Epidemic risk from friendship network data: an equivalence with a non-uniform sampling of contact networks

Contacts between individuals play an important role in determining how infectious diseases spread. Various methods to gather data on such contacts co-exist, from surveys to wearable sensors. Comparisons of data obtained by different methods in the same context are however scarce, in particular with respect to their use in data-driven models of spreading processes. Here, we use a combined data set describing contacts registered by sensors and friendship relations in the same population to address this issue in a case study. We investigate if the use of the friendship network is equivalent to a sampling procedure performed on the sensor contact network with respect to the outcome of simulations of spreading processes: such an equivalence might indeed give hints on ways to compensate for the incompleteness of contact data deduced from surveys. We show that this is indeed the case for these data, for a specifically designed sampling procedure, in which respondents report their neighbors with a probability depending on their contact time. We study the impact of this specific sampling procedure on several data sets, discuss limitations of our approach and its possible applications in the use of data sets of various origins in data-driven simulations of epidemic processes

Taking a critical look at holographic critical matter

Despite a recent flurry of applications of the broadly defined ('non-AdS/non-CFT') holographic correspondence to a variety of condensed matter problems, the status of this intriguing, yet speculative, approach remains largely undetermined. This note exposes a number of potential inconsistencies between the previously made holographic predictions and advocates for a compelling need to systematically contrast the latter against the results of alternate, more conventional, approaches as well as experimental data. It is also proposed to extend the list of computed observables and utilize the general relations between them as a further means of bringing the formal holographic approach into a closer contact with the physical realm.Comment: Contains a refined scaling analysis and comments on the recent Ref.16. None of the conclusions have change

Kinematic precision of gear trains

Kinematic precision is affected by errors which are the result of either intentional adjustments or accidental defects in manufacturing and assembly of gear trains. A method for the determination of kinematic precision of gear trains is described. The method is based on the exact kinematic relations for the contact point motions of the gear tooth surfaces under the influence of errors. An approximate method is also explained. Example applications of the general approximate methods are demonstrated for gear trains consisting of involute (spur and helical) gears, circular arc (Wildhaber-Novikov) gears, and spiral bevel gears. Gear noise measurements from a helicopter transmission are presented and discussed with relation to the kinematic precision theory

Imagine a Better World: Two Studies of Imagined Intergroup Contact

A growing body of evidence indicates that positive contact with outgroups improves attitudes towards those outgroups. Unfortunately, those with the most negative attitudes towards outgroups often have the fewest opportunities to meaningfully interact with members of those groups. These studies investigate the effects of imagining intergroup contact with a Muslim person on measures of explicit (Studies 1 and 2) and implicit (Study 2) anti-Muslim prejudice among the most ideologically intolerant individuals. Local and national participants were asked to complete a short imaginative exercise followed by a brief online questionnaire. Results indicate that imagined intergroup contact was effective in improving attitudes towards Muslims, even among those who were the most prejudiced and ideologically intolerant. We discuss the implications of these findings, as well as potential applications for imagined intergroup contact interventions, including international relations/diplomacy, and classroom diversity initiatives

Multigrid Solution of the 3D Elastic Subsurface Stress Field for Heterogeneous Materials in Contact Mechanics

The need to increase efficiency, stimulates the development of new materials tailored to specific applications and thermal/mechanical loading conditions, e.g. by controlling the property variations on a local scale: layered, graded, granular, porous and fibre-reinforced. For design and optimization of such materials the response to specific load conditions must be predicted which requires computer simulations. For applications in contact mechanics and lubrication failure criteria need to be developed which require the stress fields inside the (strongly heterogeneous) material induced by surface loading. The geometrical complexity of the subsurface topography and the need of an accurate solution require the use of a very fine discretization with a large number of elements, especially for three-dimensional problems. This requires optimally efficient numerical algorithms. In this paper the authors demonstrate the capability of Multigrid techniques to compute displacement and stress fields with great detail in strongly heterogeneous materials subject to surface loading, and in a contact mechanics application. Results are presented for a ceramic application and a contact problem of material with multiple inclusions. The efficiency of the method will allow extensive parameter studies with limited computational means. Moreover, it can efficiently be used to derive macroscopic stress-strain relations by simulations of microscopic problems. Also the method can be used for computational diagnostics of materials with specific heterogeneitie

Beyond scratching the surface : intrinsic tribological performance of polymers

Quantifying and understanding friction and wear behaviour of any type of material remains a challenge to this day. This is also true for polymers that are used frequently in sliding applications. This thesis focuses on the development of quantitative measurement techniques that can be used to understand friction and wear of polymers. To understand the influence of material properties on friction and wear behaviour it is necessary to zoom in on the relevant processes in a sliding contact. A macroscopic contact between two surfaces typically consists of multiple contacts between roughness peaks. These micro-contacts make up the real contact area which is usually a small fraction of the apparent contact area, and which depends on the mechanical properties of both surfaces as well as on the loading conditions. The friction force measured in experiments is the product of this real contact area and an average effective shear stress. Because the real contact area is difficult to control and measure for macroscopic contacts such contacts are not very useful in separating the contributions to the friction force. In contrast, single asperity techniques offer the possibility to control independently the contact area and normal load and therefore offer a way forward to a critical interpretation of measured friction forces. In the work described in this thesis microscopic tribological single–asperity experiments are used to study structure-property relations. These single asperity experiments are performed using the Lateral Force Apparatus that was drastically modified to better suit this purpose. A new driving system was developed that allows friction measurements in which the sliding velocity may be varied across 5 orders of magnitude with accurate position control. This combination makes it possible to perform single–asperity measurements at widely differing speeds which are shown to be important for the interpretation of sliding friction on polymers. Accurate position control is shown to be crucial in developing advanced wear measurement techniques. In sliding friction distinction between the contribution of contact area and effective xi xii SUMMARY shear stress to the friction force is a key issue. Depending on mechanical properties and loading conditions, all materials exhibit creep on a characteristic time scale. In polymers creep is especially relevant since the associated timescales are relatively short. In single asperity friction the asperity radius and sliding speed set a contact time, during which the contact area may evolve by creep. It is shown that the contributions of contact area and effective shear stress can be distinguished from one another using single–asperity measurements at widely differing sliding velocities. In the study of wear the interpretation of measurements on macroscopic multi– asperity contacts also pose problems since they consist of a collection of microcontacts between deformed asperities. Since the strain at failure of a polymer is expected to be an important factor in determining the wear of polymers the unambiguous strain distribution of a single asperity contact is an advantage in the study of structure-wear relations. In this thesis a novel single–asperity technique to measure wear rate is developed. In this method the wear rate is measured in real time. The method is fast, uses very little material, and yet gives good statistics and a strong correlation with macroscopically measured wear rates. In a study on PE it is found that the wear rate is related to the molecular weight. Quantitative single–asperity measurements are a critical step in understanding structure-tribology relations. While macroscopic tribological experiments can only scratch the surface of structure-tribology relations, single–asperity techniques probe the material properties lying underneath