Ultrasonic tracking of a sinking ball in a vibrated dense granular suspension

Observing and understanding the movement of an intruder through opaque dense suspensions such as quicksand remains a practical and conceptual challenge. Here we use an ultrasonic probe to investigate the dynamics of a steel ball sinking in a 3D dense glass bead packing saturated by water. We show that the frictional model developed for dry granular media can be used to describe the ball motion induced by horizontal vibration. From this rheology we infer the static friction coefficient and effective viscosity that decrease when increasing the vibration intensity. Our main finding is that the vibration-induced reduction of the yield stress and increase of the sinking depth are presumably due to induced slipping at the grain contacts but without visible plastic rearrangements of grains, in contrast to dry granular packings. To explain these results, we propose a mechanism of acoustic lubrication that reduces the inter-particle friction and leads to a decrease of the yield stress. This scenario is different from the mechanism of liquefaction usually invoked in loosely packed quicksands where the vibration-induced compaction increases the pore pressure and decreases the confining pressure on the solid skeleton, thus reducing the granular resistance to external load.Comment: 9 pages and 5 figures, plus the supplemental information (1 page, 2 movies, 1 figure

Regression sampling in statistical auditing

Auditing;Regression Analysis;accountancy

Compliance With Legal Age Restrictions on Adolescent Alcohol Sales for Alcohol Home Delivery Services (AHDS)

Purpose: Alcohol availability is an important predictor of alcohol use in adolescents and its negative consequences. Within this study, we zoomed in on availability through alcohol home delivery services (AHDS) by measuring compliance with the legal age limit in this sector. Methods: Two methods were used: (1) mystery shopping in which 21 underage AHDS beer orders were placed and received and (2) mystery calls in which 30 AHDS were called and questioned. Results: In the mystery shopping study, all 21 deliveries were approved and executed by the AHDS (compliance 0%). Within the mystery calls, 23 AHDS said they would not sell alcohol to 15-year-olds (76.7% self-reported compliance), but 19 of them approved secondary purchasing. In total, self-reported compliance was only found in 4 out of the 30 AHDS (13.3%). Conclusions: In addition to supermarkets, liquor stores, and the catering industry, AHDS should not be overlooked. Low (or no) compliance with age limits in AHDS is a weak link in the system of underage alcohol availabilit

Overcoming status quo bias in the human brain

Humans often accept the status quo when faced with conflicting choice alternatives. However, it is unknown how neural pathways connecting cognition with action modulate this status quo acceptance. Here we developed a visual detection task in which subjects tended to favor the default when making difficult, but not easy, decisions. This bias was suboptimal in that more errors were made when the default was accepted. A selective increase in subthalamic nucleus (STN) activity was found when the status quo was rejected in the face of heightened decision difficulty. Analysis of effective connectivity showed that inferior frontal cortex, a region more active for difficult decisions, exerted an enhanced modulatory influence on the STN during switches away from the status quo. These data suggest that the neural circuits required to initiate controlled, nondefault actions are similar to those previously shown to mediate outright response suppression. We conclude that specific prefrontal-basal ganglia dynamics are involved in rejecting the default, a mechanism that may be important in a range of difficult choice scenarios