Rotor-Stator Emulsification in the Turbulent Inertial Regime: Experiments toward a Robust Correlation for the Droplet Size.

The Sauter mean diameter, d32, is a representative parameter in emulsions that indicates the average size of the oil droplets once the emulsion becomes stable. Several mathematical and physical approaches have been employed in the literature to seek expressions for d32 under different conditions. The present work sheds light on this rich literature and emphasizes that the characterization of emulsions is still a fertile field for investigation. In this paper, a new Π-theorem-based model to predict the normalized Sauter mean diameter for the specific case of rotor-stator emulsification is sought by applying a multiple regression analysis on experimental data of oil-in-water (O-W) emulsions produced using three different oils: paraffin, soybean oil, and isopropyl myristate, at different oil-to-water (O/W) ratios and rotor speeds. The proposed model quantifies the roles of the viscous, inertial, and interfacial tension forces, besides the O/W ratio, in the emulsification process within the turbulent inertial subrange. The developed empirical correlation is then contrasted with relevant literature models for reliability assessment; predictions of the present explicit model are proven to be more accurate for the fluid properties and the experimental conditions under study.

Microstructural changes of concentrated Newtonian suspensions in the first oscillation cycles probed with linear and non-linear rheology.

Carotenuto et al. (Rheol Acta, 2021, 60, 309) recently showed that the complex viscosity of a Newtonian non-Brownian suspension is smaller than the steady shear one, whatever the imposed strain amplitude. Oscillatory shear can alter the microstructure through a shear induced particle diffusion mechanism. This mechanism needs time to show its effect and cannot be invoked to explain the observed mismatch between the steady shear and the complex viscosity. Moreover, in the limit of vanishing strain amplitudes and of very large ones, where the oscillatory shear is equivalent to consecutive steady flow reversals, the oscillatory shear should not alter the microstructure and the Cox-Merz rule should hold. With a combination of approaches exploiting the Lissajous-Bowditch plots, the Fourier transform rheology and the Sequence of Physical Processes, we investigate the microstructure changes induced in the first oscillatory cycles. The results from the different analyses agree with the microstructure rearranging mechanisms proposed by Carotenuto et al.: at small amplitudes, the oscillatory shear rotates couples of touching particles towards the flow direction, at medium amplitudes it breaks particle clusters and at very large amplitudes it reshuffles and reorients all the particles. We show that the vast majority of the microstructure rearrangement occurs soon after the flow inversion of the first cycle, while before it the microstructure is not altered. This allows us to suggest a procedure to "recover" the Cox-Merz rule: a single cycle of oscillation must be imposed and the stress response of the sole first quarter of oscillation must be analysed.

Analysis of Carbon Nanoparticle Coatings via Wettability.

Wettability, typically estimated through the contact angle, is a fundamental property of surfaces with wide-ranging implications in both daily life and industrial processes. Recent scientific interest has been paid to the surfaces exhibiting extreme wettability: superhydrophobic and superhydrophilic surfaces, characterized by high water repellency and exceptional water wetting, respectively. Both chemical composition and morphology play a role in the determination of the wettability "performance" of a surface. To tune surface-wetting properties, we considered coatings of carbon nanoparticles (CNPs) in this study. They are a new class of nanomaterials synthesized in flames whose chemistry, dimension, and shape depend on combustion conditions. For the first time, we systematically studied the wettability of CNP coatings produced in a controlled rich ethylene/air flame stabilized over a McKenna burner. A selected substrate was intermittently inserted in the flame at 15 mm above the burner to form a thin coating thanks to a thermophoretic-driven deposition mechanism. The chemical-physical quality and the deposed quantity of the CNPs were varied by opportunely combing the substrate flame insertion number (from 1 to 256) and the carbon-to-oxygen ratio, C/O (from 0.67 to 0.87). The wettability of the coatings was evaluated by measuring the contact angle, CA, with the sessile drop method. When the C/O = 0.67, the CNPs were nearly spherical, smaller than 8 nm, and always generated hydrophilic coatings (CA < 35°). At higher C/O ratios, the CNPs reached dimensions of 100 nm, and fractal shape aggregates were formed. In this case, either hydrophilic (CA < 76°) or superhydrophobic (CA ~166°) behavior was observed, depending on the number of carbon nanoparticles deposed, i.e., film thickness. It is known that wettability is susceptible to liquid surface tension, and therefore, tests were conducted with different fluids to establish a correlation between the flame conditions and the nanostructure of the film. This method offers a fast and simple approach to determining mesoscale information for coating roughness and topographical homogeneity/inhomogeneity of their surfaces.

