This paper investigates the nano-fluidic contact angle measurement by performing molecular dynamics simulations. The contact angle between a nanowater droplet and a platinum surface is important for the design of the porous catalyst layer in low-temperature fuel cells. The measurement can generally be conducted by an atomic force microscope (AFM). However, the interaction force between the water droplet and the probe tip of the microscope may influence the measurement results. This paper employs the molecular dynamics technique to investigate the offset of the contact angle measurement. Calculations are in two sets, one simulated the water molecules clustering on the platinum surface, and the other involved the AFM measurement of the contact angle. The former case presents the original contact angle between the nano-scale water droplet and the platinum surface; the offset of the contact angle measurement due to intrusion of the AFM probe is predictable from the latter case. For engineering purposes, we present a correlation between the offset angle and the AFM measurement locations.

The removal of carbon dioxide (CO 2 ) at the anode of a micro direct methanol fuel cell (lDMFC) is critical. The bubbles are generated at the anode and may block part of the catalyst/diffusion layer, causing the lDMFC to malfunction. This work discusses the CO 2 bubble dynamics of microfluidics in a lDMFC from a microscopic perspective. A two-dimensional, nine-velocity lattice-Boltzmann model was adopted in this work to simulate the two-component (CO 2 bubble plus methanol solution) two-phase (gaseous and liquid) micro flow in a microchannel. The liquid-gas surface tension, the buoyancy force and the fluid-solid wall interaction force play the major roles in the bubble dynamics. They are treated as source terms in the lattice momentum equation. Simulation results indicate that the methanol stream flow rate, the pore size and the channel incline angle significantly affect the removal of CO 2 bubbles. The effect of the incline angle is substantial at low stream flow rates. The critical pore size in the microchannel for removing bubbles at all angles under various flow conditions has been predicted quantitatively.

By means of microfluidic analysis with a thermal lattice-Boltzmann method, we investigated the hydrophilic, thermal and geometric effects on the dynamics of CO 2 bubbles at anode microchannels (e.g., porous layers and flow channels) of a micro-direct methanol fuel cell. The simulation results show that a more hydrophilic wall provides an additional attractive force to the aqueous methanol in the flow direction and that moves the CO 2 bubble more easily. The bubble propagates quicker in the microchannel with a positive temperature gradient imposed from the inlet to the exit, mainly due to the Marangoni effect. Regarding the geometric effect of the microchannel, the bubble moves more rapidly in a divergent microchannel than in a straight or convergent channel. On the basis of the quantitative evaluation of hydrophilic, thermal and geometric effects, we are able to design the bubble-removal technique in micro fuel cells.Keywords Thermal lattice-Boltzmann method Á Bubble dynamics Á Two-phase flow Á Micro-direct methanol fuel cell List of symbols e lattice velocity vector e lattice speed (cm/s) f density distribution function (g/cm 3 ) g thermal distribution function (g K/cm 3 ) G rr 0 interaction strength between the species r and the other species r 0 (cm 3 /g s)

This paper describes the fundamental theory, algorithm and computation methods to predict the performance of proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) using a simplified computational fluid dynamics (CFD) approach. Based on the common transport phenomenon inside both fuel cells, the mass, momentum, energy and species equations were derived. Darcy laws were employed to simplify the momentum equation and also to linearize the species equation. The mathematical model was solved in various flow channel designs and some membrane electrode assembly (MEA) options. The major concern is mainly on the cathode side, in the PEMFC case, that dominates the performance deterioration due to potential loss in the flow field. In the case of DMFCs, both anode and cathode sides are simulated. The methanol crossover effect is also included. This virtual performance test bench plays an important role in the prototype fuel cell design. The computer aided design tool is proved to be useful in configuration designs. Additionally, it provides the detailed transport phenomenon inside the fuel cell stack.