Convective Assembly of a Particle Monolayer.

Recently, the steady-state process of convective assembly has emerged as a viable production route for colloidal monolayers. The present study models the regions of particle assembly: Region I comprises convective concentration of a particle suspension in a liquid below a meniscus, Region II comprises permeation of fluid through the dense particle monolayer, and Region III comprises capillary densification. For each region, the dominant physics and nondimensional groups are identified, and quantitative models are derived to describe the evolution of microstructure in terms of the main process parameters. The concentration profile within the assembly zone of Region I is predicted, including the role of a concentration-dependent diffusion constant and the shape of the meniscus. The fluid flow through the assembled monolayer is treated in Region II, along with a stability calculation to reveal that isolated particle clusters do not survive on top of the monolayer. The physics of capillary crystallization is addressed in Region III, with an emphasis on the density of cracks that emerge. The Peclet number and Capillary number both play important roles but in different regions of the assembly process.Part of this work was performed during Norman Fleck’s stay at INM that was supported by the Alexander von Humboldt Foundation. The authors acknowledge Eduard Arzt’s continuing support of this project.This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acs.langmuir.5b0363

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

Building micro-soccer-balls with evaporating colloidal fakir drops

Evaporation-driven particle self-assembly can be used to generate three-dimensional microstructures. We present a new method to create these colloidal microstructures, in which we can control the amount of particles and their packing fraction. To this end, we evaporate colloidal dispersion droplets on a special type of superhydrophobic micro-structured surface, on which the droplet re- mains in Cassie-Baxter state during the entire evaporative process. The remainders of the droplet consist of a massive spherical cluster of the microspheres, with diameters ranging from a few tens up to several hundreds of microns. We present scaling arguments to show how the final particle packing fraction of these balls depends on the dynamics of the droplet evaporation.Comment: Manuscript Submitted to Physical Review Letters, 29th February 201

Three-dimensional nanoscopy of colloidal crystals.

We demonstrate the direct three-dimensional imaging of densely packed colloidal nanostructures using stimulated emission depletion microscopy. A combination of two de-excitation patterns yields a resolution of 43 nm in the lateral and 125 nm in the axial direction and an effective focal volume that is by 126-fold smaller than that of a corresponding confocal microscope. The mapping of a model system of spheres organized by confined convective assembly unambiguously identified face-centered cubic, hexagonal close-packed, random hexagonal close-packed, and body-centered cubic structures. An increasing need for noninvasive visualization on the nanoscale has fueled the development of far-field optical microscopy with resolution far below the wavelength of light.1,2 In materials science, structural studies with length scales of interest in the (sub-) micrometer range have typically been conducted either by collective scattering-based techniques or electron and scanning probe microscopes. Far-field optical methods however retain the advantage of simultaneously providing local, dynamic, and noninvasiv

Photocatalytic oxidation of ethanol using macroporous titania

Photocatalytic oxidation (PCO) using TiO2 is a potential means of remediating poor indoor air quality that is attributed to low levels of volatile organic compounds (VOC). In this work, ethanol is chosen as a simple compound representative of VOC’s. The aim of this research is to establish a baseline for the photocatalytic activity of TiO2 in ethanol PCO as well as the photonic efficiency of the photoreactor. The PCO conversion could then be enhanced by using photocatalyst having a macroporous structure. A flat plate photoreactor, UV light delivery and a flow system was designed in this work to accomplish ethanol PCO. Three kinds of photocatalysts were evaluated: 1) commercial Degussa P25 (in powder and slurry form), 2) unstructured sol-gel TiO2 and 3) macroporous TiO2 deposited on two substrates (optic fiber and glass slide). Titania from sol-gel hydrolysis was found to be a better photocatalyst than the commercial Degussa P25. Maximum PCO conversion found is 61% using an optimum TiO2 surface loading of 0.403 mg/cm2. A quantum efficiency of 2.3% was obtained for the photoreactor. Kinetic analysis of the experimental rate data gave an apparent reaction order of 0.45 and an approximate rate constant of 0.00144 (mol/cm3)0.55 (cm3/ gcat-s) for ethanol PCO. The photocatalyst samples were characterized using XRD and it was found that during sol-gel hydrolysis, only anatase crystalline phase was formed. From SEM images it was confirmed that the dipcoating method at low TiO2 weights resulted to a macroporous structure but only short range ordering is apparent. It was also found that colloidal crystals made from convective assembly have very good long range order and with clearly visible (111) symmetric plane. The available surface areas were measured from adsorption isotherms, for the unstructured sol-gel TiO2 it was found to have a surface area of 50 m2/g which is comparable to Degussa P25. The pore size distributions were generated from desorption isotherms, for the unstructured sol-gel TiO2 it was found to have an average pore size of 3.9 nm. A porosity of 0.21 and bulk density of 3.07 cm3/g was also found, indicating a much denser structure than Degussa P25 slurry. Lastly, an effort was made to attain higher PCO conversion for the macroporous TiO2 through higher TiO2 weights at ideal TiO2:PS weight ratio, using three different colloidal crystal templating methods and four variations of sol-gel infiltration techniques. However, no evidence that a macroporous structure was formed. Comparable PCO conversion values to unstructured sol-gel TiO2 were obtained. Additional work is needed to improve the methodology used in the fabrication of the macroporous structure

