Force distributions and force chains in random stiff fiber networks

We study the elasticity of random stiff fiber networks. The elastic response of the fibers is characterized by a central force stretching stiffness as well as a bending stiffness that acts transverse to the fiber contour. Previous studies have shown that this model displays an anomalous elastic regime where the stretching mode is fully frozen out and the elastic energy is completely dominated by the bending mode. We demonstrate by simulations and scaling arguments that, in contrast to the bending dominated \emph{elastic energy}, the equally important \emph{elastic forces} are to a large extent stretching dominated. By characterizing these forces on microscopic, mesoscopic and macroscopic scales we find two mechanisms of how forces are transmitted in the network. While forces smaller than a threshold $F_c$ are effectively balanced by a homogeneous background medium, forces larger than $F_c$ are found to be heterogeneously distributed throughout the sample, giving rise to highly localized force-chains known from granular media.Comment: 7 pages, 7 figures, final version as publishe

Non-universality of elastic exponents in random bond-bending networks

We numerically investigate the rigidity percolation transition in two-dimensional flexible, random rod networks with freely rotating cross-links. Near the transition, networks are dominated by bending modes and the elastic modulii vanish with an exponent f=3.0\pm0.2, in contrast with central force percolation which shares the same geometric exponents. This indicates that universality for geometric quantities does not imply universality for elastic ones. The implications of this result for actin-fiber networks is discussed.Comment: 4 pages, 3 figures, minor clarifications and amendments. To appear in PRE Rap. Com

A volume-preserving sharpening approach for the propagation of sharp phase boundaries in multiphase lattice Boltzmann simulations

Lattice Boltzmann models that recover a macroscopic description of multiphase flow of immiscible liquids typically represent the boundaries between phases using a scalar function, the phase field, that varies smoothly over several grid points. Attempts to tune the model parameters to minimise the thicknesses of these interfaces typically lead to the interfaces becoming fixed to the underlying grid instead of advecting with the fluid velocity. This phenomenon, known as lattice pinning, is strikingly similar to that associated with the numerical simulation of conservation laws coupled to stiff algebraic source terms. We present a lattice Boltzmann formulation of the model problem proposed by LeVeque and Yee [J. Comput. Phys. 86, 187] to study the latter phenomenon in the context of computational combustion, and offer a volume-conserving extension in multiple space dimensions. Inspired by the random projection method of Bao and Jin [J. Comput. Phys. 163, 216] we further generalise this formulation by introducing a uniformly distributed quasi-random variable into the term responsible for the sharpening of phase boundaries. This method is mass conserving and the statistical average of this method is shown to significantly delay the onset of pinning

Tools for reactor evaluation in bioprocesses

Aerobic microbial cultivations are industrially important group of processes and pose challenges for the reactor design. In particular, estimation of industrial scale conditions is difficult from laboratory and pilot scale data. Due to complex interaction of gas/liquid phase hydrodynamics, mass transfer parameters and microbial metabolism, both improvement of modelling tools and reactor design are desired. We present an approach to estimate growth conditions in industrial scale reactor by combining black-box metabolic models with CFD-model. The reactor type used here is Outotec OKTOP9000®, which is used in the industrial hydrometallurgical processes at 900 m3 scale. It is adopted to a laboratory setting and compared to stirred tank reactor (STR) in gas dispersion, mass transfer and yeast cultivation experiments. In addition, a kinetic model for the yeast growth is developed based on literature sources and validated by the laboratory scale batch cultivations. This kinetic model is used along with CFD-model that is developed to describe the flow and mass transfer conditions in the industrial scale reactor. The laboratory scale experiments show the feasibility of OKTOP9000® reactor when compared to STR, particularly with improved gas handling capacity. The modelling approach shows qualitatively similar behavior in the large scale simulations when compared to laboratory scale cultivations

Simulation of Flow of Mixtures Through Anisotropic Porous Media using a Lattice Boltzmann Model

We propose a description for transient penetration simulations of miscible and immiscible fluid mixtures into anisotropic porous media, using the lattice Boltzmann (LB) method. Our model incorporates hydrodynamic flow, diffusion, surface tension, and the possibility for global and local viscosity variations to consider various types of hardening fluids. The miscible mixture consists of two fluids, one governed by the hydrodynamic equations and one by diffusion equations. We validate our model on standard problems like Poiseuille flow, the collision of a drop with an impermeable, hydrophobic interface and the deformation of the fluid due to surface tension forces. To demonstrate the applicability to complex geometries, we simulate the invasion process of mixtures into wood spruce samples.Comment: Submitted to EPJ

Strain-controlled criticality governs the nonlinear mechanics of fibre networks

Disordered fibrous networks are ubiquitous in nature as major structural components of living cells and tissues. The mechanical stability of networks generally depends on the degree of connectivity: only when the average number of connections between nodes exceeds the isostatic threshold are networks stable (Maxwell, J. C., Philosophical Magazine 27, 294 (1864)). Upon increasing the connectivity through this point, such networks undergo a mechanical phase transition from a floppy to a rigid phase. However, even sub-isostatic networks become rigid when subjected to sufficiently large deformations. To study this strain-controlled transition, we perform a combination of computational modeling of fibre networks and experiments on networks of type I collagen fibers, which are crucial for the integrity of biological tissues. We show theoretically that the development of rigidity is characterized by a strain-controlled continuous phase transition with signatures of criticality. Our experiments demonstrate mechanical properties consistent with our model, including the predicted critical exponents. We show that the nonlinear mechanics of collagen networks can be quantitatively captured by the predictions of scaling theory for the strain-controlled critical behavior over a wide range of network concentrations and strains up to failure of the material

