Helsinki University of Technology

About: Helsinki University of Technology is a based out in . It is known for research contribution in the topics: Artificial neural network & Finite element method. The organization has 8962 authors who have published 20136 publications receiving 723787 citations. The organization is also known as: TKK & Teknillinen korkeakoulu.

Assessing the nature of lipid raft membranes.

Abstract: The paradigm of biological membranes has recently gone through a major update. Instead of being fluid and homogeneous, recent studies suggest that membranes are characterized by transient domains with varying fluidity. In particular, a number of experimental studies have revealed the existence of highly ordered lateral domains rich in sphingomyelin and cholesterol (CHOL). These domains, called functional lipid rafts, have been suggested to take part in a variety of dynamic cellular processes such as membrane trafficking, signal transduction, and regulation of the activity of membrane proteins. However, despite the proposed importance of these domains, their properties, and even the precise nature of the lipid phases, have remained open issues mainly because the associated short time and length scales have posed a major challenge to experiments. In this work, we employ extensive atom-scale simulations to elucidate the properties of ternary raft mixtures with CHOL, palmitoylsphingomyelin (PSM), and palmitoyloleoylphosphatidylcholine. We simulate two bilayers of 1,024 lipids for 100 ns in the liquid-ordered phase and one system of the same size in the liquid-disordered phase. The studies provide evidence that the presence of PSM and CHOL in raft-like membranes leads to strongly packed and rigid bilayers. We also find that the simulated raft bilayers are characterized by nanoscale lateral heterogeneity, though the slow lateral diffusion renders the interpretation of the observed lateral heterogeneity more difficult. The findings reveal aspects of the role of favored (specific) lipid–lipid interactions within rafts and clarify the prominent role of CHOL in altering the properties of the membrane locally in its neighborhood. Also, we show that the presence of PSM and CHOL in rafts leads to intriguing lateral pressure profiles that are distinctly different from corresponding profiles in nonraft-like membranes. The results propose that the functioning of certain classes of membrane proteins is regulated by changes in the lateral pressure profile, which can be altered by a change in lipid content.

Impurity and alloying effects on interfacial reaction layers in Pb-free soldering

Abstract: The objective of this review is to study the effect of minor alloying and impurity elements, typically present in electronics manufacturing environment, on the interfacial reactions between Sn and Cu, which is the base system for Pb-free soldering. Especially, the reasons leading to the observed interfacial reaction layers and their microstructural evolution are analysed. The following conclusions have been reached. Alloying and impurity elements can have three major effects on the reactions between the Sn-based solder and the conductor metal: Firstly, they can increase or decrease the reaction/growth rate. Secondly, additives can change the physical properties of the phases formed (in the case of Cu and Sn, ɛ and η). Thirdly they can form additional reaction layers at the interface or they can displace the binary phases that would normally appear and form other reaction products instead. Further, the alloying and impurity elements can be divided roughly into two major categories: (i) elements (Ni, Au, Sb, In, Co, Pt, Pd, and Zn) that show marked solubility in the intermetallic compound (IMC) layer (generally take part in the interfacial reaction in question) and (ii) elements (Bi, Ag, Fe, Al, P, rare-earth elements, Ti and S) that are not extensively soluble in IMC layer (only change the activities of species taking part in the interfacial reaction and do not usually participate themselves). The elements belonging to category (i) usually have the most pronounced effect on IMC formation. It is also shown that by adding appropriate amounts of certain alloying elements to Sn-based solder, it is possible to tailor the properties of the interfacial compounds to exhibit, for example, better drop test reliability. Further, it is demonstrated that if excess amount of the same alloying elements are used, drastic decrease in reliability can occur. The analysis for this behaviour is based on the so-called thermodynamic–kinetic method.

A family of mixed finite elements for the elasticity problem

Abstract: A new mixed finite element formulation for the equations of linear elasticity is considered. In the formulation the variables approximated are the displacement, the unsymmetric stress tensor and the rotation. The rotation act as a Lagrange multiplier introduced in order to enforce the symmetry of the stress tensor. Based on this formulation a new family of both two-and three-dimensional mixed methods is defined. Optimal error estimates, which are valid uniformly with respect to the Poisson ratio, are derived. Finally, a new postprocessing scheme for improving the displacement is introduced and analyzed.

Multiscale modeling of emergent materials: biological and soft matter

Abstract: In this review, we focus on four current related issues in multiscale modeling of soft and biological matter. First, we discuss how to use structural information from detailed models (or experiments) to construct coarse-grained ones in a hierarchical and systematic way. This is discussed in the context of the so-called Henderson theorem and the inverse Monte Carlo method of Lyubartsev and Laaksonen. In the second part, we take a different look at coarse graining by analyzing conformations of molecules. This is done by the application of self-organizing maps, i.e., a neural network type approach. Such an approach can be used to guide the selection of the relevant degrees of freedom. Then, we discuss technical issues related to the popular dissipative particle dynamics (DPD) method. Importantly, the potentials derived using the inverse Monte Carlo method can be used together with the DPD thermostat. In the final part we focus on solvent-free modeling which offers a different route to coarse graining by integrating out the degrees of freedom associated with solvent.

Dynamic thermal modelling of power transformers

Abstract: The aim of this paper is to introduce hot-spot and top-oil temperature thermal models for more accurate temperature calculations during transient states based on data received in a normal heat run test (i.e., the top oil in the tank of the transformer and the average winding-to-average oil gradient). Oil viscosity changes and loss variation with temperature are taken into account. The new thermal models will be validated using experimental (fiber-optic test) results obtained at varying load current on a 250-MVA-ONAF-cooled unit, a 400-MVA-ONAF-cooled unit and a 605-MVA-OFAF-cooled unit. The results are also compared with the IEEE-Loading guide (1995) Annex G method.