입자계 필름의 건조 공정과 대류자가조립 공정의 코팅 영역 지도에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부, 2014. 2. 안경현.Particulate coatings are commonly used for many industrial products such as paints, adhesives, paper coatings, anti-reflective films, batteries, fuel cells, optical devices, data storage devices and other applications. A typical particulate coating is composed of particles, solvent, polymer binder and other additives. Although the compositions of these coatings are identical, the difference in the microstructure of the coatings established during drying is influential in the properties and performances of the final products. Therefore, it is important to understand the microstructural change during drying which can be characterized by the particle distribution, the degree of alignment and the coating thickness. In general, as a particulate coating dries, the close-packed region is formed from the boundary of the coating. The close-packed region is the region where the particles are closely packed with the solvent filling in the interparticle void space. The capillary pressure of the meniscus in the close-packed region and the fluid flow into or through the close-packed region have a strong influence on the film formation. Therefore, predicting the formation of the close-packed region and understanding the role of the close-packed region give useful information on the microstructure such as the degree of alignment and the coating thickness as well as the particle distribution. Two particulate coating methods, drying of colloidal films and convective assembly, were investigated by modeling the formation of the close-packed region. In addition, the coating regime maps that predict the microstructural change depending on the coating conditions in terms of the dimensionless variables were created. In the case of the drying of colloidal films, the effect of evaporation, diffusion and sedimentation on the formation of the close-packed region (particle surface accumulation or sediment) was predicted by solving the particle conservation equation. From these results, the drying regime maps in terms of two dimensionless variables, the Peclet number and the sedimentation number, were created to predict evaporation, diffusion or sedimentation dominance for a given drying conditions. The particle distribution during drying of a model system comprised of monodisperse silica particles in water was observed using cryoSEM. There was a good agreement between the experimental observations and the model predictions. In the case of the convective assembly, the role of the close-packed region in developing the colloidal crystal films was proposed, and the length of the close-packed region was predicted by solving mass balance equations. The dimensionless coating thickness as well as the dimensionless length of the close-packed region was found to be the functions of only three dimensionless variables: two capillary numbers and the initial volume fraction. From the modeling results, the coating process regime maps that predict the coating thickness for a given coating condition were created. In addition, using the model system of monodisperse silica particles in alcohol, the length of the close-packed region was measured under various coating conditions to validate the model predictions. The experiments firmly supported the model predictions.Abstract Contents List of Figures List of Tables I. Introduction 1.1. Particulate coatings 1.2. Drying of colloidal films 1.3. Convective assembly II. Theory 2.1. Drying of colloidal films 2.2. Convective assembly III. Methods 3.1. Drying of colloidal films 3.1.1. Numerical method 3.1.2. Experimental methods 3.2. Convective assembly 3.2.1. Experimental methods IV. Results and Discussion 4.1. Drying of colloidal films 4.1.1. Numerical results 4.1.2. Drying regime maps 4.1.3. Experimental results 4.2. Convective assembly 4.2.1. Modeling results 4.2.2. Coating process regime maps 4.2.3. Experimental results V. Conclusion Nomenclature Bibliography 국문 초록Docto