Universal power law governing pedestrian interactions

Human crowds often bear a striking resemblance to interacting particle systems, and this has prompted many researchers to describe pedestrian dynamics in terms of interaction forces and potential energies. The correct quantitative form of this interaction, however, has remained an open question. Here, we introduce a novel statistical-mechanical approach to directly measure the interaction energy between pedestrians. This analysis, when applied to a large collection of human motion data, reveals a simple power-law interaction that is based not on the physical separation between pedestrians but on their projected time to a potential future collision, and is therefore fundamentally anticipatory in nature. Remarkably, this simple law is able to describe human interactions across a wide variety of situations, speeds, and densities. We further show, through simulations, that the interaction law we identify is sufficient to reproduce many known crowd phenomena.

Why is the bulk resistivity of topological insulators so small?

As-grown topological insulators (TIs) are typically heavily doped n-type crystals. Compensation by acceptors is used to move the Fermi level to the middle of the band gap, but even then TIs have a frustratingly small bulk resistivity. We show that this small resistivity is the result of band bending by poorly screened fluctuations in the random Coulomb potential. Using numerical simulations of a completely compensated TI, we find that the bulk resistivity has an activation energy of just 0.15 times the band gap, in good agreement with experimental data. At lower temperatures activated transport crosses over to variable range hopping with a relatively large localization length.

Coulomb gap triptych in a periodic array of metal nanocrystals

The Coulomb gap in the single-particle density of states (DOS) is a universal consequence of electron-electron interaction in disordered systems with localized electron states. Here we show that in arrays of monodisperse metallic nanocrystals, there is not one but three identical adjacent Coulomb gaps, which together form a structure that we call a "Coulomb gap triptych." We calculate the DOS and the conductivity in two- and three-dimensional arrays using a computer simulation. Unlike in the conventional Coulomb glass models, in nanocrystal arrays the DOS has a fixed width in the limit of large disorder. The Coulomb gap triptych can be studied via tunneling experiments.

The problem of shot selection in basketball

In basketball, every time the offense produces a shot opportunity the player with the ball must decide whether the shot is worth taking. In this article, I explore the question of when a team should shoot and when they should pass up the shot by considering a simple theoretical model of the shot selection process, in which the quality of shot opportunities generated by the offense is assumed to fall randomly within a uniform distribution. Within this model I derive an answer to the question “how likely must the shot be to go in before the player should take it?” and I show that this lower cutoff for shot quality depends crucially on the number of shot opportunities remaining (say, before the shot clock expires), with larger demanding that only higher-quality shots should be taken. The function is also derived in the presence of a finite turnover rate and used to predict the shooting rate of an optimal-shooting team as a function of time. The theoretical prediction for the optimal shooting rate is compared to data from the National Basketball Association (NBA). The comparison highlights some limitations of the theoretical model, while also suggesting that NBA teams may be overly reluctant to shoot the ball early in the shot clock.

Theory of volumetric capacitance of an electric double-layer supercapacitor

Electric double-layer supercapacitors are a fast-rising class of high-power energy storage devices based on porous electrodes immersed in a concentrated electrolyte or ionic liquid. As yet there is no microscopic theory to describe their surprisingly large capacitance per unit volume (volumetric capacitance) of ~100 F/cm(3), nor is there a good understanding of the fundamental limits on volumetric capacitance. In this paper we present a non-mean-field theory of the volumetric capacitance of a supercapacitor that captures the discrete nature of the ions and the exponential screening of their repulsive interaction by the electrode. We consider analytically and via Monte Carlo simulations the case of an electrode made from a good metal and show that in this case the volumetric capacitance can reach the record values. We also study how the capacitance is reduced when the electrode is an imperfect metal characterized by some finite screening radius. Finally, we argue that a carbon electrode, despite its relatively large linear screening radius, can be approximated as a perfect metal because of its strong nonlinear screening. In this way the experimentally measured capacitance values of ~100 F/cm(3) may be understood.

