surfaces with a desirable wetting state during drop impact.