Combining fracture mechanics and rheology to investigate the impact of micro-aeration on chocolate oral processing.

This study presents a rigorous mechanical characterisation investigation on milk chocolate with varying porosities, at different temperatures and strain rate levels. Uniaxial compression tests at temperatures varying from 20 °C to 30 °C were performed to measure the bulk properties of chocolate as a function of porosity and temperature. Fracture experiments were also conducted to compute the fracture energy at temperature levels between 20 °C and 30 °C for all tested samples. Additionally, rheological experiments are conducted to compute the viscosity of the different chocolates at 37 °C. This combined experimental analysis of solid mechanics, fracture mechanics, and rheology aims to define the impact of temperature and chocolate's phase change from solid to liquid on its mechanical properties. Moreover, the impact of micro-aeration on the relationship between material properties and temperature is discussed. The results demonstrate a significant impact of both temperature and micro-aeration on the chocolate's material properties; fracture stresses decrease with micro-aeration due to the presence of micro-pores creating weak links in the chocolate matrix, the critical strain energy release rate decreases with micro-aeration at temperatures up to 25 °C and increases at temperatures above 30 °C. Finally, the viscosity at 37 °C increases with increasing porosity due to the obstruction of the flow by micro-pores acting as "solid" particles. The results highlight how the impact of micro-aeration on the material properties of chocolate alters as the testing temperature rises and the material changes phase. The relationships between the micro-aeration and material properties and the dependence of temperature on the different mechanical properties are used to explain the difference in textural attributes as obtained from temporal dominance sensation tests. This study seeks to contribute valuable insights into the field of chocolate technology, emphasizing the need for a combined engineering approach to understand the structural breakdown of chocolate during oral processing as mechanisms such as chewing, melting, mixing and shearing occur.

Experimental and numerical evaluation of the effect of micro-aeration on the thermal properties of chocolate.

Thermal properties, such as thermal conductivity, specific heat capacity and latent heat, influence the melting and solidification of chocolate. The accurate prediction of these properties for micro-aerated chocolate products with varying levels of porosity ranging from 0% to 15% is beneficial for understanding and control of heat transfer mechanisms during chocolate manufacturing and food oral processing. The former process is important for the final quality of chocolate and the latter is associated with sensorial attributes, such as grittiness, melting time and flavour. This study proposes a novel multiscale finite element model to accurately predict the temporal and spatial evolution of temperature across chocolate samples. The model is evaluated via heat transfer experiments at temperatures varying from 16 °C to 45 °C. Both experimental and numerical results suggest that the rate of heat transfer within the micro-aerated chocolate is reduced by 7% when the 15% micro-aerated chocolate is compared to its solid counterpart. More specifically, on average, the thermal conductivity decreased by 20% and specific heat capacity increased by 10% for 15% micro-aeration, suggesting that micro-pores act as thermal barriers to heat flow. The latter trend is unexpected for porous materials and thus the presence of a third phase at the pore's interface is proposed which might store thermal energy leading to a delayed release to the chocolate system. The developed multiscale numerical model provides a design tool to create pore structures in chocolate with optimum melting or solidifying response.

Effect of micro-aeration on the mechanical behaviour of chocolates and implications for oral processing.

Aeration in foods has been widely utilised in the food industry to develop novel foods with enhanced sensorial characteristics. Specifically, aeration at the micron-sized scale has a significant impact on the microstructure where micro-bubbles interact with the other microstructural features in chocolates. This study aims to determine the effect of micro-aeration on the mechanical properties of chocolate products, which are directly correlated with textural attributes such as hardness and crumbliness. Uniaxial compression tests were performed to determine the mechanical properties such as Poisson's ratio, Young's modulus and macroscopic yield strength together with fracture tests to estimate the fracture toughness. In vivo mastication tests were also conducted to investigate the link between the fracture properties and fragmentation during the first two chewing cycles. The uniaxial stress-strain data were used to calibrate a viscoplastic constitutive law. The results showed that micro-aeration significantly affects mechanical properties such as Young's modulus, yield and fracture stresses, as well as fracture toughness. In addition, it enhances the brittle nature of the chocolate, as evidenced by lower fracture stress but also lower fracture toughness leading to higher fragmentation, in agreement with observations in the in vivo mastication tests. As evidenced by the XRT images and the stress-strain measurements micro-aeration hinders the re-arrangement of the microscopic features inside the chocolate during the material's deformation. The work provides a new insight of the role of bubbles on the bulk behaviour of complex multiphase materials, such as chocolates, and defines the mechanical properties which are important input parameters for the development of oral processing simulations.

