66 research outputs found
Comment on ``Large Slip of Aqueous Liquid Flow over a Nanoengineered Superhydrophobic Surface'' by C-H Choi and C Kim
In a recent Letter (Phys. Rev. Lett. vol 96, 066001 (2006), ref [1]), Choi
and Kim reported slip lengths of a few tens of microns for water on
nanoengineered superhydrophobic surfaces, on the basis of rheometry
(cone-and-plate) measurements. We show that the experimental uncertainty in the
experiment of Ref. [1], expressed in term of slip lengths, lies in the range 20
- 100 micrometers, which is precisely the order of magnitude of the reported
slip lengths. Moreover we point out a systematic bias expected on the
superhydrophobic surfaces. We thus infer that it is not possible to draw out
any conclusion concerning the existence of huge slip lengths in the system
studied by Choi and Kim.Comment: to appear in Physical Review Letter
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
Micro-evaporators for kinetic exploration of phase diagrams
We use pervaporation-based microfluidic devices to concentrate species in
aqueous solutions with spatial and temporal control of the process. Using
experiments and modelling, we quantitatively describe the advection-diffusion
behavior of the concentration field of various solutions (electrolytes,
colloids, etc) and demonstrate the potential of these devices as universal
tools for the kinetic exploration of the phases and textures that form upon
concentration
Near-wall nanovelocimetry based on Total Internal Reflection Fluorescence with continuous tracking
The goal of this work is to make progress in the domain of near-wall
velocimetry. The technique we use is based on the tracking of nanoparticles in
an evanescent field, close to a wall, a technique called TIRF (Total Internal
Reflection Fluorescence)-based velocimetry. At variance with the methods
developed in the literature, we permanently keep track of the light emitted by
each particle during the time the measurements of their positions ('altitudes')
and speeds are performed. By performing the Langevin simulation, we quantified
effect of biases such as Brownian motion, heterogeneities induced by the walls,
statistical biases, photo bleaching, polydispersivity and limited depth of
field. Using this method, we obtained slip length on hydrophilic surfaces of 15 nm for sucrose solution, and 910 nm for water; On hydrophobic
surface, 325 nm for sucrose solution, and 559 nm for water. The
errors (based on 95% confidence intervals) are significantly smaller than the
state-of-the-art, but more importantly, the method demonstrates for the first
time a capacity to measure slippage with a satisfactory accuracy, while
providing a local information on the flow structure with a nanometric
resolution. Our study confirms the discrepancy already pointed out in the
literature between numerical and experimental slip length estimates. With the
progress conveyed by the present work, TIRF based technique with continuous
tracking can be considered as a quantitative method for investigating flow
properties close to walls, providing both global and local information on the
flow.Comment: 24 pages, 13 figure
Reaction-diffusion dynamics: confrontation between theory and experiment in a microfluidic reactor
We confront, quantitatively, the theoretical description of the
reaction-diffusion of a second order reaction to experiment. The reaction at
work is \ca/CaGreen, and the reactor is a T-shaped microchannel, 10 m
deep, 200 m wide, and 2 cm long. The experimental measurements are
compared with the two-dimensional numerical simulation of the
reaction-diffusion equations. We find good agreement between theory and
experiment. From this study, one may propose a method of measurement of various
quantities, such as the kinetic rate of the reaction, in conditions yet
inaccessible to conventional methods
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