Paulo H. S. Santos

Streptococcus mutans-derived extracellular matrix in cariogenic oral biofilms

Biofilms are highly structured microbial communities that are enmeshed in a self-produced extracellular matrix. Within the complex oral microbiome, Streptococcus mutans is a major producer of extracellular polymeric substances including exopolysaccharides (EPS), eDNA, and lipoteichoic acid (LTA). EPS produced by S. mutans-derived exoenzymes promote local accumulation of microbes on the teeth, while forming a spatially heterogeneous and diffusion-limiting matrix that protects embedded bacteria. The EPS-rich matrix provides mechanical stability/cohesiveness and facilitates the creation of highly acidic microenvironments, which are critical for the pathogenesis of dental caries. In parallel, S. mutans also releases eDNA and LTA, which can contribute with matrix development. eDNA enhances EPS (glucan) synthesis locally, increasing the adhesion of S. mutans to saliva-coated apatitic surfaces and the assembly of highly cohesive biofilms. eDNA and other extracellular substances, acting in concert with EPS, may impact the functional properties of the matrix and the virulence of cariogenic biofilms. Enhanced understanding about the assembly principles of the matrix may lead to efficacious approaches to control biofilm-related diseases.

Rheological and Thermal Behavior of Gelled Hydrocarbon Fuels

Rocket propulsion systems are bound to energetic propellant combinations to provide the best performance in conjunction with smallest possible storage volume. The application of a gelled fuel and a gelled oxidizer potentially combines the advantages of conventional solid and liquid propellants without some specific disadvantages of both individual systems. Gelled JP-8 and RP-1 fuels have been used to study the rheological behavior of gelled hydrocarbons. For all investigations fumed silica was used as a gelling agentwith 4 to 7wt%.Alongside a description of the gel mixing process, the paper discusses viscosity, stability, thixotropic behavior, and the viscoelastic properties of the gels through their storage and loss moduli as a function of gelling-agent amount. An extended Herschel– Bulkley model was applied to describe the viscosity characteristics of the hydrocarbon gels. Differential scanning calorimetry measurements showed only a slight influence of the gelling-agent amount on the heat of vaporization of the gels. The ungelled hydrocarbons featured a higher heat of vaporization than the gels.

Mechanically modified xanthan gum: Rheology and polydispersity aspects

Xanthan gum solutions were treated with high-pressure homogenization (HPH) in order to provide alternative treatments to enzymatic and chemical modification of this carbohydrate. Rheological properties of the treated and control samples were investigated in detail to gain an understanding of functional consequences of physical modification. The molecular structural properties were investigated via Size exclusion chromatography (SEC) coupled with Multi-angle laser light scattering (MALLS) and Circular dichroism (CD). Structured network of xanthan gum solutions was lost gradually depending on the severity of the HPH treatment as evidenced by the observed changes in the viscosity and viscoelasticity of the treated solutions. Reduction in molecular weight and a significant increase in polydispersity of the polymer were the expected causes of these rheological changes. Observed increase in hydrodynamic volume upon HPH treatment was not surprising and attributed to the loss of structured networks. Changes in the rheological and structural characteristics of biopolymer were irreversible and significant recovery was not detected over a period of 11 weeks.

Effect of Shear History on Rheology of Time-Dependent Colloidal Silica Gels

This paper presents a rheological study describing the effects of shear on the flow curves of colloidal gels prepared with different concentrations of fumed silica (4%, 5%, 6%, and 7%) and a hydrophobic solvent (Hydrocarbon fuel, JP-8). Viscosity measurements as a function of time were carried out at different shear rates (10, 50, 100, 500, and 1000 s−1), and based on this data, a new structural kinetics model was used to describe the system. Previous work has based the analysis of time dependent fluids on the viscosity of the intact material, i.e., before it is sheared, which is a condition very difficult to achieve when weak gels are tested. The simple action of loading the gel in the rheometer affects its structure and rheology, and the reproducibility of the measurements is thus seriously compromised. Changes in viscosity and viscoelastic properties of the sheared material are indicative of microstructural changes in the gel that need to be accounted for. Therefore, a more realistic method is presented in this work. In addition, microscopical images (Cryo-SEM) were obtained to show how the structure of the gel is affected upon application of shear.

Fluid properties of organic-nanoparticle and fumed silica systems for gelled materials

Gelled materials combine mechanical properties of liquids and solids. Knowing the rheological behavior of these systems is essential in many technological areas. The present paper presents a study on fluid properties of silica gels and HPC water systems containing aluminum oxide nanoparticles. Viscosity measurements for silica‐hydrocarbon gels showed qualitatively evidence of thixotropicity in the fluid with no viscosity recovery. Steady state and small amplitude oscillatory stress tests were performed for HPC water systems with the addition of metal nanoparticles, showing the particles effect on the viscosity and viscoelasticity of the fluid. No time dependence or thixotropy was observed for these samples.

Rheology of JP‐8/SiO2and RP‐1/SiO2Gels

Gelled propellants can be a promising replacement for next generation propulsion systems. For rocket engine operation, the application of a gelled fuel and gelled oxidizer can combine the advantages of liquid propellants (e.g., high performance, throttled operation, and multiple starts) without many of the specific disadvantages of the individual systems. A study is presented that describes the rheological behavior of gelled JP‐8 turbine fuel and gelled rocket propellant RP‐1 when fumed silica is used as a gelling agent. Along with the determination of an optimal gel mixing process, gel stability and rheological parameters showed a significant dependence on the added silica amount.

Rheology of JP-8∕SiO[sub 2] and RP-1∕SiO[sub 2] Gels

Gelled propellants can be a promising replacement for next generation propulsion systems. For rocket engine operation, the application of a gelled fuel and gelled oxidizer can combine the advantages of liquid propellants (e.g., high performance, throttled operation, and multiple starts) without many of the specific disadvantages of the individual systems. A study is presented that describes the rheological behavior of gelled JP‐8 turbine fuel and gelled rocket propellant RP‐1 when fumed silica is used as a gelling agent. Along with the determination of an optimal gel mixing process, gel stability and rheological parameters showed a significant dependence on the added silica amount.

Rheology of JP‐8/SiO2 and RP‐1/SiO2 Gels

Gelled propellants can be a promising replacement for next generation propulsion systems. For rocket engine operation, the application of a gelled fuel and gelled oxidizer can combine the advantages of liquid propellants (e.g., high performance, throttled operation, and multiple starts) without many of the specific disadvantages of the individual systems. A study is presented that describes the rheological behavior of gelled JP‐8 turbine fuel and gelled rocket propellant RP‐1 when fumed silica is used as a gelling agent. Along with the determination of an optimal gel mixing process, gel stability and rheological parameters showed a significant dependence on the added silica amount.