DGGE analysis of buffalo manure eubacteria for hydrogen production: effect of pH, temperature and pretreatments.

Buffalo dung is a low-cost substrate with plenty of carbohydrates, an optimal carbon/nitrogen ratio, and a rich microbial flora, and could become a valuable source of biogas. Therefore, in the present study we compared the type and amount of specific eubacteria to the different configurations of pH, temperature and thermal pretreatment after fermentation in batch reactors in order to understand the suitability of buffalo manure for hydrogen production. The phylogenetic structure of the microbial community in fermentation samples was studied using denaturing gradient gel electrophoresis to generate fingerprints of 16S rRNA genes. The sequences analysis revealed abundance of the phyla Bacteroidetes and Firmicutes, and in particular of the order Clostridiales. Very active hydrogen producing bacteria belonging to Clostridium cellulosi species were identified demonstrating the suitability of this substrate to produce hydrogen. Moreover, a large fraction of 16S-rDNA amplicons could not be assigned to lower taxonomic ranks, demonstrating that numerous microorganisms involved in anaerobic fermentation in digesters or bioreactors are still unclassified or unknown.

HPMC Hydrogel Formation Mechanisms Unveiled by the Evaluation of the Activation Energy.

Aqueous solutions of hydroxypropyl methylcellulose (HPMC) show inverse thermoreversible gelation, i.e., they respond to small temperature variations exhibiting sol-gel transition during heating, and reversibly gel-sol transition during cooling. According to the pertinent literature on HPMC aqueous systems, at room temperature, the loss modulus (G") is higher than the storage modulus (G'). During the heating ramp, the viscoelastic response follows a peculiar path: initially, G" and G' smoothly decrease, then drop to a minimum and finally increase. Eventually, G' overcomes G", indicating the gel formation. A recent explanation of this behaviour considers a two-step mechanism: first, phase separation occurs, then fibrils form from a polymer-rich phase and entangle, leading to a three-dimensional network. Based on this, our research focuses on the rheological analysis of the different steps of the sol-gel transition of an HPMC aqueous solution. We perform different viscoelastic tests: thermal ramps, time sweeps, and frequency sweeps at selected characteristic temperatures. We couple classical analysis of the SAOS experiments with an innovative approach based on the evaluation of the activation energy (Ea), made possible by the instrument intrinsic temperature oscillations around the target value. Results show that Ea can be a valid tool that contributes to further clarifying the peculiar microstructural evolution occurring in this kind of thermoreversible gel.

The peculiar role of C/N and initial pH in anaerobic digestion of lactating and non-lactating water buffalo manure.

Manure from lactating and non-lactating water buffaloes was separately collected from a single dairy farm and anaerobically digested under mesophilic conditions in batch mode to produce biogas. This substrate, scarcely studied in the literature, showed two peculiarities regarding two fundamental parameters in the digestion processes: C/N ratio and initial pH. Typically, optimal C/N varies from 20 to 30, but in this work an almost negligible role of this ratio is observed. We demonstrated it by investigating a very large C/N interval, from 9.7 to 50.1, not by adding selected nutrients to the system, but exploiting the natural variation of the substrate. Concerning the pH, we show that also typically considered unfavorable conditions are feasible for this substrate. In fact, though neutral-basic initial pH is proved to be optimal to run the digestion process, in line with many other kinds of dungs, also acid initial pH leads to satisfactory CH4 yield. This is principally related to the capability of water buffalo manure of auto-modifying the pH to neutrality during the digestion, when initial pH of 5.0 and 6.0 are considered. This aspect may be relevant in co-digestion processes with acid wastes, since it may allow not adding neither a buffer, nor a pH regulator to the system. All the digestion conditions are separately tested with lactating and non-lactating water buffaloes and no statistical meaningful differences exist between the two kinds of cattle.