Mean free path

About: Mean free path is a research topic. Over the lifetime, 4412 publications have been published within this topic receiving 114418 citations.

Modeling of Knudsen Layer Effects in Micro/Nanoscale Gas Flows

Abstract: We propose a power-law based effective mean free path (MFP) model so that the Navier-Stokes-Fourier equations can be employed for the transition-regime flows typical of gas micro/nanodevices. The effective MFP model is derived for a system with planar wall confinement by taking into account the boundary limiting effects on the molecular free paths. Our model is validated against molecular dynamics simulation data and compared with other theoretical models. As gas transport properties can be related to the mean free path through kinetic theory, the Navier-Stokes-Fourier constitutive relations are then modified in order to better capture the flow behavior in the Knudsen layers close to surfaces. Our model is applied to fully developed isothermal pressure-driven (Poiseuille) and thermal creep gas flows in microchannels. The results show that our approach greatly improves the near-wall accuracy of the Navier-Stokes-Fourier equations, well beyond the slip-flow regime.

Toward a Realistic Diffusion Tensor for Galactic Cosmic Rays

Abstract: We present analytical expressions for the mean free path parallel to the background magnetic field, calculated from quasi-linear theory for a composite slab/two-dimensional geometry model of turbulence. Diffusion perpendicular to the field is assumed to be anisotropic and proportional to parallel diffusion. For the variation in the magnetic field and the correlation length throughout the heliosphere, recent results of Zank et al. are used. We emphasize the role of the power spectrum of magnetic field fluctuations in determining the rigidity dependence of the parallel diffusion coefficient, and show that this rigidity dependence is unlikely to remain constant throughout the heliosphere. While we concentrate on Galactic protons at medium to high rigidities, we briefly discuss electron parallel mean free paths at low rigidities. Drifts are also included in the numerical modulation model, the results of which are compared with Ulysses, IMP 8, and Pioneer 10 measurements. Good agreement is found for energy spectra and latitudinal gradients in the inner heliosphere, but not for radial gradients farther away from the Sun in the ecliptic plane. We suggest that the combination of a turbulence, a scattering, and a modulation model presents a formalism that should prove useful for further studies.

Surface impact ionization in silicon devices

Abstract: In the past, carrier ionization rates in the silicon bulk have been measured and reported extensively. We present experiments and accurate electric field calculations for deriving the surface impact ionization rate of electrons. It is given by \alpha_{n}(surface) = 2.45 \cdot 10^{6} \cdot \exp(-1.92.10^{6}/E) [cm-1] Due to the lower mean free path at the surface, this ionization rate is much smaller then the well known bulk values and falls-off more steeply for low electric fields. The consequences for the simulation of MOS substrate currents will be shown.

Rod-like colloids and polymers in shear flow: a multi-particle-collision dynamics study

Abstract: The effect of the hydrodynamic interaction on the dynamics of flexible and rod-like polymers in solution is investigated. The solvent is simulated by the multi-particle-collision dynamics (MPCD) algorithm, a mesoscale simulation technique. The dynamics of the solvent is studied and the self-diffusion coefficient is calculated as a function of the mean free path of a particle. At small mean free paths, the hydrodynamic interaction strongly influences the dynamics of the fluid particles. This solvent model is then coupled to a molecular dynamics simulation algorithm. We obtain excellent agreement between our simulation results for a flexible polymer and the predictions of Zimm theory. The study of the translational diffusion coefficient of rod-like polymers confirms the predicted chain-length dependence. In addition, we study the influence of shear on the structural properties of rod-like polymers. For shear rates exceeding the rotational relaxation time, the rod-like molecule aligns with the shear flow, leading to an orientational symmetry breaking transverse to the flow direction. The comparison of the obtained shear rate dependencies with theoretical predictions exhibits significant deviations. The properties of the orientational tensor and the rotational velocity are discussed in detail as a function of shear rate.

Length dependent thermal conductivity measurements yield phonon mean free path spectra in nanostructures.

Abstract: Thermal conductivity measurements over variable lengths on nanostructures such as nanowires provide important information about the mean free paths (MFPs) of the phonons responsible for heat conduction. However, nearly all of these measurements have been interpreted using an average MFP even though phonons in many crystals possess a broad MFP spectrum. Here, we present a reconstruction method to obtain MFP spectra of nanostructures from variable-length thermal conductivity measurements. Using this method, we investigate recently reported length-dependent thermal conductivity measurements on SiGe alloy nanowires and suspended graphene ribbons. We find that the recent measurements on graphene imply that 70% of the heat in graphene is carried by phonons with MFPs longer than 1 micron.