Arrested demixing opens route to bigels

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.

Structure determination of an amorphous drug through large-scale NMR predictions

Martins Balodis, Pierrick Berruyer, Manuel Cordova, David Lyndon Emsley, Albert Hofstetter, Federico Maria Paruzzo, Bruno Simões de Almeida

Knowledge of the structure of amorphous solids can direct, for example, the optimization of pharmaceutical formulations, but atomic-level structure determination in amorphous molecular solids has so far not been possible. Solid-state nuclear magnetic resonance (NMR) is among the most popular methods to characterize amorphous materials, and molecular dynamics (MD) simulations can help describe the structure of disordered materials. However, directly relating MD to NMR experiments in molecular solids has been out of reach until now because of the large size of these simulations. Here, using a machine learning model of chemical shifts, we determine the atomic-level structure of the hydrated amorphous drug AZD5718 by combining dynamic nuclear polarization-enhanced solid-state NMR experiments with predicted chemical shifts for MD simulations of large systems. From these amorphous structures we then identify H-bonding motifs and relate them to local intermolecular complex formation energies. Determining the structure of amorphous solids is important for optimization of pharmaceutical formulations, but direct relation of molecular dynamics (MD) simulations and NMR to achieve this is challenging. Here, the authors use a machine learning model of chemical shifts to solve the atomic-level structure of the hydrated amorphous drug AZD5718 by combining dynamic nuclear polarization-enhanced solid-state NMR with predicted shifts for MD simulations of large systems.

NATURE RESEARCH2021

Shear Thickening Fluid Impregnated Foam Composites for Vibration Control and Impact Protection

Mathieu Soutrenon

Highly concentrated suspensions of silica particles present shear thickening properties, shifting from a low to a high viscosity state, instantly and reversibly at a critical shear rate. Their viscosity increases by up to 3 orders of magnitude, changing from a honey-like liquid to a brittle solid material. This sharp rise in viscosity is associated with a large absorption of energy that can be used in damping applications. These shear thickening fluids (STF) are among the few passive materials that can simultaneously bring both an increase in damping and in stiffness properties. In mechanical design, this leads potentially to a weight reduction of the overall system. This thesis work is aimed to develop and test a composite material solution for vibration control and impact protection, using STF as the damping element. A main challenge was to overcome the fact that STF are liquid at rest, and very sensitive to their environment. The proposed damping material was therefore constituted of foam impregnated with STF and encapsulated with silicone (STF/foam). A comparison of properties with other damping materials such as Smactane®was emphasized along the thesis. Storage and processing conditions of STF constituted of highly concentrated suspensions were shown to affect their rheological properties similarly to other sensitive parameters such as particle concentration. Rigorous processing methods were thus required to produce STF with defined and stable rheological properties. The use of sonication was preferred to disperse the aggregates of particles in concentrated colloidal suspensions due to the packing of the dry powder. Contact with air or humidity deteriorates the shear thickening properties of STF. Storage of STF in a freezer at −24 ◦C or under nitrogen prevents this. The use of a physical barrier to air and humidity such as silicone to protect STF is also shown to greatly reduce the ageing problem. The energy dissipated during a cyclic strain cycle was used as a measure of the damping property of the STF, and compared to that of other materials. It was measured with dynamic rheological measurements performed on suspensions with variations of particle concentration, size and test frequency. STF were shown to dissipate a large amount of energy during their transition, which increases with the increase of frequency and particle concentration and decrease with the increase of particle size. In specific cases of large strain amplitude, they potentially dissipate more energy than viscoelastic materials such as rubber. An open cell melamine resin foam was used as a lightweight scaffold to provide a three dimensional structure to the STF. The foam distributes the load and locally shears the STF. The shape of the foam is easily adjustable. To impregnate the foam with the STF, a system similar to vacuum infusion was used. The foam was placed under vacuum in a mold and the STF heated at 70 ◦C to reduce its viscosity and shear thickening properties was sucked in the foam with the depressurization. The STF/foam systems were then encapsulated in silicone to protect the STF/foam damping pads from contamination and avoid outgassing. During compression tests at low frequencies and large deformations, we demonstrated that STF could be activated in this type of structure, with mechanical properties comparable to those of a viscous honey-like fluid at rest and silicone when the STF is in the solid state. The same behavior was observed during impact solicitations. STF/foam presents a very low coefficient of restitution and absorbs the shock wave associated with impacts. Shock tests done separately confirm the shock absorption properties of STF/foam even if the STF is not activated. The combination of mechanical and damping properties of the foam/STF systems remain lower than for very high performance damping materials like Smactane®. The reasons are their low mechanical compression and shear properties at rest and the need to overcome a threshold strain or stress to activate the STF. However, the large dissimilarity between the states before and after the transition makes these materials interesting for impact or large strain amplitude applications such as sole for running shoes or several space structures.

EPFL2013

SAMARIS D26 - Modelling of UHPFRC in composite structures

Emmanuel Denarié, Hamid Sadouki

The extremely low permeability of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) associated with their outstanding mechanical properties make them especially suitable to locally "harden" reinforced concrete structures in critical zones subjected to an aggressive environment and to significant mechanical stresses. UHPFRC materials can be applied on new structures, or on existing ones for rehabilitation, as thin watertight overlays in replacement of waterproofing membranes, as reinforcement layers combined with reinforcement bars, or as prefabricated elements such as kerbs. The successful rehabilitation of existing structures is a major challenge for civil engineers. When existing concrete needs to be replaced, a new composite structure formed of the new material cast on the existing substrate will result from the intervention. Both the protective function and the mechanical performance of the composite system have to be guaranteed over the planned service life. These requirements can be fulfilled by a sound understanding of the origin of deteriorations, and by proper design and application. Further, to take the full benefit of the potential of UHPFRC to rehabilitate or reinforce structures, it is often needed to get better insights of the performance of composite UHPFRC-Concrete members by means of numerical models such as finite element simulations. The numerical modelling of multiple layer systems involves comprehensive models of the time dependent behaviour of the materials at early-age and long term. UHPFRC require special consideration of their tensile hardening behaviour. This document gives the theoretical background, and provides validations of a constitutive model for UHPFRC under tensile loading. This model is then applied to the inverse analysis of test results in bending, and to the simulation of various configurations of application of UHPFRC in composite structural members.