Showing papers by "Sylwester J. Rzoska published in 2015"

Unique effects of thermal and pressure histories on glass hardness: Structural and topological origin

Abstract: The properties of glass are determined not only by temperature, pressure, and composition, but also by their complete thermal and pressure histories. Here, we show that glasses of identical composition produced through thermal annealing and through quenching from elevated pressure can result in samples with identical density and mean interatomic distances, yet different bond angle distributions, medium-range structures, and, thus, macroscopic properties. We demonstrate that hardness is higher when the density increase is obtained through thermal annealing rather than through pressure-quenching. Molecular dynamics simulations reveal that this arises because pressure-quenching has a larger effect on medium-range order, while annealing has a larger effect on short-range structures (sharper bond angle distribution), which ultimately determine hardness according to bond constraint theory. Our work could open a new avenue towards industrially useful glasses that are identical in terms of composition and density, but with differences in thermodynamic, mechanical, and rheological properties due to unique structural characteristics.

Indentation deformation mechanism of isostatically compressed mixed alkali aluminosilicate glasses

Abstract: Oxide glasses can be permanently densified through application of high pressure at room or elevated temperature. Such treatment allows for modification of macroscopic glass properties. However, the structural origins of the pressure-induced property changes are not yet fully understood. In this study, we investigate the ability of a glass network to resist densification under pressure at both ambient and elevated temperatures. We study the detailed deformation mechanisms (densification and shear flow) that occur during indentation of series of as-prepared and isostatically compressed mixed Na/K aluminosilicate glasses, which exhibit a pronounced nonlinear scaling in glass properties due to the mixed alkali effect. Following pressure treatment at elevated temperature, an increase in Vickers hardness is observed due to a significant decrease in densification under the indenter. In contrast, the volume of glass developed in the pileup regions due to shear flow is unaffected by the pressure treatment. This change in the relative contributions of these plastic deformation mechanisms can explain the decrease in crack resistance of the glasses induced by the isostatic compression treatment.

Fragility and basic process energies in vitrifying systems.

Abstract: The concept of ‘fragility’ constitutes a central point of the glass transition science serving as the ‘universal’ metric linking previtreous dynamics of qualitatively distinct systems. Finding the fundamental meaning of fragility is the ‘condicio sine qua’ for reaching the long expected conceptual breakthrough in this domain. This report shows that fragility is determined by the ratio between two fundamental process energies, viz. the activation enthalpy and activation energy. The reasoning, avoiding any underlying physical model, is supported by the experimental evidence ranging from low molecular weight liquids and polymers to plastic crystals and liquid crystals. All these lead to the new general scaling plot for dynamics of arbitrary glass former. The limited adequacy of broadly used so far semi-empirical relationships between fragility and the activation energy is shown. Results presented remain valid for an arbitrary complex system and collective phenomena if their dynamics is described by the general super-Arrhenius relation.

Divergent dynamics and the Kauzmann temperature in glass forming systems

Abstract: In the last decade the challenging analysis of previtreous behavior of relaxation time (τ(T)) in ultraviscous low molecular weight liquids led to the conceptual shift of the glass transition physics toward theories not predicting a “finite-temperature” divergence. This “breakthrough” experimental finding was strengthened by the discovery that “dynamic” (i.e. from τ(T) fitting) and “thermodynamic” estimations of the “ideal glass” (Kauzmann) temperature do not match, what in fact questioned its existence. In this report, due to the novel way of analysis based on the transformation of τ(T) experimental data to the activation energy temperature index form, the clear prevalence of the “finite-temperature” divergence is proved. The obtained “dynamic” singular temperatures clearly coincide with “thermodynamic” estimations of the Kauzmann temperature, thus solving also the second mystery. The comprehensive picture was obtained due to the analysis of 55 experimental data-sets, ranging from low molecular weight liquids and polymers to liquid crystal and plastic crystals.

Temperature-dependent densification of sodium borosilicate glass

Abstract: Densified glasses recovered from a high-pressure state are of potential technological interest due to their modified physical and chemical properties. Here we investigate the temperature-dependent densification behavior of a sodium borosilicate glass in a gas pressure chamber at 1 GPa. The temperature is varied for a 30 min treatment between 0.6Tg and 1.15Tg, where Tg is the glass transition temperature, and the treatment duration is varied between 10 and 10 000 min for compression at 0.9Tg. Permanent densification occurs for temperatures above 0.7Tg and the degree of densification increases with increasing compression temperature and time, until attaining an approximately constant value for temperatures above Tg. The same temperature and time dependence is also found for the glass mechanical properties (hardness and brittleness) and the network structure, i.e., fraction of three-fold versus four-fold coordinated boron atoms and ring versus non-ring trigonal boron atoms, and the extent of mixing of Si and B. The results provide insights into the temperature-dependence of the network densification and the relative roles of viscous flow and more localized rearrangements.

