11 research outputs found
Pair correlation function of charge-stabilized colloidal systems under sheared conditions
No full textThe pair correlation function of charge stabilized colloidal particles under strongly sheared conditions is studied using the analytical intermediate asymptotics method recently developed in Banetta and Zaccone (Phys. Rev. E 99, 052606 (2019) to solve the steady-state Smoluchowski equation for medium to high values of the Péclet number; the analytical theory works for dilute conditions. A rich physical behaviour is unveiled for the pair correlation function of colloids interacting via the repulsive Yukawa (or Debye-Hückel) potential, in both the extensional and compressional sectors of the solid angle. In the compression sector, a peak near contact is due to the advecting action of the flow and decreases upon increasing the coupling strength parameter Γ of the Yukawa potential. Upon increasing the screening (Debye) length κ− 1, a secondary peak shows up, at a larger separation distance, slightly less than the Debye length. While this secondary peak grows, the primary peak near contact decreases. The secondary peak is attributed to the competition between the advecting (attractive-like) action of the flow in the compressions sector, and the repulsion due to the electrostatics. In the extensional sectors, a depletion layer (where the pair-correlation function is identically zero) near contact is predicted, the width of which increases upon increasing either Γ or κ− 1
Radial distribution function of Lennard-Jones fluids in shear flows from intermediate asymptotics
No full textDetermining the microstructure of colloidal suspensions under shear flows has been a challenge for theoreticaland computational methods due to the singularly perturbed boundary-layer nature of the problem. Previousapproaches have been limited to the case of hard-sphere systems and suffer from various limitations in theirapplicability. We present an alternative analytic scheme based on intermediate asymptotics which solves theSmoluchowski diffusion-convection equation including both intermolecular and hydrodynamic interactions. Themethod is able to recover previous results for the hard-sphere fluid and to predict the radial distribution function(rdf) of attractive fluids such as the Lennard-Jones (LJ) fluid. In particular, a new depletion effect is predictedin the rdf of the LJ fluid under shear. This method can be used for the theoretical modeling and understandingof real fluids subjected to flow, with applications ranging from chemical systems to colloids, rheology, plasmas,and atmospherical science
Solution to the two-body Smoluchowski equation with shear flow for charge-stabilized colloids at low to moderate P\'eclet numbers
No full textWe developed an analytical theoretical method to determine the microscopical
structure of weakly to moderately sheared colloidal suspensions in dilute
conditions. The microstructure is described by the static structure factor,
obtained by solving the stationary two-body Smoluchowski advection-diffusion
equation. The singularly perturbed PDE problem is solved by performing an
angular averaging over the extensional and compressing sectors and by the
rigorous application of boundary-layer theory (intermediate asymptotics). This
allows us to expand the solution to a higher order in P\'eclet with respect to
previous methods. The scheme is independent of the type of interaction
potential. We apply it to the example of charge-stabilized colloidal particles
interacting via the repulsive Yukawa potential and study the distortion of the
structure factor. It is predicted that the distortion is larger at small
wavevectors and its dependence on is a simple power law. At increasing
, the main peak of the structure factor displays a broadening and shift
towards lower in the extensional sectors, which indicates shear-induced
spreading out of particle correlations and neighbor particles locally being
dragged away from the reference one. In the compressing sectors, instead, a
narrowing and shift towards high is predicted, reflecting shear-induced
ordering near contact and concomitant depletion in the medium-range. An overall
narrowing of the peak is also predicted for the structure factor averaged over
the whole solid angle. Calculations are also performed for hard spheres,
showing good overall agreement with experimental data. It is also shown that
the shear-induced structure factor distortion is orders of magnitude larger for
the Yukawa repulsion than for the hard spheres
Extended Charge-on-Particle Optimized Potentials for Liquid Simulation Acetone Model: The Case of Acetone-Water Mixtures
No full textIt is well known that classical molecular dynamics (MD) simulations of acetone-water mixtures lead to a strong phase separation when using most of the standard all-atom force fields, despite the well-known experimental fact that acetone is miscible with water in any proportion at room temperature. We describe here the use of a charge-on-particle (COP) model for accounting for the induced polarization effect in acetone-water mixtures which can solve the de-mixing problem at all acetone molar fractions. The polarizability effect is introduced by means of a Virtual Site (VS) on the carbonyl group of the acetone molecule, which increases its dipole moment and leads to a better affinity with water molecules. The VS parameter is set by fitting the density of the mixture, at different acetone molar fractions. The main novelty of the VS approach lies on the transferability and universality of the model, since the polarizability can be controlled without modifying the force field adopted, like previous efforts did. The results are satisfactory also in terms of transport properties, i.e. diffusivity and viscosity coefficients of the mixture
IMPACT OF TURBULENCE MODELING ON FLUID/SOLID HEAT TRANSFER INSIDE INDUSTRIAL AUTOCLAVES
No full textThis work is centred on the analysis of the impact of different turbulence
modeling approaches on the fluid/solid heat exchange inside a commercial size autoclave.
