20 research outputs found
Rheology and dynamics of simple and complex liquids in mesoporous matrices
Subject of my thesis is a study of rheologic and dynamic properties of fluids confined in an isotropic pore network with pore radii of approx. 5nm embedded in a monolithic silica matrix (porous Vycor). The experimental technique bases on the capillary rise of a wetting liquid in a porous substrate, also known as spontaneous imbibition. A crucial part of the conducted experiments centers on the increasing relevance of the liquid-substrate interface in the mesopore confinement. Detailed analyses of the measurements carried out with water, silicon oils, and a series of hydrocarbons result in precise information on the boundary conditions expressed in terms of the velocity slip length. Systematic variations of the chain-length of the used hydrocarbons also allow for an assessment of the influence of the shape of the liquid\u27;s building blocks on the nanoscopic flow behavior. Supplemental forced throughput experiments additionally address the impact of the liquid-substrate interaction. Furthermore, the influences of spatial confinement on the surface freezing transition of the linear hydrocarbon n-tetracosane as well as on the mesophase transitions of the liquid crystal 8OCB are investigated. Finally, a third, more general study focuses on the kinetics of the invasion front, which is supposed to be influenced significantly by the random environment of the pore space considered.Das Themengebiet meiner Dissertation umfasst rheologische und dynamische Eigenschaften von Flüssigkeiten in einem isotropen Porennetzwerk einer monolithischen Glasmatrix (poröses Vycor). Die Porenradien betragen dabei etwa 5 nm. Die durchgeführten Experimente beruhen auf dem Prinzip des Kapillarsteigens einer benetzenden Flüssigkeit in einem porösen Substrat (spontane Imbibition). Ein Schwerpunkt der Untersuchungen liegt auf dem zunehmenden Einfluss der Grenzfläche zwischen Fluid und Substrat als Folge der extremen, räumlichen Beschränkung. Analysen der Experimente mit Wasser, Silikonölen und einer Reihe Kohlenwasserstoffe liefern präzise Angaben zur hydrodynamischen Randbedingung beschrieben durch die sog. Schlupflänge. Systematische Variationen der Kettenlänge der verwendeten Kohlenwasserstoffe erlauben die Beurteilung der Bedeutsamkeit der Molekülform für das mikroskopische Fließverhalten. Dank einer alternativen Experimentführung kann man sich auch mit dem Einfluss der Benetzbarkeit der Flüssigkeit befassen. Darüber hinaus wird der Einfluss der räumlichen Beschränkung auf Phasenübergänge behandelt. Im Speziellen werden das Oberflächengefrieren eines Alkans und die Mesophasen eines Flüssigkristalls untersucht. Schließlich wird die Aufrauung der voranschreitenden Benetzungsfront beim Kapillarsteigen in dem mesoporösen Glas untersucht. Man erwartet, dass deren Bewegung durch die zufällige Netzwerk-Topologie entscheidend beeinflusst wird
Dynamics of bubbles in microchannels : theoretical, numerical and experimental analysis
This thesis aims at contributing to the characterization of the dynamics of bubbles in microfluidics through modeling and experiments. Two flow regimes encountered in microfluidics are studied, namely, the bubbly flow regime and the Taylor flow regime (or slug flow). In particular, the first part of this thesis focuses on the dynamics of a bubbly flow inside a horizontal, cylindrical microchannel in the presence of surfactants using numerical simulations. A numerical method allowing to simulate the transport of surfactants along a moving and deforming interface and the Marangoni stresses created by an inhomogeneous distribution of these surfactants on this interface is implemented in the Level set module of the research code. The simulations performed with this code regarding the dynamics of a bubbly flow give insights into the complexity of the coupling of the different phenomena controlling the dynamics of the studied system. Fo example it shows that the confinement imposed by the microchannel walls results in a significantly different distribution of surfactants on the bubble surface, when compared to a bubble rising in a liquid of infinite extent. Indeed, surfactants accumulate on specific locations on the bubble surface, and create local Marangoni stresses, that drastically influence the dynamics of the bubble. In some cases, the presence of surfactants can even cause the bubble to burst, a mechanism that is rationalized through a normal stress balance at the back of the bubble. The numerical method implemented in this thesis is also used for a practical problem, regarding the artisanal production of Mezcal, an alcoholic beverage from Mexico. The second part of the thesis deals with the dynamics of a Taylor flow regime, through experiments and analytical modeling. An experimental technique that allows to measure the thickness of the lubrication film forming between a pancake-like bubble and the microchannel wall is developed. The method requires only a single instantaneous bright-field image of a pancake-like bubble translating inside a microchannel. In addition to measuring the thickness of the lubrication film, the method also allows to measure the depth of a microchannel. Using the proposed method together with the measurment of the bubble velocity allows to infer the surface tension of the interface between the liquid and the gas. In the last chapter of this thesis, the effect of buoyancy on the dynamics of a Taylor flow is quantified. Though often neglected in microfluidics, it is shown that buoyancy effects can have a significant impact on the thickness of the lubrication film and consequently on the dynamics of the Taylor flow. These effects are quantified using experiments and analytical modeling. This work was performed at Princeton University with Professor Howard A. Stone during a seven month stay
Light scattering studies on lecithin micellar solution
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1991.Includes bibliographical references.by Richard Addison Chyamberlin.Ph.D
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Astigmatism and Pseudoaccommodation in Pseudophakic Eyes
noAdvanced IOLs with circumferential zones of different power provide pseudoaccommodation. We investigated the potential for power variation with meridian, namely astigmatism, to provide pseudo-accommodation. With appropriate power and axis orientations, acceptable pseudo-accommodation can be achieved
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The friction coefficient of soft contact lens surfaces in relation to comfort and performance
The soft contact lenses of today are made from a variety of hydrogel materials. These materials have different properties in terms of water content, monomers, hardness and other tensile characteristics. It is likely that the frictional properties also vary between materials. It is known that constituents of the tear film interact with contact lens materials to form a biofilm on the lens surface. The hypothesis of this research is that although the frictional properties of lens materials may vary these properties do not affect the comfort and performance of the lenses in vivo.
A tribometer is a device to measure the coefficient of friction of materials. There was no commercially available tribometer designed specifically for use with contact lens materials, so one was constructed and validated against standard solid materials. The same equipment was used to determine the friction coefficients of five contemporary soft lens materials under different conditions of lubrication but, unlike other tribometers, this unique design simulated human blinking as far as possible. The experimental friction coefficients varied widely from 0.27 to 5.89 under different conditions of lubrication. The largest variation between materials was seen using the most viscous lubricant.
For the in vivo studies the author coordinated the manufacture of 250 contact lenses, which were lathe cut and polished to a standard design, achieving exceptionally tight tolerances, using the same five materials. This rigourous process was carried out to minimise variations in the geometry of each contact lens. Subjects were screened to minimise ocular heterogeneities between subjects. Clinical performance of each lens was assessed using comfort, contrast sensitivity, visual acuity, entoptic phenomena, non-invasive tear break-up time and lens movement on the eye. In a clinical environment none of these parameters showed any associations with the coefficients of friction found in vitro, apart from a moderate correlation (rho = 0.5) between lens movement and the coefficient of friction under borderline friction conditions. In conclusion, the findings of this research support the hypothesis that frictional properties of soft lenses do not affect comfort and performance in vivo