Mechanisms of Atomization from Rotary Dental Instruments and Its Mitigation.

Since the onset of coronavirus disease 2019, the potential risk of dental procedural generated spray emissions (including aerosols and splatters), for severe acute respiratory syndrome coronavirus 2 transmission, has challenged care providers and policy makers alike. New studies have described the production and dissemination of sprays during simulated dental procedures, but findings lack generalizability beyond their measurements setting. This study aims to describe the fundamental mechanisms associated with spray production from rotary dental instrumentation with particular focus on what are currently considered high-risk components-namely, the production of small droplets that may remain suspended in the room environment for extended periods and the dispersal of high-velocity droplets resulting in formites at distant surfaces. Procedural sprays were parametrically studied with variables including rotation speed, burr-to-tooth contact, and coolant premisting modified and visualized using high-speed imaging and broadband or monochromatic laser light-sheet illumination. Droplet velocities were estimated and probability density maps for all laser illuminated sprays generated. The impact of varying the coolant parameters on heating during instrumentation was considered. Complex structured sprays were produced by water-cooled rotary instruments, which, in the worst case of an air turbine, included droplet projection speeds in excess of 12 m/s and the formation of millions of small droplets that may remain suspended. Elimination of premisting (mixing of coolant water and air prior to burr contact) resulted in a significant reduction in small droplets, but radial atomization may still occur and is modified by burr-to-tooth contact. Spatial probability distribution mapping identified a threshold for rotation speeds for radial atomization between 80,000 and 100,000 rpm. In this operatory mode, cutting efficiency is reduced but sufficient coolant effectiveness appears to be maintained. Multiple mechanisms for atomization of fluids from rotatory instrumentation exist, but parameters can be controlled to modify key spray characteristics during the current crisis.

Novel method for the measurement of liquid film thickness during fuel spray impingement on surfaces.

This paper describes the development and application of a novel optical technique for the measurement of liquid film thickness formed on surfaces during the impingement of automotive fuel sprays. The technique makes use of the change of the light scattering characteristics of a metal surface with known roughness, when liquid is deposited. Important advantages of the technique over previously established methods are the ability to measure the time-dependent spatial distribution of the liquid film without a need to add a fluorescent tracer to the liquid, while the measurement principle is not influenced by changes of the pressure and temperature of the liquid or the surrounding gas phase. Also, there is no need for non-fluorescing surrogate fuels. However, an in situ calibration of the dependence of signal intensity on liquid film thickness is required. The developed method can be applied to measure the time-dependent and two-dimensional distribution of the liquid fuel film thickness on the piston or the liner of gasoline direct injection (GDI) engines. The applicability of this technique was evaluated with impinging sprays of several linear alkanes and alcohols with different thermo-physical properties. The surface temperature of the impingement plate was controlled to simulate the range of piston surface temperatures inside a GDI engine. Two sets of liquid film thickness measurements were obtained. During the first set, the surface temperature of the plate was kept constant, while the spray of different fuels interacted with the surface. In the second set, the plate temperature was adjusted to match the boiling temperature of each fuel. In this way, the influence of the surface temperature on the liquid film created by the spray of different fuels and their evaporation characteristics could be demonstrated.

Revealing the complex conduction heat transfer mechanism of nanofluids.

