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PubMedDate not supplied

CT of intrasellar colloid cyst.

Colloid cysts are congenital lesions considered to be derived from the primitive neuroepithelium and are most frequently located in the anterior third ventricle. In this paper a case of colloid cyst with a intra/suprasellar location is presented. Computed tomographic characteristics of this lesion were strongly consistent with the classic features of colloid cysts, particularly when interpreted retrospectively after surgical and histopathological data became available. The unusual occurrence of a colloid cyst in this location supports the theory that epithelial cysts related to sella turcica are derived not solely from Rathke cleft but also from neuroepithelium.

Adult↗
PubMed1996

Stereotactic microsurgical craniotomy for the treatment of third ventricular colloid cysts.

The treatment of colloid cysts remains controversial. This article reports on 18 patients with colloid cysts of the third ventricle who presented to the University of Michigan between January 1990 and June 1995. Two patients had conventional transcortical-transventricular approaches; the remaining 16 patients underwent a stereotactic modification of the transcortical approach, using a specially designed cylindrical retractor, for the resection of their colloid cysts. Minor morbidity and no mortality have been associated with this technique. There have been no instances of recurrence during the period of this report. The mean operative time for the two conventional craniotomies was 233 minutes, compared with 129 minutes for the stereotactic craniotomies. Our technique allows for a limited craniotomy and limited cortical disruption. It is not dependent on ventricular size. Localization of the colloid cyst is trivial because of modern stereotactic technology, and all standard microsurgical techniques can be used. The retractor is readily adaptable to any Leksell system. This technique shortens operative time, compared with conventional craniotomy, and allows for definitive treatment of the lesion.

Adolescent↗
PubMed2002

Imaging of abdominal infection using 99m Tc stannous fluoride colloid labelled leukocytes.

Radiolabelling of leukocytes using labelled phagocytosed technetium-99m (99mTc) colloidal radiopharmaceuticals has been reported as a method for imaging infection. This in vivo study compares the use of leukocytes labelled using 99mTc stannous fluoride colloid with leukocytes labelled using indium-111 (111In) oxinate. A total of 26 patients (10 male, 16 female; mean age 52 years, range 23-88 years) referred for the investigation of possible infection were studied using both leukocyte labelling methods simultaneously. Images were acquired 4h and 24h after re-injection of the labelled cells. The images were evaluated qualitatively by two nuclear medicine physicians. The results show a high degree of concordance between the techniques: 11 of the 28 images showed a focus of leukocyte accumulation with both techniques at 24h, and 13 out of 28 showed a normal appearance at 24h with both methods. In four cases the results were discordant; the 99mTc stannous fluoride colloid labelled leukocytes gave a false positive appearance at 24h in three patients and a false negative in one. In conclusion, colloid labelling of leukocytes offers a sensitive method for the detection of infective foci coupled with the high resolution imaging offered by 99mTc. It has the advantage over other in vitro labelling methods of being a simpler, non-labour-intensive procedure employing whole blood, and its use should be considered by departments that have limited facilities for in vitro leukocyte labelling.

Abdominal Abscess↗
PubMed2000

Surprisingly short-ranged interactions in highly charged colloidal suspensions.

The interaction potential between colloidal particles in a suspension has been the topic of much research recently. Digital video microscopy techniques have come into wide use, as this method yields direct information about the structure of such systems. However, two main problems have plagued researchers working with this technique. First, what one sees through a microscope is actually a projection of a three-dimensional sample onto a two-dimensional image plane. Second, in order to achieve long-range interactions between particles, the ionicity of the surrounding medium must be as low as possible. In order to address the first problem, researchers have created quasi-two-dimensional samples by confining the system between two glass plates. However, this geometry makes it difficult to control the ionicity, and it also makes the analysis more difficult since one is dealing with an anisotropic system for which established theories of colloidal interactions formulated for the bulk do not apply. We have developed techniques to effectively address each of these two problems. Our sample cell is large enough to allow direct contact of the suspension with ion exchange resin, and allows one to make bulk measurements of the structure. In addition, we have developed techniques to handle the projection effects. We have used these methods to measure the radial distribution function of dilute suspensions of highly charged unconfined polystyrene microspheres in a density matched mixture of H2O and D2O. We found that the interaction potential between the colloidal particles was much shorter ranged than would be expected based on the Derjaguin-Landau-Verwey-Overbeek theory of colloidal interactions.

Journal Article↗
PubMed2002

Colloids, polymers, and needles: demixing phase behavior.

