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Experimental Apparatus for the Study of micro Heat Exchangers with Inlet Temperatures between -200 and 200 °C and Elevated Pressures
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The current paper presents a test bench for micro-fabricated Recuperative Counter Flow Heat
Exchanger (RCFHE). The bench is suitable for up to 200 K difference between inlets temperatures and
operating pressures up to 32 MPa. The experimental setup allows controlling the physical state of the gas
(i.e. temperature, pressure and flow rate) at the RCFHE inlets. The bench has 5 controlled parameters and 5
more that are monitored and enables studying each of the hot and cold channels separately. We demonstrate
a steady supply of liquid nitrogen into the device for 10 minutes without thermal insulation of the specimen.
Another run is a steady state experiment with a temperature difference of about 20-30 K between inlets.
These show that the apparatus is capable of characterizing heat exchangers and serve as preliminary results
Electrostatic and electrokinetic contributions to the elastic moduli of a driven membrane
We discuss the electrostatic contribution to the elastic moduli of a cell or
artificial membrane placed in an electrolyte and driven by a DC electric field.
The field drives ion currents across the membrane, through specific channels,
pumps or natural pores. In steady state, charges accumulate in the Debye layers
close to the membrane, modifying the membrane elastic moduli. We first study a
model of a membrane of zero thickness, later generalizing this treatment to
allow for a finite thickness and finite dielectric constant. Our results
clarify and extend the results presented in [D. Lacoste, M. Cosentino
Lagomarsino, and J. F. Joanny, Europhys. Lett., {\bf 77}, 18006 (2007)], by
providing a physical explanation for a destabilizing term proportional to
\kps^3 in the fluctuation spectrum, which we relate to a nonlinear ()
electro-kinetic effect called induced-charge electro-osmosis (ICEO). Recent
studies of ICEO have focused on electrodes and polarizable particles, where an
applied bulk field is perturbed by capacitive charging of the double layer and
drives flow along the field axis toward surface protrusions; in contrast, we
predict "reverse" ICEO flows around driven membranes, due to curvature-induced
tangential fields within a non-equilibrium double layer, which hydrodynamically
enhance protrusions. We also consider the effect of incorporating the dynamics
of a spatially dependent concentration field for the ion channels.Comment: 22 pages, 10 figures. Under review for EPJ
Controlling nanoslot overlimiting current with the depth of a connecting microchamber
The overlimiting ion flux, in excess of the limiting-value stipulated by diffusion, across a wide nanoslot (of fixed depth) is shown to be sensitively dependent on the depth of the connecting microchamber at one end of the nanoslot, which controls the onset of a vortex instability that specifies the dimension of the concentration polarization layer responsible for overlimiting behavior. Simple scaling arguments relating the microchamber depth to the effective fluid viscosity produce experimentally verified scaling dependence of the polarization layer length, the onset voltage for overlimiting behavior and the overlimiting current on the microchamber depth
Understanding electrokinetics at the nanoscale: A perspective
Electrokinetics promises to be the microfluidic technique of choice for portable diagnostic chips and for nanofluidic molecular detectors. However, despite two centuries of research, our understanding of ion transport and electro-osmotic flow in and near nanoporous membranes, whose pores are natural nanochannels, remains woefully inadequate. This short exposition reviews the various ion-flux and hydrodynamic anomalies and speculates on their potential applications, particularly in the area of molecular sensing. In the process, we revisit several old disciplines, with some unsolved open questions, and we hope to create a new one