Thermally activated flow in models of amorphous solids

Amorphous solids yield at a critical value Sigma(c) of the imposed stress Sigma through a dynamical phase transition. While sharp in athermal systems, the presence of thermal fluctuations leads to the rounding of the transition and thermally activated flow even below Sigma(c). Here we study the steady-state thermal flow of amorphous solids using a mesoscopic elastoplastic model. In the Hebraud-Lequex (HL) model we provide an analytical solution of the thermally activated flow at low temperature. We then propose a general scaling law that also describes the transition rounding. Finally, we find that the scaling law holds in numerical simulations of the HL model, a two-dimensional (2D) elastoplastic model, and previously published molecular dynamics simulations of 2D Lennard-Jones glass.

Towards the Glass Transition in Vibrated Granular Matter

Alessandro Luigi Sellerio

Granular materials are large sets of macroscopic particles that interact solely via contact forces. The static behavior depends on the contact network and on the surface friction forces between grains; when they are set in motion (typically by vibrations) their dynamics is dominated by inelastic collisions. For these reasons granular media show an extremely rich phenomenology, ranging from fluid-like properties (if strongly vibrated), to "jamming", glassy, behavior (if weakly vibrated), to aging and hysteretical phenomena observed when they become trapped in frozen, amorphous states. The objective of this work is to study these states and transitions, and to characterize the analogies found between the dynamic behavior of vibrated granular media and the glass transition observed in thermal glass-formers. These analogies justify the interest in granular materials, because granular media can be seen as simplified model systems useful in the study of out of equilibrium thermodynamics, and, in general, to the larger framework known as "complexity". The granular materials considered here are composed of spheric, polished glass spheres. Since the surface state plays an important role in the grain-grain interaction, some measurements were also performed with acid etched beads, having different surface roughness. The samples are vertically vibrated to achieve vibrofluidization. Different kinds of vibration are used, to highlight different properties of the system. We first consider the transition between the fluid and the subcooled glassy phase, using different experimental techniques. The most important one is a torsion oscillator, that interacts with the granular media via immersed probes. The torsion oscillator can be used in forced mode. A torque is applied on the probe, and we measure the mechanical response function (complex susceptibility). In general, a relaxation is found and it is interpreted as the signature of the irreversible energy loss (damping) in granular collisions. This relaxation has an intrinsic time scale, and systematic analysis of it shows that a clear parallel can be traced to the behavior of "strong" glasses. In particular, it is found that (i) the relaxation time is a function of a unified control parameter, proportional to the square root of the average vibration, and phenomenologically equivalent to an effective temperature; (ii) the functional form with which the relaxation times approach the final "frozen" state has an Arrhenius, or Vögel-Fulcher-Tamman (VFT) behavior. The same torsion oscillator is employed in free mode. In this case, no external torque is applied, and the probe moves adapting its position under the effect of the continuous rearrangements in the sample. The system is studied by computing the power spectral density of the (angular position) time series. The resulting spectra represent a "configurational noise" as the system randomly hops from one configuration to the following. This allows to define, using a completely different approach, the same intrinsic time scale observed in forced mode measurements. The comparison of the two techniques allows to obtain a more complete and detailed picture of the dynamics in the jamming region. From this comparison, it was inferred that the system is also influenced by an effective vibration frequency, and that the relaxation time has indeed a non-Arrhenius behavior as a function of a control parameter defined as as = √ Γ/ωs. A model was developed combining rheological observations to a statistic approach describing extremal phenomena. This model justifies the appearance of both the control parameter and the VFT evolution of the relaxation. Furthermore, the model is predictive and exposes the effect of a few other rheologic properties of granular system. The effect of surface roughness are considered, showing that the static and dynamic surface friction coefficients are well described by the model. A second relevant part of this work is devoted to an explicit verification that macroscopic probes act as Brownian objects. This fact is often used to interpret experimental data (also in the present work) and to propose theoretical model. However, no explicit evidence has ever been discussed. This is hard to do, using a constrained system such as the torsion oscillator, because the restoring coefficient influences the dynamics of diffusion. To overcome the problem we built a different apparatus, called "Brownian motor", where the probes are mounted on ball bearings, so that they are free to turn without constraint. The properties of the time series of the position of the free turning probe and of the torsionally constrained oscillator can finally be analyzed and compared with simple simulations. The data show an overall diffusion-like behavior, that is influenced by the presence of constraints. Using fractal analysis we estimate the diffusion, or Hurst exponent. This allows to verify that a "macroscopic" object (the probe) immersed in the "microscopic" granular medium indeed behaves as a Brownian object, and that its dynamics can be studied in detail, showing that it undergoes anomalous diffusion. This work is concluded with a discussion on a few possible developments. The most promising idea is a novel approach to the study of the geometrical properties of the contact network of granular assemblies, that is responsible for many of the properties of the granular sample. By using Magnetic Resonance Imaging, the static 3-D structure of granular media can be reconstructed with unprecedented accuracy, resolution and ease of reproducibility. From the spatial information we can extract all the properties of static granular media: the compaction factor, the grain-grain correlation function, the free volume and other observables. Systematic studies could allow experimental confirmations of the many theoretical models that have been proposed in the last years and that still lack a thorough comparison with experiments. This idea does not conclude the perspectives of this work, that are vast and intriguing. A few promising subjects are reviewed more into detail in the corresponding Perspective section. To name a few we cite: measurements of induced aging in non-vibrated samples, the Brownian motor, stick and slip phenomena and their comparison with earthquakes.

