Biomedical subjects

A J Archer

Publications and source records attributed to A J Archer.

12 recordsLinked to original sources

PubMed2007

Model colloidal fluid with competing interactions: bulk and interfacial properties.

Using a simple mean field density functional theory (DFT), the authors investigate the structure and phase behavior of a model colloidal fluid composed of particles interacting via a pair potential which has a hard core of diameter sigma, is attractive Yukawa at intermediate separations, and is repulsive Yukawa at large separations. The authors analyze the form of the asymptotic decay of the bulk fluid correlation functions, comparing results from DFT with those from the self-consistent Ornstein-Zernike approximation (SCOZA). In both theories the authors find rich crossover behavior, whereby the ultimate decay of correlation functions changes from monotonic to long wavelength damped oscillatory decay on crossing certain lines in the phase diagram or sometimes from oscillatory to oscillatory with a longer wavelength. For some choices of potential parameters the authors find, within the DFT, a lambda line at which the fluid becomes unstable with respect to periodic density fluctuations. SCOZA fails to yield solutions for state points near such a lambda line. The propensity towards clustering of particles, which is reflected by the presence of a long wavelength (>>sigma) slowly decaying oscillatory pair correlation function, and a structure factor that exhibits a very sharp maximum at small but nonzero wave numbers, is enhanced in states near the lambda line. The authors present density profiles for the planar liquid-gas interface and for fluids adsorbed at a planar hard wall. The presence of a nearby lambda transition gives rise to pronounced long wavelength oscillations in the one-body density profiles at both types of interface.

Journal Article↗

PubMed2006

Screening in Yukawa fluid mixtures.

The effective pair potential between mesoscopic charged particles in a neutralizing background medium takes a Yukawa form exp(-lambda r)/r with screening length lambda(-1). We consider a dilute suspension of such Yukawa particles dispersed in a solvent with correlation length xi<lambda(-1) and show that the Yukawa interaction is screened if the pair potentials between solvent particles exhibit Yukawa decay with the same screening length lambda(-1). However, if the solvent pair potentials are shorter ranged than the solute Yukawa potentials, then the effective potential between pairs of solute particles is unscreened, i.e., the effective potential between the solute particles is equal to the bare potential at large particle separations.

Journal Article↗

PubMed2006

Structure, phase behavior, and inhomogeneous fluid properties of binary dendrimer mixtures.

The effective pair potentials between different kinds of dendrimers in solution can be well approximated by appropriate Gaussian functions. We find that in binary dendrimer mixtures the range and strength of the effective interactions depend strongly upon the specific dendrimer architecture. We consider two different types of dendrimer mixtures, employing the Gaussian effective pair potentials, to determine the bulk fluid structure and phase behavior. Using a simple mean field density functional theory (DFT) we find good agreement between theory and simulation results for the bulk fluid structure. Depending on the mixture, we find bulk fluid-fluid phase separation (macrophase separation) or microphase separation, i.e., a transition to a state characterized by undamped periodic concentration fluctuations. We also determine the inhomogeneous fluid structure for confinement in spherical cavities. Again, we find good agreement between the DFT and simulation results. For the dendrimer mixture exhibiting microphase separation, we observe a rather striking pattern formation under confinement.

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PubMed2006

Pair-correlation functions and phase separation in a two-component point Yukawa fluid.

We investigate the structure of a binary mixture of particles interacting via purely repulsive point Yukawa pair potentials with a common inverse screening length lambda. Using the hypernetted chain closure to the Ornstein-Zernike equations, we find that for a system with "ideal" (Berthelot mixing rule) pair-potential parameters for the interaction between unlike species, the asymptotic decay of the total correlation functions crosses over from monotonic to damped oscillatory on increasing the fluid total density at fixed composition. This gives rise to a Kirkwood line in the phase diagram. We also consider a "nonideal" system, in which the Berthelot mixing rule is multiplied by a factor (1+delta). For any delta>0 the system exhibits fluid-fluid phase separation and remarkably the ultimate decay of the correlation functions is now monotonic for all (mixture) state points. Only in the limit of vanishing concentration of either species does one find oscillatory decay extending to r=infinity. In the nonideal case the simple random-phase approximation provides a good description of the phase separation and the accompanying Lifshitz line.

