Spelling suggestions: "subject:"dammed"" "subject:"jammed""
1 
Structural Characteristics and Applications of Hardparticle Packings via Eventdriven Molecular Dynamics SimulationsJanuary 2014 (has links)
abstract: In this dissertation, the results of our comprehensive computational studies of disordered jammed (i.e., mechanically stable) packings of hard particles are presented, including the family of superdisks in 2D and ellipsoids in 3D Euclidean space. Following a very brief introduction to the hardparticle systems, the event driven molecular dynamics (EDMD) employed to generate the packing ensembles will be discussed. A large number of 2D packing configurations of superdisks are subsequently analyzed, through which a relatively accurate theoretical scheme for packingfraction prediction based on local particle contact configurations is proposed and validated via additional numerical simulations. Moreover, the studies on binary ellipsoid packing in 3D are briefly discussed and the effects of different geometrical parameters on the final packing fraction are analyzed. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2014

2 
Nonlocal rheology of soft glassy materials / Rhéologie non locale des matériaux vitreux mousMansard, Vincent 10 September 2012 (has links)
Les matériaux vitreux mous (émulsion concentrée, mousse, suspension concentrée...) présentent un comportement rhéologique entre solide et liquide. Aux petites contraintes le système reste élastique, mais audessus d’une contrainte seuil, le système s’écoule comme un liquide visqueux. Ce comportement se trouve dans de nombreux fluides industriels comme dans les cosmétiques, l’agroalimentaire ou encore le béton. Une contrainte seuil n’apparait que au dessus d’une certaine fractions volumique. Au dessus de cette fraction les particules se bloquent entre elle, la relaxation n’est plus possible et l’écoulement devient fortement coopératif.Cette coopérativité influe sur la rhéologie à petite échelle, Quand le confinement devient de l’ordre de quelques particules, la viscosité ne dépend plus uniquement, comme habituellement, de la contrainte locale mais aussi de la contrainte au voisinage. C’est ce qu’on appelle rhéologie nonlocaleJ’ai étudié expérimentalement ce comportement en utilisant les outils de micro fluidiques. J’ai étudié une microémulsion concentrée s’écoulant dans un microcanal en observant directement l’écoulement des gouttes avec un microscope confocal. Les résultats sont comparés au model “KineticElastoPlastic” de Bocquet et al. 2009 et à des simulations de dynamique moléculaire. / Soft glassy materials (concentrated emulsion, foams, concentrated suspension…) present rheological properties between solids and liquid. Under small stress they stay elastic but at stress higher than a yield stress they begin to flow as a liquid. Those fluids are used in cosmetics, food industry or building materials as concrete. The yield stress behavior only appears when the volume fraction is high enough, where the particles are blocked by their neighbors. So the systems cannot relax and the flow become highly cooperative.This cooperativity impacts the rheology at small scale. When the confinement is of the order of few particles, the viscosity does not only depend on the local stress as usually but also on the stress in the neighborhood. This is called nonlocal rheology.I studied experimentally this behavior by flowing concentrated emulsion in a microchannel and observing directly the flow of the droplet with a confocal microscopy. The results from these microfluidics experiments are compared to predictions of the Kinetic Elasto Plastic model of Bocquet et al. 2009 and molecular dynamic simulation of jammed soft particles.