We consider a ternary mixture of hard colloidal spheres, ideal polymer spheres, and rigid vanishingly thin needles, which model stretched polymers or colloidal rods. For this model, we develop a geometry-based density functional theory, apply it to bulk fluid phases, and predict demixing phase behavior. In the case of no polymer-needle interactions, two-phase coexistence between colloid-rich and colloid-poor phases is found. For hard needle-polymer interactions, we predict rich phase diagrams, exhibiting three-phase coexistence, and reentrant demixing behavior.

Journal Article↗
PubMed2002

Viscosity of bimodal and polydisperse colloidal suspensions.

We present a theoretical framework for the viscosity of bimodal and polydisperse colloidal suspensions. For colloidal dispersions both interparticle forces between pairs of particles and many-particle effects such as depletion forces can have a significant effect on rheology. As hydrodynamic interactions are also important for colloidal systems, a theoretical description that includes hydrodynamic and thermodynamic interactions is required. An integral equation theory for multicomponent systems accounts for the contribution of thermodynamic interactions to the viscosity of dispersions. Introduction of small particles into a system of larger particles causes depletion forces between the large particles that increase the viscosity, while replacing large particles with an equal volume fraction of small particles increases the free volume in the system and decreases the viscosity. The integral equations model both of these effects in concentrated suspensions and provide a microscopic interpretation of free volume changes as changes in radial distribution functions. For a bimodal mixture they predict a dependence of the viscosity on size ratio, composition, and total volume fraction. Polydispersity is modeled by a small number of components whose sizes and weights are chosen to match the moments of the size distribution. This theory predicts a reduction in viscosity due to polydispersity and explains conflicting experimental measurement of the viscosity of hard-sphere colloids. Existing theoretical approaches that neglect the multiparticle correlations, included through the integral equations, yield qualitatively incorrect results for the change in the viscosity relative to monodisperse systems.

Journal Article↗
PubMed2003

Simulation study of nonergodicity transitions: gelation in colloidal systems with short-range attractions.

Computer simulations were used to study the gel transition occurring in colloidal systems with short-range attractions. A colloid-polymer mixture was modeled and the results were compared with mode coupling theory (MCT) expectations and with the results for other systems (hard-spheres system and Lennard-Jones system). The self-intermediate scattering function and the mean squared displacement were used as the main dynamical quantities. Two different colloid packing fractions have been studied. For the lower packing fraction, alpha-scaling holds and the wave-vector analysis of the correlation function shows that gelation is a regular nonergodicity transition within MCT. The leading mechanism for the novel nonergodicity transition is identified as the bond formation caused by the short-range attraction. The time scale and diffusion coefficient also show qualitatively the expected behavior, although different exponents are found for the power-law divergences of these two quantities. The non-Gaussian parameter was also studied and a very large correction to Gaussian behavior was found. The system with higher colloid packing fraction shows indications of a nearby high-order singularity, causing alpha scaling to fail, but the general expectations for nonergodicity transitions still hold.

Journal Article↗
PubMed2003

Effect of the range of attractive interactions on crystallization, metastable phase transition, and percolation in colloidal dispersions.

The equilibrium as well as nonequilibrium phase behaviors of colloidal dispersions have been investigated using statistical-mechanical theories of fluids and solids in complement with the renormalization-group (RG) theory. It is shown that the osmotic second virial coefficient at the critical point of the fluid-fluid transition varies with the range of attractions and is sensitive to specific forms of the attractive potential in contrast to a common speculation that it remains practically constant. However, for colloids with short-ranged forces, the critical temperature of the fluid-fluid phase transition is well correlated with the range of attractions in good agreement with an earlier empirical correlation based on simulation results. A comparison of the relative positions of the fluid-fluid coexistence curve, freezing, melting, and percolation lines in the phase diagram indicates that the gelation in colloidal systems has significant effects on the equilibrium phase transitions and crystallization, especially when the attractions between colloidal particles are short ranged.

Journal Article↗
PubMed2005

Strain softening, yielding, and shear thinning in glassy colloidal suspensions.

A microscopic theory for the dependence on external strain, stress, and shear rate of the transient localization length, elastic modulus, alpha relaxation time, shear viscosity, and other dynamic properties of glassy colloidal suspensions is formulated and numerically applied. The approach is built on entropic barrier hopping as the elementary physical process. The concept of an ideal glass transition plays no role, and dynamical slowing down is a continuous, albeit precipitous, process with increasing colloid volume fraction. The relative roles of mechanically driven motion versus thermally activated barrier hopping and transport have been studied. Various scaling behaviors are found for the relaxation time and shear viscosity in both the controlled stress and shear rate mode of rheological experiments. Apparent power law and/or exponential dependences of the elastic modulus and perturbative and absolute yield stresses on colloid volume fraction are predicted. A nonmonotonic dependence of the absolute yield strain on volume fraction is also found. Qualitative and quantitative comparisons of calculations with experiments on high volume fraction glassy colloidal suspensions show encouraging agreement, and multiple testable predictions are made. The theory is generalizable to treat nonlinear rheological phenomena in other soft glassy complex fluids including depletion gels.

