Understanding the electroosmotic flow in microchannels is of both fundamental and practical significance for the design and optimization of various microfluidic devices to control fluid motion. In this paper, a lattice Boltzmann equation, which recovers the nonlinear Poisson-Boltzmann equation, is used to solve the electric potential distribution in the electrolytes, and another lattice Boltzmann equation, which recovers the Navier-Stokes equation including the external force term, is used to solve the velocity fields. The method is validated by the electric potential distribution in the electrolytes and the pressure driven pulsating flow. Steady-state and pulsating electroosmotic flows in two-dimensional parallel uniform and nonuniform charged microchannels are studied with this lattice Boltzmann method. The simulation results show that the heterogeneous surface potential distribution and the electroosmotic pulsating flow can induce chaotic advection and thus enhance the mixing in microfluidic systems efficiently.
The existing experimental data in the literature on hydrodynamics for liquid flow in microchannels are analyzed and the reasons causing the diversities are discussed and summarized. The present experimental data for deionized water flow in glass microtubes with diameters ranging from 50 to 530 µm show that the friction factors and transition Reynolds numbers from laminar to turbulent flow are in good agreement with the conventional theoretical predictions. However, the friction factors in stainless steel microtubes with diameters of 119 and 172 µm are much higher than the conventional theoretical predictions. This discrepancy is attributed to the large surface relative roughness or dense roughness distribution in the stainless steel tubes. Numerical simulations taking into account the electroviscous effect are carried out by using the lattice Boltzmann method. The simulation results show that the electroviscous effect does not play a significant role in the flow characteristics for channel dimensions of the order of microns and hence it can be neglected in engineering applications for moderate electrical conductivity of the liquid and conductivity of the walls. From the literature review and the present test data, it is validated that for liquid flow in smooth microchannels the conventional theoretical prediction for flow characteristics should still be applied.
While it is expected that inhomogeneity negatively affects battery performance, a quantitative understanding of the influence of inhomogeneity has remained elusive due to the difficulty of measuring it in a precise and rapid manner. Here, the ability of high-energy synchrotron X-rays to effectively probe the inhomogeneity in battery cathode films is demonstrated both for fundamental studies of single-layer cathode films and for improving manufacturing processes for industrially relevant multilayer stacks. High-energy lateral mapping studies were performed for very high energy density batteries (∼300 Wh/kg) made from NMC622 cathodes and Li metal anodes, where NMC622 denotes Li-(Ni 0.6 Mn 0.2 Co 0.2 )O 2 . It was first demonstrated for a multilayer pouch cell (7 layers, ∼3 mm thick) that both local and long-range variations in the NMC loading can be precisely quantified, allowing the quality of the coating process to be assessed. Next, it was shown that for a single cathode layer extracted from a pouch cell battery cycled to failure that local variations in the cathode state-of-charge (SOC) can be mapped with a sensitivity of about 0.1%. In this manner it was possible to identify three hot spots in which the local performance was much worse than for the rest of the cell as well as to gain insights into the specific failure mechanisms affecting both these local regions and the cell as a whole.
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