Anomalously large capacitance of an ionic liquid described by the restricted primitive model

We use Monte Carlo simulations to examine the simplest model of a room-temperature ionic liquid (RTIL), called the "restricted primitive model," at a metal surface. We find that at moderately low temperatures the capacitance of the metal-RTIL interface is so large that the effective thickness of the electrostatic double layer is up to three times smaller than the ion radius. To interpret these results we suggest an approach which is based on the interaction between discrete ions and their image charges in the metal surface and which therefore goes beyond the mean-field approximation. When a voltage is applied across the interface, the strong image attraction causes counterions to condense onto the metal surface to form compact ion-image dipoles. These dipoles repel each other to form a correlated liquid. When the surface density of these dipoles is low, the insertion of an additional dipole does not require much energy. This leads to a large capacitance C that decreases monotonically with voltage V, producing a "bell-shaped" curve C(V). We also consider what happens when the electrode is made from a semimetal rather than a perfect metal. In this case, the finite screening radius of the electrode shifts the reflection plane for image charges to the interior of the electrode, and we arrive at a "camel-shaped" C(V). These predictions seem to be in qualitative agreement with experiment.

Non-mean-field theory of anomalously large double layer capacitance

Mean-field theories claim that the capacitance of the double layer formed at a metal/ionic conductor interface cannot be larger than that of the Helmholtz capacitor, whose width is equal to the radius of an ion. However, in some experiments the apparent width of the double layer capacitor is substantially smaller. We propose an alternate non-mean-field theory of the ionic double layer to explain such large capacitance values. Our theory allows for the binding of discrete ions to their image charges in the metal, which results in the formation of interface dipoles. We focus primarily on the case where only small cations are mobile and other ions form an oppositely charged background. In this case, at small temperature and zero applied voltage dipoles form a correlated liquid on both contacts. We show that at small voltages the capacitance of the double layer is determined by the transfer of dipoles from one electrode to the other and is therefore limited only by the weak dipole-dipole repulsion between bound ions so that the capacitance is very large. At large voltages the depletion of bound ions from one of the capacitor electrodes triggers a collapse of the capacitance to the much smaller mean-field value, as seen in experimental data. We test our analytical predictions with a Monte Carlo simulation and find good agreement. We further argue that our "one-component plasma" model should work well for strongly asymmetric ion liquids. We believe that this work also suggests an improved theory of pseudocapacitance.

Capacitance of the double layer formed at the metal/ionic-conductor interface: how large can it be?

The capacitance of the double layer formed at a metal/ionic-conductor interface can be remarkably large, so that the apparent width of the double layer is as small as 0.3 A. Mean-field theories fail to explain such large capacitance. We propose an alternate theory of the ionic double layer which allows for the binding of discrete ions to their image charges in the metal. We show that at small voltages the capacitance of the double layer is limited only by the weak dipole-dipole repulsion between bound ions, and is therefore very large. At large voltages the depletion of bound ions from one of the capacitor electrodes triggers a collapse of the capacitance to the mean-field value.

Ionic conductivity on a wetting surface

Recent experiments measuring the electrical conductivity of DNA molecules highlight the need for a theoretical model of ion transport along a charged surface. Here we present a simple theory based on the idea of unbinding of ion pairs. The strong humidity dependence of conductivity is explained by the decrease in the electrostatic self-energy of a separated pair when a layer of water (with high dielectric constant) is adsorbed to the surface. We compare our prediction for conductivity to experiment and discuss the limits of its applicability.

