Rapid diffusion of magic-size islands by combined glide and vacancy mechanism.

Using molecular dynamics, nudged elastic band, and embedded atom methods, we show that certain 2D Ag islands undergo extremely rapid one-dimensional diffusion on Cu(001) surfaces. Indeed, below 300 K, hopping rates for "magic-size" islands are orders of magnitude faster than hopping rates for single Ag adatoms. This rapid diffusion requires both the c(10 x 2) hexagonally packed superstructure typical of Ag on Cu(001) and appropriate "magic sizes" for the islands. The novel highly cooperative diffusion mechanism presented here couples vacancy diffusion with simultaneous core glide.

Pb nanoprecipitates in Al: magic-shape effects due to elastic strain.

We present a theory for size-dependent shapes of Pb nanoprecipitates in Al, introducing the concept of "magic shapes," i.e., shapes having near-zero homogeneous elastic strains. Our quantitative atomistic calculations of edge energies show their effect on precipitate shape to be negligible, thus it appears that shapes must be due to the combined effect of strain and interface energies. By employing an algorithm for generating magic shapes, we replicate the experimental observations by selecting magic-shape precipitates with interfacial energies less than a cutoff value.

Serial factitious disorder and Munchausen by proxy in pregnancy.

Factitious disorder, including Munchausen syndrome, is seldom documented among pregnant patients but can have powerful consequences. We report on a 44-year-old woman who, over a period of two decades, self-induced labour and delivery in five consecutive pregnancies. She precipitated labour by rupturing her own amniotic sac with a fingernail or cervical manipulation, or misappropriating and self-administering prostaglandin suppositories from the hospital unit on which she worked as a nurse. Preterm deliveries resulted in fetal demise in one case and in neonatal intensive care treatment for two of the offspring. One of the surviving children has cerebral palsy attributable to the mother's factitious illness behaviour, which raises the spectre of Munchausen by proxy maltreatment. The patient sought attention and care through the ruses, which have never been uncovered by her obstetric and gynaecologic caregivers. Indeed, she underwent an unnecessary hysterectomy because of the illusion of heavy menstrual bleeding. Most recently, the patient has been engaging in surreptitious autophlebotomy to force blood transfusions.

Finite-size effects on the structure of grain boundaries.

We present a combined experimental and theoretical analysis of the structure of finite-sized Sigma 3 [112] grain boundaries in Au. High-resolution electron microscopy shows lattice translations at the grain boundary, with the magnitude of the translation varying along the finite-sized grain boundaries. The presence of this structural profile is explained using continuum elasticity theory and first-principles calculations as originating from a competition between elastic energy and the energy cost of forming continuous [111] planes across the boundary. This competition leads to a structural transition between offset-free and nontrivial grain boundary structures at a critical grain boundary size, in agreement with the experiments. We also provide a method to estimate the energy barrier of the gamma surface.

Atomistic and lattice model of a grain boundary defaceting phase transition.

Recent calculations have shown that grain boundary (GB) stress is too small to stabilize finite GB facets, suggesting that the existing theory of GB defaceting phase transitions is incomplete. We perform molecular dynamics calculations, which show a reversible phase transition at approximately 400 K with a concerted shuffle of two atoms at the facet junction as the elementary excitation. Based on this excitation we formulate an appropriate lattice model, perform Monte Carlo simulations, and establish an analytical relationship between the elementary excitation energy and the transition temperature.

Strain relief through heterophase interface reconstruction: Ag(111)/Ru(0001).

We report an experimental (scanning tunneling microscopy) and theoretical (embedded atom method) study of a heterophase interface reconstruction between Ag(111) and Ru(0001). Despite the large 7% mismatch, the second layer of Ag from the Ru exhibits a hexagonal structure with Ag bulk spacing, providing a close match to bulk Ag. The first layer of Ag (next to Ru) is reconstructed in a highly symmetrical and regular structure containing monolayer long threading dislocations. We argue that this structure may generally occur to relieve strain in a certain class of heterophase interfaces.

Understanding the magnetocatalytic effect: magnetism as a driving force for surface segregation.

The magnetocatalytic (or Hedvall) effect refers to a change in the rate of a chemical reaction on a magnetic surface at the Curie point T(C). For Ni catalysts, experiments suggest the effect is related to a sudden increase in segregated surface C, a strong catalytic poison, at temperatures below T(C). However, the connection between magnetism and surface segregation is not understood. Using density functional theory and spin-dynamics simulations, we show that the solubility of C in Ni is significantly reduced in the ferromagnetic state, because C suppresses Ni magnetism and thereby increases the heat of solution. This explains the observed increase in C segregation and the reduced catalytic activity below T(C).

