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S Dietrich

Publications and source records attributed to S Dietrich.

At least 37 records · Page 2Linked to original sources
PubMed2004

Liquid-crystalline Casimir effect in the presence of a patterned substrate.

We consider a nematic liquid crystal confined by two parallel planar interfaces, one being laterally homogeneous and the other provided by a substrate endowed with a periodic chemical stripe pattern. Based on continuum theory we analyze the influence of the lateral pattern on the liquid-crystalline Casimir force acting on the interfaces of the nematic cell at distance d due to the thermal fluctuations of the nematic director. For d much larger than the pattern periodicity, the influence of the patterned substrate can be described by a homogeneous, effective anchoring energy. By tuning this parameter we recover previous results for the liquid-crystalline Casimir force. For the general case, i.e., smaller separations, we present numerical results.

Journal Article↗
PubMed2004

Colloidal hard-rod fluids near geometrically structured substrates.

Density functional theory is used to study colloidal hard-rod fluids near an individual right-angled wedge or edge as well as near a hard wall, which is periodically patterned with rectangular barriers. The Zwanzig model, in which the orientations of the rods are restricted to three orthogonal orientations but their positions can vary continuously, is analyzed by numerical minimization of the grand potential. Density and orientational order profiles, excess adsorptions, as well as surface and line tensions are determined. The calculations exhibit an enrichment [depletion] of rods lying parallel and close to the corner of the wedge (edge). For the fluid near the geometrically patterned wall, complete wetting of the wall-isotropic liquid interface by a nematic film occurs as a two-stage process in which first the nematic phase fills the space between the barriers until an almost planar isotropic-nematic liquid interface has formed separating the higher-density nematic fluid in the space between the barriers from the lower-density isotropic bulk fluid. In the second stage, a nematic film of diverging film thickness develops upon approaching bulk-isotropic-nematic coexistence.

Journal Article↗
PubMed2004

Exactly solvable model of monomer-monomer reactions on a two-dimensional random catalytic substrate.

We study the equilibrium properties of a monomer-monomer A+B--> reaction on a two-dimensional substrate containing randomly placed catalytic bonds. Interacting A and B species undergo continuous exchanges with particle reservoirs and react as soon as a pair of unlike particles appears on sites connected by a catalytic bond. For annealed disorder in the placement of catalytic bonds the model is mapped onto a general spin S=1 model and solved exactly for the pressure in a particular case. At equal activities of the two species a second order phase transition is revealed.

Carbon Monoxide↗
PubMed2004

Model for spreading of liquid monolayers.

Manipulating fluids at the nanoscale within networks of channels or chemical lanes is a crucial challenge in developing small scale devices to be used in microreactors or chemical sensors. In this context, ultrathin (i.e., monolayer) films, experimentally observed in spreading of nanodroplets or upon extraction from reservoirs in capillary rise geometries, represent an extreme limit which is of physical and technological relevance since the dynamics is governed solely by capillary forces. In this work we use kinetic Monte Carlo (KMC) simulations to analyze in detail a simple, but realistic model proposed by Phys. Rev. Lett. 76, 86 (1996)]] for the two-dimensional spreading on homogeneous substrates of a fluid monolayer which is extracted from a reservoir. Our simulations confirm the previously predicted time dependence of the spreading, X ( t--> infinity ) =A square root of t, with X (t) as the average position of the advancing edge at time t, and they reveal a nontrivial dependence of the prefactor A on the strength U0 of interparticle attraction and on the fluid density C0 at the reservoir as well as an U0 -dependent spatial structure of the density profile of the monolayer. The asymptotic density profile at long time and large spatial scale is carefully analyzed within the continuum limit. We show that including the effect of correlations in an effective manner into the standard mean-field description leads to predictions both for the value of the threshold interaction above which phase segregation occurs and for the density profiles in excellent agreement with KMC simulation results.

Computer Simulation↗
PubMed2004

Depletion potential in colloidal mixtures of hard spheres and platelets.

The depletion potential between two hard spheres in a solvent of thin hard disclike platelets is investigated by using either the Derjaguin approximation or density functional theory. Particular attention is paid to the density dependence of the depletion potential. A second-order virial approximation is applied, which yields nearly exact results for the bulk properties of the hard-platelet fluid at densities two times smaller than the density of the isotropic fluid at isotropic-nematic phase coexistence. As the platelet density increases, the attractive primary minimum of the depletion potential deepens and an additional small repulsive barrier at larger sphere separations develops. Upon decreasing the ratio of the radius of the spheres and the platelets, the primary minimum diminishes and the position of the small repulsive barrier shifts to smaller values of the sphere separation.

Biophysical Phenomena↗
PubMed2004

Phase diagram of a model for 3He-4He mixtures in three dimensions.

