Structuration and deformation of colloidal hydrogels.

The aim of the present paper is to determine the optimum conditions for the formation of homogeneous colloidal silica hydrogels by aggregation and drying processes, avoiding mechanical instabilities at the surface. Aggregation is controlled by adding monovalent salt to the silica nano-particle suspension while the drying of the sol is also modulated by changing the evaporation rate. A phase diagram reveals two regions in the parameter plane, ionic strength evaporation rate: a region where the drop undergoes an isotropic shrinkage and forms the required homogeneous gel and a region where mechanical instabilities appear due to the formation of a solid skin at the gel surface.

Adverse Mechanical Ventilation and Pneumococcal Pneumonia Induce Immune and Mitochondrial Dysfunctions Mitigated by Mesenchymal Stem Cells in Rabbits.

Background: Mechanical ventilation for pneumonia may contribute to lung injury due to factors that include mitochondrial dysfunction, and mesenchymal stem cells may attenuate injury. This study hypothesized that mechanical ventilation induces immune and mitochondrial dysfunction, with or without pneumococcal pneumonia, that could be mitigated by mesenchymal stem cells alone or combined with antibiotics.

Methods: Male rabbits underwent protective mechanical ventilation (8 ml/kg tidal volume, 5 cm H2O end-expiratory pressure) or adverse mechanical ventilation (20 ml/kg tidal-volume, zero end-expiratory pressure) or were allowed to breathe spontaneously.

Synergy between the crack pattern and substrate elasticity in colloidal deposits.

Desiccation cracks in colloidal deposits occur to release the excess strain energy arising from the competition between the drying induced shrinkage of the deposit and its adhesion to the substrate. Here we report remarkably different morphology of desiccation cracks in the dried patterns formed by the evaporation of sessile drops containing colloids on elastomer (soft) or glass (stiff) substrates. The change in the crack pattern, i.

Cracking to curling transition in drying colloidal films.

Drying-induced cracking is widely encountered in nature and is of fundamental interest in industrial applications. During desiccation, the evolution of water content is nonlinear. Considering the inhomogeneous procedure of desiccation, it is worth considering whether water content will affect the crack pattern formation.

Imbibition and structure of silica nanoporous media characterized by neutron imaging.

Hypothesis: Colloidal silica dispersions dried under controlled conditions form solid gels that display mechanical properties similar to those observed in several practical processes. An understanding of their structural characteristics and liquid flow properties can therefore help establish these gels as an alternative family of model materials to study practical porous systems.

Experiments: Neutron radiography is a non-destructive technique well-adapted to study hydrogen-rich domains in porous materials due to the high attenuation power of hydrogen.

The solute mechanical properties impact on the drying of dairy and model colloidal systems.

The evaporation of colloidal solutions is frequently observed in nature and in everyday life. The investigation of the mechanisms taking place during the desiccation of biological fluids is currently a scientific challenge with potential biomedical and industrial applications. In the last few decades, seminal works have been performed mostly on dried droplets of saliva, urine and plasma.

Influence of Bénard-Marangoni instability on the morphology of drying colloidal films.

The drying of colloidal suspensions is a very complex process leading to a sol-gel transition induced by solvent evaporation. The resulting film can even crack and delaminate. In this study, we investigate the drying process of a colloidal suspension with a highly volatile solvent and we show for initially millimeter-thick layers that the resulting pattern of delaminated plates considerably differs from what is usually observed for aqueous colloidal suspensions.

Imbibition on a porous layer: dynamical and mechanical characterization.

Solvent penetration in porous layers was analyzed using dynamical and mechanical characterization. Spreading dynamics of a solvent drop in a porous substrate provided parameters of the porous medium such as permeability and porosity. These measurements are relevant for many porous systems, for example paintings or porous varnishes and resins… We present direct visualizations of the drop as well as of the wet zone during the imbibition process and we evidence three distinct regimes.

Structuration of the Surface Layer during Drying of Colloidal Dispersions.

During evaporative drying of a colloidal dispersion, the structural behavior at the air-dispersion interface is of particular relevance to the understanding of the consolidation mechanism and the final structural and mechanical properties of the porous media. The drying interface constitutes the region of initial drying stress that, when accumulated over a critical thickness, leads to crack formation. This work presents an experimental study of top-down drying of colloidal silica dispersions with three different sizes (radius 5, 8, and 13 nm).

Drying colloidal systems: Laboratory models for a wide range of applications.

The drying of complex fluids provides a powerful insight into phenomena that take place on time and length scales not normally accessible. An important feature of complex fluids, colloidal dispersions and polymer solutions is their high sensitivity to weak external actions. Thus, the drying of complex fluids involves a large number of physical and chemical processes.

Mechanical ventilation and Streptococcus pneumoniae pneumonia alter mitochondrial homeostasis.

Required mechanical ventilation (MV) may contribute to bacterial dissemination in patients with Streptococcus pneumoniae pneumonia. Significant variations in plasma mitochondrial DNA (mtDNA) have been reported in sepsis according to the outcome. The impact of lung stretch during MV was addressed in a model of pneumonia.

