Percolation and phase behavior in cellulose nanocrystal suspensions from nonlinear rheological analysis.

We examine the influence of surface charge on the percolation, gel-point and phase behavior of cellulose nanocrystal (CNC) suspensions in relation to their nonlinear rheological material response. Desulfation decreases CNC surface charge density which leads to an increase in attractive forces between CNCs. Therefore, by considering sulfated and desulfated CNC suspensions, we are comparing CNC systems that differ in their percolation and gel-point concentrations relative to their phase transition concentrations. The results show that independently of whether the gel-point (linear viscoelasticity, LVE) occurs at the biphasic - liquid crystalline transition (sulfated CNC) or at the isotropic - quasi-biphasic transition (desulfated CNC), the nonlinear behavior appears to mark the existence of a weakly percolated network at lower concentrations. Above this percolation threshold, nonlinear material parameters are sensitive to the phase and gelation behavior as determined in static (phase) and LVE conditions (gel-point). However, the change in material response in nonlinear conditions can occur at higher concentrations than identified through polarized optical microscopy, suggesting that the nonlinear deformations could distort the suspensions microstructure such that for example a liquid crystalline phase (static) suspension could show microstructural dynamics similar to a biphasic system.

Shear and extensional rheology of commercial thickeners used for dysphagia management.

People who suffer from swallowing disorders, commonly referred to as dysphagia, are often restricted to a texture-modified diet. In such a diet, the texture of the fluid is modified mainly by the addition of gum or starch-based thickeners. For optimal modification of the texture, tunable rheological parameters are shear viscosity, yield stress, and elasticity. In this work, the flow properties of commercial thickeners obtained from major commercial suppliers were measured both in shear and extensional flow using a laboratory viscometer and a newly developed tube viscometry technique, termed Pulsed Ultrasound Velocimetry plus Pressure Drop (PUV + PD). The two methods gave similar results, demonstrating that the PUV + PD technique can be applied to study flow during the swallowing process in geometry similar to that of the swallowing tract. The thickeners were characterized in relation to extensional viscosity using the Hyperbolic Contraction Flow method, with microscopy used as a complementary method for visualization of the fluid structure. The gum-based thickeners had significantly higher extensional viscosities than the starch-based thickeners. The rheological behavior was manifested in the microstructure as a hydrocolloid network with dimensions in the nanometer range for the gum-based thickeners. The starch-based thickeners displayed a granular structure in the micrometer range. In addition, the commercial thickeners were compared to model fluids (Boger, Newtonian, and Shear-thinning) set to equal shear viscosity at 50/s and it was demonstrated that their rheological behavior could be tuned between highly elastic, extension-thickening to Newtonian. PRACTICAL APPLICATIONS: Thickeners available for dysphagia management were characterized for extensional viscosity to improve the understanding of these thickeners in large scale deformation. Extensional deformation behavior was further explained by using microcopy as corresponding technique for better understanding of structure/rheology relationship. Moreover, the major challenge in capturing human swallowing process is the short transit times of the bolus flow (<1 s). Therefore, the ultrasound-based rheometry method; PUV+PD which measures the real-time flow curve in ∼50 ms was used in addition to classical shear rheometry. The two methods complimented each other indicating that the PUV+PD method can be applied to study the transient swallowing process which is part of our future research, where we are studying the flow properties of fluids in an in vitro swallowing tract.

Impact of solvent quality on the network strength and structure of alginate gels.

The influence of the mixture of water and alcohols on the solubility and properties of alginate and its calcium-induced gels is of interest for the food, wound care and pharmaceutical industries. The solvent quality of water with increasing amounts of ethanol (0-20%) on alginate was studied using intrinsic viscosity. The effect of ethanol addition on the rheological and mechanical properties of calcium alginate gels was determined. Small-angle X-ray scattering and transmission electron microscopy were used to study the network structure. It is shown that the addition of ethanol up to 15% (wt) increases the extension of the alginate chain, which correlates with increased moduli and stress being required to fracture the gels. The extension of the polymer chain is reduced at 20% (wt) ethanol, which is followed by reduced moduli and stress at breakage of the gels. The network structure of gels at high ethanol concentrations (24%) is characterized by thick and poorly connected network strands.

Interplay between flow and diffusion in capillary alginate hydrogels.

