Frédéric Ayela

Rheological properties of nanofluids flowing through microchannels

Results on the viscosity of SiO2 nanofluids submitted to very strong shear rates are reported. Experiments were conducted with micromachined capillary viscometers equipped with local pressure probes. For particle sizes of 35, 94, and 190nm, and solid volume fractions from 1.4% to 7%, a Newtonian behavior has been observed up to 5×104s−1. The increase in the nanofluid viscosity obeys a classical model but with a crowding factor which is a function of the diameter of the particles. This size dependence is explained by the influence of the shearing motion on the aggregates aspect ratio.

An experimental study and modelling of roughness effects on laminar flow in microchannels

Three different approaches were used in the present study to predict the influence of roughness on laminar flow in microchannels. Experimental investigations were conducted with rough microchannels 100 to 300μm in height ( H ). The pressure drop was measured in test-sections prepared with well-controlled wall roughness (periodically distributed blocks, relative roughness k * = k /0.5 H ≈0.15) and in test-sections with randomly distributed particles anchored on the channel walls ( k * ≈0.04–0.13). Three-dimensional numerical simulations were conducted with the same geometry as in the test-section with periodical roughness (wavelength L ). A one-dimensional model (RLM model) was also developed on the basis of a discrete-element approach and the volume-averaging technique. The numerical simulations, the rough layer model and the experiments agree to show that the Poiseuille number Po increases with the relative roughness and is independent of Re in the laminar regime ( Re Po observed during the experiments is predicted well both by the three-dimensional simulations and the rough layer model. The RLM model shows that the roughness effect may be interpreted by using an effective roughness height k eff . k eff / k depends on two dimensionless local parameters: the porosity at the bottom wall; and the roughness height normalized with the distance between the rough elements. The RLM model shows that k eff / k is independent of the relative roughness k * at given k / L and may be simply approximated by the law: k eff / k = 1 − ( c (ϵ)/2π)( L / k ) for k eff / k >0.2, where c decreases with the porosity ϵ.

Experimental characterization of water flow through smooth rectangular microchannels

This article presents experimental results obtained in water flows through smooth rectangular microchannels. The experimental setup used in the present study enabled the investigation of both very small length scales (21–4.5μm) and a wide range of Reynolds numbers (0.1–300). The evolution of the friction coefficient was inferred from pressure drop versus flow-rate measurements for two types of water with different electrical conductivities. The channels were made of a silicon engraved substrate anodically bonded to a Pyrex cover. In these structures, pressure losses were measured internally with micromachined Cu–Ni strain gauges. When compared to macroscale correlations, the results demonstrate that in smooth silicon-Pyrex microchannels larger than 4μm in height, the friction law is correctly predicted by the Navier-Stokes equations with the classical no-slip boundary conditions, regardless of the water electrical conductivity (>0.1μScm−1).

An Experimental Study of Water Flow in Smooth and Rough Rectangular Micro-Channels

This paper presents experimental results concerning water flow in smooth and rough rectangular micro-channels. It is part of a work intended to test the classical fluid mechanics laws when the characteristic length scale of inner liquid flows falls below 500μm. The method consists in determining experimental friction coefficients as a function of the Reynolds number. This implies simultaneous measurements of pressure drop and flow rates in microstructures. The two experimental apparatus used in this study enabled us to explore a wide range of length scales (7μm to 300μm) and of Reynolds number (0.01 to 8,000). Classical machining technologies were used to make micro-channels of various heights down to a scale of 100μm. Smaller silicon-Pyrex micro-channels were also made by means of silicon-based micro technologies. In these structures, friction coefficients have been measured locally with Cu -Ni strain gauges. For every height tested, both smooth and rough walls were successively used. When compared to macro-scale correlation the results demonstrate that i) In the smooth case, friction is correctly predicted by the Navier-Stokes equations with the classical kinematic boundary conditions, ii) For 200μm high channels, visualizations show transition to turbulence at Reynolds number of about 3,000. The presence of roughness elements did not significantly influence this result and iii) Roughness considerably increases the friction coefficient in the laminar regime. However, the Poiseuille number remains independent of the Reynolds number.Copyright

Microfluidic on chip viscometers

We present the design and the process of fabrication of micromachined capillary on chip rheometers which have performed wall shear stress and shear rate measurements on silicon oil and ethanol-based nanofluids. The originality of these devices comes from the fact that local pressure drop measurements are performed inside the microchannels. Thus, the advantage over existing microviscometers is that they can be used with the fluid under test alone; no reference fluid nor posttreatment of the data are needed. Each on chip viscometer consists of anodically bonded silicon-Pyrex derivative microchannels equipped with local probes. The anodic bonding allows to reach relatively high pressure levels (up to approximately 10 bars) in the channels, and a broad range of shear stress and shear rate values is attainable. Dielectrophoretic and electrorheological effects can be highlighted by employing alternate microstripe electrodes patterned onto the inner side of the Pyrex wall.

