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.

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Molecular momentum transport at fluid-solid interfaces in MEMS/NEMS: a review.

The value of tangential momentum accommodation coefficient (TMAC) is required while prescribing the boundary condition for flow of gases in the slip and transition flow regimes. The precise determination of its value is important for several other applications as well. This article reviews the experimental techniques employed by researchers over the decades to measure this coefficient and the values reported in the literature, with relevance to calculation of the slip velocity. The review shows that the value of TMAC is dependent on a number of parameters including nature of the gas, pressure of the gas, material of the surface, surface cleanliness and roughness, and surface temperature. For monatomic gases, the TMAC at about 0.93 is almost constant with respect to the Knudsen number, and this value can be employed for most commonly available surface materials. However, for nonmonatomic gases, TMAC decreases with an increase in Knudsen number; a correlation of TMAC with Knudsen number for this class of ga...

Survey on measurement of tangential momentum accommodation coefficient

Laser-driven high-harmonic generation provides the only demonstrated route to generating stable, tabletop attosecond x-ray pulses but has low flux compared to other x-ray technologies. We show that high-harmonic generation can produce higher photon energies and flux by using higher laser intensities than are typical, strongly ionizing the medium and creating plasma that reshapes the driving laser field. We obtain high harmonics capable of supporting attosecond pulses up to photon energies of 600 eV and a photon flux inside the water window (284 to 540 eV) 10 times higher than previous attosecond sources. We demonstrate that operating in this regime is key for attosecond pulse generation in the x-ray range and will become increasingly important as harmonic generation moves to fields that drive even longer wavelengths.

https://advances.sciencemag.org/content/advances/4/5/eaar3761.full.pdf

High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses.

Rarefied gas flow through a thin orifice is studied on the basis of the direct simulation Monte Carlo method. The mass flow rate and the flow field are calculated over the whole range of the Knudsen number for various values of the pressure ratio. It is found that at all values of the pressure ratio a significant variation of the flow rate occurs in the transition regime between the free-molecular and hydrodynamic regimes. In the hydrodynamic regime the flow rate tends to a constant value. In the case of finite pressure ratio the flow field qualitatively differs from that for outflow into vacuum, namely vortices appear in the downflow container on approaching the hydrodynamic regime. Then, in the hydrodynamic regime the gas flow forms a strong jet. A comparison of the numerical results with experimental data available in the open literature has been performed.

Numerical simulation of rarefied gas flow through a thin orifice

The temperature dependence of the tangential momentum accommodation coefficient (TMAC) is investigated by examining gas flows in a submicron channel using molecular dynamics simulations. The results show that the TMAC decreases with the increasing temperature following an exponential decay law, and is more sensitive to lower temperatures than to higher ones. The molecular trapping-desorption behaviors near the channel surface are found to be responsible for this dependence.

Temperature dependence of the tangential momentum accommodation coefficient for gases

A rarefied gas flow into vacuum through a tube of finite length is investigated over the whole range of gas rarefaction by the direct simulation Monte Carlo method. The nonequilibrium effects at the inlet and outlet of the tube have been considered by including in the computational domain large volumes of the upstream and downstream reservoirs. Results for the dimensionless flow rate and for the flow field are presented for a wide range of the gas rarefaction and for various values of the length to radius ratio in the range from 0 to 10. The influence of the gas-surface interaction model, as well as the effect of the intermolecular potential model on the gas flow, is examined. A good agreement has been obtained between the present numerical results and the corresponding experimental ones available in the literature.

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Rarefied gas flow through short tubes into vacuum

Experimental data on the accommodation coefficient of the noble gases on various surfaces are presented. The accommodation coefficient is measured via the mass flow rate of the gas through a capillary in the free-molecular regime. Three types of the surface are studied: atomically clean silver, atomically clean titanium and titanium coated by oxygen. It is shown that for the atomically clean surfaces the accommodation coefficient depends significantly on the species of the gas. For the surface coated by oxygen the accommodation coefficient is close to unity.

Accommodation coefficient of tangential momentum on atomically clean and contaminated surfaces

Erratum: “Accommodation coefficient of tangential momentum on atomically clean and contaminated surfaces” [J. Vac. Sci. Technol. A 19, 2499 (2001)]

This paper numerically studies a pressure-driven flow of rarefied gas through a slit using a direct simulation Monte Carlo method. A gas mass flow rate through a slit and a gas flow field were calculated. The calculations were carried out for various values of the pressure ratio over a wide range of gas rarefaction. The dependency of the mass flow rate and the flow field characteristics on the pressure ratio value and gas rarefaction degree was determined. In addition, the paper presents a discussion on the existence of the shock waves within a flow field in a hydrodynamic regime of the gas flow.

Pressure-driven flow of rarefied gas through a slit at a various pressure ratios

An experimental study of the gas molecule conductivity for cylindrical channel with the surface prepared by metal spray technique has been carried out. The dramatically change of a gas conductivity value is observed at the process of residual gas adsorption on a clean metal surface. Vast experimental material on molecular beam scattering and theoretical approach for description of a gas molecules scattering on the solid state surface are used for comparison and interpretation of the received experimental data. A satisfactory agreement between experimental results and numerical simulation of the effect is achieved.

Oleg V. Sazhin

Papers

Rarefied gas flow through short tubes into vacuum

Accommodation coefficient of tangential momentum on atomically clean and contaminated surfaces

Erratum: “Accommodation coefficient of tangential momentum on atomically clean and contaminated surfaces” [J. Vac. Sci. Technol. A 19, 2499 (2001)]

Pressure-driven flow of rarefied gas through a slit at a various pressure ratios

Surface composition influence on internal gas flow at large Knudsen numbers