Lionel Rousseau

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.

Selective nucleation in silicon moulds for diamond MEMS fabrication

We present a new and original approach for the fabrication of diamond MEMS using MPCVD. This process does not rely on diamond etching using conventional techniques such as e.g. RIE: here our MEMS structures are geometrically defined using silicon moulds in which diamond is grown selectively. The moulds can be prepared from silicon using DRIE and enabling a wide range of geometries. The critical point is the selectivity of diamond growth which dramatically depends on the nucleation process. Two nucleation methods for selective diamond growth inside silicon moulds were explored in parallel and compared, namely, the bias enhanced nucleation (BEN) and the nano-seeding technique. With both methods, MEMS structures were successfully fabricated and characterized, leading to values for the Young modulus above 830 GPa, thus comparing well with literature values. We believe our approach will ease the routine fabrication of large area diamond MEMS wafers for improved advanced device fabrication.

Microfabrication of new microelectrode arrays equipped with a ground surface configuration for focal neural microstimulation

Extracellular electrical stimulation of the central nervous system has been used empirically for decades, with both fundamental and clinical goals. Currently, microelectrode arrays (MEAs) offer new possibilities for CNS microstimulation, allowing in principle to activate only neurons located in the vicinity of the stimulation sites. To overcome the lack of focality of monopolar stimulations, multipolar approaches are commonly used, multiplying therefore the number of electrodes of the arrays and the complexity of the connection system behind. To overcome these limitations, we developed a ground surface configuration consisting in surrounding all the electrodes with a conductive surface laying over the MEA substrate, and using it for the stimulation current return. We first report the microfabrication of a prototype of MEA equipped with this configuration. We also perform experimental recordings of the potential field induced by microstimulations and confirm the expected increased focality with the ground surface configuration. This will open the way to focal pixel-like microstimulation of neural networks using MEAs.

Continuous Intra Ocular Pressure Measurement Sensor for Glaucoma Diagnostic

Glaucoma is an ocular pathology usually associated with an increase in Intra Ocular Pressure (IOP). In this study, we are developing disposable eye lenses including a specific micro fabricated pressure sensor to measure IOP all day long. The information data will be wireless transferred via magnetic coupling to an external receiver. Our first work deals with the sensor design and fabrication. Simulation results based on classical electronic circuit tools will be presented and lead to several sensor solutions working at different RF frequencies. The fabrication process of the first sensors will also be described and an early IOP characterisation set-up will be presented to try to later quantify the sensor sensitivity, with IOP variations measured in the range 20 to 70mmHg.

A Thermally-Driven Micromixer Based on Fluid Volume Variation

In microfluidics, flows are laminar due to low Reynolds number (<1). Consequently, mixing between two liquids is mainly obtained by natural diffusion which may take a long time or equivalently requires centimetre length channels. However, it is possible to generate chaotic-like flows either by modifying the channel geometry or by creating an external perturbation of the flow. In this paper, an active micromixer is presented consisting on thermal actuation with heating resistors. In order to disturb the liquid flow, an oscillating transverse flow is generated by heating the liquid. Depending on the value of boiling point, either bubble expansion or volumetric dilatation controlled the transverse flow amplitude. The configuration is identical to the one of Dodge et al. [7], but the transversal oscillating flow is created by thermal actuation instead of pneumatic ones. A chaotic like mixing is then induced under particular conditions depending on volume expansion, liquid velocity, frequency of actuation[[ellipsis]] This solution presents the advantage to achieve mixing in a very short time (1 s) and along a short channel distance (channel width). It can also be integrated in a more complex device due to actuator integration with microfluidics.Copyright

Probing Vortex Density Fluctuations in Superfluid Turbulence

Quantum fluids -such as superfluid helium- are recognized as a model-fluids for turbulence research because their dynamics result from the tangle of quantized vortex lines. We report a time-resolved measurements of the local vortex line density in a turbulent quantum flow, here 4He at 1.6 K. The sensor is a micromachined second sound resonator inserted across the flow. Over one decade of inertial range, the spectrum exhibits a -1.6 power law scaling, compatible with a -5/3 exponent.

Surface Acoustic Love Waves Sensor for Chemical and Electrochemical Detection

The present work is an experimental study of shear horizontal surface acoustic wave (SH-SAW) miniaturized sensors which offer a high potential for electrochemical applications in liquid environments and in real-time. Our devices consist of a 42° rotYX lithium tantalate (LiTaO3 ) substrate coated with an SU8 photoresist polymer in order to produce acoustic waveguides supporting a Love–wave. The sensors architecture and fabrication techniques are presented. Standard techniques employing continuous wave system and pulse mode measurements have shown the propagation of both surface skimming bulk waves (SSBW) and leaky SH-SAW (LSAW) on 42°rot YXLiTaO3 . A numerical calculation using a simple balanced summation waves model is presented. Taking into account waves reflections and our measured velocity values, the simulation is in accordance with measurement. A copper’s electrodeposition experiment was performed to estimate the sensitivity of SAW devices. The measured sensitivity of 0.38 cm2 .g−1 is discussed in the framework of previously published works concerning Love wave devices.Copyright