Why do grain boundaries exhibit finite facet lengths?

Uniform finite facets are frequently observed at grain boundaries (GBs) and are usually attributed to equilibrium stabilization by GB stress. We report calculations for an aluminum twin GB using density functional theory, the embedded-atom method, and continuum elasticity theory. These methods show that GB stress is much too small to stabilize finite facets, suggesting that the usual explanation is incorrect.

Overlayer strain relief on surfaces with square symmetry: phase diagram for a 2D Frenkel-Kontorova model.

Overlayers on surfaces with square symmetry exhibit a huge variety of strain relief mechanisms. I present a simple 2D Frenkel-Kontorova model and calculate the associated zero temperature phase diagram which shows a transition from overlayers with square symmetry (and possible square dislocation patterns) to hexagonal symmetry. The phase diagram includes the experimentally observed clock-rotated phase. Local density approximation calculations suggested by the model show that a clean Ni(100) surface reconstructs from a bulk-terminated to a clock-rotated structure at biaxial compressive strains above 2.5%.

Dislocation emission around nanoindentations on a (001) fcc metal surface studied by scanning tunneling microscopy and atomistic simulations.

We present a combined study by scanning tunneling microscopy and atomistic simulations of the emission of dissociated dislocation loops by nanoindentation on a (001) fcc surface. The latter consist of two stacking-fault ribbons bounded by Shockley partials and a stair-rod dislocation. These dissociated loops, which intersect the surface, are shown to originate from loops of interstitial character emitted along the <110> directions and are usually located at hundreds of angstroms away from the indentation point. Simulations reproduce the nucleation and glide of these dislocation loops.

Surface step effects on nanoindentation.

Atomistic simulation is used to examine nanoindentation of a Au(111) crystal both near and far from a surface step. While the load needed to nucleate dislocations decreases significantly when indenting close to the step, the extent of the step's influence is not as great as seen experimentally. This behavior is explained by measuring the contact area from the simulation data. A new metric, the slip vector, shows material slip coinciding with the <112> directions of a lowest unstable stacking fault barrier. The slip vector is used to calculate an atomic critical resolved shear stress, which is shown to be a good dislocation nucleation criterion.

A clinical evaluation of air-abrasion treatment of questionable carious lesions. A 12-month report.

BACKGROUND: The efficacy of treating questionable incipient lesions early with air abrasion, a modality used by many practitioners, has not been adequately demonstrated. METHODS: The authors enrolled 223 teeth, each with a questionable incipient pit-and-fissure carious lesion, from 93 dental patients in a projected five-year randomized clinical trial. Caries was defined as softness, decalcification or cavitation at the base of a pit or fissure or radiographic evidence of caries. Each tooth was randomly assigned to either a treatment group (n = 113 teeth) or a control group (n = 110 teeth) (which was observed but left untreated until the definition of caries was met). Each tooth in the treatment group was air-abraded and restored with a flowable resin-based composite. The authors re-examined teeth in both groups every six months; they evaluated the restorations using a modified set of Ryge criteria and inspected teeth for caries using radiographs, mirrors and standardized explorers. RESULTS: Of the 113 teeth with questionable incipient carious lesions air-abraded in the treatment group, 50 had caries extending into dentin. After 12 months of clinical service, there were three sealants that exhibited a partial loss of sealant which did not require any re-treatment. Two restorations with penetrating staining were re-treated. In the control group at the end of 12 months, only nine of the 86 recalled teeth were diagnosed with pit-and-fissure caries and were treated with air abrasion and restored with flowable resin-based composite. There was no statistically significant difference between the volume of the treatment and control preparations. CONCLUSION: After 12 months of clinical service, two preventive resin-based composite restorations in the treatment group required re-treatment. Fewer teeth than expected in the control group were diagnosed as having caries and were treated. CLINICAL IMPLICATIONS: The merit of treating questionable incipient pit-and-fissure carious lesions early with air abrasion has not been demonstrated after 12 months in this clinical study.

Effect of variable light intensity on composite shrinkage.

