The Role of Friction in Compaction and Segregation of Granular Materials

We investigate the role of friction in compaction and segregation of granular materials by combining Edwards' thermodynamic hypothesis with a simple mechanical model and mean-field based geometrical calculations. Systems of single species with large friction coefficients are found to compact less. Binary mixtures of grains differing in frictional properties are found to segregate at high compactivities, in contrary to granular mixtures differing in size, which segregate at low compactivities. A phase diagram for segregation vs. friction coefficients of the two species is generated. Finally, the characteristics of segregation are related directly to the volume fraction without the explicit use of the yet unclear notion of compactivity.Comment: 9 pages, 6 figures, submitted to Phys. Rev.

Accuracy of Sonographic Chorionicity Classification in Twin Gestations

To evaluate the accuracy of sonographic classification of chorionicity in a large cohort of twins and investigate which factors may be associated with sonographic accuracy

Maternal-fetal medicine fellows' perception and comfort with obstetrical ultrasound and prenatal diagnosis.

BackgroundUltrasound training is a vital component of maternal-fetal medicine fellowships in the United States. Of the 18 months of core clinical training, the American Board of Obstetrics and Gynecology currently requires a minimum of 3 months to be dedicated to ultrasound to be eligible for board certification. However, the experience and degree of hands-on training differ among the fellowship programs and have not been reassessed for nearly a decade.ObjectiveTo assess regional heterogeneity in the ultrasound training experience during maternal-fetal medicine fellowship in the United States.Study designA survey was distributed to postgraduate year (PGY)-6 maternal-fetal medicine fellows registered to attend an annual ultrasound training course before the conference (n=114). For programs with >1 fellow attending (n=39), only 1 of them completed the survey to represent the program. The questions included demographics of the program, ultrasound training structure, the fellows' self-perception of ultrasound capabilities, research, mentorship, and technical aspects of sonography.ResultsSeventy two postgraduate year 6 fellows with a wide geographic distribution as follows completed the survey (96% response rate): 10 (14%) from the West, 16 (22%) from the Midwest, 17 (24%) from the South, and 29 (40%) from the Northeast. Respondents undergoing training in the South were less likely to report feeling comfortable performing nuchal translucency and detailed anatomic surveys than those from other regions (nuchal translucency: P=.046; anatomy: P=.011). Most of the respondents reported feeling comfortable performing growth (78%) and umbilical artery Doppler (58%) and feeling uncomfortable with three-dimensional ultrasound, neurosonography, and fetal echocardiography. Respondents in the Northeast were more likely to report feeling comfortable performing chorionic villus sampling (P=.001). There was no difference among fellowship programs in the presence or absence of ultrasound curriculum, bedside teaching, ultrasound-focused research mentorship, or months of ultrasound training.ConclusionDespite the standardization of ultrasound training structure across the United States, there remains regional heterogeneity in fellow self-reported comfort with specific ultrasound techniques and chorionic villus sampling at a midpoint in their fellowship training. The maternal-fetal medicine attending involvement at the bedside did not affect the fellow self-reported comfort with ultrasound surveys. This study highlights the need for further optimization of maternal-fetal medicine fellowship ultrasound training, especially in advanced sonography and diagnostic procedures

Prepregnancy Obesity and Risks of Stillbirth.

We examined the association of maternal obesity with risk of stillbirth, focusing on whether the pattern of results varied by gestational age or maternal race-ethnicity or parity.Analyses included 4,012 stillbirths and 1,121,234 liveborn infants delivered in California from 2007-2010. We excluded stillbirths due to congenital anomalies, women with hypertensive disorders or diabetes, and plural births, to focus on fetuses and women without these known contributing conditions. We used Poisson regression to estimate relative risks (RR) and 95% confidence intervals (CI). Separate models were run for stillbirths delivered at 20-23, 24-27, 28-31, 32-36, 37-41 weeks, relative to liveborn deliveries at 37-41 weeks.For stillbirth at 20-23 weeks, RRs were elevated for all race-ethnicity and parity groups. The RR for a 20-unit change in BMI (which reflects the approximate BMI difference between a normal weight and an Obese III woman) was 3.5 (95% CI 2.2, 5.6) for nulliparous white women and ranged from 1.8 to 5.0 for other sub-groups. At 24-27 weeks, the association was significant (p<0.05) only for multiparous non-Hispanic whites; at 28-31 weeks, for multiparous whites and nulliparous whites and blacks; at 32-36 weeks, for multiparous whites and nulliparous blacks; and at 37-41 weeks, for all groups except nulliparous blacks. The pattern of results was similar when restricted to stillbirths due to unknown causes and somewhat stronger when restricted to stillbirths attributable to obstetric causes.Increased risks were observed across all gestational ages, and some evidence of heterogeneity of the associations was observed by race-ethnicity and parity