Herve Willaime

Second-order slip laws in microchannels for helium and nitrogen

We perform gas flow experiments in a shallow microchannel, 1.14±0.02 μm deep, 200 μm wide, etched in glass and covered by an atomically flat silicon wafer. The dimensions of the channel are accurately measured by using profilometry, optical microscopy and interferometric optical microscopy. Flow-rate and pressure drop measurements are performed for helium and nitrogen, in a range of averaged Knudsen numbers extending up to 0.8 for helium and 0.6 for nitrogen. This represents an extension, by a factor of 3 or so, of previous studies. We emphasize the importance of the averaged Knudsen number which is identified as the basic control parameter of the problem. From the measurements, we estimate the accommodation factor for helium to be equal to 0.91±0.03 and that for nitrogen equal to 0.87±0.06. We provide estimates for second-order effects, and compare them with theoretical expectations. We estimate the upper limit of the slip flow regime, in terms of the averaged Knudsen number, to be 0.3±0.1, for the two gases.

Monodisperse Colloids Synthesized with Nanofluidic Technology

Limitations in the methods employed to generate micrometric colloidal droplets hinder the emergence of key applications in the fields of material science and drug delivery. Through the use of dedicated nanofluidic devices and by taking advantage of an original physical effect called capillary focusing, we could circumvent some of these limitations. The nanofluidic (i.e., submicrometric) devices introduced herein are made of soft materials, and their fabrication relies upon rapid technologies. The objects that we have generated are simple droplets, multiple droplets, particles, and Janus particles whose sizes lie between 900 nm and 3 microm (i.e., within the colloidal range). Colloidal droplets have been assembled on-chip into clusters and crystals, yielding discrete diffraction patterns. We illustrate potential applications in the field of drug delivery by demonstrating the ability of multiple droplets to be phagocytosed by murine macrophage-type cells.

Wettability Patterning by UV-Initiated Graft Polymerization of Poly(acrylic acid) in Closed Microfluidic Systems of Complex Geometry

Many microfluidic applications require modified surface wettability of the microchannels. Patterning of wettability within enclosed microfluidic structures at high spatial resolution has been challenging in the past. In this paper, we report an improved method for altering the surface wettability in poly(dimethylsiloxane) (PDMS) microchannels by UV-induced graft polymerization of poly(acrylic acid). Our method presents significant improvements in terms of wettability contrast and spatial resolution of the patterned structures as compared to recent literature and is in particular applicable to complex microfluidic structures with a broad range of channel sizes and aspect ratios. A key part of our work is the clear description of the surface treatment process with the identification of key parameters, some of which have been overlooked, neglected, or misinterpreted in previous works. We have studied these key parameters in detail and provide recommended values for each parameter supported by experimental results. This detailed understanding of the treatment process and the effects of the critical parameters on it allowed us to significantly improve quality and reliability of the treatment process.

Design and characterization of bubble phononic crystals

We report the practical realization of phononic crystals with gas inclusions, using soft lithography techniques. Ultrasonic experiments from 0.3 to 5 MHz confirm the existence of deep and wide minima of transmission through the crystal. We show that the first gap is due to the combined effects of Bragg reflections and bubble resonances. We propose a simple layered model that gives a reasonable prediction of the ultrasonic transmission.

Vortex density spectrum of quantum turbulence

The fluctuations of the vortex density in a turbulent quantum fluid are deduced from local second-sound attenuation measurements. These measurements are performed with a micromachined open-cavity resonator inserted across a flow of turbulent He-II near 1.6 K. The frequency power spectrum of the measured vortex line density is compatible with a (−5/3) power law. The physical interpretation is discussed.

