1,721,738 research outputs found
Encapsulation of emulsion droplets by organo–silica shells
No full textSurfactant-stabilized emulsion droplets were used as templates for the synthesis of hollow colloidal particles. Monodisperse silicone oil droplets were prepared by hydrolysis and polymerization of dimethyldiethoxysiloxane monomer, in the presence of surfactant: sodium dodecyl sulphate (SDS, anionic) or Triton X-100 (non-ionic). A sharp decrease in the average droplet radius with increasing surfactant concentration was found, with a linear dependence of the droplet radius on the logarithm of the surfactant concentration. The surfactant-stabilized oil droplets were then encapsulated with a solid shell using tetraethoxysilane, and hollow particles were obtained by exchange of the liquid core. The size and polydispersity of the oil droplets and the thickness of the shell were determined using static light scattering, and hollow particles were characterized by electron microscopy. Details on the composition of the shell material were obtained from energy-dispersive X-ray analysis. In the case of sodium dodecyl sulphate, the resulting shells were relatively thin and rough, while when Triton X-100 was used, smooth shells were obtained which could be varied in thickness from very thick (≈150 nm) to very thin shells (≈17 nm). Finally, hexane droplets were encapsulated using the same procedure, showing that our method can in principle be extended to a wide range of emulsions
Quantitative real-space analysis of colloidal supraparticles self-assembled from monodisperse nanoparticles
No full textDefects and Diffusion in Colloidal Crystals
No full textColloids are microscopic particles whose size ranges from a nanometer to several micrometers, that are dispersed in a solvent. One striking characteristic of colloidal systems is that their constituent particles are in a constant state of random motion due to incessant collisions with the fast-moving molecules of the solvent. This allows colloids to spontaneously explore all space available to them, and self-assemble, i.e. organize themselves, into a wide range of phases, such as gases, liquids, crystals, and even quasicrystals - just like atoms and molecules. Hence, to some extent colloids behave like “big atoms”. In this thesis we focus on crystalline phases formed by colloids, often referred to as “colloidal crystals”, and in particular on their crystallographic defects. Even when the concentrations of these defects are low, they can significantly alter the mechanical and transport properties of the crystal. For example, even though particles in a crystal phase are typically orders of magnitude less mobile than those in a fluid, diffusion is still possible via the diffusion of defects. Therefore, understanding the motion of these defects is key to understanding transport processes in crystals. We examine crystal defects in a wide variety of systems, ranging from the archetypical model system of hard spheres to hard cubes, to mixtures of active and passive particles. The chapters in this thesis are organized as follows. We start off in Chapter 2 by looking into the diffusion and interactions of the simplest of defects, namely vacancies and interstitials, in arguably the simplest model system: monodisperse hard spheres. In Chapters 3 and 4 we study binary mixtures of hard spheres. Specifically, in Chapter 3, we investigate the diffusion and interactions of small interstitial particles in a hard-sphere interstitial solid solution. We show how transition state theory can be used to accurately predict diffusion rates in these systems. In Chapter 4, we investigate the role of defects in the colloidal Laves phase. We find a high equilibrium concentration of antisite defects to be present in the Laves phase, thus shedding new light on its self-assembly. In Chapter 5, we study vacancies in a variety of repulsive systems forming simple cubic crystals. In particular, we show that for all these systems the vacancies are “delocalized” along a row of particles, suggesting this to be an inherent feature of simple cubic crystals of repulsive particles. In Chapters 6-8, we study mixtures of passive and active colloids. In Chapters 6 and 7, we show that active particles can provide an elegant new route to removing grain boundaries in polycrystals, in two and three dimensions, respectively. Chapter 8 focusses on a fundamental question: is it possible to predict quantitatively whether two phases of active particles coexist? We show that for a torque-free active system the phase diagram can be predicted by measuring the pressure and a chemical potential-like variable
Large dispersive effects near the band edges of photonic crystals
Full text linkWe have used phase-sensitive ultrashort-pulse interferometry to study the modification of optical pulse propagation near the photonic band edges in colloidal crystals consisting of polystyrene spheres in water. A strong suppression of the group velocity is found at frequencies near the L gap of the fcc lattice. The group velocity dispersion diverges at the band edges and shows branches of both normal dispersion and anomalous dispersion, which can be interpreted as large changes in the effective mass, both positive and negative. We obtain excellent agreement with the dynamical diffraction theory
Small asymmetric Brownian objects self-align in nanofluidic channels