This project proposes itself to be a first step towards the optimization of the turbulent
flow inside this kind of machinery to improve the curing treatment of Carbon-Fiber Reinforced
Plastics (CFRP). The setup of the CFD simulations includes the presence of
a metallic sample object inside the autoclave, where air will be recirculated with velocity,
pressure and temperature typically adopted for this type of treatments. The analysis
takes advantage of parallel CFD simulations, conducted by using the open-source software
openFOAM v2106. Two turbulence models have been adopted: one is the well-known
Reynolds-Average Navier-Stokes approach (RANS), which is currently used to model the
turbulence inside this type of machinery. The second one is the Delayed Detached Eddy
Simulations (DDES), which allows the full resolution of the majority of turbulent scales
around the sample object. First, we propose the difference between the local heat flux distribution
at the air/solid interface computed by using RANS and DDES, next we analyse
the overall heat flux entering the sample object: the resolution of the turbulent scales does
not influence the local heat flux only, but also the overall heat flux entering the object; an
average increase of 35% is reported when the velocity fluctuations are neglected. Future
steps of the research foresee the analysis of the heat flux and temperature distributions on
the surface of realistic shapes and common-use CFRP. Afterwards, the autoclave design
will be optimized by adding multiple inlets and aerodynamic devices to guarantee a more
homogeneous heat flux distribution on the surface of realistic shapes of actual CFRP
Predictive model of polymer reaction kinetics and coagulation behavior in seeded emulsion co- and ter-polymerizations
No full textA mathematical model to describe the emulsion polymerization kinetics of co- and ter-polymerizations is developed. The model uses the well-known pseudo-homopolymerization approach together with recently developed models for radical entry and desorption in order to monitor crucial kinetic variables such as conversion and latex composition. The model includes a series of unknown parameters related to monomer-specific gel-effect coefficients, that are needed to compute the bimolecular termination reaction rates. The unknown parameters are determined through extensive calibration of the model on literature data for homo- and co-polymerizations of n-butyl acrylate (n-BA) and methyl methacrylate (MMA). The so-obtained predictive model is then applied to the modelling of the ter-polymerization of n-BA and MMA with 2-hydroxyethyl methacrylate (2-HEMA) with sodium persulphate (SPR) as initiator: predictions for the time-evolution of particle size and conversion are in excellent agreement with experimental measurements using Dynamic Light Scattering (DLS) and Gas Chromatography (GC), upon tuning the gel-effect coefficient related to 2-HEMA. The developed model is used to quantify the surfactant surface coverage of the particles as well as the total concentration of counterions in the system throughout the entire polymerization process. This key information provides a way to rationalize and control the coagulation behavior during the whole polymerization process
Homogenization of turbulent flows inside industrial environments: an application to the curing of Carbon Fiber Reinforced Polymers
No full textThis work proposes a study of turbulence homogenization inside large industrial environments, with a first application to the curing treatment of Carbon-Fiber-Reinforced Polymers, which are composite materials widely spread in the aerospace industry. The state-of-the-art design of these machineries causes a highly anisotropic turbulent flow, that leads to an heterogeneous heat exchange between the air and the mold containing the material to be treated, which causes the curing procedure to be inhomogeneous.
