Fabrication of high quality periodic structures through convective assembly procedures

Techniques aimed at scalable realization of periodic structures from self-assembly of constituent building blocks, an approach that could supplant microfabrication procedures, are often constrained by the lack of diversity in packing arrangements achievable with assembly of simple constituents (e.g., spherical particles). In this work, we present a strategy to effectively pattern colloidal crystalline assemblies at two characteristic scales; achieving extensive non-classical particle packing amidst fully periodic, banded structural defects. We first introduce a scalable and robust approach to fabricate non-hexagonal crystals comprised of mono-sized spherical particles through introduction of periodically oscillating flow-fields during convective particle deposition. Through this technique, we report the discovery of extensive and tunable square-packed arrangements of monosized particles i.e., (100) fcc facets oriented parallel to the underlying substrate in self-assembled colloidal structures. Besides forming large (100) fcc crystalline domains with relatively few defects, the process also results in colloidal crystals having negligible variation in thickness while simultaneously yielding controlled proportions of both hexagonal and square-packed arrangements. The formation of domains of (100) fcc symmetry structures as a result of added vibration is robust across a range of micron-scale monosized spherical colloidal suspensions (e.g., polystyrene, silica) as well as substrate surface chemistries (e.g., hydrophobic, hydrophilic). In-situ visualization during self-assembly process as well as colloidal-crystal fabrication realized at varying frequency and amplitudes of vibration gives clues toward the mechanism of this flow-driven self-assembly method.In the second part of the work, we explore the introduction of volume defects in the uniformly-packed particle assemblies. Here, unlike randomly generated defects in packing structures, we demonstrate the formation of continuous, periodic banded defects comprised of particles with an fcc (110) packing configuration, and with tunable band periodicity. Studies aimed at discerning the specific effects of vibration conditions and meniscus properties help establish a mechanistic picture of the formation of fcc (110) banded structures based on stress relaxation in crystals through generation and movement of dislocations. The final chapters of the dissertation discuss how the convective assembly techniques could be efficiently used towards fabricating various devices for energy conversion and storage

Relation between flame speed and stretch for a premixed flame in a stagnation point flow

Displacement speed of a flame is a hydrodynamic concept, and it is defined as the normal component of the velocity of the incoming flow evaluated on the unburned side of the flame. Except for a steadily propagating flame with zero stretch (planar flame), this definition is ambiguous because the mass flux and, consequently, the gas velocity vary across the thickness of the flame represented by the flame/thermo-diffusive/pre-heat zone. In other words, the flame displacement speed has different values when evaluated at different locations within the flame zone. This has led to confusion about the position within the flame that must be used for measuring flame speed. Additionally, according to the hydrodynamic theory, the flame speed of weakly stretched flames is dependent on the flame stretch and the position/isotherm chosen within the flame zone through a parameter known as the Markstein Length. It is the objective of this study to provide a recommendation for the position/isotherm within the flame zone for measuring gas velocity in experiments that provides a flame speed value consistent with the hydrodynamic theory. In this regard, a premixed flame in an axisymmetric laminar stagnation point flow is studied using the hydrodynamic theory. First, the outer or hydrodynamic solution is obtained by solving modified Euler equations along with jump conditions resulting from conservation laws. Thereafter, the structure of the flame zone is resolved by rescaling the coordinate and the inner solution obtained using asymptotic theory is presented. Having obtained the outer and inner solutions, a uniformly valid composite expression for the mass flux across the flame is derived and presented. Finally, flame speeds are evaluated at different isotherms/positions within the flame zone, and a recommendation for the position to measure gas velocity in experiments is provided

Cryogenics in the pharmaceutical industry: drug design and bioavailability improvement

The commercial deployment of cryogenic solutions has expanded significantly in recent decades, from their early adoption to maintain the integrity of the cold chain to their use to induce changes in the physicochemical properties of materials. Compared to mechanical cooling-based cryogenic technologies that rely on vapor-compression cycles, the use of liquid cryogens (such as liquid nitrogen, helium, argon) offers the benefits of higher cooling rates, lower attainable temperature levels, uniform temperature distribution, absence of mechanical components, and reduced maintenance requirements. Various technologies have been developed in recent decades exploiting the potential of these liquid cryogens, namely cryogenic freezers that can guarantee uniform and fast cooling rates, cryo-based spray freezing and micronization processes that improve the bio-assimilation of pharmaceutical drugs, as well as novel routes for the production of active pharmaceutical ingredients

Nanocellulose, a Versatile Green Platform: From Biosources to Materials and Their Applications

With increasing environmental and ecological concerns due to the use of petroleum-based chemicals and products, the synthesis of fine chemicals and functional materials from natural resources is of great public value. Nanocellulose may prove to be one of the most promising green materials of modern times due to its intrinsic properties, renewability, and abundance. In this review, we present nanocellulose-based materials from sourcing, synthesis, and surface modification of nanocellulose, to materials formation and applications. Nanocellulose can be sourced from biomass, plants, or bacteria, relying on fairly simple, scalable, and efficient isolation techniques. Mechanical, chemical, and enzymatic treatments, or a combination of these, can be used to extract nanocellulose from natural sources. The properties of nanocellulose are dependent on the source, the isolation technique, and potential subsequent surface transformations. Nanocellulose surface modification techniques are typically used to introduce either charged or hydrophobic moieties, and include amidation, esterification, etherification, silylation, polymerization, urethanization, sulfonation, and phosphorylation. Nanocellulose has excellent strength, high Young’s modulus, biocompatibility, and tunable self-assembly, thixotropic, and photonic properties, which are essential for the applications of this material. Nanocellulose participates in the fabrication of a large range of nanomaterials and nanocomposites, including those based on polymers, metals, metal oxides, and carbon. In particular, nanocellulose complements organic-based materials, where it imparts its mechanical properties to the composite. Nanocellulose is a promising material whenever material strength, flexibility, and/or specific nanostructuration are required. Applications include functional paper, optoelectronics, and antibacterial coatings, packaging, mechanically reinforced polymer composites, tissue scaffolds, drug delivery, biosensors, energy storage, catalysis, environmental remediation, and electrochemically controlled separation. Phosphorylated nanocellulose is a particularly interesting material, spanning a surprising set of applications in various dimensions including bone scaffolds, adsorbents, and flame retardants and as a support for the heterogenization of homogeneous catalysts.Initiative d'excellence de l'Université de Bordeau