3 
Dynamics of dense nonBrownian suspensions under impact / 衝撃を受ける高密度非ブラウン系懸濁液のダイナミクスPRADIPTO 26 September 2022 (has links)
京都大学 / 新制・課程博士 / 博士(理学) / 甲第24167号 / 理博第4858号 / 新制理1695(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 早川 尚男, 教授 佐々 真一, 教授 山本 潤 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

4 
Dynamique lente de systèmes colloïdaux modèles / Slow dynamics in model colloidal systemsBrambilla, Giovanni 09 February 2010 (has links)
Ce travail est consacré à l'étude, par des techniques originales de diffusion dynamique de la lumière, de la dynamique lente de deux systèmes colloïdaux modèles. On a utilisée une technique de Corrélation Résolue dans le Temps (TRC) et des simulations numériques pour sonder, sur une grande plage de concentrations et sur sept décades en temps de relaxation, la dynamique d'un système de sphères dures colloïdales approchant de la transition vitreuse. A faible fraction volumique les temps de relaxation peuvent être représentés par une loi de puissance critique comme prévu par la Théorie de Couplage des Modes (MCT). A des concentrations élevées le temps de relaxation augmente selon une inattendue loi exponentielle critique avec une fraction volumique critique supérieure à celle prévue par MCT. L'étude des fluctuations de la dynamique confirme la présence d'un régime de dynamique activée: ce scénario est le même que l'on retrouve pour les systèmes vitreux moléculaires. Le deuxième sujet étudié est le comportement d'une suspension de colloïdes attractifs soumis à la contrainte gravitationnelle. Initialement, des agrégats de particules sédimentent en formant un gel sur le fond de la cellule. Les techniques spéciales employées nous ont permis de mesurer l'évolution des profils de concentration, des profils de vitesse et la dynamique locale dans la phase gel: la sédimentation peut être complètement décrite par le taux de déformation locale qui permet aussi une remise à l'échelle de la dynamique locale. Le rôle de la friction aux parois des cellules a été sondé par des expériences de polarimétrie et par la résolution numérique d'un modèle poroélastique pour la sédimentation du gel. / We investigated, using original light scattering techniques, the slow dynamics of two different model colloidal systems. We used a Time Resolved Correlation (TRC) technique and numerical simulations to study the dynamics of systems of colloidal hard spheres over a broad range of densities and seven decades in relaxation time. At low volume fraction the equilibrium relaxation time is described by the algebraic divergence predicted by modecoupling theory (MCT). At higher density the relaxation time increases with an unexpected critical exponential form, whose the critical volume fraction is much larger then the MCT singularity. The fluctuations of the dynamics confirm the presence of an activated regime, as founded in molecular glass formers close to the glass transition. The second research subject concerns the behaviour of suspensions of attractive colloidal particles under gravitational stress. Initially, clusters fall forming a gelled deposit at the cell bottom. Our apparatus allows us to follow over time the concentration profile, the velocity profile and the local dynamics in the gel phase: the settling may be fully described by the local strain rate and dynamics exhibits remarkable scaling properties when time is normalized by strain rate. The role of the solid friction at the cell walls has been investigated by polarimetry experie nces and solving numerically a poroelastic model of gel sedimentation.

5 
Studie chování ručičky rychloměru při dopravní nehodě / Study of Speedometer's Hand Behaviour by Traffic AccidentPolnický, Václav January 2013 (has links)
This diploma thesis deals with the ability to determine impact speed of vehicles in traffic accidents. Thesis tries to find out whether jammed needle of speedometer and tachometer, as well as needle's imprint, could provide relevant informations for traffic accident analysis. The outcome is formulation, whether these informations are trustworthy, or under which circumstances are valid.

6 
Flow Induced Instabilities, ShearThickening And Fluctuation Relations In Sheared Soft MatterMajumdar, Sayantan 11 1900 (has links) (PDF)
In day to day life we encounter many different materials which are intermediate between crystalline solids and simple liquids that include paints , glues , suspensions, polymers, surfactants, food and cosmetic products and so on. ‘Soft condensed matter’ is an emerging field of science that aims to generalize the flow and various deformation mechanisms in this apparent diverse class of materials from a ‘mesoscopic’ point of view (important length scales for these systems is usually 10nm1μm) where the actual atomic and molecular details governed by various quantum mechanical laws are not very important. These soft systems are held together by weaken tropic forces and therefore can be perturbed easily (the typical elastic modulus of these materials is many orders of magnitude lower compared to metallic solids). Moreover, very long relaxation times in these systems(∼10−3 to 1 s) have made them ideal candidates to study nonequilibrium physics. The present Thesis is an endeavor to understand linear and nonlinear flow behavior and low Reynolds number instabilities in various soft matter systems like suspensions of flocculated carbon nanotubes and carbon black, surfactant gels, colloidal glasses, Langmuir monolayers etc probed mainly by bulk and interfacial rheology, insitu light scattering, particle image velocimetry(PIV) techniques and Fourier transform rheology. We also use dynamic light scattering techniques for particle sizing and characterization of Brownian systems.
