Search results for "Spheres"
showing 10 items of 329 documents
Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition
2007
We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to…
Polymer-enforced crystallization of a eutectic binary hard sphere mixture
2010
We prepared a buoyancy matched binary mixture of polydisperse polystyrene microgel spheres of size ratio 0.785 and at a volume fraction of 0.567 just below the kinetic glass transition. In line with theoretical expectations, a eutectic phase behavior was observed, but only a minor fraction of the samples crystallized at all. By adding a short non-adsorbing polymer we enforce inter-species fractionation into coexisting pure component crystals, which in turn also shows signs of intra-species fractionation. We show that in formerly inaccessible regions of the phase diagram binary hard sphere physics is made observable using attractive hard spheres. Ancillary files: Correction to Soft Matter 20…
Crystallization of silica opals onto patterned silicon wafer
2006
We report on fabrication of high quality opaline photonic crystals from large silica spheres, self-assembled in hydrophilic trenches of silicon wafers by using a drawing apparatus with a combination of stirring. The achievements here reported comprise a spatial selectivity of opal crystallization without special treatment of the wafer surface, a filling of the trenches up to the top, leading to a spatially uniform film thickness, particularly an absence of cracks within the size of the trenches, and finally a good three-dimensional order of the opal lattice even in trenches with a complex confined geometry, verified using optical measurements. The opal lattice was found to match the pattern…
Mode expansion for the density profiles of crystal–fluid interfaces: hard spheres as a test case
2012
We present a technique for analyzing the full three-dimensional density profiles of a planar crystal-fluid interface in terms of density modes. These density modes can also be related to crystallinity order parameter profiles which are used in coarse-grained, phase field type models of the statics and dynamics of crystal-fluid interfaces and are an alternative to crystallinity order parameters extracted from simulations using local crystallinity criteria. We illustrate our results for the hard sphere system using finely-resolved, three-dimensional density profiles from density functional theory of fundamental measure type.
Response charactterization of ammonium tartrate solid state pellets for ESR dosimetry with radiotherapeutic photon and electron beams.
2001
Solid state pellets (1 mm thick) for electron spin resonance (ESR) dosimetry were made using ammonium tartrate as the radiation-sensitive substance. Their behaviour was experimentally investigated as a function of dose with 60Co gamma rays. The calibration function obtained permits measurements of absorbed dose in the 2-50 Gy range, with a combined uncertainty of +/-4%. The lowest detectable dose was about 0.5 Gy. These properties are comparable with or even better than those of ESR dosimeters made from other materials. The time stability of the ESR signal of ammonium tartrate dosimeters at different storage conditions after irradiation was studied. A rather complex behaviour was observed, …
Spherical microgel colloids - Hard spheres from soft matter
1998
While gels are usually considered to be soft materials, we demonstrate that it is possible to model hard sphere behaviour when the gel structure is confined to spherical objects of sizes in the colloidal range. We have measured the static structure factor of microgel spheres dispersed in good solvents, differing in size, crosslink density and swelling behaviour, by light scattering and small angle neutron scattering. Comparing with theoretical calculations for polydisperse hard spheres we show how the interactions in highly concentrated dispersions of spherical microgel colloids are determined by an interplay of the relative length of dangling polymer ends at the surface (determined by the …
A neutron tomography study: Probing the spontaneous crystallization of randomly packed granular assemblies
2018
We study the spontaneous crystallization of an assembly of highly monodisperse steel spheres under shaking, as it evolves from localized icosahedral ordering towards a packing reaching crystalline ordering. Towards this end, real space neutron tomography measurements on the granular assembly are carried out, as it is systematically subjected to a variation of frequency and amplitude. As expected, we see a presence of localized icosahedral ordering in the disordered initial state (packing fraction around 0.62). As the frequency is increased for both the shaking amplitudes (0.2 and 0.6 mm) studied here, there is a rise in packing fraction, accompanied by an evolution to crystallinity. The ext…
Stacking faults in close-packed clusters
2002
Ground state geometries of small hard sphere clusters were studied using two different type of contact interaction, a pair-potential and a many-atom interaction. Monte Carlo method in an FCC lattice with all possible (111) stacking faults was used to obtain the minimum energy geometries for clusters up to 59 atoms. Due to the surface energy, FCC packing is generally favoured as opposite to the HCP structure. However, in most cluster sizes the ground state obtained with the many-atom interaction has one or more stacking faults. The most symmetric geometry is usually not the ground state. Clusters with 59 and 100 atoms were studied due the possibility of a high symmetry cluster with stacking …
Microsphere tensiometry to measure advancing and receding contact angles on individual particles
1999
In this paper, a method to measure the advancing and receding contact angles on individual colloidal spheres is described. For this purpose, the microspheres were attached to atomic force microscope cantilevers. Then the distance to which the microsphere jumps into its equilibrium position at the air-liquid interface of a drop or an air bubble was measured. From these distances the contact angles were calculated. To test the method, experiments were done with silanized silica spheres (4.1 μm in diameter). From the experiments with drops, an advancing contact angle of 101 ± 4° was determined. A receding contact angle of 101 ± 2° was calculated from the jump-in distance into a bubble. Both ex…