Capillary filling with pseudo-potential binary Lattice-Boltzmann model

We present a systematic study of capillary filling for a binary fluid by using a mesoscopic lattice Boltzmann model for immiscible fluids describing a diffusive interface moving at a given contact angle with respect to the walls. The phenomenological way to impose a given contact angle is analysed. Particular attention is given to the case of complete wetting, that is contact angle equal to zero. Numerical results yield quantitative agreement with the theoretical Washburn law, provided that the correct ratio of the dynamic viscosities between the two fluids is used. Finally, the presence of precursor films is experienced and it is shown that these films advance in time with a square-root law but with a different prefactor with respect to the bulk interface.Comment: 13 pages, 8 figures, accepted for publication on The European journal of physics

Lanthanide-based time-resolved luminescence immunoassays

The sensitive and specific detection of analytes such as proteins in biological samples is critical for a variety of applications, for example disease diagnosis. In immunoassays a signal in response to the concentration of analyte present is generated by use of antibodies labeled with radioisotopes, luminophores, or enzymes. All immunoassays suffer to some extent from the problem of the background signal observed in the absence of analyte, which limits the sensitivity and dynamic range that can be achieved. This is especially the case for homogeneous immunoassays and surface measurements on tissue sections and membranes, which typically have a high background because of sample autofluorescence. One way of minimizing background in immunoassays involves the use of lanthanide chelate labels. Luminescent lanthanide complexes have exceedingly long-lived luminescence in comparison with conventional fluorophores, enabling the short-lived background interferences to be removed via time-gated acquisition and delivering greater assay sensitivity and a broader dynamic range. This review highlights the potential of using lanthanide luminescence to design sensitive and specific immunoassays. Techniques for labeling biomolecules with lanthanide chelate tags are discussed, with aspects of chelate design. Microtitre plate-based heterogeneous and homogeneous assays are reviewed and compared in terms of sensitivity, dynamic range, and convenience. The great potential of surface-based time-resolved imaging techniques for biomolecules on gels, membranes, and tissue sections using lanthanide tracers in proteomics applications is also emphasized

Liikunnan merkitys alakoululaisen oppimiseen : Luento neljäsluokkalaisten huoltajille vanhempainillassa

Liikunnan on todettu useissa tutkimuksissa edistävän lasten kognitiivista kehitystä ja sitä kautta parantavan oppimisvalmiuksia. Tämä opinnäytetyö tehtiin yhteistyössä Törnävän koulun kanssa, osana ILOA-hyvinvointiprojektia. Opinnäytetyön tarkoituksena oli esitellä liikunnan ja oppimisen välistä yhteyttä Törnävän koulun neljäsluokkalaisten huoltajille vanhempainillassa. Opinnäytetyön tavoitteena oli lisätä huoltajien tietämystä liikunnan merkityksestä alakoululaisen oppimiseen. Tämän myötä huoltajat voivat motivoitua tukemaan enemmän lastensa fyysistä aktiivisuutta, ja siten lasten liikunnallisuus saattaa lisääntyä. Fyysisen aktiivisuuden lisääntyminen edistää lasten terveyttä sekä nor-maalien kognitiivisten taitojen ja oppimisvalmiuksien kehittymistä. Lisäksi tavoit-teena oli parantaa omaa tietämystämme liikunnan merkityksestä alakoululaisen oppimiseen. Opinnäytetyö toteutettiin toiminnallisena opinnäytetyönä järjestämällä luento Tör-nävän koulun neljäsluokkien vanhempainillassa lokakuussa 2014. Vanhempainillan luentomateriaali eli PowerPoint-esitys laadittiin opinnäytetyön teoreettiseen viitekehykseen pohjautuen. Teoreettisessa viitekehyksessä esiteltiin alakoululaisen liikuntasuositukset, tarkasteltiin oppimista eri näkökulmista sekä selvitettiin liikunnan ja oppimisen välistä yhteyttä viimeaikaisen, kansainvälisen tutkimustiedon pohjalta. Yhtenä asiakokonaisuutena oli lapsen tukeminen ja kannustaminen liikunnallisuuteen. Neljäsluokkalaisten huoltajilta ja opettajilta saadun palautteen mukaan opinnäytetyö oli hyödyllinen. Kehittämishaasteena voisi antaa tietoa liikunnan ja oppimisen välisestä yhteydestä isommallekin kohderyhmälle – lapsille, nuorille ja heidän huoltajilleen. Lisäksi jatkotutkimuksena voisi kartoittaa terveydenhoitajien tietämystä liikunnan ja oppimisen välisestä yhteydestä.Several studies indicate that physical activity promotes children’s cognitive devel-opment and this way improves their learning readiness. This thesis was done in cooperation with Törnävä primary school as a part of ILOA-welfare project. The intention of this thesis was to introduce the link between physical activity and learning for the parents of fourth-graders at Törnävä primary school. The goal of the thesis was to increase awareness of the effects that sports have on learning. Thereby parents are more motivated to support their children’s physi-cal activities, which may have a positive influence on their physical activeness, thus promoting health, cognitive skills and learning readiness. One of our goals was also to improve our own knowledge about the subject. The thesis was presented at a parents’ meeting in October 2014. In the presenta-tion we had PowerPoint slides, which contained information based on the theoretical frame of reference. The theoretical frame of reference includes physical activity recommendations, looking at learning from different views, and information about the connection between physical activity and learning, based on the latest international research. This thesis as a whole is about supporting and motivating children to be physically active. Based on the feedback from the parents and teachers of the fourth-graders, our thesis has been useful. In the future a challenge is to spread information to reach a bigger target group including children, young people and their guardians. It would also be recommended to chart public health nurses’ knowledge about the connection between physical activity and learning