Molecular Momentum Transport at Fluid-Solid Interfaces in MEMS/NEMS: A Review

This review is focused on molecular momentum transport at fluid-solid interfaces mainly related to microfluidics and nanofluidics in micro-/nano-electro-mechanical systems (MEMS/NEMS). This broad subject covers molecular dynamics behaviors, boundary conditions, molecular momentum accommodations, theoretical and phenomenological models in terms of gas-solid and liquid-solid interfaces affected by various physical factors, such as fluid and solid species, surface roughness, surface patterns, wettability, temperature, pressure, fluid viscosity and polarity. This review offers an overview of the major achievements, including experiments, theories and molecular dynamics simulations, in the field with particular emphasis on the effects on microfluidics and nanofluidics in nanoscience and nanotechnology. In Section 1 we present a brief introduction on the backgrounds, history and concepts. Sections 2 and 3 are focused on molecular momentum transport at gas-solid and liquid-solid interfaces, respectively. Summary and conclusions are finally presented in Section 4

Numerical solution of thermo-solutal mixed convective slip flow from a radiative plate with convective boundary condition

A mathematical model for mixed convective slip flow with heat and mass transfer in the presence of thermal radiation is presented. A convective boundary condition is included and slip is simulated via the hydrodynamic slip parameter. Heat generation or absorption effects are also incorporated. The Rosseland diffusion flux model is employed. The governing partial differential conservation equations are reduced to a system of coupled, ordinary differential equations via Lie group theory methods. The resulting coupled equations are solved using shooting method. The influences of the emerging parameters on dimensionless velocity, temperature and concentration distributions are investigated. Increasing radiative-conductive parameter accelerates the boundary layer flow and increase temperatures whereas it depresses concentration. An elevation in convection-conduction parameter also accelerates the flow and temperatures whereas it reduces concentrations. Velocity near the wall is considerably boosted with increasing momentum slip parameter although both temperature and concentration boundary layer thicknesses are decreased. The presence of a heat source is found to increase momentum and thermal boundary layer thicknesses but reduces concentration boundary layer thickness. Excellent correlation of the numerical solutions with previous non-slip studies is demonstrated. The current study has applications in bio-reactor diffusion flows and high-temperature chemical materials processing systems

A micro silicon hot-wire anemometer

A new micromachined hot-wire anemometer has been developed. Extensive tests of the anemometer's dependence on ambient temperature, over-heat ratio, and sensor length have been finalized. Due to its extremely small size, micromachined hot-wire has better sensitivity, temporal and spatial resolutions than the conventional hot-wire. Because of the large bandwidth (Mhz), velocity fluctuations at high Reynolds number flow can be measured

Effective slip and friction reduction in nanograted superhydrophobic microchannels

Enabled by a technology to fabricate well-defined nanogrates over a large area (2x2 cm(2)), we report the effect of such a surface, in both hydrophilic and hydrophobic conditions, on liquid slip and the corresponding friction reduction in microchannels. The grates are designed to be dense (similar to 230 nm pitch) but deep (similar to 500 nm) in order to sustain a large amount of air in the troughs when the grates are hydrophobic, even under pressurized liquid flow conditions (e.g., more than 1 bar). A noticeable slip (i.e., slip length of 100-200 nm, corresponding to 20%-30% reduction of pressure drop in a similar to 3 mu m high channel) is observed for water flowing parallel over the hydrophobic nanogrates; this is believed to be an "effective" slip generated by the nanostrips of air in the grate troughs under the liquid. The effective slip is clearer and larger in flows parallel to the nanograting patterns than in transverse, suggesting that the nanograted superhydrophobic surfaces would not only reduce friction in liquid flows under pressure but also enable directional control of the slip. This paper is the first to use nanoscale grating patterns and to measure their effect on liquid flows in microchannels. (c) 2006 American Institute of Physics

Molecular effects on boundary condition in micro∕nanoliquid flows

We experimentally investigated molecular effects of the slip∕no-slip boundary condition of Newtonian liquids in micro- and nanochannels as small as 350 nm. The slip was measurable for channels smaller than approximately 2 μm. The amount of slip is found to be independent of the channel size, but is a function of the shear rate, the type of liquid (polar or nonpolar molecular structure), and the morphology of the solid surface (molecular-level smoothness)

Effective slip and friction reduction in nanograted superhydrophobic microchannels

Enabled by a technology to fabricate well-defined nanogrates over a large area (2x2 cm(2)), we report the effect of such a surface, in both hydrophilic and hydrophobic conditions, on liquid slip and the corresponding friction reduction in microchannels. The grates are designed to be dense (similar to 230 nm pitch) but deep (similar to 500 nm) in order to sustain a large amount of air in the troughs when the grates are hydrophobic, even under pressurized liquid flow conditions (e.g., more than 1 bar). A noticeable slip (i.e., slip length of 100-200 nm, corresponding to 20%-30% reduction of pressure drop in a similar to 3 mu m high channel) is observed for water flowing parallel over the hydrophobic nanogrates; this is believed to be an "effective" slip generated by the nanostrips of air in the grate troughs under the liquid. The effective slip is clearer and larger in flows parallel to the nanograting patterns than in transverse, suggesting that the nanograted superhydrophobic surfaces would not only reduce friction in liquid flows under pressure but also enable directional control of the slip. This paper is the first to use nanoscale grating patterns and to measure their effect on liquid flows in microchannels. (c) 2006 American Institute of Physics.open11256292sciescopu