Journal Article↗
PubMed2005

Kinetics of ergodic-to-nonergodic transitions in charged colloidal suspensions: aging and gelation.

There are two types of isotropic disordered nonergodic states in colloidal suspensions: colloidal glasses and gels. In a recent paper [H. Tanaka, J. Meunier, and D. Bonn, Phys. Rev. E 69, 031404 (2004)], we discussed the static aspect of the differences and the similarities between the two. In this paper, we focus on the dynamic aspect. The kinetics of the liquid-glass transition is called "aging," while that of the sol-gel transition is called "gelation." The former is primarily governed by repulsive interactions between particles, while the latter is dominated by attractive interactions. Slowing down of the dynamics during aging reflects the increasing cooperativity required for the escape of a particle from the cage formed by the surrounding particles, while that during gelation reflects the increase in the size of particle clusters towards the percolation transition. Despite these clear differences in the origin of the slowing down of the kinetics between the two, it is not straightforward experimentally to distinguish them in a clear manner. For an understanding of the universal nature of ergodic-to-nonergodic transitions, it is of fundamental importance to elucidate the differences and the similarities in the kinetics between aging and gelation. We consider this problem, taking Laponite suspension as an explicit example. In particular, we focus on the two types of nonergodic states: (i) an attractive gel formed by van der Waals attractions for high ionic strengths and (ii) a repulsive Wigner glass stabilized by long-range Coulomb repulsions for low ionic strengths. We demonstrate that the aging of colloidal Wigner glass crucially differs not only from gelation, but also from the aging of structural and spin glasses. The aging of the colloidal Wigner glass is characterized by the unique cage-forming regime that does not exist in the aging of spin and structural glasses.

Journal Article↗
PubMed2005

Wall-fluid and liquid-gas interfaces of model colloid-polymer mixtures by simulation and theory.

We perform a study of the interfacial properties of a model suspension of hard sphere colloids with diameter sigma(c) and nonadsorbing ideal polymer coils with diameter sigma(p) . For the mixture in contact with a planar hard wall, we obtain from simulations the wall-fluid interfacial free energy, gamma(wf) , for size ratios q =sigma(p)/sigma(c) =0.6 and 1, using thermodynamic integration, and study the (excess) adsorption of colloids, Gamma(c) , and of polymers, Gamma(p) , at the hard wall. The interfacial tension of the free liquid-gas interface, gamma(lg) , is obtained following three different routes in simulations: (i) from studying the system size dependence of the interfacial width according to the predictions of capillary wave theory, (ii) from the probability distribution of the colloid density at coexistence in the grand canonical ensemble, and (iii) for state points where the colloidal liquid wets the wall completely, from Young's equation relating gamma(lg) to the difference of wall-liquid and wall-gas interfacial tensions, gamma(wl)-gamma(wg) . In addition, we calculate gamma(wf) ,Gamma(c) , and Gamma(p) using density functional theory and a scaled particle theory based on free volume theory. Good agreement is found between the simulation results and those from density functional theory, while the results from scaled particle theory quantitatively deviate but reproduce some essential features. Simulation results for gamma(lg) obtained from the three different routes are all in good agreement. Density functional theory predicts gamma(lg) with good accuracy for high polymer reservoir packing fractions, but yields deviations from the simulation results close to the critical point.

Journal Article↗
PubMed2005

Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and nonideal polymers.

An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and nonadsorbing polymers is studied with grand canonical Monte Carlo simulations and density functional theory. Polymer nonideality is taken into account through a repulsive step-function pair potential between polymers. Simulation results validate previous theoretical findings for the shift of the bulk fluid demixing binodal upon increasing strength of polymer-polymer repulsion, indicating suppression of phase separation. For increasing strength of the polymer-polymer repulsion, simulation and theory consistently predict the interfacial tension of the free interface between the colloidal liquid and the colloidal gas phase to decrease significantly for fixed colloid density difference in the coexisting phases, and to increase for fixed polymer reservoir packing fraction.

Journal Article↗
PubMed2005

Lattice density functional for colloid-polymer mixtures: comparison of two fundamental measure theories.

