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

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

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

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

Effect of oxygen transfer on yeast growth:growth kinetic and reactor model to estimate scale-up effects in bioreactors

Abstract Large scale fermentations face challenges in mixing and mass transfer as well as in the design and construction of the equipment. Scale-up from laboratory and pilot scale experiments is difficult because different phenomena — such as mixing times and mass transfer conditions — scale in a different way. We study the effect of mass transfer, reactor type and scale on the growth of Pichia pastoris yeast. Batch cultivation experiments monitoring the cell growth and ethanol formation are conducted in laboratory scale in two reactor types — stirred tank and an Outotec OKTOP®9000 draft tube reactor. Model for the yeast growth — including respirative and fermentative metabolism and the effect of dissolved oxygen — is formed based on literature. For scale-up studies, the growth model is used along with one dimensional reactor model that accounts for liquid mixing, gas phase dynamics and local gas hold-up and mass transfer coefficient. By using a realistic growth model along with the reactor model, the simulated effects of scale-up are presented in terms of cell yield. A decrease in yield is noticed due to oxygen depletion in gas and insufficient liquid mixing. Potential improvements are related to the gas handling capacity and liquid mixing of the reactor

Mass transfer, gas hold-up and cell cultivation studies in a bottom agitated draft tube reactor and multiple impeller Rushton turbine configuration

Abstract Gas–liquid mass transfer is an important phenomenon in aerobic microbial cultivations, and the mass transfer performance of an industrial reactor strongly affects the overall process economics. Traditionally, industrial and laboratory bioreactors have been agitated with flat disc turbines (Rushton turbines) although there are many variants to this design. In addition, pneumatically agitated reactors such as bubble columns and airlift reactors have been studied and used by the industry. In this study we utilize an agitated draft tube reactor in cell cultivation and mass transfer studies. A standard reactor geometry agitated with three Rushton turbines was compared to Outotec OKTOP®9000 reactor which is a draft tube reactor agitated with a single impeller located just below the draft tube. The experiments included cell cultivation with Pichia pastoris yeast, determination of overall mass transfer coefficient by dynamic gassing in method and measurement of local gas hold-up by electrical impedance tomography (EIT). In addition, agitation power was estimated from the power consumption of the DC-motor. OKTOP®9000 reactor was found to have higher kL a values than the STR with similar agitation power and gas flowrate. The overall gas hold-up was similar in both geometries at same power inputs and gas flow rates. However, some significant differences were detected in the distribution of gas phase between the two geometries especially in the axial direction. Also changes in the gas dispersion regime can be detected from the spatial distribution of the gas hold-up measured by EIT. The cell cultivation experiments showed the applicability of this type of agitated draft tube reactor to bioprocesses although a direct comparison with Rushton geometry is not straightforward

Numerical simulation of biomass growth in OKTOP®9000 reactor at industrial scale

Abstract Computational fluid dynamics is a powerful method for scale-up of reactors although it is still challenging to fully embrace hydrodynamics and biological complexities. In this article, an aerobic fermentation of Pichia pastoris cells is modeled in a batch OKTOP®9000 reactor. The 800 m³ industrial scale reactor is equipped with a radial impeller, designed by Outotec Oy for gas dispersion in the draft tube reactor. Measured Np of the impeller is used in hydrodynamics validation. The resolved energy dissipation rate is compensated, and its influence on mass transfer is analyzed and discussed. Gas–liquid drag force is modified to simulate effects of liquid turbulence and bubble swarms. Resolved steady state multiphase hydrodynamics is used to simulate the fermentation process. Temporal evolution of species concentrations is compared to experimental data measured in a small copy of the reactor at lab scale (14 L). The effect of oxygenation on the P. pastoris cells cultivation is considered