Aim of this work is to propose innovative methods to homogenize the turbulence inside a 16 m3 autoclave and analyse their impact on the air/mold heat exchange under different operating conditions. The first designs include the addition of random (both in location and number) velocity perturbations generated at the walls of the chamber. The impact of these sources has been examined by conducting hybrid DNS-LES simulations in an empty chamber where the circulating flow has a Reynolds number Re = O(106); the computational analysis has been carried out by using the open-source software PLUTO 4.4.2. Aim of this analysis is the mapping of the kinetic energy and enstrhophy inside the system,together with the distribution of a tracer within the chamber.
Afterwards, the impact of the velocity perturbations is analysed by simulating different stages of a realistic curing scenario, where a rectangular mold made of steel is located inside the chamber. It will be showed that a more homogeneous turbulence leads to an improvement of the heat flux distribution uniformity on the surface of the solid sample
Direct observation of heterogeneous formation of amyloid spherulites in real-time by super-resolution microscopy
No full textReal-time super-resolution microscopy analysis reveals the growth kinetics, morphology, and abundance of human insulin amyloid spherulites with different growth pathways.Protein misfolding in the form of fibrils or spherulites is involved in a spectrum of pathological abnormalities. Our current understanding of protein aggregation mechanisms has primarily relied on the use of spectrometric methods to determine the average growth rates and diffraction-limited microscopes with low temporal resolution to observe the large-scale morphologies of intermediates. We developed a REal-time kinetics via binding and Photobleaching LOcalization Microscopy (REPLOM) super-resolution method to directly observe and quantify the existence and abundance of diverse aggregate morphologies of human insulin, below the diffraction limit and extract their heterogeneous growth kinetics. Our results revealed that even the growth of microscopically identical aggregates, e.g., amyloid spherulites, may follow distinct pathways. Specifically, spherulites do not exclusively grow isotropically but, surprisingly, may also grow anisotropically, following similar pathways as reported for minerals and polymers. Combining our technique with machine learning approaches, we associated growth rates to specific morphological transitions and provided energy barriers and the energy landscape at the level of single aggregate morphology. Our unifying framework for the detection and analysis of spherulite growth can be extended to other self-assembled systems characterized by a high degree of heterogeneity, disentangling the broad spectrum of diverse morphologies at the single-molecule level.</p
Live-cell super-resolution microscopy reveals a primary role for diffusion in polyglutamine-driven aggresome assembly.
No full textThe mechanisms leading to self-assembly of misfolded proteins into amyloid aggregates have been studied extensively in the test tube under well-controlled conditions. However, to what extent these processes are representative of those in the cellular environment remains unclear. Using super-resolution imaging of live cells, we show here that an amyloidogenic polyglutamine-containing protein first forms small, amorphous aggregate clusters in the cytosol, chiefly by diffusion. Dynamic interactions among these clusters limited their elongation and led to structures with a branched morphology, differing from the predominantly linear fibrils observed in vitro Some of these clusters then assembled via active transport at the microtubule-organizing center and thereby initiated the formation of perinuclear aggresomes. Although it is widely believed that aggresome formation is entirely governed by active transport along microtubules, here we demonstrate, using a combined approach of advanced imaging and mathematical modeling, that diffusion is the principal mechanism driving aggresome expansion. We found that the increasing surface area of the expanding aggresome increases the rate of accretion caused by diffusion of cytosolic aggregates and that this pathway soon dominates aggresome assembly. Our findings lead to a different view of aggresome formation than that proposed previously. We also show that aggresomes mature over time, becoming more compacted as the structure grows. The presence of large perinuclear aggregates profoundly affects the behavior and health of the cell, and our super-resolution imaging results indicate that aggresome formation and development are governed by highly dynamic processes that could be important for the design of potential therapeutic strategies
Two Years of Arc-Tunnel Experience at Centrospazio&quot;, 3 rd European Symposium on Aerothermodynamics for Space Vehicles
No full textCentrospazio has recently developed a small archeated wind tunnel (High Enthalpy Arc-heated Tunnel, HEAT) whose flexible configuration is particularly suited for cost-effective experimentation and research on a wide variety of aerothermodynamic phenomena occurring in high speed flows. The design and operation of the HEAT has involved a number of critical aspects, ranging from the feasibility and realisation of the proposed concept, to the effectiveness and performance of pulsed arc tunnels for hypersonic aerothermodynamic testing and the applicability of the necessary instrumentation in the hostile environment of the tunnel test section. The present article reviews some of the experience and results recently got a
