33 research outputs found
Knudsen Diffusion in Silicon Nanochannels
Measurements on helium and argon gas flow through an array of parallel,
linear channels of 12 nm diameter and 200 micrometer length in a single
crystalline silicon membrane reveal a Knudsen diffusion type transport from
10^2 to 10^7 in Knudsen number Kn. The classic scaling prediction for the
transport diffusion coefficient on temperature and mass of diffusing
species,D_He ~ sqrt(T), is confirmed over a T range from 40 K to 300 K for He
and for the ratio of D_He/D_Ar ~ sqrt(m_Ar/m_He). Deviations of the channels
from a cylindrical form, resolved with transmission electron microscopy down to
subnanometer scales, quantitatively account for a reduced diffusivity as
compared to Knudsen diffusion in ideal tubular channels. The membrane
permeation experiments are described over 10 orders of magnitude in Kn,
encompassing the transition flow regime, by the unified flow model of Beskok
and Karniadakis.Comment: 4 pages, 3 figure
Hydraulic Transport Across Hydrophilic and Hydrophobic Nanopores: Flow Experiments with Water and n-Hexane
We experimentally explore pressure-driven flow of water and n-hexane across
nanoporous silica (Vycor glass monoliths with 7 or 10 nm pore diameters,
respectively) as a function of temperature and surface functionalization
(native and silanized glass surfaces). Hydraulic flow rates are measured by
applying hydrostatic pressures via inert gases (argon and helium, pressurized
up to 70 bar) on the upstream side in a capacitor-based membrane permeability
setup. For the native, hydrophilic silica walls, the measured hydraulic
permeabilities can be quantitatively accounted for by bulk fluidity provided we
assume a sticking boundary layer, i.e. a negative velocity slip length of
molecular dimensions. The thickness of this boundary layer is discussed with
regard to previous capillarity-driven flow experiments (spontaneous imbibition)
and with regard to velocity slippage at the pore walls resulting from dissolved
gas. Water flow across the silanized, hydrophobic nanopores is blocked up to a
hydrostatic pressure of at least 70 bar. The absence of a sticking boundary
layer quantitatively accounts for an enhanced n-hexane permeability in the
hydrophobic compared to the hydrophilic nanopores.Comment: 15 pages, 7 figures, in press, Physical Review E 201
Capillary rise of water in hydrophilic nanopores
We report on the capillary rise of water in three-dimensional networks of
hydrophilic silica pores with 3.5nm and 5nm mean radii, respectively (porous
Vycor monoliths). We find classical square root of time Lucas-Washburn laws for
the imbibition dynamics over the entire capillary rise times of up to 16h
investigated. Provided we assume two preadsorbed strongly bound layers of water
molecules resting at the silica walls, which corresponds to a negative velocity
slip length of -0.5nm for water flow in silica nanopores, we can describe the
filling process by a retained fluidity and capillarity of water in the pore
center. This anticipated partitioning in two dynamic components reflects the
structural-thermodynamic partitioning in strongly silica bound water layers and
capillary condensed water in the pore center which is documented by sorption
isotherm measurements.Comment: 4 pages, 3 figure
Imbibition in mesoporous silica: rheological concepts and experiments on water and a liquid crystal
We present, along with some fundamental concepts regarding imbibition of
liquids in porous hosts, an experimental, gravimetric study on the
capillarity-driven invasion dynamics of water and of the rod-like liquid
crystal octyloxycyanobiphenyl (8OCB) in networks of pores a few nanometers
across in monolithic silica glass (Vycor). We observe, in agreement with
theoretical predictions, square root of time invasion dynamics and a sticky
velocity boundary condition for both liquids investigated.
Temperature-dependent spontaneous imbibition experiments on 8OCB reveal the
existence of a paranematic phase due to the molecular alignment induced by the
pore walls even at temperatures well beyond the clearing point. The ever
present velocity gradient in the pores is likely to further enhance this
ordering phenomenon and prevent any layering in molecular stacks, eventually
resulting in a suppression of the smectic phase in favor of the nematic phase.Comment: 18 pages, 8 figure
EuPRAXIA - A Compact, Cost-Efficient Particle and Radiation Source
Plasma accelerators present one of the most suitable candidates for the development of more compact particle acceleration technologies, yet they still lag behind radiofrequency (RF)-based devices when it comes to beam quality, control, stability and power efficiency. The Horizon 2020-funded project EuPRAXIA (âEuropean Plasma Research Accelerator with eXcellence In Applicationsâ) aims to overcome the first three of these hurdles by developing a conceptual design for a first international user facility based on plasma acceleration. In this paper we report on the main features, simulation studies and potential applications of this future research infrastructure
Compact All-Optical Precision-Tunable Narrowband Hard Compton X-Ray Source
Readily available bright X-ray beams with narrow bandwidth and tunable energy promise to unlock novel developments in a wide range of applications. Among emerging alternatives to large-scale and costly present-day radiation sources which severely restrict the availability of such beams, compact laser-plasma-accelerator-driven inverse Compton scattering sources show great potential. However, these sources are currently limited to tens of percent bandwidths, unacceptably large for many applications. Here, we show conceptually that using active plasma lenses to tailor the electron bunch-photon interaction, tunable X-ray and gamma beams with percent-level bandwidths can be produced. The central X-ray energy is tunable by varying the focusing strength of the lens, without changing electron bunch properties, allowing for precision-tuning the X-ray beam energy. This method is a key development towards laser-plasma-accelerator-driven narrowband, precision tunable femtosecond photon sources, enabling a paradigm shift and proliferation of compact X-ray applications
Design Study of a Laser-driven X-ray Source for Medical Imaging
The combination of laser-wakefield acceleration and Thomsonscattering (TS) driven by one high-power laser allows for all-opticalbrilliant X-ray sources of compact design. Such source designs enablethe clinical implementation of X-ray-based medical imaging modalitiessuch as X-ray fluorescence imaging (XFI). XFI offers the in-vivolocalization of functionalized gold nanoparticles (GNPs) with highsensitivity, temporal and spatial resolution at low dose exposure. Thisrequires hard X-ray sources (~100 keV) of specific design parameters,i.e. a pencil-shaped X-ray beam within a small bandwidth and a highphoton flux to achieve feasible exposure times. Thus, optimization ofthe TS process is required to enhance the X-ray source to fulfill thegiven criteria. In this poster we show the techniques employed for thedetermination of the best laser and electron parameters. The designconcept and simulation results for a first proof-of-principle experimentare presented. Electron focusing via an active plasma lens allows forsingle-element symmetric focusing on the one hand. On the other handX-ray bandwidth control for broadband electron energy spectra (no energyspread limit) is achieved