A nonlinear mixed model framework for item response theory.

Mixed models take the dependency between observations based on the same cluster into account by introducing 1 or more random effects. Common item response theory (IRT) models introduce latent person variables to model the dependence between responses of the same participant. Assuming a distribution for the latent variables, these IRT models are formally equivalent with nonlinear mixed models. It is shown how a variety of IRT models can be formulated as particular instances of nonlinear mixed models. The unifying framework offers the advantage that relations between different IRT models become explicit and that it is rather straightforward to see how existing IRT models can be adapted and extended. The approach is illustrated with a self-report study on anger. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Nonlinear model order reduction and control of particle size distribution in a semibatch vinyl acetate/butyl acrylate emulsion copolymerization reactor

This paper addresses the control of the full particle size distribution (PSD) in a semibatch emulsion copolymerization reactor. The numerical approximation of a fundamental population balance model results in a high order system to accurately describe the distribution of particle size; therefore, model order reduction is required. Pseudo random input signals are input to the mechanistic model to generate a data set which covers the reachable region of the system, on the basis of which the transformation matrices are calculated by principal component analysis (PCA). A linear time varying model with reduced order obtained from the transformation matrices is augmented in the prediction equation of linear model predictive control. The performance of the controller is evaluated to drive the particle size distribution at the final time of the batch to the desired distribution in the presence of disturbances. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from the Seoul National University.     

Coagulation of humic acid by aluminium sulphate in saline water conditions

Chemical coagulation has been applied in practice for the pre-treatment of saline waters in the process of water treatment by membrane desalination. The relative magnitude of the ionic strength and individual ion concentrations (cations and anions) in saline waters leads to differences in the nature of coagulation by hydrolysing metal salts compared to the treatment of freshwaters with respect to colloid destabilisation, coagulation mechanisms, and colloidal removal. In this study, the coagulation of saline solutions containing humic acid by aluminium sulphate has been examined systematically. Laboratory experiments have been undertaken over a wide range of pH (4 to 10) and alum dose (3-1000 μM Al) conditions. Coagulation has been assessed principally in terms of colloidal particle removal (change in UV-absorbance) and colloid surface charge (zeta potential). The experimental results have shown that the coagulation ofhumic substances in seawater has some distinctly different characteristics compared to the well-established coagulation behaviour in surface freshwaters. These differences can be attributed to the solution chemistry and its modification of the aluminium hydrolysis reactions and the nature of the precipitates formed. Based on the results, coagulation performance domains (Al dose vs. pH) are defined, and the mechanisms of coagulation are discussed.     

Development and measurement of stress in polymer coatings

This paper reviews stress development mechanisms and stress measurement techniques for polymer coatings. Most polymeric coatings shrink during and after solidification due to chemical reaction, solvent evaporation, phase separation, or some combination thereof. Coating adhesion, however, prevents shrinkage from occurring freely; this frustration of in-plane shrinkage leads to a tensile stress in the plane of the coating. At the same time, stress accumulates, it may be relaxed by processes such as molecular motion. The measured stress at any time is the result of the competition between stress buildup from frustrated shrinkage and stress relief from relaxation. Accumulation of stress is a problem because it can lead to defects such as cracks. An understanding of stress development in various types of polymeric coating systems will lead to strategies for material selection, process optimization, and defect elimination. In this paper, background on stress development is provided, followed by an overview of stress measurement methods for polymer coatings. The remainder of the paper focuses on stress development during the drying and curing of polymer coatings, drawing many examples from previous stress measurement studies.     

Investigation into the formation and mechanical properties of a bioactive material based on collagen and calcium phosphate

A biomaterial composite was formed by the room-temperature precipitation of calcium phosphate (in the form of brushite) on to collagen. It was found that the addition of 1 mM O-phosphoserine (Ser P) causes the morphology of the brushite crystals to change from large plates to small needles. An increase in the surface coverage and weight fraction of brushite incorporated into the collagen was observed. The mechanical properties of this composite were tested in the wet state. The ultimate tensile strength (UTS) was 45 MPa compared with 34 MPa for the wet collagen. Osteoblastic differentiation was promoted on the surface of the material and new bone formed.     