A lattice model of 3He-4He mixtures which takes into account the continuous rotational symmetry O(2) of the superfluid degrees of freedom of 4He is studied in the molecular-field approximation and by Monte Carlo simulations in three dimensions. In contrast to its two-dimensional version, for reasonable values of the interaction parameters the resulting phase diagram resembles that observed experimentally for 3He-4He mixtures, for which phase separation occurs as a consequence of the superfluid transition. The corresponding continuum Ginzburg-Landau model with two order parameters describing 3He-4He mixtures near tricriticality is derived from the considered lattice model. All coupling constants appearing in the continuum model are explicitly expressed in terms of the mean concentration of 4He, the temperature, and the microscopic interaction parameters characterizing the lattice system.

Journal Article↗
PubMed2004

Bulk and interfacial properties of binary hard-platelet fluids.

Interfaces between demixed fluid phases of binary mixtures of hard platelets are investigated using density-functional theory. The corresponding excess free energy functional is calculated within a fundamental measure theory adapted to the Zwanzig model, in which the orientations of the particles of rectangular shape are restricted to three orthogonal orientations. Density and orientational order parameter profiles at interfaces between coexisting phases as well as the interfacial tension are determined. A density inversion, oscillatory density profiles, and a Fisher-Widom line have been found in a mixture of large thin and small thick platelets. The lowest interfacial tension corresponds to the mean bulk orientation of the platelets being parallel to the interface. For a mixture of large and small thin platelets, complete wetting of an isotropic-nematic interface by a second nematic phase is found.

Blood Platelets↗
PubMed2004

Activity of MMP-19 inhibits capillary-like formation due to processing of nidogen-1.

Matrix metalloproteinase 19 (MMP-19) is able to process various proteins of the basement membrane. To investigate the impact of MMP-19 activity on endothelial cells in the context of tumor extracellular matrix (ECM), we treated Matrigel matrix with an active recombinant MMP-19 and analyzed its effect on capillary-like formation. Human microvascular endothelial cells (HMEC-1) could not form capillary-like formation on Matrigel treated with recombinant MMP-19. Analyzing the Matrigel proteins, we found that MMP-19 preferentially cleaved nidogen-1. The cleavage site of nidogen-1 was mapped to Thr867-Leu868. This cleavage separates the G3 globular domain containing the binding site for the gamma1 chain of laminin-1 and collagen IV and thus abolishes the capacity of nidogen-1 to cross-link ECM proteins. Anti-nidogen antibodies directed against the G3 domain of nidogen-1 inhibited the capillary-like structure formation to a similar extent as MMP-19. Since nidogen-1 is thought to stabilize microvessels, MMP-19 might be one of the enzymes that interferes with stabilization or maturation of nascent vasculature.

Cell Division↗
PubMed2004

Adjustable gastric banding surgery in morbidly obese adolescents: experiences with eight patients.

BACKGROUND: Excessive obesity in children and adolescents is increasing in western countries all over the world. Treatment of those young subjects is extremely difficult. In cases who do not respond in any regard to conservative treatment regimens, it is worthwhile to also offer bariatric surgical procedures. METHODS: In eight young subjects with a mean age of 16.0+/-1.3 y, mean BMI 49.1+/-5.2 who where totally resistant against different therapeutic regimens, we performed adjustable laparoscopic banding surgery. Psychological tests were carried out to find out if there were any psychological disturbances. In all patients, the depression inventory for adolescents (DIKJ) and clinical interviews were carried out. RESULTS: No complications were seen; mean follow-up of 10.5+/-6.0 months showed a weight loss of 25+/-6.5 kg. All adolescents felt for the first time that it was possible to lose weight and were very happy about their body attitude. They were able to adhere to a strict dietary regimen, which allowed them to eat without any hunger problems. It was observed that most of these patients have depressive symptoms and a very low self-esteem. Some patients suffer from nervous anxiety, poor peer acceptance, less athletic competence or forms of victimisation too. CONCLUSIONS: From these results, we conclude that laparoscopic gastric banding surgery seems to be a method that could be offered to morbidly obese adolescents. However, long-term follow-up is essential and psychological problems should be carefully studied.

Adolescent↗
PubMed2004

Quality of life in persons with schizophrenia in out-patient treatment with first- or second-generation antipsychotics.

OBJECTIVE: Effects of first in comparison with second-generation antipsychotics on the subjective quality of life (QoL) of patients with schizophrenia under routine treatment conditions were examined. METHOD: In a prospective naturalistic trial the QoL, social, clinical, and treatment-related characteristics and type of antipsychotic medication of 307 patients with schizophrenia (ICD-10 F 20) were assessed five times with 6-month intervals over 2.5 years. Longitudinal and cross-sectional effects of antipsychotic medication were assessed by hierarchical regression models. Selection bias was controlled by propensity scores. RESULTS: While positive effects of first-generation antipsychotics on subjective QoL in comparison with no antipsychotic treatment were found, second-generation antipsychotics caused no better QoL than first-generation antipsychotics. CONCLUSION: The hypothesis that second-generation in comparison with first-generation antipsychotics have a better effect on the improvement of subjective QoL of people with schizophrenia in routine out-patient treatment was not supported.

Adolescent↗
PubMed2004

Earliest historic reference of 'tinnitus' is controversial.