STATEMENT OF PROBLEM: Polymerization shrinkage is a critical limitation of dental composites and may contribute to postoperative pain, tooth fracture, microleakage, and secondary caries. Polymerization with high-intensity light sources has been related to increased depth of cure and improved mechanical properties. However, high-intensity light initiation has also been associated with greater polymerization shrinkage. PURPOSE: The purpose of this study was to investigate the effect of sequentially increasing light intensity on the polymerization shrinkage of 2 composites, a hybrid and a microfil. A Knoop hardness test was used to evaluate effectiveness of the cure with each intensity increase. MATERIAL AND METHODS: Four groups of 12 samples were measured for polymerization shrinkage by using a linometer. Light intensity curing sequences were as follows: full-intensity control (100% intensity for 40 seconds), low-intensity control (25% intensity for 40 seconds), test group 1 (25% intensity for 20 seconds, 50% for 10 seconds, 100% for 10 seconds), and test group 2 (25% intensity for 10 seconds, 50% for 10 seconds, 100% for 20 seconds). Statistical comparisons were made using a 1-factor ANOVA and a Tukey multiple comparisons test within each material. RESULTS: Results showed a significant difference (P<. 05) in mean linear shrinkage between the full-intensity control group and the other 3 sequences for both composites. No difference existed within the other 3 groups for either composite. Knoop hardness was similar for the full-intensity control and test group 2. The low-intensity control group and test group 1 were also similar but significantly lower. CONCLUSION: Curing composites for 10 seconds at 25% intensity, 10 seconds at 50%, and 20 seconds at 100% significantly reduced polymerization shrinkage while not compromising depth of cure.

Identifying and reducing risk factors related to trainee-client sexual misconduct.

Sexual misconduct involving therapists-in-training and their clients is addressed. Personal and situational factors that may constitute risk factors for the development of inappropriate sexual activity between trainees and their clients are identified. Although there may be certain characteristics that put particular students at risk for such involvement, the authors believe this risk is more strongly related to systemic, programmatic, and pedagogic characteristics of the environments in which students train. Examples include, respectively, the decline of concern over transference and countertransference, failure to include education about client-therapist sexual attraction and the consequences of sexual misconduct in graduate psychology curricula, and the reluctance of supervisors to deal straightforwardly with trainees' sexual feelings. Suggestions for reducing risks for client-therapist sexual misconduct are directed toward these situational factors.

Evaluation of custom occlusal matrix technique for posterior light-cured composites.

Posterior composite restorations are difficult to contour and polish due to their occlusal anatomy and opposing occlusion. Recently a new technique utilizing a clear custom occlusal matrix showed promise in reducing the chair time necessary to place posterior composite restorations. Sixty specially molded plastic mandibular right second molar teeth with a mesio-occlusal composite preparation were restored with a light-cured composite in a typodont, using either the custom occlusal matrix (experimental) or the standard technique (control) as taught at the University of Michigan School of Dentistry. The times necessary to complete the procedures were recorded. The teeth were evaluated in a blind manner by two evaluators using a modified Ryge criteria. The times to place and finish the control and experimental restorations were subjected to a t-test. The evaluations of surface smoothness, margin adaptation, occlusal anatomy, and axial form were compared using a Fisher exact test. The custom occlusal matrix significantly reduced placement and finishing time and improved surface smoothness of mesio-occlusal posterior composite restorations placed in vitro. Due to the time needed to construct the custom occlusal matrix, there was no significant difference in total mean time of the two procedures.

"The Flemish Giant", reflections on the defense against endometriosis, inspired by Professor Emeritus Ivo A. Brosens.

Reproductive research benefits from combining animal and clinical studies. In Leuven, rabbits have constituted an animal model for many reproductive disorders, especially for those that involved surgical treatment. Much of what has been learned from animal experiments has been applied to human clinical reproductive research soon after. In this manuscript we wish to address the problem of the constant intra-abdominal battle between the menstrual aggressor and the peritoneal defense. From all published evidence we may conclude that endometriosis appears to be a dynamic disease, especially in the early phase, with subtle, atypical lesions emerging and vanishing again. In the end however the peritoneal defense system will prevail and the disease will be contained in the majority of patients. When doing repeat laparoscopies in young patients one should be prepared to encounter more advanced histological types of lesions, which not necessarily do have to indicate more advanced stages of the disease: the classical, blue and black powderburn spots and blueberry lesions reflect the extinguishing phase of the dynamic endometriotic process, and herald its inactivated histological end-stage. The dynamic phase of the disease may involve a varying interval of each patient's life, and medical suppression of the activity of the implants during this interval may lead one to conclude erroneously that treatment has been effective. If subsequently (after the end of medical suppression of the activity of the lesions) ovarian activity resumes and the lesions are stimulated again by ovarian steroids, their productive activity returns. Recurrence of disease may be diagnosed if at that stage a laparoscopy would be performed, whereas in reality only reactivation of temporarily obscured lesions did occur. The suppressed, dormant (but never absent) lesions produce mucus again, desquamation occurs, and reaction by the surrounding tissue. The inflammatory response, the local hyperemia and the neogenesis of vessels accentuate the presence of previously invisible endometriosis lesions and make them visible again. Endometriosis resumes its temporarily halted natural course of development, tissue remodeling occurs again, the battle between the aggressor and the defense resumes and waxing and waning of the several types of lesions, red, white and black, can be found again.