Nanofluids are two-phase mixtures consisting of small percentages of nanoparticles (sub 1-10 %vol) inside a carrier fluid. The typical size of nanoparticles is less than 100 nm. These fluids have been exhibiting experimentally a significant increase of thermal performance compared to the corresponding carrier fluids, which cannot be explained using the classical thermodynamic theory. This study deciphers the thermal heat transfer mechanism for the conductive heat transfer mode via a molecular dynamics simulation code. The current findings are the first of their kind and conflict with the proposed theories for heat transfer propagation through micron-sized slurries and pure matter. The authors provide evidence of a complex new type of heat transfer mechanism, which explains the observed abnormal heat transfer augmentation. The new mechanism appears to unite a number of popular speculations for the thermal heat transfer mechanism employed by nanofluids as predicted by the majority of the researchers of the field into a single one. The constituents of the increased diffusivity of the nanoparticle can be attributed to mismatching of the local temperature profiles between parts of the surface of the solid and the fluid resulting in increased local thermophoretic effects. These effects affect the region surrounding the solid manifesting interfacial layer phenomena (Kapitza resistance). In this region, the activity of the fluid and the interactions between the fluid and the nanoparticle are elevated. Isotropic increased nanoparticle mobility is manifested as enhanced Brownian motion and diffusion effects.

Hydrogen abstraction from n-butanol by the methyl radical: high level ab initio study of abstraction pathways and the importance of low energy rotational conformers.

Hydrogen abstraction reactions by the methyl radical from n-butanol have been investigated at the ROCBS-QB3 level of theory. Reaction energies and product geometries for the most stable conformer of n-butanol (ROH) have been computed, the reaction energies order α < γ < ß < δ < OH. The preference for n-butane to favour H-abstraction at C(ß) and C(γ) while, in contrast, n-butanol favours radical reactions at the C(α) carbon is rationalised. Transition state (TS) barriers and geometries for the most stable conformer of n-butanol are presented, and discussed with respect to the Hammond postulate. The reaction barriers order as α < OH < γ < ß < δ. This relative ordering is not consistent with product radical stability, C-H bond dissociation energies or previous studies using O[combining dot above]H and HO[combining dot above](2) radicals. We provide a molecular orbital based rationalisation for this ordering and answer two related questions: Why is the γ-channel more stable than the ß-channel? Why do the two C(γ)-H H-abstraction TS differ in energy? The method and basis set dependence of the TS barriers is investigated. The Boltzmann probability distribution for the n-butanol conformers suggests that low energy conformers are present in approximately equal proportions to the most stable conformer at combustion temperatures where CH(3) radicals are present. Thus, the relative significance of the various H-abstraction channels has been assessed for a selection of higher energy conformers (ROH'). Key results include finding that higher energy n-butanol conformers (E(ROH') > E(ROH)) can generate lower energy product radicals, E(ROH') < E(ROH). Moreover, higher energy conformers can also have a globally competitive TS energy for H-abstraction.

Spatial distribution of fluorescence intensity within large droplets and its dependence on dye concentration.

The dependence of fluorescence intensity distributions within droplets on added dye concentration has been calculated by extension of the geometrical-optics approximation and verified by experimental observations. With rising dye concentration, surface plots of the equatorial fluorescence pattern show decreasing relevance of intensity enhancement at focusing points of internal light rays and increasing effects of linear absorption on the characteristic features of the distribution. For comparison with experimentally obtained images of the fluorescence intensity distribution within droplets, a method for calculating volume-integrated intensity distributions was developed in which image distortion at the fluid-air interface is included. A comparison of the calculated and the experimentally determined fluorescence intensity distributions within a droplet confirmed the accuracy of the geometrical-optics approach at high dye concentrations. However, discrepancies from experimental results are visible at low dye concentrations owing to nonlinear optical effects.

Shadow Doppler technique for sizing particles of arbitrary shape.

The output from a linear diode array is used in a modified laser Doppler velocimeter to measure the size and shape of irregular particles. The sizing accuracy for transparent and opaque particles between 30 and 140 µm is better than 10%. The inaccuracy caused by trajectories that lay at angles of less than 24° to the axis of the array was less than +5%, and a further inaccuracy of +5% was caused by defocusing of the particle from the center of the velocimeter measuring volume by up to ±500 µm. The advantages of the shadow Doppler technique over other techniques for sizing irregular particles, such as amplitude systems with pointer volumes, are that the shadow Doppler technique records shape, the optical arrangement is more robust, less precise alignment is required, and the equipment can be constructed at low cost.