Drying drops : Drying drops containing solutes: From hydrodynamical to mechanical instabilities.

The drying of complex fluids involves a large number of microscopic phenomena (transport and organization of non-volatile solutes) as well as hydrodynamic and mechanical instabilities. These phenomena can be captured in drying sessile drops where different domains can be identified: strong concentration gradients, formation of a glassy or porous envelope that withstands mechanical stress, and consolidation of a layer strongly adhering to the substrate at the drop edge. In colloidal systems, we quantify the evolution of the particle volume fraction at a nanometric scale and microscopic scale and identify the conditions for the envelope formation at the free surface by balancing the effect of diffusion and evaporation.

Linezolid and atorvastatin impact on pneumonia caused by Staphyloccocus aureus in rabbits with or without mechanical ventilation.

Pneumonia may involve methicillin-resistant Staphylococcus aureus (MRSA), with elevated rates of antibiotics failure. The present study aimed to assess the effect of statins given prior to pneumonia development. Spontaneously breathing (SB) or mechanically ventilated (MV) rabbits with pneumonia received atorvastatin alone, linezolid (LNZ) alone, or a combination of both (n = 5 in each group).

Crack opening: from colloidal systems to paintings.

Shrinkage cracks are observed in many materials, particularly in paintings where great interest lies in deducing quantitative information on the material with the aim of proposing authentication methods. We present experimental measurements on the crack opening induced by the drying of colloidal layers and compare these results to the case of a pictorial layer. We propose a simple model to predict the crack width as a function of the thickness of the drying layer, based on the balance between the drying stress buildup and the shear frictional stress with the substrate.

Mechanical Ventilation Alters the Development of Staphylococcus aureus Pneumonia in Rabbit.

Ventilator-associated pneumonia (VAP) is common during mechanical ventilation (MV). Beside obvious deleterious effects on muco-ciliary clearance, MV could adversely shift the host immune response towards a pro-inflammatory pattern through toll-like receptor (TLRs) up-regulation. We tested this hypothesis in a rabbit model of Staphylococcus aureus VAP.

Striped patterns induced by delamination of drying colloidal films.

The drying of a dispersion of nanoparticles on a solid substrate can result in the formation of spontaneous well-ordered stripe patterns left on the substrate. The evaporation of solvent yields large stresses in the material which usually cause crack formation and delamination from the substrate. The formation of these stripes results from a balance between the drying stress which drives the delamination crack front propagation and the cohesive properties of the material.

Surface patterns in drying films of silica colloidal dispersions.

We report an experimental study on the drying of silica colloidal dispersions. Here we focus on surface instability occurring in a drying paste phase before crack formation which affects the final film quality. Observations at macroscopic and microscopic scales reveal the occurrence of instability, and the morphology of the film surface.

Elapsed time for crack formation during drying.

The drying of colloidal films usually leads to mechanical instabilities that affect the uniformity of the final deposit. The resulting patterns are the signature of the mechanical stress, and reveal the way the system consolidates. We report experimental results on the crack patterns induced by the drying of sessile drops of concentrated dispersions.

Impact of the prone position in an animal model of unilateral bacterial pneumonia undergoing mechanical ventilation.

Background: The prone position (PP) has proven beneficial in patients with severe lung injury subjected to mechanical ventilation (MV), especially in those with lobar involvement. We assessed the impact of PP on unilateral pneumonia in rabbits subjected to MV.

Methods: After endobronchial challenge with Enterobacter aerogenes, adult rabbits were subjected to either "adverse" (peak inspiratory pressure = 30 cm H2O, zero end-expiratory pressure; n = 10) or "protective" (tidal volume = 8 ml/kg, 5 cm H2O positive end-expiratory pressure; n = 10) MV and then randomly kept supine or turned to the PP.

Direct observation of concentration profiles induced by drying of a 2D colloidal dispersion drop.

We report experimental results on the drying of a colloidal dispersion drop in a circular thin cell. This confined geometry is well adapted to quantify concentration profiles inside the drop using fluorescence microscopy. Two stages have been identified in the drop evolution.

Influence of mechanical properties of nanoparticles on macrocrack formation.

We present an experimental investigation of drying suspensions of both hard and soft nanolatex spheres. The crack formation is examined as a function of the proportion of hard and soft deformable particles, leading to tunable elastic properties of the drying film. In our experimental systems, no crack formation could be observed below an onset value of the proportion in hard spheres phi approximately 0.

When a crack is oriented by a magnetic field.

Upon drying, colloidal suspensions undergo a phase transformation from a "liquid" to a "gel" state. With further solvent evaporation, tensile stresses develop in the gel, which ultimately leads to fractures. These generally manifest themselves in regular cracking patterns which reflect the physical conditions of the drying process.

Alternating crack propagation during directional drying.

The propagation of fractures during the drying of a colloidal silica suspension confined in a vertical microtube is investigated. During the drying process, the particle concentration increases until gel formation. In the gelled region, the tensile stresses increase and lead to the formation of two vertical perpendicular cracks propagating in the drying direction, dividing the tube into four equivalent regions.