Velocity gradient distributions in fully developed turbulence: An experimental study

We report measurements of the probability distribution function of the velocity derivatives, and the corresponding hyperflatness factors, up to order 6, as a function of the microscale Reynolds number Rλ. The measurements are performed in a flow produced between counter-rotating disks, using low-temperature helium gas as the working fluid, in a range of microscale Reynolds numbers lying between 150 and 2300. Consistently with previous studies, a transitional behavior is found around Rλ≈700. We determine a simple scaling law, in terms of Rλ, which allows the collapse of the tails of the pdf of the velocity derivatives onto a single curve, below the transition. We find well-defined relative power laws for the hyperflatness factors Hp and Hp*, throughout the entire range of variation of Rλ: H4=F=(0.99±0.05)H60.376±0.015 and H5*=(0.95±0.05)H60.67±0.022. These results are compared to those of previous investigators and to various theoretical approaches both statistical (multifractal model) and structural (i.e....

Exponents of the structure functions in a low temperature helium experiment

Abstract We report new measurements of longitudinal structure function exponents in fully developed turbulence. The measurements are performed in low temperature helium, for a wide range of microscale Reynolds numbers (from 150 to 5040), using large velocity records (up to 1010 points for the largest one, i.e. 3 × 106 integral time scales). Exponents of the longitudinal structure function of the velocity (denoted by ξ p ∗ ) are determined, in general, up to tenth order, and in one case, up to twelfth order. They are slightly smaller than previously reported values and it is safe to say that, within the accuracy of the present measurements, they do not seem to evolve significantly with the Reynolds number. High order values of ξ p ∗ , obtained by extrapolating the distributions of the velocity increments, are determined for the largest record; they are found to decrease with p above p>18.

Multifractal asymptotic modeling of the probability density function of velocity increments in turbulence

Moments and probability density functions (PDF) of (absolute value) velocity increments jv.xC‘/ v.x/j in turbulence are linked by simple integral relations. It is shown that the steepest descent method can be applied to evaluate the integrals if the moments (the absolute value structure functions) obey multifractal scaling laws of the type hjv.x C ‘/ v.x/j n iD An‘ n .A double asymptotic relation then relates the moments to the PDF. The dominant (exponential) terms of the asymptotic relation naturally yield the Legendre transform that is at the core of the Parisi‐Frisch model of inertial-range intermittency. Using the asymptotic relation, the PDF can be reconstructed from the multifractal exponent spectrum n and the statistics of large scale moments. On the basis of experimental results, it is shown that moments are quantitatively represented by multifractal scaling laws and large scale Gaussian (or quasi-Gaussian) statistics. The large scale at which the statistics are Gaussian (or quasi-Gaussian) is determined from inertial-range data alone and is of the order of the integral scale for Taylor-scale Reynolds numbers R in the range (300‐2200). This representation of moments together with the double asymptotic relations is able to reconstruct quantitatively the experimental inertial-range PDF. Analytic expressions (She-Leveque and Log-normal) of scaling exponents are both shown to lead to reconstructed PDF with systematic deviations from experiment. ©1999 Published by Elsevier Science B.V. All rights reserved.

Structures and transition in a high Reynolds number experiment

Abstract We present a brief review of experimental results obtained in high Reynolds number flows using Helium gas as the working fluid. We focus on measurements of the skewness and the flatness of the velocity derivative, and in particular their evolution with the Reynolds number. A transition, observed at Rλ ≈ 700, is discussed. In the second part, we analyze the structures associated with high velocity gradients, which we identify as “worms”; we finally present a detailed analysis of the evolution of their mean size, their number and their velocity increment with Reynolds number.

Development of a Plasma Process for Microfluidic Devices in the Prospect of Cell Attachment

Plasma processing has been developed to produce selective chemistry in the inner surface of a microfluidic system. This dry process is an alternative solution to the Chemical Vapor Deposition (CVD) process that allows us to work at low temperatures thus avoiding the degradation of the substrate by heat. The present study focused on the surface modification of PDMS in order to make them more hydrophilic and capable to exhibit a high percentage of COOH functions which will provide a good asset for future cell attachment.