EPFL2012

Arrested demixing opens route to bigels

Maxim Belushkin, Nicolas Dorsaz, Giuseppe Foffi, Francesco Varrato

Understanding and, ultimately, controlling the properties of amorphous materials is one of the key goals of material science. Among the different amorphous structures, a very important role is played by colloidal gels. It has been only recently understood that colloidal gels are the result of the interplay between phase separation and arrest. When short-ranged attractive colloids are quenched into the phase-separating region, density fluctuations are arrested and this results in ramified amorphous space-spanning structures that are capable of sustaining mechanical stress. We present a mechanism of aggregation through arrested demixing in binary colloidal mixtures, which leads to the formation of a yet unexplored class of materials-bigels. This material is obtained by tuning interspecies interactions. Using a computer model, we investigate the phase behavior and the structural properties of these bigels. We show the topological similarities and the geometrical differences between these binary, interpenetrating, arrested structures and their well-known monodisperse counterparts, colloidal gels. Our findings are supported by confocal microscopy experiments performed on mixtures of DNA-coated colloids. The mechanism of bigel formation is a generalization of arrested phase separation and is therefore universal.

Natl Acad Sciences2012

Molecular dynamics simulations of nucleation and phase transition in halide perovskites

Paramvir Ahlawat

Perovskite solar cells have emerged as the most promising cheaper photovoltaic technology. Besides solar cells, halide perovskites have a wide range of applications due to their remarkable optoelectronic properties. Starting from 3.8% in 2009, solar to the power conversion efficiency of perovskite solar cells is now >25%, exceeding market leader polycrystalline silicon. These advancements are mainly due to the improvement in their manufacturing process targeted to make better morphology. To this objective, tens of thousands of experimental studies are conducted where a wide range of additives, compositions, and processing conditions are optimized based on a trial and error methodology. Although this approach increased the efficiency, however, perovskite electronics suffer from the problem of stability and reproducibility. Therefore, to industrialize highly efficient perovskite electronics, one needs to understand better and rationalize their fabrication process, i.e., crystallization of halide perovskites. In this thesis, I perform molecular simulations to understand the formation process of halide perovskites. First, I try to lay out the atomic details of the nucleation process of perovskite from the solution. I find that nucleation proceeds in a two-step manner where an intermediate phase forms before the emergence of perovskite crystals from the solution. I observe that monovalent cations can play an essential role in the nucleation process. On this information, simulation-driven experiments are performed by experimental collaborators to demonstrate that the grain size and crystallinity can be tuned by only varying the concentration of monovalent cations in solution. I also attempted to study the effects of different solvents and pseudo-halide additives used to make high-efficiency solar cells. I calculated that by modulating the solute-solvent interactions, one could minimize raw material losses during the coating process. In comparison, pseudo-halide additives are found to passivate the ionic defects and may slow down the growth process that helps to improve the final morphology. Second, I study the solid-solid phase transition in the popular two-step process. I discover a new path to the low-temperature formation of highly efficient metastable cubic phase of formamidinium lead iodide perovskite. I first validate my simulations with in-situ experiments performed by experimental collaborators. Then simulations are employed to design a practical methodology to successfully make thin films of phase pure cubic formamidinium lead iodide at lower temperatures. Moreover, I study the intrinsic stability of this material. I calculate the energy barrier between the metastable photo-active phase and its photo-inactive thermodynamically stable hexagonal phase. I find that once formed; the photo-active phase might be kinetically stable. Additionally, I identify new polymorphs (perovskitoids) of halide perovskites and also study the structural insights of low-dimensional halide perovskites. Overall, I present a fundamental understanding of the crystallization of perovskites which is helpful to design better experiments to make high efficiency and stable perovskite electronics.