Experimental studies were carried out on fully developed and steady electro-osmotic flow in a rectangular channel where the channel height $h$ is comparable to its width and the thickness of the electric double layer characterized by the Debye length is much less than $h$. The nano-particle image velocimetry technique was used to measure the two components of the velocity field parallel to and within about 100 nm of the channel wall for $h\,{\leq}\,25\,\umu$m. The mobility of the particle tracers was calculated from averaged velocity data for various electric field strengths. The experimentally determined mobility values are compared with analytical predictions for dilute aqueous solutions of sodium tetraborate.

Highly-porous, lightweight , and inexpensive three-dimensional (3D) sponges composed of interconnected carbon nanotubes (CNTs) without base materials synthesize with a facile and scalable one-step chemical vapor deposition process, and test as anode of microbial fuel cells (MFCs). The MFCs generates higher power densities of 2150 W m-3 (per anode volume) or 170 W m-3 (per anode chamber volume), comparable to those of commercial 3D carbon felt electrodes test under the same conditions. The high performances attribute to excellent charge transfer between CNTs and microbes owing to 13 times lower charge transfer resistance compared to that of carbon felt. The material cost of producing these CNT sponge estimates to be ~$0.1/g CNT , significantly lower than that of other methods. In addition, the high production rate of about 3.6 g h-1 compared to typical production rate of 0.02 g h-1 of other CNT-based materials makes this process economically viable. The one-step synthesis method allowing selfassembly of 3D CNT sponges as they grow is low cost and scalable, making this a promising method for manufacturing high-performance anodes of MFCs, with broad applicability to microbial electrochemical systems in general.

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The Brownian fluctuations of the colloidal tracers often used in microscale velocimetry are typically isotropic in the bulk. In the near-wall region, however, these fluctuations are strongly affected by the hydrodynamic interaction with the wall and by the no-flux condition imposed by the wall. These wall effects can, under appropriate conditions, bias measurements based on colloidal tracers, potentially leading to significant overestimation of near-wall velocities. We use a Fokker–Planck description to generate probability density functions of the distances from a single wall sampled by the matched particles that are present in the same window at both the start and end of a time interval. The importance of the resulting bias for experimental parameters is then quantified in terms of the size of the imaged region and measurement interval. We conclude with a brief discussion of the implications for near-wall velocimetry measurements.

Based upon high resolution LDA measurements over a range of momentum deficit thickness Reynolds numbers (Rθ=U∞θ/ν) from 1430 to 31 000, DeGraaff and Eaton [J. Fluid Mech. 422, 319 (2000)] propose a new mixed scaling for the near-wall region profile of the axial turbulent stress, u2¯. The present results support the validity of this scaling over an extended Reynolds number range 1000⩽Rθ⩽5×106.