Tinnitus, that is defined as 'ringing in the ear', is and has probably always been a very common phenomenon in the health history of mankind. A variety of pathomechanisms for its onset has been proposed in the past and this trend sees no stopping. Precise pathomechanisms still remain unclear. From the historical point of view, tinnitus is a very interesting topic but there is a lack of scientific enquiries. As its earliest historic reference, the Papyrus Ebers is often cited. By reviewing the original source, however, it is very unlikely that this contains the earliest historic reference of tinnitus. It is rather likely indeed that tinnitus was a known symptom in the ancient Egypt but it has never been exclusively been written down. This article reviews data provided by this fascinating historical period on the questionable reference and highlights the impact of history on the modern status of tinnitus.

Egypt, Ancient↗
PubMed2003

Hard-sphere fluids in contact with curved substrates.

The properties of a hard-sphere fluid in contact with hard-spherical and cylindrical walls are studied. Rosenfeld's density functional theory (DFT) is applied to determine the density profile and surface tension gamma for wide ranges of radii of the curved walls and densities of the hard-sphere fluid. Particular attention is paid to investigate the curvature dependence and the possible existence of a contribution to gamma which is proportional to the logarithm of the radius of curvature. Moreover, by treating the curved wall as a second component at infinite dilution, we provide an analytical expression for the surface tension of a hard-sphere fluid close to arbitrary hard convex walls. The agreement between the analytical expression and DFT is good. Our results show no signs for the existence of a logarithmic term in the curvature dependence of gamma.

Journal Article↗
PubMed2003

Equilibrium properties of a monomer-monomer catalytic reaction on a one-dimensional chain.

We study the equilibrium properties of a lattice-gas model of an A+B-->0 catalytic reaction on a one-dimensional chain in contact with a reservoir for the particles. The particles of species A and B are in thermal contact with their vapor phases acting as reservoirs, i.e., they may adsorb onto empty lattice sites and may desorb from the lattice. If adsorbed A and B particles appear at neighboring lattice sites, they instantaneously react and both desorb. For this model of a catalytic reaction in the adsorption-controlled limit, we derive analytically the expression of the pressure and present exact results for the mean densities of particles and for the compressibilities of the adsorbate as a function of the chemical potentials of the two species.

Journal Article↗
PubMed2003

Universality of the thermodynamic Casimir effect.

Recently a nonuniversal character of the leading spatial behavior of the thermodynamic Casimir force has been reported [X. S. Chen and V. Dohm, Phys. Rev. E 66, 016102 (2002)]. We reconsider the arguments leading to this observation and show that there is no such leading nonuniversal term in the systems with short-ranged interactions if one treats properly the effects generated by a sharp momentum cutoff in the Fourier transform of the interaction potential. We also conclude that lattice and continuum models then produce results in mutual agreement independent of the cutoff scheme, contrary to the aforementioned report. All results are consistent with the universal character of the Casimir force in the systems with short-ranged interactions. The effects due to dispersion forces are discussed for the systems with periodic or realistic boundary conditions. In contrast to the systems with short-ranged interactions, for L/xi>>1, one observes leading finite-size contributions governed by power laws in L due to the subleading long-ranged character of the interaction, where L is the finite system size and xi is the correlation length.

Journal Article↗
PubMed2002

Wetting and capillary nematization of binary hard-platelet and hard-rod fluids.

Density-functional theory is used to investigate the phase behavior of colloidal binary hard-platelet and hard-rod fluids near a single hard wall or confined in a slit pore. The Zwanzig model, in which the orientations of the particles of rectangular shape are restricted to three orthogonal orientations, is analyzed by numerical minimization of the grand potential functional. The density and orientational profiles as well as the surface contributions to the grand potential are determined. The calculations exhibit a wall-induced continuous surface transition from uniaxial to biaxial symmetry for the hard-rod fluid. Complete wetting of the wall-isotropic liquid interface by a biaxial nematic film for rods and a uniaxial nematic film for platelets is found. For the fluids confined by two parallel hard walls, we determine a first-order capillary nematization transition for large slit widths, which terminates in a capillary critical point upon decreasing the slit width.

Journal Article↗
PubMed2002

Entropic torque.

Quantitative predictions are presented of a depletion-induced torque and force acting on a single colloidal hard rod immersed in a solvent of hard spheres close to a planar hard wall. This torque and force, which are entirely of entropic origin, may play an important role for the key-lock principle, according to which a biological macromolecule (the key) is functional only in a particular orientation with respect to a cavity (the lock).

Journal Article↗
PubMed2002

Geometrically-controlled twist transitions in nematic cells.

We study geometrically controlled twist transitions of a nematic confined between a sinusoidal grating and a flat substrate. In these cells, the transition to the twisted state is driven by surface effects. We have identified the mechanisms responsible for the transition analytically and used exact numerical calculations to study the range of surface parameters where the twist instability occurs. Close to these values, the cell operates under minimal external fields or temperature variations.

Journal Article↗