Dilatational rheology of water-in-diesel fuel interfaces: effect of surfactant concentration and bulk-to-interface exchange.

Micrometer-sized water droplets dispersed in diesel fuel are stabilized by the fuel's surface-active additives, such as mono-olein and poly(isobutylene)succinimide (PIBSI), making the droplets challenging for coalescing filters to separate. Dynamic material properties found from interfacial rheology are known to influence the behavior of microscale droplets in coalescing filters. In this work, we study the interfacial dilatational properties of water-in-fuel interfaces laden with mono-olein and PIBSI, with a fuel phase of clay-treated ultra-low sulphur diesel (CT ULSD). First, the dynamic interfacial tension (IFT) is measured using pendant drop tensiometry, and a curvature-dependent form of the Ward and Tordai diffusion equation is applied for extracting the diffusivity of the surfactants. Additionally, Langmuir kinetics are applied to the dynamic IFT results to obtain the maximum surface concentration (Γ∞) and ratio of adsorption to desorption rate constants (κ). We then use a capillary pressure microtensiometer to measure the interfacial dilatational modulus, and further extract the characteristic frequency of surfactant exchange (ω0) by fitting a model assuming diffusive exchange between the interface and bulk. In this measurement, 50-100 µm diameter water droplets are pinned at the tip of a glass capillary in contact with the surfactant-containing fuel phase, and small amplitude capillary pressure oscillations over a range of frequencies from 0.45-20 rad s-1 are applied to the interface, inducing changes in interfacial tension and area to yield the dilatational modulus, E*(ω). Over the range of concentrations studied, the dilatational modulus of CT ULSD with either mono-olein or PIBSI increases with a decrease in bulk concentration and plateaus at the lowest concentrations of mono-olein. Characteristic frequency (ω0) values extracted from the fit are compared with those calculated using equilibrium surfactant parameters (κ and Γ∞) derived from pendant drop tensiometry, and good agreement is found between these values. Importantly, the results imply that diffusive exchange models based on the equilibrium relationships between surfactant concentration and interfacial tension can be used to infer the dynamic dilatational behavior of complex surfactant systems, such as the water-in-diesel fuel interfaces in this study.

Insights into the Microscale Coalescence Behavior of Surfactant-Stabilized Droplets Using a Microfluidic Hydrodynamic Trap.

Coalescence of micrometer-scale droplets is impacted by several parameters, including droplet size, viscosities of the two phases, droplet velocity, angle of approach, as well as interfacial tension and surfactant coverage. The thinning dynamics of films between coalescing droplets can be particularly complex in the presence of surfactants, due to the generation of Marangoni stresses and reduced film mobility. Here, a microfluidic hydrodynamic "Stokes" trap is used to gently steer and trap surfactant-laden micrometer-sized droplets at the center of a cross-slot. Water droplets are formed upstream of the cross-slot using a microfluidic T-junction, in heavy and light mineral oils and stabilized using SPAN 80, an oil-soluble surfactant. Incoming droplets are made to coalesce with the trapped droplet, yielding measurements of the film drainage time. Film drainage times are measured as a function of continuous phase viscosity, incoming droplet speed, trapped droplet size, and surfactant concentrations above and below the critical micelle concentration (CMC). As expected, systems with higher surfactant concentrations and slower incoming droplet speed exhibit longer film drainage times. At low surfactant concentrations, the drainage time is longer for the more viscous heavy mineral oil in the continuous phase, whereas at high surfactant concentrations, the dependence on continuous phase viscosity vanishes. Perhaps more surprisingly, larger droplets and high confinement also result in longer film drainage times, potentially due to deformation of the droplet interfaces. The results are used here to determine critical conditions for coalescence, including both an upper and a lower critical capillary number. Moreover, it is shown that induced surfactant concentration gradient effects enable coalescence events after the droplets had originally flocculated, at surfactant concentrations above the CMC. The microfluidic hydrodynamic trap provides new insights into the role of surfactants in film drainage and opens avenues for controlled coalescence studies at micrometer length scales and millisecond time scales.

PA practice analysis: Multidisciplinary tasks, knowledge, and skills.