Alginate gels with naturally occurring macroscopic capillaries have been used as a model system to study the interplay between laminar flow and diffusion of nanometer-sized solutes in real time. Calcium alginate gels that contain homogeneously distributed parallel-aligned capillary structures were formed by external addition of crosslinking ions to an alginate sol. The effects of different flow rates (0, 1, 10, 50 and 100 µl min(-1)) and three different probes (fluorescein, 10 kDa and 500 kDa fluorescein isothiocyanate-dextran) on the diffusion rates of the solutes across the capillary wall and in the bulk gel in between the capillaries were investigated using confocal laser scanning microscopy. The flow in the capillaries was produced using a syringe pump that was connected to the capillaries via a tube. Transmission electron microscopy revealed an open aggregated structure close to the capillary wall, followed by an aligned network layer and the isotropic network of the bulk gel. The most pronounced effect was observed for the 1 nm-diameter fluorescein probe, for which an increase in flow rate increased the mobility of the probe in the gel. Fluorescence recovery after photobleaching confirmed increased mobility close to the channel, with increasing flow rate. Mobility maps derived using raster image correlation spectroscopy showed that the layer with the lowest mobility corresponded to the anisotropic layer of ordered network chains. The combination of microscopy techniques used in the present study elucidates the flow and diffusion behaviors visually, qualitatively and quantitatively, and represents a promising tool for future studies of mass transport in non-equilibrium systems.

Mesoporous silica nanoparticles with controllable morphology prepared from oil-in-water emulsions.

Mesoporous silica nanoparticles are an important class of materials with a wide range of applications. This paper presents a simple protocol for synthesis of particles as small as 40nm and with a pore size that can be as large as 9nm. Reaction conditions including type of surfactant, type of catalyst and presence of organic polymer were investigated in order to optimize the synthesis. An important aim of the work was to understand the mechanism behind the formation of these unusual structures and an explanation based on silica condensation in the small aqueous microemulsion droplets that are present inside the drops of an oil-in-water emulsion is put forward.

Bacteria-triggered degradation of nanofilm shells for release of antimicrobial agents.

Due to an increase in lifestyle diseases in the developed world, the number of chronic wounds is increasing at a fast pace. Chronic wound infections are common and systemic antibiotics are usually used as a treatment. In this paper we describe an approach to encapsulate antimicrobial agents in hollow microcapsules covered with a nanofilm shell that degrades through the action of a virulence factor from Pseudomonas aeruginosa. The shell was assembled using the layer-by-layer (LbL) technique with poly-l-lysine and hyaluronic acid. The microcapsules were loaded with a model substrate or a drug. By crosslinking the components in the nanofilm, the film remained intact when exposed to human wound proteases. However, the film was degraded and the drug exposed when in contact with Pseudomonas aeruginosa's Lys-X specific protease IV. The antimicrobial efficacy of the drug-loaded microcapsules was confirmed by exposure to virulent Pseudomonas aeruginosa. The current study contributes to the establishment of a release platform for targeted treatment of topical infections with the aim of minimizing both overexposure to drugs and development of bacterial resistance.

Correction: Magnetically induced structural anisotropy in binary colloidal gels and its effect on diffusion and pressure driven permeability.

Correction for 'Magnetically induced structural anisotropy in binary colloidal gels and its effect on diffusion and pressure driven permeability' by Christoffer Abrahamsson et al., Soft Matter, 2014, 10, 4403-4412.

Meso-ordered PEG-based particles.

We report on the formation of meso-ordered hydrogel particles by cross-linking poly(ethylene glycol) diacrylate (PEG-DA) in the presence of surfactants in a confined environment. The results demonstrated that well-ordered mesoporous hydrogel particles having a pore size of about 5 nm could be formed. It is suggested that these meso-ordered hydrogel particles might have unique drug-delivery capabilities.

Magnetically induced structural anisotropy in binary colloidal gels and its effect on diffusion and pressure driven permeability.

We report on the synthesis, microstructure and mass transport properties of a colloidal hydrogel self-assembled from a mixture of colloidal silica and nontronite clay plates at different particle concentrations. The gel-structure had uniaxial long-range anisotropy caused by alignment of the clay particles in a strong external magnetic field. After gelation the colloidal silica covered the clay particle network, fixing the orientation of the clay plates. Comparing gels with a clay concentration between 0 and 0.7 vol%, the magnetically oriented gels had a maximum water permeability and self-diffusion coefficient at 0.3 and 0.7 vol% clay, respectively. Hence the specific clay concentration resulting in the highest liquid flux was pressure dependent. This study gives new insight into the effect of anisotropy, particle concentration and bound water on mass transport properties in nano/microporous materials. Such findings merit consideration when designing porous composite materials for use in for example fuel cell, chromatography and membrane technology.

Microstructural, mechanical and mass transport properties of isotropic and capillary alginate gels.

