Quantitative Prediction of Tip-Sample Repulsive Forces and Sample Deformation in Tapping-Mode Frequency and Force Modulation Atomic Force Microscopy

The ability to predict sample deformation and the resultant interaction forces is a vital component to preventing sample damage and acquiring accurate height traces in atomic force microscopy (AFM). By using the recently developed frequency and force modulation (FFM) control scheme, a prediction method is developed by coupling previously developed analytical work with numerical integration of the equation of motion for the AFM tip. By selecting a zero resonance frequency shift, the sample deformation is found to depend only on those parameters defining the tip-sample interaction forces. The results are represented graphically and through a multiple regression model so that the user can predict the tip penetration and maximum repulsive force with knowledge of the maximum attractive force and steepness of the repulsive regime in the tip-sample interaction force curve. The prediction model is shown to be accurate for a wide range of imaging conditions

Subcortical volumes across the lifespan: Data from 18,605 healthy individuals aged 3–90 years

Age has a major effect on brain volume. However, the normative studies available are constrained by small sample sizes, restricted age coverage and significant methodological variability. These limitations introduce inconsistencies and may obscure or distort the lifespan trajectories of brain morphometry. In response, we capitalized on the resources of the Enhancing Neuroimaging Genetics through Meta‐Analysis (ENIGMA) Consortium to examine age‐related trajectories inferred from cross‐sectional measures of the ventricles, the basal ganglia (caudate, putamen, pallidum, and nucleus accumbens), the thalamus, hippocampus and amygdala using magnetic resonance imaging data obtained from 18,605 individuals aged 3–90 years. All subcortical structure volumes were at their maximum value early in life. The volume of the basal ganglia showed a monotonic negative association with age thereafter; there was no significant association between age and the volumes of the thalamus, amygdala and the hippocampus (with some degree of decline in thalamus) until the sixth decade of life after which they also showed a steep negative association with age. The lateral ventricles showed continuous enlargement throughout the lifespan. Age was positively associated with inter‐individual variability in the hippocampus and amygdala and the lateral ventricles. These results were robust to potential confounders and could be used to examine the functional significance of deviations from typical age‐related morphometric patterns

Cortical thickness across the lifespan: Data from 17,075 healthy individuals aged 3-90 years

Delineating the association of age and cortical thickness in healthy individuals is critical given the association of cortical thickness with cognition and behavior. Previous research has shown that robust estimates of the association between age and brain morphometry require large‐scale studies. In response, we used cross‐sectional data from 17,075 individuals aged 3–90 years from the Enhancing Neuroimaging Genetics through Meta‐Analysis (ENIGMA) Consortium to infer age‐related changes in cortical thickness. We used fractional polynomial (FP) regression to quantify the association between age and cortical thickness, and we computed normalized growth centiles using the parametric Lambda, Mu, and Sigma method. Interindividual variability was estimated using meta‐analysis and one‐way analysis of variance. For most regions, their highest cortical thickness value was observed in childhood. Age and cortical thickness showed a negative association; the slope was steeper up to the third decade of life and more gradual thereafter; notable exceptions to this general pattern were entorhinal, temporopolar, and anterior cingulate cortices. Interindividual variability was largest in temporal and frontal regions across the lifespan. Age and its FP combinations explained up to 59% variance in cortical thickness. These results may form the basis of further investigation on normative deviation in cortical thickness and its significance for behavioral and cognitive outcomes

Modelling human choices: MADeM and decision‑making

Research supported by FAPESP 2015/50122-0 and DFG-GRTK 1740/2. RP and AR are also part of the Research, Innovation and Dissemination Center for Neuromathematics FAPESP grant (2013/07699-0). RP is supported by a FAPESP scholarship (2013/25667-8). ACR is partially supported by a CNPq fellowship (grant 306251/2014-0)

Capturing the effects of free surfaces on void strengthening with dislocation dynamics

Void strengthening in crystalline materials refers to the increase in yield stress due to the impediment of dislocation motion by voids. Dislocation dynamics (DD) is a modeling method well suited to capture the physics, length scales, and time scales associated with void strengthening. However, previous DD simulation of dislocation–void interactions have been unable to accurately account for the strong image forces acting on the dislocation due to the void’s free surface. In this article, we employ a finite-element-based DD method to determine the obstacle strength of voids, defined as the critical resolved shear stress for a dislocation to glide past an array of voids. Our results demonstrate that the attractive image forces between the dislocation and free surface significantly reduce the obstacle strength of voids. Effects of surface mobility and stress concentrations around the void are also explored and are shown to have minimal effect on the critical stress. Finally, a new model relating void size and spacing to obstacle strength is proposed

The role of free surfaces on the formation of prismatic dislocation loops

Prismatic punching is a process where voids grow through the nucleation and emission of prismatic dislocation loops (PDLs). In this work we employ dislocation dynamics to determine the effect of image stresses produced by the void’s free surface on PDL formation in a face-centered cubic lattice. We find that image stresses cause PDL formation to fall into two distinct pressure regimes. In the low pressure regime, image stresses dominate dislocation cross-slip, reducing the PDL’s size and formation rate