Biomedical subjects

A B Schofield

Publications and source records attributed to A B Schofield.

9 recordsLinked to original sources

PubMed2007

Hypersonic acoustic excitations in binary colloidal crystals: big versus small hard sphere control.

The phononic band structure of two binary colloidal crystals, at hypersonic frequencies, is studied by means of Brillouin light scattering and analyzed in conjunction with corresponding dispersion diagrams of the single colloidal crystals of the constituent particles. Besides the acoustic band of the average medium, the authors' results show the existence of narrow bands originating from resonant multipole modes of the individual particles as well as Bragg-type modes due to the (short-range) periodicity. Strong interaction, leading to the occurrence of hybridization gaps, is observed between the acoustic band and the band of quadrupole modes of the particles that occupy the largest fractional volume of the mixed crystal; the effective radius is either that of the large (in the symmetric NaCl-type crystalline phase) or the small (in the asymmetric NaZn(13)-type crystalline phase) particles. The possibility to reveal a universal behavior of the phononic band structure for different single and binary colloidal crystalline suspensions, by representing in the dispersion diagrams reduced quantities using an appropriate length scale, is discussed.

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PubMed2006

Stable jets of viscoelastic fluids and self-assembled cylindrical capsules by hydrodynamic focusing.

We demonstrate formation of long-lived cylindrical jets of a viscoelastic fluid using hydrodynamic focusing. A solution of polyacrylamide in water is driven coaxially with immiscible oil and subjected to strong extensional flow. At high flow rates, the aqueous phase forms jets that are 4-90 microm in diameter and several centimeters long. The liquid surfaces of these jets are then used as templates for assembly of microspheres into novel rigid and hollow cylinders.

Acrylic Resins↗

PubMed2005

Stability of the binary colloidal crystals AB2 and AB13.

Suspensions of binary mixtures of hard-sphere poly-methylmethacrylate colloidal particles were studied at six different size ratios alpha. The main aim was to determine the range of size ratios over which the binary colloidal crystals AB2 and AB13 are stable. Combining these results with those of earlier work, we found stability of AB2 for 0.60 approximately > alpha approximately > 0.425, in good agreement with theoretical predictions by computer simulation and cell model methods. AB13 was observed for 0.62 approximately > alpha approximately > 0.485, the lower limit being significantly smaller than the theoretical prediction of about 0.525. Rough measurements of crystallization rates showed that AB2 tended to crystallize fastest at small size ratios, whereas the opposite was true for AB13. These findings should provide a guide to the optimum conditions for materials applications of these binary colloidal crystals.

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PubMed2004

Phonons in suspensions of hard sphere colloids: volume fraction dependence.

The propagation of sound waves in suspensions of hard sphere colloids is studied as a function of their volume fraction up to random close packing using Brillouin light scattering. The rich experimental phonon spectra of up to five phonon modes are successfully described by theoretical calculations based on the multiple scattering method. Two main types of phonon modes are revealed: Type A modes are acoustic excitations which set up deformations in both the solid (particles) and the liquid (solvent) phases; for type B modes the stress and strain are predominantly localized near the interface between the solid particles and the surrounding liquid (interface waves). While the former become harder (increase their effective sound velocity) as the particle volume fraction increases the latter become softer (the corresponding sound velocity decreases).

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PubMed2002

Structural aging of crystals of hard-sphere colloids.

We report a detailed experimental study of the aging of the (initial) random hexagonal close-packed (rhcp) crystals formed in suspensions of hard-sphere colloids near the melting point. By suspending the same colloidal particles in two different mixtures of solvents we are able to tune the strength of the gravitational forces acting on the particles. The crystal structure is deduced from diffraction patterns measured by the light scattering equivalent of powder x-ray crystallography. A spontaneous aging of the structure is observed over long periods of time, consisting of a fraction of pure face-centered cubic (fcc) crystals growing at the expense of the randomly stacked crystallites. The rate of growth of the new crystals is small and consistent with the predictions by Pronk and Frenkel [13]. Gravity is also revealed to affect the crystals and favor fcc order but through a slow gradual rearrangement of the random stacking. An important new observation is that small mechanical perturbations can strongly affect the structure of the colloidal crystals, promoting fcc growth and interfering with the spontaneous aging process. Previous experimental results are also discussed in the light of these new findings.

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PubMed2002

Multiple glassy states in a simple model system.

Experiments, theory, and simulation were used to study glass formation in a simple model system composed of hard spheres with short-range attraction ("sticky hard spheres"). The experiments, using well-characterized colloids, revealed a reentrant glass transition line. Mode-coupling theory calculations and molecular dynamics simulations suggest that the reentrance is due to the existence of two qualitatively different glassy states: one dominated by repulsion (with structural arrest due to caging) and the other by attraction (with structural arrest due to bonding). This picture is consistent with a study of the particle dynamics in the colloid using dynamic light scattering.

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PubMed2001

Binary hard-sphere crystals with the cesium chloride structure.

The possibility of binary hard-sphere colloids crystallizing with the cesium chloride (CsCl) structure was examined experimentally using poly (methyl methacrylate) particles dispersed in organic solvents. Towards this end, two dispersions were prepared that contained particles with a radius ratio alpha=R(B)/R(A), where A refers to the large particles and B the small, of 0.736. This is close to the optimum ratio of 0.732 at which this structure is predicted to form as determined by space-filling calculations. The phases found within the binary mixture were examined using laser light crystallography and scanning electromicroscopy, and some were shown to have the CsCl structure. Over a period of time some of the CsCl crystals disappeared indicating that they were metastable and that this structure may not be the most enduring phase at this size ratio.

Journal Article↗

PubMed2001

Glasslike kinetic arrest at the colloidal-gelation transition.

We show that gelation of weakly attractive colloids is remarkably similar to the colloidal glass transition. Like the glass transition, dynamic light scattering functions near gelation scale with scattering vector, and exhibits a two-step decay with a power-law divergence of the final decay time. Like the glass transition, static light scattering does not change upon gelation. These results suggest that, like the glass transition, gelation results from kinetic arrest due to crowding of clusters, and that both gelation and the glass transition are manifestations of a more general jamming transition.

Journal Article↗

PubMed2001

Phase separation in star-polymer-colloid mixtures.

We examine the demixing transition in star-polymer-colloid mixtures for star arm numbers f=2,6,16,32 and different star-polymer-colloid size ratios 0.18< or =q< or =0.50. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecular computer simulations. The numerical results show a spinodal instability. The demixing binodals are approximately calculated and found to be consistent with experimental observations.

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