Chapter 1 gives a general introduction to soft condensed matter, particularly, the important length and time scales, various interactions and the rich phase behavior emerged from the delicate balance between energy and entropy in these systems. In this context, We describe the detailed phase behavior of two such systems studied in this thesis. We next describe briefly a few important concepts which motivate the main problems studied in the present thesis like the shearthickening in suspensions of Brownian and nonBrownian particles, nonequilibrium steady state fluctuation relations in driven systems, elasticity driven instabilities in complex fluids, jamming transitions and aging behavior. This is followed by a discussion of the experimental techniques like linear and nonlinear rheology, including the Fourier transform rheology.
Chapter 2 discusses the experimental techniques used by us in detail. We first describe the different components and mode of operations of the MCR300 stresscontrolled rheometer (Paar Physica, Germany) and various experimental geometries. Next we discuss the set up for two dimensional rheological measurements. The homebuilt imaging set up for insitu polarized light scattering and direct imaging studies is described along with the insitu particle image velocimetry (PIV) to map out the exact spatially resolved velocity profiles in 2D systems. We give a brief account of the techniques of Fourier transform rheology. At the end of this chapter, we briefly describe the angle resolved dynamic light scattering (DLS) set up (Brookhaven Instruments, USA).
In Chapter 3, we study colossal discontinuous shearthickening transition in confined suspensions of fractal clusters formed by multiwall carbon nanotubes (MWNT) by rheology and insitu imaging experiments. Monotonic decrease in viscosity with increasing shear stress, known as shear thinning, is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. In the present experiments we demonstrate a discontinuous shear thickening transition where the viscosity jumps sharply above a critical shear stress by four to six orders of magnitude in flocculated suspensions of MWNT even at very low weight fractions(∼0.5%). Rheooptical observations reveal the shearthickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the nonBrownian MWNT dispersions revealing a starting jammed state followed by shearthinning and shearthickened states. The present study further suggests that the shearthickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear thickening phenomena in confined geometries is pertinent for flow controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues. We try to understand the flow of jammed and shearthickened states under constant applied strain rate by studying the building up and relaxation of individual stress fluctuation events similar to the flow in dense granular materials. We also characterize the metastable shear thickened states by superposing a small sinusoidal stress component on a steady applied stress as well as by studying the a thermal entropy consuming fluctuations which are also observed for other jammed systems under an applied steady shear stress as described in the next chapter.
Chapter 4 reports the study of nonequilibrium fluctuations in concentrated gels and glassy systems(in jammed state), the nature of fluctuations and their systemsize dependence in the framework of fluctuation relation and Generalized Gumbel distribution. In the first part, we show that the shear rate at a fixed shear stress in a micellar gel in a jammed state exhibits large fluctuations, showing positive and negative values, with the mean shear rate being positive. The resulting probability distribution functions (PDFs) of the global power flux to the system vary from Gaussian to nonGaussian, depending on the driving stress and in all cases show similar symmetry properties as predicted by GallavottiCohen steady state fluctuation relation. The fluctuation relation allows us to determine an effective temperature related to the structural constraints of the jammed state. We have measured the stress dependence of the effective temperature. Further, experiments reveal that the effective temperature and the standard deviation of the shear rate fluctuations increase with the decrease of the systemsize. In the second part of this chapter, we report a universal large deviation behavior of spatially averaged global injected power just before the rejuvenation of the jammed state formed by an aging suspension of laponite clay under an applied stress. The probability distribution function (PDF) of these entropy consuming strongly nonGaussian fluctuations follow an universal large deviation functional form described by the Generalized Gumbel (GG) distribution like many other equilibrium and nonequilibrium systems with high degree of correlations but do not obey GallavottiCohen Steady State Fluctuation Relation (SSFR). However, far from the unjamming transition (for smaller applied stresses) SSFR is satisfied for both Gaussian as well as nonGaussian PDF. The observed slow variation of the mean shear rate with system size supports a recent theoretical prediction for observing GG distribution. We also establish the universality of the observations reported in this chapter in the light of other jammed systems under shear.
