Conventional ion imaging techniques utilized grid electrodes to extract and to accelerate ions toward the detector. The disadvantages of grid electrodes caused transmission reduction, severe image distortions and image blur due to the non-point source geometry. All these problems can be solved by the utilization of an open lens electrode assembly. In velocity mapping, the extracting electric field of an open electrostatic lens that projects the ion cloud onto the detector. The major advantage of the combination of ion lens optics and two-dimensional detection is that ions from different positions with the same initial velocity vector would be mapped onto the same position on the detector, which was named ¡§ velocity map imaging .¡¨ The kinetic energy resolutions achievable with this method are not generally considered as being competitive with the best photofragment translational spectroscopy technique. But Ashfold and co-workers have demonstrated that velocity imaging methods can provide dissociation energy with one wavenumber resolution, i.e., it compares favourably with all rival photofragment translational spectroscopy techniques. We construct an ion imaging apparatus and the pressure inside can be maintained at ~ 10-6 Torr with differential pumping when the pulsed nozzle is shut off. The pressure in the source chamber raises from 2.1¡Ñ10-6 to 1.0¡Ñ10-5 Torr and the pressure in the photolysis chamber raises from 2.4¡Ñ10-7 to 3.6¡Ñ10-7 Torr, when the pulsed nozzle is turned on with a stagnation pressure at 3 bar. Because reactive chemicals attack the piezo disk translator and ruin the Viton O-ring, a modified pulsed nozzle and Teflon O-ring are adopted to overcome these problems. This pulsed nozzle is mounted on a three-dimensional translational stage such that the nozzle can be aligned inside vacuum. The homogeneity of the accelerating electric field is crucial to the performance of the ion imaging apparatus. To meet this requirement, parallel electrodes of identical dimensions have been assured in the manufacture of the ion lens assembly.

Abstract The subject of this thesis is flow imaging by methods based on the nuclear magnetic resonance (NMR) phenomenon. The thesis consists of three related topics: In the first one the feasibility of measuring velocity maps and distributions inside a microfluidic device by pulsed field gradient (PFG) NMR has been demonstrated. The second topic was to investigate microfluidic gas flow using a combination of a special detection technique and a powerful signal enhancement method. The third topic is related to the unambiguous determination of velocities under challenging experimental conditions and introduces a new, improved velocity imaging sequence. In the first part, well established imaging methods have been used to study water flow inside a micromixer. A surface coil matching the region of interest of the mixer was home built and used in the measurements in order to gain a better signal-to-noise ratio. Velocities inside the mixer have been measured by phase-encoding velocity, with unprecedented spatial resolution. Two dimensional NMR imaging and velocity maps revealed clogging and different manufacturing qualities of the mixers. In addition to the velocity maps, which display an average velocity for spins within one pixel, complete velocity distributions (so called average propagators) were measured. It was found that in the absence of spatial resolution in the third dimension, the propagator data can provide valuable insight to the flow system by revealing overlapping flow passages. The next topic was gas flow inside a microfluidic device. It was investigated by time-of-flight flow imaging. The measurement of the weak gas signal was enabled by the use of two signal enhancement techniques: remote detection NMR and parahydrogen induced polarization (PHIP). The results demonstrate that a very significant signal enhancement can be achieved by this technique. In the future it may enable the investigation of interesting chemical reactions inside microreactors. The third and last topic of the thesis deals with measuring flow by the so called multiecho sequences. When multiecho sequences are used in combination with phase encoding velocity, an error may be introduced: the multiecho sequence may produce a cumulative error to the phase of the magnetization, if it is sensitive to RF pulse imperfections. The problem has been elaborately explained and various solutions discussed, among the newly proposed one. Experimental results demonstrate the performance of the new velocity imaging sequence and show that the new sequence enables the unambiguous determination of velocities even in challenging experimental conditions resulting from inhomogeneous radio frequency fields of the measurement coils.

Solides élastiques au repos, les fluides à seuil s’écoulent comme un liquide au-delà d’une certaine contrainte. Plusieurs applications industrielles concernent l’écoulement de ces fluides dans des milieux poreux. On peut citer par exemple les émulsions dans le processus de récupération du pétrole, les opérations de cimentation dans le sol, ou le nettoyage d’un sol contaminé par une boue. Pour ces applications, il est nécessaire de connaitre la pression nécessaire pour un débit voulu à la sortie du milieu poreux. Dans de tels cas, l’écoulement est perturbé par la complexité de la géométrie. Les modèles développés pour décrire la loi de Darcy supposent une loi rhéologique appliquée localement, mais ces modèles décrivent mal ce type d’écoulement. De plus, des effets complexes peuvent s’ajouter comme le glissement à la paroi ou la thixotropie. Dans cette thèse, nous étudions l’écoulement de carbopol (ETD 2050) à travers différentes géométries. Tout d’abord au rhéomètre, nous montrons que le fluide, sous certaines conditions, correspond bien à un fluide à seuil modèle. Nous démontrons que le protocole expérimental utilisé est très important et qu’un comportement thixotropique peut apparaitre s’il n’est pas respecté. Ce comportement apparait notamment lorsque le fluide reste sous le seuil, l’impact augmentant avec le temps d’attente. Ensuite, nous comparons la loi d’écoulement obtenue au rhéomètre à l’écoulement dans un canal droit obtenu par microfabrication. Nous montrons alors l’importance du glissement proche du seuil et ses conséquences sur la loi d’écoulement. Enfin nous étudions l’écoulement du carbopol dans un milieu poreux. Le milieu poreux de 5x5cm est obtenu par microfabrication. La largeur moyenne des canaux est égale à celle du canal droit. Nous avons développé une nouvelle méthode de mesure des champs de vitesse. Nous montrons l’apparition d’une chenalisation de l’écoulement à travers quelques canaux du milieu poreux. Nous comparons ensuite la loi d’écoulement du milieu poreux à celle obtenue dans le canal droit. On remarque que la vitesse d’écoulement est plus faible dans le milieu poreux que dans le canal droit. / Elastic solids at rest, yield stress fluids flow like a liquid beyond a certain stress. Many industrial applications required the flow of these fluids in porous media, for example: the emulsion flow in oil recovery processes, the cementing operations in the ground, or the cleaning of sludge in a contaminated soil. For many applications, it could be interesting to know the pressure required for a desired flow rate. In such cases, the flow behavior of the fluid is complicated by the complexity of the geometry. The models developed to describe Darcy's law assume a rheological law applied locally, but these models poorly describe this type of flow. Furthermore, complex effects can be added like the wall slip or the thixotropy. In this thesis, we study the flow of carbopol (ETD 2050) through different geometries. First we show that the fluid, for some conditions, corresponds to model yield stress fluids. The experimental protocol used is very important and a thixotropic behavior can appear if it is not respected. This behavior appears especially when the fluid remains below the yield stress, the impact increases with the waiting time. We then compare the flow law obtained by rheometer in a straight channel obtained by microfabrication. We show the importance of the wall slip near the yield stress and the impact on the flow law. Finally, using a new method to measure the velocity fields developed during this thesis, we study the flow of carbopol in a porous medium. This porous medium of 5x5cm is obtained by microfabrication. The mean width of the channels is equivalent to the one of the straight channel. We show the emergence of a channeling flow through some channels of the porous medium. We then compare the flow law of the porous medium to the one obtained in the straight channel. It can be observed that the flow rate is lower in the porous medium than in the straight channel.