Journal Article↗

PubMed2006

Soft core fluid in a quenched matrix of soft core particles: a mobile mixture in a model gel.

We present a density-functional study of a binary phase-separating mixture of soft core particles immersed in a random matrix of quenched soft core particles of larger size. This is a model for a binary polymer mixture immersed in a cross-linked rigid polymer network. Using the replica "trick" for quenched-annealed mixtures we derive an explicit density functional theory that treats the quenched species on the level of its one-body density distribution. The relation to a set of effective external potentials acting on the annealed components is discussed. We relate matrix-induced condensation in bulk to the behavior of the mixture around a single large particle. The interfacial properties of the binary mixture at a surface of the quenched matrix display a rich interplay between capillary condensation inside the bulk matrix and wetting phenomena at the matrix surface.

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PubMed2005

Density functional theory for the freezing of soft-core fluids.

We present a simple density functional theory for the solid phases of systems of particles interacting via soft-core potentials. In particular, we apply the theory to particles interacting via repulsive point Yukawa and Gaussian pair potentials. We find qualitative agreement with the established phase diagrams for these systems. The theory is able to account for the bcc-fcc solid transitions of both systems and the re-entrant melting that the Gaussian system exhibits.

Journal Article↗

PubMed2005

Asymptotic decay of pair correlations in a Yukawa fluid.

We analyze the r-->infinity asymptotic decay of the total correlation function h (r) for a fluid composed of particles interacting via a (point) Yukawa pair potential. Such a potential provides a simple model for dusty plasmas. The asymptotic decay is determined by the poles of the liquid structure factor in the complex plane. We use the hypernetted-chain closure to the Ornstein-Zernike equation to determine the line in the phase diagram, well removed from the freezing transition line, where crossover occurs in the ultimate decay of h (r) , from monotonic to damped oscillatory. We show that (i) crossover takes place via the same mechanism (coalescence of imaginary poles) as in the classical one-component plasma and in other models of Coulomb fluids and (ii) leading-order pole contributions provide an accurate description of h (r) at intermediate distances r as well as at long range.

Journal Article↗

PubMed2005

Solvent mediated interactions close to fluid-fluid phase separation: microscopic treatment of bridging in a soft-core fluid.

Using density functional theory we calculate the density profiles of a binary solvent adsorbed around a pair of big solute particles. All species interact via repulsive Gaussian potentials. The solvent exhibits fluid-fluid phase separation, and for thermodynamic states near to coexistence the big particles can be surrounded by a thick adsorbed "wetting" film of the coexisting solvent phase. On reducing the separation between the two big particles we find there can be a "bridging" transition as the wetting films join to form a fluid bridge. The effective (solvent mediated) potential between the two big particles becomes long ranged and strongly attractive in the bridged configuration. Within our mean-field treatment the bridging transition results in a discontinuity in the solvent mediated force. We demonstrate that accounting for the phenomenon of bridging requires the presence of a nonzero bridge function in the correlations between the solute particles when our model fluid is described within a full mixture theory based upon the Ornstein-Zernike equations.

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PubMed2004

Dynamical density functional theory and its application to spinodal decomposition.

We present an alternative derivation of the dynamical density functional theory for the one-body density profile of a classical fluid developed by Marconi and Tarazona [J. Chem. Phys. 110, 8032 (1999)]. Our derivation elucidates further some of the physical assumptions inherent in the theory and shows that it is not restricted to fluids composed of particles interacting solely via pair potentials; rather it applies to general, multibody interactions. The starting point for our derivation is the Smoluchowski equation and the theory is therefore one for Brownian particles and as such is applicable to colloidal fluids. In the second part of this paper we use the dynamical density functional theory to derive a theory for spinodal decomposition that is applicable at both early and intermediate times. For early stages of spinodal decomposition our nonlinear theory is equivalent to the (generalized) linear Cahn-Hilliard theory, but for later times it incorporates coupling between different Fourier components of the density fluctuations (modes) and therefore goes beyond Cahn-Hilliard theory. We describe the results of calculations for a model (Yukawa) fluid which show that the coupling leads to the growth of a second maximum in the density fluctuations, at a wave number larger than that of the main peak.