137. ELUCIDATION OF THE MOLECULAR MECHANISMS THAT UNDERPIN CAPACITATION-ASSOCIATED SPERM SURFACE REMODELLING

Recent research from our laboratory has provided evidence that sperm-egg interaction is mediated by a multimeric sperm receptor complex. Furthermore, we have demonstrated that this complex is assembled on the sperm surface during the final phase of their maturation, a process known as capacitation. The mechanisms underpinning this capacitation-associated surface remodelling remain poorly understood and are the subject of our current investigation. Specifically we have focused on whether this process is driven by vesicle mediated, intracellular trafficking of proteins. For this purpose we have examined the presence and physiological significance of a key part of the molecular machinery necessary for this form of trafficking, namely the enzyme dynamin. Our studies revealed that sperm are endowed with at least two isoforms of dynamin (1 and 2) both of which reside within the peri-acrosomal region of the sperm head, a location compatible with a role in sperm membrane remodelling. Consistent with this putative role, it was demonstrated that dynamin 1 was phosphorylated during capacitation, a post-translational modification that has been causally linked to both its activation and to the capacitation-associated surface remodelling of mouse spermatozoa. Furthermore, it was demonstrated that pharmacological inhibition of dynamin activity led to a concomitant reduction in the ability of spermatozoa to bind to the zona pellucida of homologous oocytes. This suppression was also correlated with reduced surface expression of a number of proteins including a subset of putative sperm-zona pellucida receptors. Collectively these data support the novel hypothesis that dynamin does participate indirectly in sperm membrane remodelling events by virtue of its ability to mediate intracellular trafficking.

The high prevalence of sexual concerns among women seeking routine gynecological care

BACKGROUND

Sexuality is an important part of health, quality of life, and general wellbeing. Studies indicate that less than half of patients' sexual concerns are known by their physicians, and physicians are unaware of how common these sexual concerns are in their practices. Our objective was to determine the prevalence and type of sexual concerns among women seeking routine gynecological care.

METHODS

We mailed the survey in waves. Of 1480 women seeking routine gynecological care from the departments of Family Practice and Obstetrics and Gynecology at Madigan Army Medical Center between August 1992 and January 1993, 964 responded. The main outcome measures were self-reported sexual concerns and their experiences with discussing these concerns with a physician.

RESULTS

A A total of 98.8% of the women we surveyed reported one or more sexual concerns. The most frequently reported concerns were lack of interest (87.2%), difficulty with orgasm (83.3%), inadequate lubrication (74.7%), dyspareunia (71.7%), body image concerns (68.5%), unmet sexual needs (67.2%), and needing information about sexual issues (63.4%). More than half reported concerns about physical or sexual abuse, and more than 40% reported sexual coercion at some point in their lives.

CONCLUSIONS

Our results suggest that sexual health concerns are prevalent for women seeking routine gynecological care. Sexual health inquiry should be a regular and important part of health care maintenance.

Changing the guard: how the Mewburn Veterans Centre received new owners

The consolidation of programs and services is currently of great interest in Canadian health care. This article describes the process that was used to transfer a 146-bed continuing care centre from one hospital organization to another. Leadership, team work, structure and communication were found to be the keys to success in dealing with the many problems and complexities of a facility transfer.

Papanicolaou smear quality assurance: providing feedback to physicians

BACKGROUND

The effective management of Papanicolaou (Pap) smears depends on the reliability and accuracy of obtaining and interpreting the specimen. Provider sampling error is one of the important factors contributing to inadequate specimens. Feedback on provider performance may be an effective way to improve the quality of Pap smears.

METHODS

A pilot study in a university-based residency program involving resident and faculty physicians was initiated to assess the impact of feedback on performance of Pap smears. After establishing adequacy and inadequacy criteria and recording adequacy rates for 3 months, individual and group feedback was implemented. No formal educational intervention on Pap smear technique was undertaken.

RESULTS

The quality of 836 Pap smears performed by 9 faculty and 13 resident physicians showed continued improvement in both sampling and slide preparation to 90% adequacy over a 9-month period. This improvement, though clinically useful, was not statistically significant owing to the relatively small numbers of smears performed by each physician. This form of feedback may be useful in both practice and educational settings.

CONCLUSIONS

Feedback without any formal educational intervention led to a clinically useful trend of improvement in the quality of Pap smears, which has been sustained since the study began. This type of simple feedback may be useful in practice settings and particularly valuable in pinpointing areas for improvement for learners in residency programs.

