Soutenance de thèse de Zhenzhen LI

Soutenance de thèse de Zhenzhen Li, Ingénieure ESPCI, réalisée au laboratoire de microfluidique, MEMs et nanostructures sous la direction de Patrick Tabeling

Microfluidique à l’échelle micrométrique et sub-micrométrique : nano-PTV (particle tracking velocimetry), formation des gouttes et modèle sub-micromtriques

In this work, we have addressed three projects with the application of Microfluidics :

1. With the technology of Total Internal Reflection Velocimetry, we realised the nano-PTV of fluid flow within 800 nm close to solid surface. We achieved unprecedented accuracy of measurement compared with the state of art, by determining precisely the wall position, and by Langevin simulation, which takes into account of the sources of biases, such as Brownian motion, shear stress, electrostatic repulsion between particles and the wall, effect of out of focus, etc. We achieved ±5 nm and ±10 nm accuracy on the slip length determination for sucrose solution and for water. The no-slip condition on hydrophilic surface is confirmed, and a positive slip length on hydrophobic surface is clearly illustrated. This result demon- strated that the nano-PTV by TIRF is a quantitative methodology for the study of fluid flow near solid surface. This technology is applied to study flow of semi diluted polymer solution close to hydrophilic solid surface, a negative slip length is observed, due to adsorp- tion of polymer chain onto the surface.

2. We collaborated with A. Leshansky to study quantitatively the mechanism of step emulsification. The dispersed fluid and continuous fluid are co-flowing in a confined Hele- Shaw channel, before going into an unconfined pool. Drops are formed at the intersection between shallow channel and the pool. Two phases - step emulsification and large drops - are distinguished based on a well defined capillary number. The phase transition oc- curs at a critical capillary number, which is a function of the aspect ratio of the shallow channel. This discovery is confirmed by the theory, which is based on Hele-Shaw dynam- ics with the effect of capillary force. We found good agreement between experiments and theory, on the step emulsification droplet size, dispersed fluid pinching dynamics, and on the shape of free interface between dispersed fluid and continuous fluid prior to pinching. This work provides important theoretical support to the rising communities who utilise the step emulsification to make mono dispersed droplets for biological and chemical application.

3. We collaborated with a group of petroleum companies (AEC), to develop a technol- ogy which has potential application to the Enhanced Oil Recovery. Nano particles syn- thesized by the AEC is supposed to perform phase transition or deliver signals once in touch with oil. The principal idea consists in sending these nano particles into the porous media underground along with the injection fluids, and recollect them on the production well side. According to the information they deliver, the distribution oil may be mapped. We constructed a micro model based on microfluidic technology, which mimics the com- plex structure of porous media of rocks. The AEC synthesized nano particles are injected into the micro model, their motion and retention can be observed in real time. This work provides important information on the particle motion in porous media, which cannot be realised in conventional core experiments.

Key words
Microfluidic design, fabrication and control, fluid mechanics, complex fluids, petroleum recovery.