Towards a complete photonic band gap in the visible

The first part of the thesis describes the fabrication and the characterization of face-centered-cubic (fcc) photonic crystals (PCs) of dielectric (core-shell) spheres in a low-dielectric host (air). We demonstrate the synthesis and optical characterization of the PC's building blocks: well-defined core-shell colloidal particles and hollow shells of zinc sulfide (ZnS) and silica (SiO2). The synthesis method allows for the production of monodisperse particles with a tunable core-to-shell size ratio and total radius. By use of the controlled drying method, we demonstrate the fabrication of large planar PCs of well-defined thickness from SiO2, ZnS, and ZnS-core-SiO2-shell colloidal particles. We demonstrate, both experimentally and theoretically, that the relative stop gap width in the (111) fcc crystallographic direction in the case of high-index core and low-index shell spheres is larger than in a PC of homogeneous spheres of either material. The second part of the thesis focuses on the preparation and characterization of photonic materials of different degree of order made of metal colloidal particles. We demonstrate the synthesis and characterization of large (R > 100 nm) silver (Ag) particles. The particles are obtained by reducing silver nitrate with ascorbic acid in aqueous solutions in the presence of a protective polymer. The resulting particles are spherical porous aggregates with a low polydispersity (< 20%) and surface roughness on the order of a few nanometers. The optical properties on a single-particle level are well described if an effective dielectric constant is used. Depending on the volume fraction and the effective polydispersity, in water these particles form charge-stabilized glasses or crystals. Under illumination with white light, these samples display bright colors. A strong modulation is found in the reflectivity of photonic glasses possessing a short-range order only. The general features in the experimental spectra are found in the theoretical reflectivity spectra for fcc crystals. From our results, it is likely that amorphous metallo-dielectric materials can be used in some applications where the spontaneous emission of light needs to be modified. The last part of the thesis deals with PCs with complex lattices and PCs made of non-spherical colloidal particles. We present the fabrication of binary colloidal crystals through a simple layer-by-layer drying process. We demonstrate the fabrication of crystals with a stoichiometry of large (L) and small (S) particles of LS2 and LS. In addition, we observed the formation of a new LS3 binary crystal. By using spheres with different composition, one component can be selectively removed. As an example, we demonstrate the synthesis of a hexagonal non-close-packed colloidal crystal. We demonstrate the fabrication of PCs of ellipsoidal colloidal particles. By use of MeV ion irradiation, we deformed spheres into oblate ellipsoids as organized in a thin 3D colloidal crystal. Both the unit cell symmetry and the particle form factor have been changed as a result of the collective deformation process, leading to an appreciable tunability in the optical properties of the PC. Crystals

Encapsulation of colorants by natural polymers for food applications

Product appearance is an important factor for consumers when determining the quality of a product, and colour is one of the most important factors which contribute to product appearance. Currently, the safety and consumer acceptance of some colorants used in food products, such as titanium dioxide and some synthetic colorants, are under discussion. Therefore, new ways to use natural colorants as alternatives to these suspect colorants for future applications are being investigated. A promising method for increasing the applicability of the often sensitive natural colorants is the encapsulation of these colorants in colloidal particles by natural polymers such as carbohydrates, lipids and proteins. In recent years, micro- and nano-encapsulation have increasingly been used for various purposes concerning several food properties such as colour, flavour and micronutrient content. This technique results in improved stability for the often sensitive natural colorants and presents the possibility of entrapping water-insoluble colorants for improved use in an aqueous system. This paper reviews the main methods that are used for encapsulation by natural polymers, discusses the different types thereof that are used for encapsulation of colorants, and provides a short overview of natural colorants successfully encapsulated in these natural polymers

Motion of a massive microsphere bound to a spherical vesicle

We study the motion of a small solid particle (a few micrometers in size) attached to the membrane of a spherical giant lipid vesicle. By means of optical manipulation, the particle is first brought near the top of the vesicle, and released. We determine the friction experienced by the particle moving along the vesicle surface under the action of gravity. From experiments with latex and glass beads, we check that SOPC membranes are fluid at room temperature (static shear modulus u = 0) and estimate the shear viscosity of SOPC bilayers: nm = 3 * 10^(-6) surface poise

Motion of a massive particle attached to a spherical interface: statistical properties of the particle path

We have studied the motion of a Brownian particle on a spherical interface under gravity, with the aim of setting up a protocol to measure the friction (f) felt by such a particle in experimental conditions. Our analysis is based on the Schmoluchowski equation for particle motion. Essentially we derive a practical criterion to find f from the average particle path. Our statements are illustrated by a few experimental and numerical examples. Numerical paths are obtained by computer simulation and experimental paths are those of micrometre-sized latex or glass particles attached to spherical giant lipid (SOPC) vesicles. From experimental values of f, we estimate the surface shear viscosity of SOPC bilayers to be in the range 3-8×10^(-6) Poise

Influence of thickeners (microfibrillated cellulose, starch, xanthan gum) on rheological, tribological and sensory properties of low-fat mayonnaises

Microfibrillated cellulose (MFC) is obtained by high-shear treatment of cellulose. MFC is suitable for use as clean-label, low-calorie thickener in semi-solid foods such as mayonnaises due to its high water holding capacity. The aim of this study was to determine the effect of type and concentration of thickener on rheological, tribological and sensory properties of low-fat mayonnaises. Low-fat mayonnaises were prepared with four types of thickeners (MFC, chemically modified starch, native waxy corn starch, xanthan gum) at three concentrations. Higher biopolymer concentrations resulted in increased shear viscosities, G′ and G″, yield stress and enhanced lubrication (i.e. lower friction coefficients). Mayonnaises with modified starch and xanthan gum generally had higher shear viscosity and yield stress compared to mayonnaises with comparable concentrations of MFC and waxy corn starch. MFC-thickened mayonnaises had highest G’, G” and boundary friction coefficients. Sensory properties of mayonnaises were determined using the Rate-All-That-Apply (RATA) method (n = 80). Addition of xanthan gum induced high sliminess and pulpiness, and low melting, creaminess and smoothness. Sensory properties of mayonnaises with MFC were generally similar to those with modified and waxy corn starch, despite differences in appearance (increased yellowness and slightly lower glossiness). Multiple Factor Analysis revealed that more shear-thinning mayonnaises were perceived as slimy. Boundary friction was negatively correlated with stickiness, while friction at the start of the hydrodynamic regime was positively correlated with melting sensations. We conclude that microfibrillated cellulose can be used as a thickener in low-fat mayonnaise as an alternative to commercially used chemically modified starch without considerably affecting its sensory texture properties