Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

Superhydrophobic surfaces (SHSs) have the potential to achieve large drag reduction for internal and external flow applications. However, experiments have shown inconsistent results, with many studies reporting significantly reduced performance. Recently, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible, by creating adverse Marangoni stresses. Yet, testing this hypothesis is challenging. Careful experiments with purified water show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases. This suggests that other physical processes, such as thermal Marangoni stresses or interface deflection, could explain the lower performance. To test the surfactant hypothesis, we perform the first numerical simulations of flows over a SHS inclusive of surfactant kinetics. These simulations reveal that surfactant-induced stresses are significant at extremely low concentrations, potentially yielding a no-slip boundary condition on the air--water interface (the "plastron") for surfactant amounts below typical environmental values. These stresses decrease as the streamwise distance between plastron stagnation points increases. We perform microchannel experiments with thermally-controlled SHSs consisting of streamwise parallel gratings, which confirm this numerical prediction. We introduce a new, unsteady test of surfactant effects. When we rapidly remove the driving pressure following a loading phase, a backflow develops at the plastron, which can only be explained by surfactant gradients formed in the loading phase. This demonstrates the significance of surfactants in deteriorating drag reduction, and thus the importance of including surfactant stresses in SHS models. Our time-dependent protocol can assess the impact of surfactants in SHS testing and guide future mitigating designs.Comment: 25 pages including supplemental information, 7 figures; videos available on reques

Beauty is in the efficient coding of the beholder

Sexual ornaments are often assumed to be indicators of mate quality. Yet it remains poorly known how certain ornaments are chosen before any coevolutionary race makes them indicative. Perceptual biases have been proposed to play this role, but known biases are mostly restricted to a specific taxon, which precludes evaluating their general importance in sexual selection. Here we identify a potentially universal perceptual bias in mate choice. We used an algorithm that models the sparseness of the activity of simple cells in the primary visual cortex (or V1) of humans when coding images of female faces. Sparseness was found positively correlated with attractiveness as rated by men and explained up to 17% of variance in attractiveness. Because V1 is adapted to process signals from natural scenes, in general, not faces specifically, our results indicate that attractiveness for female faces is influenced by a visual bias. Sparseness and more generally efficient neural coding are ubiquitous, occurring in various animals and sensory modalities, suggesting that the influence of efficient coding on mate choice can be widespread in animals

Les rues des tableaux: The Geography of the Parisian Art Market 1815-1955

Building upon a preliminary socioeconomic analysis of the art dealers in Paris between 1815 and 1955 (ARTL@S Bulletin 2, n°2), this paper presents the findings of a spatial study of the Parisian art market in this period. Using serial geographical data drawn from a single, consistent source – the Bottin du commerce – we mapped the spatial evolution of art dealers over 140 years, using a geocoding system with composite locators. The article explores the different spatial dynamics of this market, and seeks to shed light on the links between the evolution of the Parisian economy as a whole and the individual trajectories of its art dealers

Version abrégée transculturelle du Balanced Inventory of Desirable Responding (BIDR)

Le Balanced Inventory of Desirable Responding (BIDR) (Paulhus, 1984, 1991) est un des questionnaires les plus utilisés pour évaluer la désirabilité sociale. Il est composé de deux échelles de 20 énoncés, soit l'autoduperie, la tendance à se décrire de façon honnête, mais biaisée positivement, et l'hétéroduperie, qui est la tendance à présenter une image favorable de soi à autrui (Paulhus, 1986). Les deux échelles possèdent de bonnes qualités psychométriques, et ce, pour la version francophone (Cournoyer & Sabourin, 1991) et anglophone (Paulhus, 1984, 1991). Les études qui comportent plusieurs questionnaires tendent à produire plus de valeurs manquantes et un taux de refus supérieur que dans les enquêtes où le nombre est moindre (Stanton, Sinar, Balzer, & Smith, 2002). Le besoin d'une version abrégée est donc grand, - surtout pour un questionnaire tel le BIDR qui est utilisé dans le but de supporter la validité discriminante d'autres questionnaires. Le but de la présente recherche consiste à proposer une version abrégée du BIDR qui puisse être utilisée tant en français qu'en anglais. Pour ce faire, une procédure en quatre étapes est utilisée: analyse d'items, analyse factorielle exploratoire, analyse du fonctionnement différentiel d'items (FDI) et analyse factorielle confirmatoire. Un total de 2787 sujets (700 hommes anglophones, 928 femmes anglophones, 567 hommes francophones et 592 femmes francophones) a répondu aux énoncés des deux échelles du BIDR. Cet échantillon provient de quatre études différentes (une francophone et trois anglophones). La version abrégée proposée comporte 21 énoncés dont huit pour l'échelle d'autoduperie et 13 pour l'échelle d'hétéroduperie. Les analyses factorielles confirmatoires montrent que cette version abrégée présente des qualités psychométriques similaires pour les deux langues. Cependant, les coefficients KR-20 de l'échelle d'autoduperie sont légèrement sous le seuil minimal accepté en recherche de 0,60 proposé par Robinson, Shaver et Wrightsman (1991). D'autres recherches sont donc nécessaires afin de reformuler ou remplacer certains énoncés de cette échelle

Automated assessment and tracking of human body thermal variations using unsupervised clustering

The presented approach addresses a review of the overheating that occurs during radiological examinations, such as magnetic resonance imaging, and a series of thermal experiments to determine a thermally suitable fabric material that should be used for radiological gowns. Moreover, an automatic system for detecting and tracking of the thermal fluctuation is presented. It applies hue-saturated-value-based kernelled k-means clustering, which initializes and controls the points that lie on the region-of-interest (ROI) boundary. Afterward, a particle filter tracks the targeted ROI during the video sequence independently of previous locations of overheating spots. The proposed approach was tested during experiments and under conditions very similar to those used during real radiology exams. Six subjects have voluntarily participated in these experiments. To simulate the hot spots occurring during radiology, a controllable heat source was utilized near the subject’s body. The results indicate promising accuracy for the proposed approach to track hot spots. Some approximations were used regarding the transmittance of the atmosphere, and emissivity of the fabric could be neglected because of the independence of the proposed approach for these parameters. The approach can track the heating spots continuously and correctly, even for moving subjects, and provides considerable robustness against motion artifact, which occurs during most medical radiology procedures