Regulation of pyelonephritis-associated pili phase-variation in Escherichia coli: binding of the PapI and the Lrp regulatory proteins is controlled by DNA methylation

Expression of pyelonephritis-associated pili (Pap) in Escherichia coli is under a phase-variation control mechanism in which individual cells alternate between pili+ (ON) and pili- (OFF) states through a process involving DNA methylation by deoxyadenosine methylase (Dam). Methylation of two GATC sites (GATC1028 and GATC1130) within the pap regulatory region is differentially inhibited in phase ON and phase OFF cells. The GATC1028 site of phase ON cells is non-methylated and the GATC1130 site is fully methylated. Conversely, in phase OFF cells the GATC1028 site is fully methylated whereas the GATC1130 site is non-methylated. Two transcriptional activators, PapI and Lrp (leucine-responsive regulatory protein), are required for this specific methylation inhibition. DNA footprint analysis using non-methylated pap DNAs indicates that Lrp binds to a region surrounding the GATC1130 site, whereas PapI does not appear to bind to pap regulatory DNA. However, addition of Lrp and PapI together results in an additional DNaseI footprint around the GATC1028 site. Moreover, Dam methylation inhibits binding of Lrp/PapI near the GATC1028 site and alters binding of Lrp at the GATC1130 site. Our results support a model in which Dam and Lrp/PapI compete for binding near the GATC1028 site, regulating the methylation state of this GATC site and, consequently, the pap transcription state.

Development of a Pap smear quality-assurance system in family practice

BACKGROUND AND OBJECTIVES

To improve the effectiveness of cervical cancer screening, quality assurance programs can be designed to ensure that normal Pap smear results are dealt with appropriately and that Pap smears yielding inadequate specimen material are brought to a physician's attention. The objective of this study was to use a new Pap Smear Quality Assurance (PAPQA) system to determine and monitor the performance of physicians in a family practice over a two-year period.

METHODS

We developed a PAPQA system designed to gather data and report on: 1) Pap smear adequacy, 2) reporting of abnormal results to physicians, and 3) follow-up of patients who had abnormal Pap smear results. We followed these parameters for two years.

RESULTS

Over a two-year period, 2,771 cervical Pap smears were performed, of which 64% were normal. The percentage of Pap smears that yielded adequate specimen material improved from 82% to 91%, an improvement we attributed to feedback the system provided to physicians. Overall, Pap smear results and follow-up appeared in the medical record of 94% of patients who had abnormal findings. However, during the second year, the quality assurance system detected a deterioration in documentation of results and follow-up plans that coincided with moving the practice to a new facility. The operating cost for this program was approximately $950 per year.

CONCLUSIONS

Quality assurance programs can effectively monitor physicians' performance in dealing with abnormal Pap smears, can detect deteriorations in performance, and can improve some aspects of performance through feedback reporting to physicians.

Inside the chart review

The study on which this article is based was conducted at the University of North Carolina's Family Practice Center to analyze the content of teaching that occurred in the face-to-face chart review sessions. Data were collected from a sample of 276 chart reviews and processed by a microcomputer program that compiled reports of types of problems encountered by residents, topics of discussion pertinent to those problems, and learning needs defined as a result of those discussions. Reports were generated for faculty members nad residents, and summary reports were produced for the teaching encounters. This system of computerized data analysis can be used to identifying learning needs of residents and areas of teaching emphasis for faculty. The information can be used for educational planning and faculty development.

Observations on Tc-99m-erythrocyte venography in normal subjects and in patients with deep vein thrombosis

Blood pool imaging of the lower limbs with Tc-99m-erythrocytes demonstrates the deep venous channels of the leg. The radionuclide image of deep vein thrombosis, by this method, appears sufficiently distinctive to suggest that Tc-99m-erythrocyte venography may be a useful screening examination for deep vein patency.