Hydrodynamic cavitation in microsystems. I. Experiments with deionized water and nanofluids

An experimental study of hydrodynamic cavitation downstream microdiaphragms and microventuris is presented. Deionized water and nanofluids have been characterized within silicon–Pyrex micromachined devices with hydraulic diameters ranging from 51 μm to 104 μm. The input pressure could reach up to 10 bars, and the flow rate was below 1 liter per hour. The output pressure of the devices was fixed at values ranging from 0.3 bar to 2 bars, so that it was possible to study the evolution of the cavitation number as a function of the Reynolds number in the orifice of the diaphragms or in the throat of the venturis. A delay on the onset of cavitation has been recorded for all the devices when they are fed with deionized water, because of the metastability of the liquid and because of the lack of roughness of the walls. For the first time, hydrodynamic cavitation of nanofluids (nanoparticles dispersed into the liquid) has been considered. The presence of nano-aggregates in the liquid does not exhibit any noticeabl...

A two-axis micromachined silicon actuator with micrometer range electrostatic actuation and picometer sensitive capacitive detection

This article presents an innovative micromachined silicon actuator. A 50-μm-thick silicon foil is anodically bonded onto a broached Pyrex substrate. A free standing membrane and four coplanar electrodes in close proximity are then lithographied and etched. The use of phosphorus doped silicon with low electrical resistivity allows the application of an electrostatic force between one electrode and the moving diaphragm. This plane displacement and the induced interelectrode variation are capacitively detected. Due to the very low electrical resistivity of the doped silicon, there is no need to metallize the vertical trenches of the device. No piezoelectric transducer takes place so that the mechanical device is free from any hysteretic or temperature dependance. The range of the possible actuation along the x and y axis is around 5 μm. The actual sensitivity is xn=0.54 A/Hz1/2 and yn=0.14 A/Hz1/2. The microengineering steps and the electronic setup devoted to design the actuator and to perform relative capa...

Hydrodynamic cavitation in microsystems. II. Simulations and optical observations

Numerical calculations in the single liquid phase and optical observations in the two-phase cavitating flow regime have been performed on microdiaphragms and microventuris fed with deionized water. Simulations have confirmed the influence of the shape of the shrinkage upon the contraction of the jet, and so on the localisation of possible cavitating area downstream. Observations of cavitating flow patterns through hybrid silicon–pyrex microdevices have been performed either via a laser excitation with a pulse duration of 6 ns, or with the help of a high-speed camera. Recorded snapshots and movies are presented. Concerning microdiaphragms, it is confirmed that very high shear rates downstream the diaphragms are the cause of bubbly flows. Concerning microventuris, a gaseous cavity forms on a boundary downstream the throat. As a consequence of a microsystem instability, the cavity displays a high frequency pulsation. Low values Strouhal numbers are associated to such a sheet cavitation. Moreover, when the in...

Quench propagation ignition using single-mode diode laser

The stability of NbTi-based multifilamentary composite wires subjected to local heat disturbances of short durations is studied in pool boiling helium conditions. A new type of heater is being developed to characterize the superconducting to normal state transition. It relies on a single-mode Diode Laser with an optical fiber illuminating the wire surface. This first paper focuses mainly on the feasibility of this new heater technology and eventually discusses the difficulties related to it. A small overview of Diode Lasers and optical fibers revolving around our application is given. Then, we describe the experimental setup, and present some recorded voltage traces of transition and recovery processes. In addition, we present also some energy and Normal Zone Propagation Velocity data and we outline ameliorations that will be done to the system.

A micromachined silicon magnetometer

Abstract A novel magnetic sensor is under investigation. This device is a bidimensional generalization of vibrating reed magnetometers, and is intended to be sensitive to very low magnetic fields (about 10 13 THz 1/2 ) and to very low magnetic moments. This sensor uses the magnetic force which acts on a sample of total magnetization M submitted to a magnetic field gradient grad B . The deformation caused by the force applied to a thin silicon membrane can be capacitively detected. When the field gradient is an alternating one at the resonance frequency of the membrane, the deformation is enhanced by the Q -factor. With such a device, one could perform magnetic measurements with unsurpassed sensitivities over a wide range of temperature. An experimental set-up to perform Q measurements on Si membranes submitted to an alternating electrostatic force is also described.