We consider a binary mixture of colloid and polymer particles with positions on a simple cubic lattice. Colloids exclude both colloids and polymers from nearest neighbor sites. Polymers are treated as effective particles that are mutually noninteracting, but exclude colloids from neighboring sites; this is a discrete version of the (continuum) Asakura-Oosawa-Vrij model. Two alternative density functionals are proposed and compared in detail. The first is based on multioccupancy in the zero-dimensional limit of the bare model, analogous to the corresponding continuum theory that reproduces the bulk fluid free energy of free volume theory. The second is based on mapping the polymers onto a multicomponent mixture of polymer clusters that are shown to behave as hard cores; the corresponding property of the extended model in strong confinement permits direct treatment with lattice fundamental measure theory. Both theories predict the same topology for the phase diagram with a continuous fluid-fcc freezing transition at low polymer fugacity and, upon crossing a tricritical point, a first-order freezing transition for high polymer fugacities with rapidly broadening density jump.

Journal Article↗
PubMed2005

Pinning and dynamics of colloids on one-dimensional periodic potentials.

Using numerical simulations we study the pinning and dynamics of interacting colloids on periodic one-dimensional substrates. As a function of colloid density, temperature, and substrate strength, we find a variety of pinned and dynamic states including a locked smectic, pinned buckled, two-phase flow, and moving partially ordered structures. We show that for increasing colloid density, peaks in the depinning threshold occur at commensurate states. The scaling of the pinning threshold versus substrate strength changes when the colloids undergo a transition from one-dimensional chains to a buckled configuration.

Journal Article↗
PubMed2006

Fluid-mechanical and electrical fluctuation forces in colloids.

Fluctuations in fluid velocity and fluctuations in electric fields may both give rise to forces acting on small particles in colloidal suspensions. Such forces in part determine the thermodynamic stability of the colloid. At the classical statistical thermodynamic level, the fluid velocity and electric field contributions to the forces are comparable in magnitude. When quantum fluctuation effects are taken into account, the electric-fluctuation-induced van der Waals forces dominate those induced by purely fluid-mechanical motions. The physical principles are applied in detail for the case of colloidal particle attraction to the walls of the suspension container and more briefly for the case of forces between colloidal particles.

Journal Article↗
PubMed2006

Critical behavior of a colloid-polymer mixture confined between walls.

We investigate the influence of confinement on phase separation in colloid-polymer mixtures. To describe the particle interactions, the colloid-polymer model of Asakura and Oosawa [J. Chem. Phys. 22, 1255 (1954)] is used. Grand canonical Monte Carlo simulations are then applied to this model confined between two parallel hard walls, separated by a distance D = 5 colloid diameters. We focus on the critical regime of the phase separation and look for signs of crossover from three-dimensional (3D) Ising to two-dimensional (2D) Ising universality. To extract the critical behavior, finite size scaling techniques are used, including the recently proposed algorithm of Kim et al [Phys. Rev. Lett. 91, 065701 (2003)]. Our results point to "effective" critical exponents that differ profoundly from 3D Ising values, and that are already very close to 2D Ising values. In particular, we observe that the critical exponent of the order parameter in the confined system is smaller than in 3D bulk, yielding a "flatter" binodal. Our results also show an increase in the critical colloid packing fraction in the confined system with respect to the bulk. The latter seems consistent with theoretical expectations, although subtleties due to singularities in the critical behavior of the coexistence diameter cannot be ruled out.

Journal Article↗
PubMed2000

Simulation method of colloidal suspensions with hydrodynamic interactions: fluid particle dynamics

We develop a new simulation method of colloidal suspensions, which we call a "fluid particle dynamics" (FPD) method. This FPD method, which treats a colloid as a fluid particle, removes the difficulties stemming from a solid-fluid boundary condition in the treatment of hydrodynamic interactions between the particles. The importance of interparticle hydrodynamic interactions in the aggregation process of colloidal particles is demonstrated as an example. This method can be applied to a wide range of problems in colloidal science.

Journal Article↗
PubMed1991

Albumin, HES 120 and dextran 70 as adjuvants to red blood cell concentrates: a study on colloid osmotic pressure changes in vitro.

An in vitro model of surgical bleeding was developed to simulate continuous blood loss and replacement therapy with plasma substitutes and red cell concentrates. The model was used to determine the lowest colloid concentration in vitro for each of the plasma substitutes that sustains colloid osmotic pressure above 2.4 kPa (or 18 mmHg) when used up to the recommended maximal total dose. Plasma, supernatant separated from red cell concentrates and dextran 70, hydroxyethyl starch 120 or albumin were mixed to create dilutions imitating plasma composition in the course of clinical blood loss and replacement therapy. The relative volume of each component was calculated according to the model when the bleeding was equal to multiples of 10% of blood volume up to a blood loss of 120%. Our measurements indicate that the colloid concentrations of 5.0% for albumin, 4.0% for hydroxyethyl starch 120 and 3.5% for dextran 70 preserve colloid osmotic pressure above 2.4 kPa.

Albumins↗