In conducting its practice analysis, the National Commission on Certification of Physician Assistants incorporated new approaches in 2015. Twelve groups of PAs identified knowledge, tasks, and skills required for practice not only in primary care, but also in 11 practice focus areas (specialties). In addition, a list of diseases and disorders likely to be encountered was identified for each specialty. A representative sample of 15,771 certified PAs completed an online algorithm-driven survey and data were analyzed to determine how practice is the same for all PAs and how it differs depending on the practice focus. Larger differences between PAs in different specialties were seen in the frequency ratings of diseases and disorders encountered than for ratings of task, knowledge, and skill statements. Distinctions between PAs in primary care and those in other specialties were more pronounced, as the specialty was more divergent from primary care.

A Retrospective Review of Physician Assistant Education Association End of Rotation Examinations.

The PAEA End of Rotation™ exams were developed to assess medical knowledge of the 7 core supervised clinical practice experiences, including Emergency Medicine, Family Medicine, Internal Medicine, General Surgery, Pediatrics, Women's Health, and Psychiatry and Behavioral Health. The examinations were created by experienced PA educators and national exam experts with continual review and content updates. This paper summarizes changes since the inception of the exam program, including test construction and development, reliability, validity, and scale scoring. They are built using content blueprints and topic lists that were developed by experienced PA educators and national examination experts. All examination items are peer reviewed by PA educators and statistically validated for accuracy and consistency by psychometricians who specialize in examination development. This article will review the changes since the inception of the exam program including test construction and development, reliability, validity and scale scoring. Also addressed is an update on the PAEA End of Curriculum™ examination.

Calibrating the Medical Council of Canada's Qualifying Examination Part I using an integrated item response theory framework: a comparison of models and designs.

PURPOSE: The aim of this research was to compare different methods of calibrating multiple choice question (MCQ) and clinical decision making (CDM) components for the Medical Council of Canada's Qualifying Examination Part I (MCCQEI) based on item response theory. METHODS: Our data consisted of test results from 8,213 first time applicants to MCCQEI in spring and fall 2010 and 2011 test administrations. The data set contained several thousand multiple choice items and several hundred CDM cases. Four dichotomous calibrations were run using BILOG-MG 3.0. All 3 mixed item format (dichotomous MCQ responses and polytomous CDM case scores) calibrations were conducted using PARSCALE 4. RESULTS: The 2-PL model had identical numbers of items with chi-square values at or below a Type I error rate of 0.01 (83/3,499 or 0.02). In all 3 polytomous models, whether the MCQs were either anchored or concurrently run with the CDM cases, results suggest very poor fit. All IRT abilities estimated from dichotomous calibration designs correlated very highly with each other. IRT-based pass-fail rates were extremely similar, not only across calibration designs and methods, but also with regard to the actual reported decision to candidates. The largest difference noted in pass rates was 4.78%, which occurred between the mixed format concurrent 2-PL graded response model (pass rate= 80.43%) and the dichotomous anchored 1-PL calibrations (pass rate= 85.21%). CONCLUSION: Simpler calibration designs with dichotomized items should be implemented. The dichotomous calibrations provided better fit of the item response matrix than more complex, polytomous calibrations.

Survey analysis with mixture Rasch models.

This research provides a demonstration of the utility of mixture Rasch models (MRMs) for the analysis of survey data. Specifically, a framework based on a mixture partial credit model (MPCM) will be presented. MRMs are able to provide information regarding latent classes (subpopulations without manifest grouping variables) and separate item parameter estimates for each of these latent classes. Analyses can provide insight into how a survey scale is functioning and how survey respondents differ from one another. The paper provides a detailed example with real survey data from a higher education survey administered to college seniors through all stages of model estimation and selection, description of model results, and follow-up analyses using the MRM results. The results found three distinct classes and discussed each class in terms of the pattern of item parameter estimates within class. The paper also investigated differences of class assignment based on the college the student belongs to on campus.

Amyloidosis of the nasopharynx: an unexpected cause of unilateral middle ear effusion.

Amyloidosis is an idiopathic disease that is characterized by the extracellular deposition of fibrillar proteins. The disease can be categorized as primary or secondary where deposits occur in conjunction with chronic diseases such as rheumatoid arthritis or tuberculosis. The deposits can be localized or systemically distributed. It can mimic, and also be associated with underlying malignancy. Primary amyloidosis is a rare cause of a nasopharyngeal lesion, and less so of a secondary middle ear effusion. Its association with underlying chronic and malignant disease must not be over-looked if serious complications are to be avoided. It is, therefore, important to consider this as a differential diagnosis in such patients.