Physical ageing of spreading droplets in a viscous ambient phase

Nanoscale topographic features of solid surfaces can induce complex metastable behavior in colloidal and multiphase systems. Recent studies on single microparticle adsorption at liquid interfaces have reported a crossover from fast capillary driven dynamics to extremely slow kinetic regimes that can require up to several hours or days to attain thermodynamic equilibrium. The observed kinetic regime resembling physical ageing in glassy materials has been attributed to unobserved surface features with dimensions on the order of a few nanometers. In this work, we study the spontaneous spreading of water droplets immersed in oil and report an unexpectedly slow kinetic regime not described by previous spreading models. We can quantitatively describe the observed regime crossover and spreading rate in the late kinetic regime with an analytical model considering the presence of periodic metastable states induced by nanoscale topographic features (characteristic area ~4 nm^2, height ~1 nm) observed via atomic force microscopy. The analytical model proposed in this work reveals that certain combinations of droplet volume and nanoscale topographic parameters can significantly hinder or promote wetting processes such as spreading, wicking, and imbibition

Mouillage et démouillage de surfaces hétérogènes

L'objet de cette thèse est la caractérisation expérimentale de l'influence de défauts de mouillabilité sur l'étalement et le retrait d'un liquide sur une surface hétérogène. La plus grande partie de cette thèse est consacrée à l'étude morphologique et dynamique de grosses gouttes posées alimentées à débit constant. La démarche adoptée procède du simple au complexe: nous avons étudié l'influence de défauts isolés, le couplage de deux défauts, l'influence d'un réseau périodique et l'influence de la densité sur une distribution désordonnée de défauts. Une partie moins importante mais non moins riche est consacrée à l'étude du démouillage spontané d'une couche de liquide métastable sur une surface hétérogène. Nous avons mesuré l'influence de défauts isolés et nous avons décrit le processus de nucléation de gouttelettes satellites derrière le défaut lors du retrait du liquide sur la surface solide. Les résultats obtenus montrent la possibilité de déplacer et de confiner des volumes contrôlables de liquide sur des surfaces planes.In this thesis, we report an experimental study of the influence of wettability defects on an advancing and on a receding contact line on a heterogeneous surface. The most part of the thesis is dedicated to a morphological and a dynamical study of large drops. The liquid is injected into the drop at constant flow rate. The approach of the problem goes from simplicity to complexity: we have investigated the influence of isolated defects, the coupling between two defects, the influence of a periodic array of defects and the influence of the defect density on a disordered distribution of defects. The last part of the thesis is dedicated to an experimental study of spontaneous dewetting of a metastable liquid layer deposited on a heterogeneous surface. We have measured the influence of isolated defects and we have described the process of nucleation of satellite drops behind the defect during the receding of the liquid layer on the solid surface. The obtained results show the possibility to displace and to confine selected volumes of liquid on plane solid surfaces.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Physical ageing of spreading droplets in a viscous ambient phase

Abstract In this work, we study the spontaneous spreading of water droplets immersed in oil and report an unexpectedly slow kinetic regime not described by previous spreading models. We can quantitatively describe the observed regime crossover and spreading rate in the late kinetic regime with an analytical model considering the presence of periodic metastable states induced by nanoscale topographic features (characteristic area ~4 nm2, height ~1 nm) observed via atomic force microscopy. The analytical model proposed in this work reveals that certain combinations of droplet volume and nanoscale topographic parameters can significantly hinder or promote wetting processes such as spreading, wicking, and imbibition

A methanol-tolerant gas-venting microchannel for a microdirect methanol fuel cell

As a byproduct, CO2 gas is constantly generated from the electrochemical reactions of direct methanol fuel cells (DMFCs). In the anodic channel of a DMFC, the gas forms bubbles, which leads to bubble clogging and pressure buildup if the device is miniaturized. Bubble clogging increases the flow resistance in microchannels, calling for excessive power consumption for fuel delivery. Pressure buildup aggravates the undesired crossover of methanol. In order to solve those problems, this paper introduces a gas-venting microchannel that directly removes gas bubbles from the two-phase flows of gas and methanol solution without leakage. By employing a hydrophobic nanoporous membrane, successful venting is achieved for both water and methanol fuel with a concentration of as high as 10M. The fuel is contained without leakage under overpressures of as high as 200 kPa for both water and 10-M methanol, fulfilling the requirement of the current- as well as next-generation microdirect methanol fuel cells. A 1-D venting rate model is developed and experimentally verified for elongated bubbles. The reported bubble removal approach is also useful for other microfluidic devices, in which the accidental introduction of gas bubbles is prevalent. © 2007 IEEE

CO2 dissolution in water using long serpentine microchannels

The evolution of carbon dioxide bubbles dissolving in water is experimentally examined using long microchannels. We study the coupling between bubble hydrodynamics and dissolution in confined geometries. The gas impregnation process in liquid produces significant flow rearrangements. Depending on the initial volumetric liquid fraction, three operating regimes are identified, namely saturating, coalescing, and dissolving. The morphological and dynamical transition from segmented to dilute bubbly flows is investigated. Tracking individual bubbles along the flow direction is used to calculate the temporal evolution of the liquid volumetric fraction and the average flow velocity near reference bubbles over long distances. This method allows us to empirically establish the functional relationship between bubble size and velocity. Finally, we examine the implication of this relationship during the coalescing flow regime, which limits the efficiency of the dissolution process

Flow-focusing regimes for accelerated production of monodisperse drug-loadable microbubbles toward clinical-scale applications

Ultrasound imaging often calls for the injection of contrast agents, micron-sized bubbles which echo strongly in blood and help distinguish vascularized tissue. Such microbubbles are also being augmented for targeted drug delivery and gene therapy, by the addition of surface receptors and therapeutic payloads. Unfortunately, conventional production methods yield a polydisperse population, whose nonuniform resonance and drug-loading are less than ideal. An alternative technique, microfluidic flow-focusing, is able to produce highly monodisperse microbubbles with stabilizing lipid membranes and drug-carrying oil layers. However, the published 1 kHz production rate for these uniform drug bubbles is very low compared to conventional methods, and must be improved before clinical use can be practical. In this study, flow-focusing production of oil-layered lipid microbubbles was tested up to 300 kHz, with coalescence suppressed by high lipid concentrations or inclusion of Pluronic F68 surfactant in the lipid solution. The transition between geometry-controlled and dripping production regimes was analysed, and production scaling was found to be continuous, with a power trend of exponent ~5/12 similar to literature. Unlike prior studies with this trend, however, scaling curves here were found to be pressure-dependent, particularly at lower pressure-flow equilibria (e.g. <15 psi). Adjustments in oil flow rate were observed to have a similar effect, akin to a pressure change of 1–3 psi. This analysis and characterization of high-speed dual-layer bubble generation will enable more-predictive production control, at rates practical for in vivo or clinical use