Inactivation of Staphylococcus aureus and native microflora in human milk by high pressure processing

Abstract: The storage of unpreserved food, including breast milk, is associated with the growth of microorganisms, including pathogenic bacteria. It is therefore necessary to use suitable processes to eliminate pathogenic microorganisms and reduce the total microbial count in order to ensure product safety for consumers. In the present study, samples of milk obtained from volunteers donating to the human milk bank were artificially contaminated with Staphylococcus aureus ATCC 6538. This bacteria was the model microorganism of choice, being relatively resistant to high pressure as well as posing the most serious risk to infant health. The results obtained show that high pressure processing can reduce the count of S. aureus by about 5 log units at 4°C and about 8 log units at 50°C, and totally eliminate Enterobacteriaceae after 5 min of treatment, and result in a total microbial count reduction after 10 min treatment at 500 MPa at 20°C and 50°C. This suggests the possibility of this technology being applied to ensure...

Germination and Inactivation of Alicyclobacillus acidoterrestris Spores Induced by Moderate Hydrostatic Pressure.

Abstract: Given the importance of spoilage caused by Alicyclobacillus acidoterrestris for the fruit juice industry, the objective of this work was to study the germination and inactivation of A. acidoterrestris spores induced by moderate hydrostatic pressure. Hydrostatic pressure treatment can induce the germination and inactivation of A. acidoterrestris spores. At low pH, spore germination of up to 3.59-3.75 log and inactivation of 1.85-2.04 log was observed in a low pressure window (200-300 MPa) applied at 50 degrees C for 20 min. Neutral pH suppressed inactivation, the number of spores inactivated at pH 7.0 was only 0.24-1.06 log. The pressurization temperature significantly affected spore germination and inactivation. The degree of germination in apple juice after pressurization for 30 min with 200 MPa at 20 degrees C was 2.04 log, with only 0.61 log of spores being inactivated, while at 70 degrees C spore germination was 5.94 log and inactivation 4.72 log. This temperature strongly stimulated germination and inactivation under higher (500 MPa) than lower (200 MPa) pressure. When the oscillatory mode was used, the degree of germination and inactivation was slightly higher than at continuous mode. The degree of germination and inactivation was inversely proportional to the soluble solids content and was lowest in concentrated apple juice.

DPA Release and Germination of Alicyclobacillus acidoterrestris Sporesunder High Hydrostatic Pressure

Abstract: High thermoresistance combined with the ability to grow under acidic conditions, which are unique among spore formers, make Alicyclobacillus acidoterrestris one of the most serious problems for the fruit processing industry. Dipicolinic acid (DPA) is an important factor in spore resistance to many environmental stresses, and in spore stability. The aim of the study was to determine the relationship between DPA release and the germination of A. acidoterrestris spores, initiated by high hydrostatic pressure (HHP). Samples of the spores of two A. acidoterrestris strains suspended in apple juice and pH 4.0 and pH 7.0 McIlvain buffers were treated with pressure of 100-500 MPa, at a temperature of 20-75°C for 15 min. The total amount of DPA in A. acidoterrestris spores was 50.3 μM for the TO-169/06 and 42.7 μM for the TO-117/02 strain. The amount of DPA released in apple juice treated with 300 MPa was 29.3 μM at 50°C and 35.8 μM at 75°C for the TO-169/06 strain, and 24.6 μM at 50°C and 27.8 μM at 75°C for the TO-117/02 strain. DPA release in the pH 7 buffers and at 20°C was inhibited. The amount of DPA released correlated to the amount of the germinated A. acidoterrestris spores.

Dielectric Spectroscopy of Pressurized Saccharomyces cerevisiae

Abstract: Results of broadband dielectric spectroscopy (BDS) in Saccharomyces cerevisiae (baker’s yeast), in situ as the function of pressure are presented. They show a clear evidence of a threshold to the new pattern of the pressure evolution of the static dielectric permittivity and DC electric conductivity already for P t ≈ 200MPa at T = 5 o C and P t ≈ 300MPa at T = 25 o C. BDS monitoring versus pressure tests up to P = 400MPa revealed particularly notable changes of properties after 30 minutes of compressing. Finally, the correlation between the amount of the spectrophotometric maximum absorbance and the DC electric conductivity was found. All these indicate significance of BDS as the tool for testing of pressure properties of cells assemblies, model foods etc., in situ under high pressures.

Simultaneous Impact of High Pressures and Pulsed Electric field on Saccharomyces cerevisiae Model System

Abstract: Results of simultaneous high pressure (HPP) and pulsed electric field (PEF) preservation treatments on Saccharomyces cerevisiae model suspension are presented. They are based on the novel implementation of PEF method, associated with the sequence of sine- wave pulses of the strong electric field. The simultaneous HPP + PEF enabled reaching the pasteurization threshold for pressure P = 200 MPa and the intensity of the electric field E < 10 kV/cm well below typical values for separate HPP and PEF treatments. Notable is the negligible parasitic heating for the new PEF implementation. The supplementary broad band dielectric spectroscopy (BDS) frequency scan for HPP, PEF and HPP + PEF revealed qualitative differences, particularly notable for the complex conductivity representation.