Macroscopically homogeneous and inhomogeneous calcium alginate gels are formed via internal or external addition of various amounts of calcium to an alginate solution. The externally formed gels contain parallel aligned capillary structures. The mechanical and mass transport properties and the microstructure of the differently set gels were characterized by rheological measurements, fluorescence recovery after photobleaching (FRAP) and transmission electron microscopy (TEM). TEM images show a zone of distorted anisotropic gel structure in the vicinity of the capillaries as well as indications of a lower degree of void connectivity. The diffusion rates of dextran at large distances from the capillaries were fast and capillary gels showed a plastic behaviour in comparison to the internally set gels. The results presented show large functional differences between the internally and externally set gels, which cannot be explained by the presence of capillaries alone.

Surface-directed structure formation of ß-lactoglobulin inside droplets.

The morphology of ß-lactoglobulin structures inside droplets was studied during aggregation and gelation using confocal laser scanning microscopy (CLSM) equipped with a temperature stage and transmission electron microscopy (TEM). The results showed that there is a strong driving force for the protein to move to the interface between oil and water in the droplet, and the ß-lactoglobulin formed a dense shell around the droplet built up from the inside of the droplets. Less protein was found inside the droplets. The longer the ß-lactoglobulin was allowed to aggregate prior to gel formation, the larger the part of the protein went to the interface, resulting in a thicker shell and very little material being left inside the droplets. The droplets were easily deformed because no network stabilizes them. When 0.5% emulsifier, polyglycerol polyresinoleat (PGPR), was added to the oil phase, the ß-lactoglobulin was situated both inside the droplets and at the interface between the droplets and the oil phase; when 2% PGPR was added, the ß-lactoglobulin structure was concentrated to the inside of the droplets. The possibility to use the different morphological structures of ß-lactoglobulin in droplets to control the diffusion rate through a ß-lactoglobulin network was evaluated by fluorescence recovery after photobleaching (FRAP). The results show differences in the diffusion rate due to heterogeneities in the structure: the diffusion of a large water-soluble molecule, FITC-dextran, in a dense particulate gel was 1/4 of the diffusion rate in a more open particulate ß-lactoglobulin gel in which the diffusion rate was similar to that in pure water.

Mechanical and thermal pretreatments of crushed tomatoes: effects on consistency and in vitro accessibility of lycopene.

The effects of mechanical and thermal treatments on the consistency and in vitro lycopene accessibility of crushed tomatoes were evaluated. Different crushing intensities and a subsequent heat treatment carried out as a heat shock (95 degrees C for 8 min) or a boiling step (100 degrees C for 20 min) were examined. Additional homogenization was compared with milder crushing regarding the effect on lycopene content and in vitro accessibility. Textural properties, polygalacturonase and pectinmethylesterase activity, pectin degree of methoxylation, lycopene content, and in vitro lycopene accessibility were evaluated. Microstructure was studied using both light and transmission electron microscopy. Crushing and subsequent heating affected the pectin degree of methoxylation and the consistency of the crushed tomatoes. The mechanical and thermal treatments did not affect the lycopene content to any great extent; however, in vitro accessibility seemed to improve with extensive crushing followed by heating. Crushing or homogenization in itself was not enough to increase in vitro lycopene accessibility.

Effect of confinement and kinetics on the morphology of phase separating gelatin-maltodextrin droplets.

The effect of confinement on the structure evolution and final morphology during phase separation and gelation of gelatin and maltodextrin was investigated and compared to the structures seen in bulk phase. Emulsion droplets with diameters from 4 to 300 mum were analyzed using confocal laser scanning microscopy and image analysis. With the confocal laser scanning microscope it was possible to follow the entire phase separating process inside the droplets in real-time. The samples were either quenched directly from 70 degrees C down to 20 degrees C or exposed to holding times at 40 degrees C. Different cooling procedures were studied to examine the structure evolution both before and after gelation in the restricted geometries. The concentration of the biopolymer mixture was kept constant at 4 w/w% gelatin and 6 w/w% maltodextrin. The results revealed that the size of the confinement had a great effect on both the initiation of phase separation and the final morphology of the microstructure inside the emulsion droplets. The phase separation in small droplets was observed to occur at a temperature above the phase separating temperature for bulk. Small droplets had either a microstructure with a shell of maltodextrin and core of gelatin or a microstructure where the two biopolymers had formed two separate bicontinuous halves. The initiation of phase separation in large droplets was similar to what was seen in bulk. The microstructure in large droplets was discontinuous, resembling the morphology in bulk phase. The kinetics had an effect on the character of the maltodextrin inclusions, as the cooling procedure of a direct quench gave spherical inclusions with an even size distribution, while a holding time at 40 degrees C resulted in asymmetrical and elongated inclusions.