The origin ofhigh hydraulic resistance for filter cakes ofdef ormable particles: cell-bed deformation or surface-layer effect?

This study reports a numerical approach for modeling the hydraulic resistance ofa filter cake ofdef ormable cells. First, a mechanical and osmotic model that describes the volume fraction ofsolids in a bed ofyeast cells as a function ofthe compressive pressure it experiences is presented. The effects ofpressure on the compressibility ofyeast cells beds were further investigated both by filtration experiments and by centrifugal experiments based on the multiple speed equilibrium sediment height technique. When comparing the latter measurements with compression model calculations, we observed that the method based on centrifugal experiments suffers from rapid relaxation of the compressed bed. Concerning the filtration experiments, specific resistance ofwell-defined bed ofcells were calculated by a combination of the compression model with a formulation for hydraulic resistivity developed using the Lattice Boltzmann method. We further explain the experimental values observed for the hydraulic resistance ofcell beds, assuming that the first layer ofcells in contact with the membrane partially blocks the membrane area open to flow. In such a case, the blocked area seems to be a constant fraction of the normal cell–cell contact area

Lattice Boltzmann method for colloidal dispersions with phase change.

Colloidal dispersions are known to undergo phase transition in a number of processes. This often gives rise to formation of structures in a flowing medium. In this paper, we present a model for flow of a colloidal dispersion with phase change. Two distribution functions are used. The colloid is described as a non-ideal fluid capable of phase change, but rather than taking the dispersion medium as the second fluid, a better choice is the dispersion (water plus colloid) which can be considered as an incompressible fluid. This choice allows a standard Lattice Boltzmann (LB) model for incompressible fluids to be used in combination with for the 'free-energy' LB model for the colloid. The coupling between the two fluids is the drag force on the colloid and the dependence of the viscosity of the overall fluid on the particle volume fraction. The problems raised by characteristic times and lengths have been treated. The main application considered is the growth dynamics or domain structuration of protein dispersions during dead-end filtration on a membrane surface

How a colloidal paste flows – scaling behaviors in dispersions of aggregated particles under mechanical stress –

We have developed a novel computational scheme that allows direct numerical simulation of the mechanical behavior of sticky granular matter under stress. We present here the general method, with particular emphasis on the particle features at the nanometric scale. It is demonstrated that, although sticky granular material is quite complex and is a good example of a challenging computational problem (it is a dynamical problem, with irreversibility, self-organization and dissipation), its main features may be reproduced on the basis of rather simple numerical model, and a small number of physical parameters. This allows precise analysis of the possible deformation processes in soft materials submitted to mechanical stress. This results in direct relationship between the macroscopic rheology of these pastes and local interactions between the particles

Numerical simulation of a UV photografting process for hollow-fiber membranes

A numerical model has been developed to represent the process by which hollow-fiber membranes can undergo continuous surface modification by UV photografting. The model takes into account the coupled effects of radiation, mass transfer with polymerization reaction and heat transfer with evaporation. It gives approximately correct values for the mass of polymer grafted, but no attempt is made to relate this quantity with permeability or retention. The behavior of this complex model is used to explain how operating conditions can influence the result of the grafting process

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Influence of ionic specificity on the microstructure and the strength of gelled colloidal silica suspensions

The dynamic rheological behavior of gelled Ludox suspensions in the presence of the structure-breaker ammonium counterion and the structure-maker sodium counterion has been investigated. Depending on the nature of the electrolyte and its concentration, the results highlight the crucial effect of the network microstructure on the rheological results. Within the same microstructure, the strength of gelled networks has been found to be greater in the presence of the poorly hydrated ammonium counterions at any Ludox volume fraction. The obtained results were discussed with respect to recent contradictory trends reported in the literature concerning the behavior of silica suspensions

Ion-specific repulsive interactions in colloidal silica dispersions evidenced through osmotic compression measurements and implication in frontal ultrafiltration experiments

Comparative osmotic compression experiments were performed on colloidal silica dispersions in the presence of various chloride salts at the same 0.01Mconcentration with different counterions and highlighted the influence of ionic specificity on the resistance to water removal. These results were complemented with frontal ultrafiltration measurements that demonstrate modulation of the permeate flux according to the salt used

3D Reconstruction of Ultrafiltration Cakes from Binarised Images

Transmission electron microscopy (TEM) imaging is a common technique for studying microstructure of ultrafiltration cakes. It yields local microstructural information in the form of two-dimensional gray-level images of slices a few particle diameters in thickness. This work presents a simple yet powerful particle-by-particle reconstruction scheme for simulating ultrafiltration cake microstructure from TEM images. The scheme uses binarised TEM images, thereby permitting use of lesser quality TEM images. Moreover, the reconstruction scheme embeds the multi-scale nature of ultrafiltration cake microstructure using a novel concept that consists in matching the morphology of simulated and measured structures at a number of scales identifiable in the microstructure. In the end, reconstructed microstructures are intended for bettering our understanding of the relationships between cake morphology, ultrafiltration performance and operating conditions