Preimplantation adoption: establishing pregnancy using donated oocytes and spermatozoa

The experience of transferring embryos produced through in-vitro fertilization (IVF) utilizing donated oocytes and spermatozoa is described. Recipients (n = 28; aged 38-59 years) received oral micronized oestradiol and i.m. progesterone and were synchronized to donors undergoing ovarian stimulation. Reasons for selecting therapy included advanced reproductive age (> 42 years; n = 21) or hypergonadotrophic hypogonadism (n = 7), combined with severe male factor infertility in 23 couples. Five women were single and without partners. Oocytes were fertilized by cryopreserved spermatozoa designated for use by the recipient. Up to five embryos were transferred transcervically. Supernumerary embryos were cryopreserved. A total of 36 aspirations produced 15.6 +/- 7.3 oocytes per retrieval. In 10/36 cycles (27.8%), embryos were available for cryopreservation. Using fresh embryos, the overall pregnancy rate was 38.9% (14/36), clinical pregnancy rate 33.3% (12/36), and ongoing/delivered pregnancy rate 30.6% (11/36). Three ongoing pregnancies were later established by transferring cryopreserved embryos. Adjusting for these events, the per aspiration overall pregnancy rate per retrieval was 47.2%, clinical pregnancy rate 41.7%, and ongoing/delivered pregnancy rate 38.9%. Implantation rates per individual embryo transferred were 16.6% following fresh embryo transfer. A viable pregnancy was achieved by 14 of 28 women (50% cumulative pregnancy rate). We conclude that using donor oocytes and donor spermatozoa is efficacious and allows couples of whom both members suffer from severe gamete abnormalities and single functionally agonadal women an effective means of achieving pregnancy.     

Fuzzy modelling and estimation of economic relationships

A modelling strategy based on the application of fuzzy logic is shown to provide a powerful and efficient method for the estimation of non-linear and linear economic relationships. The procedure is particularly suitable for the estimation of ill-defined systems in which there is considerable uncertainty about the nature and range of key input variables. In addition, no prior knowledge is required about the form of the underlying relationships, and trend, cyclical and irregular components of the model can all be estimated in a single pass. The potential benefits of the fuzzy logic approach are illustrated using a model of real-wage behaviour in the United States over the period 1960-1995. The results suggest that the relationships in the model are basically non-linear.     

An infrared heat source for temperature control of a smallmicroscope flow bath

A simple, inexpensive, and easily installed infrared heat source with temperature controller for small superfusion baths used in electrophysiologic experiments is described. The output of a thermal sensor in the bath is amplified and sent to a feedback circuit which in turn controls the voltage supplied to a tungsten/halogen bulb focused on the bath. Fluctuation from the set point is no more than 0.2 degrees C with good fluid level control. Judging from the lack of change in the cardiac action potential duration, heart cells do not preferentially absorb infrared radiation.     

Niagara Cantilever

The first modern metal cantilever bridge in the United States, using erection methods that were to be utilized in most future cantilever bridges, was by C. C. Schneider across the Niagara Gorge in 1883. The Niagara, saw in order, John Roebling’s Railroad Suspension Bridge, Samuel Keefer’s Honeymoon Suspension Bridge, Edward Serrell’s Lewiston-Queenston Suspension Bridge, Schneider’s cantilever, Leffert Buck’s arch bridge at the falls as well as Buck’s arch built under Roebling’s suspension bridge. Schneider’s bridge had a useful life of over 40 years during a period when rolling stock on the railroads was increasing rapidly. The speed of erection of a new style bridge coupled with its performance makes it one of the most innovative and significant bridges built in the world at the time.     

Thermal and thermomechanical effects in dry sliding

Whenever friction occurs in dry sliding of mechanical components, mechanical energy is transformed into heat through surface and volumetric processes in and around the real area of contact. This frictional heating, and the thermal and thermomechanical phenomena associated with it, can have a very important influence on the tribological behavior of the sliding components, especially at high sliding velocities. Significant developments in the study of these phenomena are reviewed in this paper. Among the topics reviewed are mechanisms of frictional heating and the distribution of heat during sliding friction, the measurement and analysis of surface and nearsurface temperatures resulting from frictional heating, thermal deformation around sliding contacts and the changes in contact geometry caused by thermal deformation and thermoelastic instability, and the thermomechanical stress distribution around the frictionally heated and thermally deformed contact spots. The paper concludes with a discussion of the influence of the thermal and thermomechanical contact phenomena on wear, thermocracking and other modes of failure of sliding mechanical components.