Full text linkAlthough the self-alignment of asymmetric macro-sized objects of a few tens of microns in size have been studied extensively in experiments and theory, access to much smaller length scales is still hindered by technical challenges. We combine molecular dynamics and stochastic rotation dynamics techniques to investigate the self-orientation phenomenon at different length scales, ranging from the micron to the nano scale by progressively increasing the relative strength of diffusion over convection. To this end, we model an asymmetric dumbbell particle in Hele-Shaw flow and explore a wide range of Péclet numbers (Pe) and different particle shapes, as characterized by the size ratio of the two dumbbell spheres (R). By independently varying these two parameters we analyse the process of self-orientation and characterize the alignment of the dumbbell with the direction of the fluid flow. We identify three different regimes of strong, weak and no alignment and we map out a state diagram in Pe versus R plane. Based on these results, we estimate dimensional length scales and flow rates for which these findings would be applicable in experiments. Finally, we find that the characteristic reorientation time of the dumbbell is a monotonically decreasing function of the dumbbell anisotropy.Accepted Author ManuscriptComplex Fluid Processin
Direct Observation of the Formation of Liquid Protrusions on Polymer Colloids and their Coalescence
No full textMonodisperse nonspherical poly (methyl methacrylate) (PMMA) particles where a central core particle had grown two extra “lobes”, or protrusions, placed opposite each other were successfully synthesized by swelling and subsequent polymerization of cross-linked PMMA spheres with methyl methacrylate and the cross-linker ethylene glycol dimethacrylate. The use of large (3 μm) seed particles allowed for real-time monitoring of the swelling and deswelling of the cross-linked particles with optical microscopy. First, a large number of small droplets of swelling monomers formed simultaneously on the surface of the seed particles, and then fused together until under certain conditions two protrusions remained on opposite sides of the seed particles. The yield of such particles could be made up to 90% with a polydispersity of 7.0%. Stirring accelerated the transfer of the swelling monomers to the seed particles. Stirring was also found to induce self-assembly of the swollen seed particles into a wide variety of n-mers, consisting of a certain number, n, of swollen seed particles. The formation of these structures is guided by the minimization of the interfacial free energy between the seed particles, liquid protrusions and aqueous phase, but stirring time and geometrical factors influence it as well. By inducing polymerization the structures could be made permanent. Some control over the topology as well as overall size of the clusters was achieved by varying the stirring time before polymerization. 3D models of possible particle structures were used to identify all projections of the structures obtained by scanning electron microscopy. These models also revealed that the seed particles inside the central coalesced body were slightly compressed after polymerization. By extending the synthesis of the monodisperse particles with n = 1 to (slightly) different monomers and/or different cores, an important class of patchy particles could be realized
Quasicrystals from nanocrystals
No full textQuasicrystals have a host of unusual physical properties. These intermediates between amorphous solids and regular crystalline materials can now be made to self-assemble from nanoparticles
Synthesis of Monodisperse, Rodlike Silica Colloids with Tunable Aspect Ratio
No full textAlthough the experimental study of spherical colloids has been extensive, similar studies on rodlike particles are rare because suitable model systems are scarcely available. To fulfill this need, we present the synthesis of monodisperse rodlike silica colloids with tunable dimensions. Rods were produced with diameters of 200 nm and greater and lengths up to 10 μm, resulting in aspect ratios from 1 to ∼25. The growth mechanism of these rods involves emulsion droplets inside which silica condensation takes place. Due to an anisotropic supply of reactants, the nucleus grows to one side only, resulting in rod formation. In concentrated dispersions, these rods self-assemble in liquid crystal phases, which can be studied quantitatively on the single particle level in three-dimensional real-space using confocal microscopy. Isotropic, paranematic, and smectic phases were observed for this system
Phase separation and self-assembly in a fluid of Mickey Mouse particles
No full textRecent developments in the synthesis of colloidal particles allow for control over shape and inter-particle interaction. One example, among others, is the so-called “Mickey Mouse” (MM) particle for which the self-assembly properties have been previously studied yielding a stable cluster phase together with elongated, tube-like structures. Here, we investigate under which conditions a fluid of Mickey Mouse particles can yield phase separation and how the self-assembly behaviour affects the gas–liquid coexistence. We vary the distance between the repulsive and the attractive lobes (bond length), and the interaction range, and follow the evolution of the gas–liquid (GL) coexistence curve. We find that upon increasing the bond length distance the binodal line shifts to lower temperatures, and that the interaction range controls the transition between phase separation and self-assembly of clusters. Upon further reduction of the interaction range and temperature, the clusters assume an increasingly ordered tube-like shape, ultimately matching the one previously reported in literature. These results are of interest when designing particle shape and particle–particle interaction for self-assembly processes
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