Liquid Metals and Liquid Crystals Subject to Flow: From Fundamental Fluid Physics to Functional Fibers

Thèse de doctorat (2019)

La recherche en technologie se tourne actuellement vers la technologie portable, qui inclut les fibres des textiles incorporant des matériaux qui répondent aux stimuli. Que l’on produise ces fibres ... [plus ▼]

La recherche en technologie se tourne actuellement vers la technologie portable, qui inclut les fibres des textiles incorporant des matériaux qui répondent aux stimuli. Que l’on produise ces fibres synthétiques en laboratoire ou de façon industrielle, le processus de fabrication engage à la fois la chimie, la physique, et les sciences des matériaux, et se complexifie quand on veut réaliser des fibres coaxiales avec des noyaux fonctionnels. Cette thèse se penche de plusieurs façons sur les aspects de la production des fibres dotées d’un noyau de matériaux fonctionnels. Dans un premier temps, en raison de l'importance de la tension superficielle pour la stabilisation des jets liquides, on présente une technique pour mesurer la tension superficielle entre deux fluides immiscibles. Cette technique utilise des capillaires en verre dans une géométrie microfluidique. L'avantage de cette technique est de permettre de mesurer la tension superficielle uniquement sur la base des observations de la déformation des gouttelettes et de la pression nécessaire pour réaliser cette déformation dans le capillaire, sans avoir à compter sur les valeurs externes, comme la densité et la viscosité. Les valeurs obtenues sont similaires à celles disponibles dans la littérature. En exploitant les informations obtenues lors des premières expériences, on présente le filage au mouillé, qui produit des fibres en polymère avec une filière construite des capillaires en verre. On a réussi à produire des fibres de polymère pur ainsi que des fibres incorporant un noyau de cristal liquide nématique ou cholésterique, et les fibres obtenues avec les cristaux liquides conservent les propriétés optiques caractéristiques. Il n’a en revanche pas été possible d’obtenir des fibres avec des noyaux continus de métal liquide ; on présente une analyse des causes de cet échec et on suggère une autre méthode pour parvenir à réaliser éventuellement des microfibres avec un noyau continu de métal liquide. [moins ▲]

Microfluidic Wet Spinning of Core-Sheath Elastomer-Liquid Crystal Fibers

Communication orale (2019, March 29)

Liquid crystals encapsulated in fibers have a wide variety of applications in sensing. In order to produce these, several methods have been explored. Electrospinning is among the better-known techniques ... [plus ▼]

Liquid crystals encapsulated in fibers have a wide variety of applications in sensing. In order to produce these, several methods have been explored. Electrospinning is among the better-known techniques with considerable successes. Only a limited range of polymers, though, has been used for electrospinning with liquid crystal cores, and the process of electrospinning has many obstacles to its utility at an industrial scale. On the other hand, wet-spinning techniques are better suited for industrial applications and are widely used in textile manufacturing, but are not commonly used for coaxial fiber production, especially with the large experimental scales that are difficult to replicate in a standard liquid crystal research laboratory. We therefore propose a method for wet-spinning coaxial core-sheath liquid crystal-filled elastomer fibers using a microfluidic set-up. Based on the flow-focusing method used for the production of liquid crystal shells and emulsions, this technique generates coaxial filaments by pumping a core-sheath flow of a liquid crystal surrounded by a rubbery polymer solution into a co-flowing coagulation bath. The coagulation bath is tuned to quickly extract the elastic polymer solution solvent, leaving behind a dry, continuous fiber. We have employed this method to produce fibers of polybutadiene and polyisoprene containing a core of a liquid crystal, such as 4-cyano-4'-pentylbiphenyl (5CB). Investigations into the choice of polymer solution, i.e. both the polymer and solvents used, will be presented in addition to discussion on parameters affecting the contiguity of the core. [moins ▲]

Microfluidic Tensiometry Technique for the Characterization of the Interfacial Tension between Immiscible Liquids.

in Langmuir (2018)

The interfacial tension between two immiscible fluids is of critical importance for understanding many natural phenomena as well as in industrial production processes; however, it can be challenging to ... [plus ▼]

The interfacial tension between two immiscible fluids is of critical importance for understanding many natural phenomena as well as in industrial production processes; however, it can be challenging to measure this parameter with high accuracy. Most commonly used techniques have significant shortcomings because of their reliance on other data such as density or viscosity. To overcome these issues, we devise a technique that works with very small sample quantities and does not require any data about either fluid, based on micropipette aspiration techniques. The method facilitates the generation of a droplet of one fluid inside of the other, followed by immediate in situ aspiration of the droplet into a constricted channel. A modified Young-Laplace equation is then used to relate the pressure needed to produce a given deformation of the droplet's radius to the interfacial tension. We demonstrate this technique on different systems with interfacial tensions ranging from sub-millinewton per meter to several hundred millinewton per meter, thus over 4 orders of magnitude, obtaining precise results in agreement with the literature solely from experimental observations of the droplet deformation. [moins ▲]

Chiral Nematic Liquid Crystal Microlenses

in Scientific Reports (2017)

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids ... [plus ▼]

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and immersed in water. On addition of chiral dopants to the liquid crystal, the films exhibit optical textures with concentric ring patterns and radial variation of the birefringence color. Both are related to a biconvex shape of the chiral liquid crystal film; the rings are due to interference. The curvature radii of the biconvex lens array are in the range of a few millimeters. This curvature leads to a radial variation of the optical axis along the plane of the film. Such a Pancharatnam-type phase lens dominates the imaging and explains the measured focal length of about one millimeter. To our knowledge, these are the first spontaneously formed Pancharatnam devices. The unwinding of the helical structure at the grid walls drives the lens shape. The relation between the lens curvature and material properties such as helical pitch, the twist elastic constant, and the interfacial tensions, is derived. This simple, novel method for spontaneously forming microlens arrays can also be used for various sensors. [moins ▲]