We examine in the first part of Chapter 5, the shearthinning behavior of a two dimensional yield stress bearing monolayer of sorbitan tristearate at air/water interface. The flow curve (stress vs shear rate) consists of a linear region at low shear stresses/shear rates, followed by a stress plateau at higher values. The velocity profile obtained from particle imaging velocimetry indicates that shear banding occurs showing coexistence of fluidized region near the rotor and solid region with vanishing shearrate away from the rotor. In the fluidized region, the velocity profile which is linear at low shear rates becomes exponential at the onset of shearthinning, followed by a time varying velocity profile in the plateau region. At low values of constant applied shear rates, the viscosity of the film increases with time, thus showing aging behavior like in soft glassy threedimensional (3D) systems. Further, at the low values of the applied stress in the yield stress regime, the shearrate fluctuations in time show both positive and negative values, similar to that observed in sheared 3D jammed systems. By carrying out a statistical analysis of these shearrate fluctuations, we estimate the effective temperature of the soft glassy monolayer using the GalavattiCohen steady state fluctuation relation. In the second part of this chapter, we study in detail the nonlinear viscoelastic behavior of Langmuir monolayers. Under oscillatory shear usually observed in many 3D metastable complex fluids with large structural relaxation times. At large strain amplitudes(γ), the storage modulus (G”) decreases monotonically whereas the loss modulus (G”) exhibits a peak above a critical strain amplitude before it decreases at higher strain amplitudes. The power law decay exponents of G” and G” are in the ratio 2:1. The peak in G” is absent at high temperatures and low concentration of sorbitan tristearate. Strainrate frequency sweep measurements on the monolayers do indicate a strainrate dependence of the structural relaxation time. The present study on sorbitan tristearate monolayers clearly indicates that the nonlinear viscoelastic behavior in 2D Langmuir monolayers is very general and exhibits many of the features observed in 3D complex fluids.
We report in the first part of Chapter 6 scattering dichroism experiments to quantify the spatiotemporal nematodynamics of shearthinning worm like micellar gels of surfactant Cetyltrimethylammonium Tosylate (CTAT) in the presence of salt sodium chloride (NaCl) enroute to rheochaos. For shear rates past the plateau onset, we observe a presence of alternating bright and dark‘ intertwined’ birefringent structures along the vorticity direction. The orientational order corresponding to these structures are predominantly oriented at +45deg and−45deg to the flow (v) in the (v,∇v) plane. The orientational dynamics of the nematics especially at the interface between the structures, has a onetoone correspondence with the temporal behavior of the stress. Experiments show that the spatial motion of the vorticity structures depend on the gap thickness of the Couette cell. We next discuss the random temporal flow behavior of this system at high values of applied shear rate/stress in the framework of elastic turbulence in the second part of this chapter. Here, we study the statistical properties of spatially averaged global injected power fluctuations for the wormlike micellar system described above. At sufficiently high Weissenberg numbers (Wi) the shear rate and hence the injected power p(t) at a constant applied stress shows large irregular fluctuations in time. The nature of the probability distribution function (PDF) of p(t) and the powerlaw decay of its power spectrum are very similar to that observed in recent studies of elastic turbulence for polymer solutions. Remarkably, these nonGaussian pdf scan be well described by an universal large deviation functional form given by the Generalized Gumbel (GG) distribution observed in the context of spatially averaged global measures in diverse classes of highly correlated systems. We show by insitu rheology and polarized light scattering experiments that in the elastic turbulent regime the flow is spatially smooth but random in time, in agreement with a recent hypothesis for elastic turbulence.
In Chapter 7, we study the vorticity banding under large amplitude oscillatory shear (LAOS) in a dilute wormlike micellar gel formed by surfactant CTAT by Fourier transform rheology and insitu polarized light scattering. Under LAOS we found the signature of a nontrivial orderdisorder transition of Taylor vortices. In the nonlinear regime, higher harmonicde composition of the resulting stress signal reveals that the third harmonic I3 shows a very prominent maximum at the strain value where the number density (nv) of the Taylor vortices is maximum for a wide range of angular frequencies both above and below the linear crossover point. Subsequent increase in applied strain results in distortions of the vortices and a concomitant decrease in nv when I3 also drops very sharply and acts like an order parameter for this orderdisorder transition. We further quantify the transition by defining an independent order parameter like quantity from the spatial correlation function of the scattered intensity and equivalently its Fourier transform which essentially captures the non monotonous third harmonic behavior. Lissajous plots indicate an intracycle strain hardening for the values of γ corresponding to the peak of I3 similar to that observed for hardsphere glasses. Our study is an important step forward to correlating the structures developed in the system under LAOS to the appearances of the higher harmonics in the nonlinear regime.
The Thesis concludes with a summary of the main results and a brief account on the scope of future work as described in Chapter 8.

Page generated in 0.9185 seconds