18,550 research outputs found
Frequency dependence of microflows upon acoustic interactions with fluids
Rayleigh surface acoustic waves (SAWs), generated on piezoelectric substrates, can interact with liquids to generate fast streaming flows. Although studied extensively, mainly phenomenologically, the effect of the SAW frequency on streaming in fluids in constrained volumes is not fully understood, resulting in sub-optimal correlations between models and experimental observations. Using microfluidic structures to reproducibly define the fluid volume, we use recent advances modeling the body force generated by SAWs to develop a deeper understanding of the effect of acoustic frequency on the magnitude of streaming flows. We implement this as a new predictive tool using a finite element model of fluid motion to establish optimized conditions for streaming. The model is corroborated experimentally over a range of different acoustic excitation frequencies enabling us to validate a design tool, linking microfluidic channel dimensions with frequencies and streaming efficiencies. We show that in typical microfluidic chambers, the length and height of the chamber are critical in determining the optimum frequency, with smaller geometries requiring higher frequencies
Dirac fermion reflector by ballistic graphene sawtooth-shaped npn junctions
We have realized a Dirac fermion reflector in graphene by controlling the
ballistic carrier trajectory in a sawtooth-shaped npn junction. When the
carrier density in the inner p-region is much larger than that in the outer
n-regions, the first straight np interface works as a collimator and the
collimated ballistic carriers can be totally reflected at the second zigzag pn
interface. We observed clear resistance enhancement around the np+n regime,
which is in good agreement with the numerical simulation. The tunable
reflectance of ballistic carriers could be an elementary and important step for
realizing ultrahigh-mobility graphene field effect transistors utilizing Dirac
fermion optics in the near future
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