Multi-Scale Modelling of Aggregation of TiO2 Nanoparticle Suspensions in Water

Titanium dioxide nanoparticles have risen concerns about their possible toxicity and the European Food Safety Authority recently banned the use of TiO2 nano-additive in food products. Following the intent of relating nanomaterials atomic structure with their toxicity without having to conduct large-scale experiments on living organisms, we investigate the aggregation of titanium dioxide nanoparticles using a multi-scale technique: starting from ab initio Density Functional Theory to get an accurate determination of the energetics and electronic structure, we switch to classical Molecular Dynamics simulations to calculate the Potential of Mean Force for the connection of two identical nanoparticles in water; the fitting of the latter by a set of mathematical equations is the key for the upscale. Lastly, we perform Brownian Dynamics simulations where each nanoparticle is a spherical bead. This coarsening strategy allows studying the aggregation of a few thousand nanoparticles. Applying this novel procedure, we find three new molecular descriptors, namely, the aggregation free energy and two numerical parameters used to correct the observed deviation from the aggregation kinetics described by the Smoluchowski theory. Ultimately, molecular descriptors can be fed into QSAR models to predict the toxicity of a material knowing its physicochemical properties, enabling safe design strategies.

A Multi-Scale Modelling of Aggregation of TiO2 Nanoparticle Suspensions in Water

Titanium dioxide nanoparticles have risen concerns about their possible toxicity and the European Food Safety Authority recently banned the use of TiO2 nano-additive in food products. Following the intent of relating nanomaterials atomic structure with their toxicity without having to conduct large scale experiments on living organisms, we investigate the aggregation of titanium dioxide nanoparticles using a multi-scale technique: starting from ab initio Density Functional Theory to get an accurate determination of the energetics and electronic structure, we switch to classical Molecular Dynamics simulations to calculate the Potential of Mean Force for the connection of two identical nanoparticles in water; the fitting of the latter by a set of mathematical equations is the key for the upscale. Lastly, we perform Brownian Dynamics simulations where each nanoparticle is a spherical bead. This coarsening strategy allows studying the aggregation of a few thousand nanoparticles. Applying this novel procedure, we find three new molecular descriptors, namely, the aggregation free energy and two numerical parameters used to correct the observed deviation from the aggregation kinetic described by the Smoluchowski theory. Molecular descriptors can be fed into QSAR models to predict the toxicity of a material knowing its physicochemical properties, without having to conduct large scale experiments on living organisms.

Every Snowflake is Not Unique

This article discusses various aspects of snowflake architectures. It is certain that every snowflake conforms to only one architecture: a flat star with six fishbones connected at the center. The latent heat of solidification, which is released by the water vapor that becomes solid at the bead surface. There comes a critical time when the spherical bead is no longer an efficient architecture for dissipating heat. The principle calls for design change, toward faster heat release and solidification. The growth of ice morphs abruptly into a ball continued in one plane by needles. Because of the configuration of the water molecule, the needles grow in six directions. The flat star transfers heat to the surroundings more easily than a spherical bead with the same diameter. In order to give credit to the view that every snowflake is unique, the actual configuration depends on many secondary effects, which are of random origin.

Thermal Simulations of Thermocouple Tips in Hot Jets

Computational fluid dynamics (CFD) simulations have been carried out for the turbulent convective heat transfer, conduction and radiation for metal thermocouple tips, accommodated in hot gas jets to study the measurement accuracy of the thermocouples. The study covers several thermocouple sizes, jet temperatures, and Reynolds numbers. The spherical bead, representing the tip, becomes so hot that it radiates some heat to the colder surrounding surfaces. This phenomenon is responsible for a gap between the jet temperature and the bead temperature. The mentioned temperature difference is significant. It grows both with bead size and gas temperatures but decreases with the Reynolds number.

Mechano-chemical dynamics of active gels: interplay between spatial bistability and volume variations

The mechano-chemical dynamics of a spherical bead of gel immersed in an autocatalytic bistable chemical reaction are examined. The spheres exhibit autonomous volume self-oscillation dynamics. We present a multi-diffusional approach to a hydrodynamic theory of gels, and also evolution equations incorporating chemical processes.

CFD Simulation of Thermocouple Measurement of Hot Gas Jets

Turbulent convective heat transfer and radiation is simulated for a hot gas jet, impinging perpendicular on a flat surface at 2 jet diameters away from the jet nozzle. A small solid spherical bead, located in the jet centre half way from the wall, represents a thermocouple sensitive point. The bead becomes so hot that it radiates some heat to the colder surrounding surfaces. This phenomenon is responsible for a gap between the jet temperature and the bead temperature. The jet Reynolds number ranged from 7.67*103 to 4.52*104. Bead sizes 1.0 and 2.0 mm are used in jets at 500°C and 900°C. The simulations show that the mentioned temperature differences are significant and grow rapidly with high temperatures but decrease with Reynolds number. The temperature gap, which can be regarded as the thermocouple measurement error, increases also with the bead size. Simulations can be conducted for specific thermocouples with other shapes and materials to assist the measurement process. The modelling methodology is found to be promising for such demanding simulations that require a fine grid for resolving the field near the bead without using excessive cells in the rest of the domain. Hence, further work in this field is envisaged using the same methodology for solving convection, conduction and radiation in one single model and at a reasonable computational cost together with validating measurements. Hopefully this study contributes to a better understanding of the measurement of hot gas jet temperature and its improvement with the aid of simulations.

The approach of a sphere to a wall at finite Reynolds number

The approach to a wall of a non-Brownian rigid spherical particle, settling in a viscous fluid with a Reynolds number of the order of unity, is studied experimentally. Far from the wall, the fluid motion around the particle is driven by inertia and viscosity forces. The particle Stokes number is also of the order of unity, so that the particle motion far from the wall is driven by inertia. In the close vicinity of the wall, however, the particle–wall hydrodynamic interaction decelerates the particle significantly. An interferometric device is used to measure the vertical displacement of a millimetric size spherical bead at distances from the wall smaller than 0.1 sphere radius, with a spatial resolution of 100 nm. For the range of impact Stokes number (St*, based on the limit velocity of the sphere in an unbounded fluid) explored here (up to St* ≅ 5), the measurements reveal that a small region of negligible particle inertia still exists just prior to contact of the sphere with the wall. In this lubrication-like region, the particle velocity decreases linearly with decreasing particle–wall distance and vanishes at contact, ruling out the possibility of a rebound. The vertical extent of this region decreases with increasing Stokes number and is e.g. only 10 μm large at impact Stokes number St* ≅ 5.