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PubMed2003

Silica accelerated systemic autoimmune disease in lupus-prone New Zealand mixed mice.

The genetic backgrounds of lupus-prone murine models are a valuable resource for studying the influence of environmental exposure on autoimmune diseases in sensitive populations. Epidemiological studies have shown associations between silica exposure and several autoimmune diseases, including scleroderma and systemic lupus erythematosus. To determine whether silica exposure can exacerbate systemic autoimmunity in genetically predisposed animals, New Zealand mixed mice were intranasally instilled twice with saline or saline suspensions of 1 mg silica or 500 micro g TiO2, a dose equivalent in surface area, and were evaluated with respect to health and immune status. Survival in silica exposed NZM mice was decreased compared to saline and TiO2 exposed mice. Proteinuria levels were elevated in silica exposed mice. Levels of circulating immune complexes, autoantibodies to nuclear antigen (ANA), histone, and double stranded DNA were measured every two weeks by ELISA. Circulating immune complexes showed a trend towards an increased acceleration in levels in the silica exposed mice compared to saline and TiO2 exposed mice. ANA levels were significantly higher in silica exposed animals compared to saline and TiO2 exposed animals (0.237 +/- 0.03 versus 0.140 +/- 0.029 and 0.125 +/- 0.03, P < 0.05) 16 weeks postexposure. Autoantibodies to histone were also significantly elevated after 16 weeks in silica exposed animals compared to saline and TiO2 exposed animals (0.227 +/- 0.03 versus 0.073 +/- 0.015 and 0.05 +/- 0.03, P < 0.05). In contrast, serum IgG levels were decreased in silica exposed NZM mice compared to the saline controls, however, IgM levels were unaffected. Lungs of the silica-exposed mice had increased inflammatory infiltrates as well as fibrotic lesions characterized by excess collagen deposition. Therefore, although NZM mice are susceptible to SLE, silica exposure significantly exacerbated the course of disease.

Animals↗

PubMed2001

Binary Gaussian core model: fluid-fluid phase separation and interfacial properties.

Using a mean-field equation of state we calculate the density-concentration phase diagrams for a binary mixture of repulsive Gaussian core particles over a range of size ratios. A simple mean-field density functional (DFT) approach, equivalent to the random phase approximation, is used to calculate the surface tension and density profiles of the interface between the demixed fluid phases of the binary mixture. For certain coexisting states oscillations are found in the density profiles on both sides of the interface, i.e., approaching both bulk phases. The form of the oscillations is determined by the asymptotic decay of the bulk total pairwise correlations, and the onset of oscillations in the interfacial density profiles depends on the location of the crossover line (Fisher-Widom line) in the bulk phase diagram where the asymptotic decay changes from monotonic to damped oscillatory. For certain particle size ratios we find another crossover line that separates a region of the phase diagram where the longest-range decay of the pairwise correlations is damped oscillatory from a region where the longest-range decay is damped oscillatory but with a different wavelength. We argue that many of the predictions of the simple DFT approach should remain valid in more refined treatments.

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PubMed1998

In vivo addition of telomeric repeats to foreign DNA generates extrachromosomal DNAs in the taxol-producing fungus Pestalotiopsis microspora.

Transformation of the taxol-producing filamentous fungus Pestalotiopsis microspora with a plasmid containing the bacterial hygromycin resistance gene fused to Aspergillus regulatory sequences resulted in the in vivo formation of extrachromosomal DNAs with telomeric repeats in the majority of transformants. Repeats of the telomeric sequence 5'-TTAGGG-3' were appended to nontelomeric transforming DNA termini. No fungal sequences other than telomeric repeats were detected in extrachromosomal DNAs. Transformants contained three to six different sizes or conformational forms of extrachromosomal DNAs. The DNAs showed no change in size or internal structure during 6 months of growth with selection, but were lost after 20 days of growth without selection. Transformation of wild-type P. microspora with a PCR-amplified extrachromosomal DNA having terminal telomeric repeats produced up to 50-fold more transformants than the original transformation vector. The addition of telomeric repeats to foreign DNA is unusual among fungi and may have important adaptive or developmental implications.

Aspergillus nidulans↗