Measuring the free energy of hard-sphere colloidal glasses

Free energy is a key thermodynamic observable that controls the elusive physics of the glass transition. However, measuring the free energy of colloidal glasses from microscopy images is challenging due to the difficulty of measuring the individual particle size in the slightly polydisperse glassy systems. In this paper, we carry out experiments and numerical simulations of colloidal glasses with the aim to find a practical approach to measure the free energy from colloidal particles at mild polydispersity. We propose a novel method which requires only the particle coordinates from a few confocal microscopy snapshots to estimate the average particle diameter and use it as an input for our experimental free energy measurements. We verify our free energy calculations from Cell Theory with the free energy obtained by Thermodynamic Integration. The excellent agreement between the free energies measured using the two methods close to the glass transition packing fraction highlights the dominant role played by \emph{vibrational} entropy in determining a colloidal glass's free energy. Finally, the noticeable free energy difference calculated from uniform and conjectured particle sizes emphasizes the sensitivity on particle free volumes when measuring free energy in the slightly polydisperse colloidal glass.

РОЗРОБЛЕННЯ ПРОГРАМНОГО ЗАБЕЗПЕЧЕННЯ ДЛЯ ОЦІНЮВАННЯ ОДНОРІДНОСТІ РОЗПОДІЛУ НАПОВНЮВАЧА В ПОЛІМЕРНІЙ МАТРИЦІ

Purpose. Creation of software for evaluating the uniformity of distribution of the filler in a polyethylene matrix.Methodology. Software development was carried out using the Python programming language and libraries: PIL, Numpy, Matplotlib, Xlsxwriter. The suitability of the developed software for use was determined by verifying it. During this verification, polyethylene compositions filled with colloidal graphite in the form of compressed films were evaluated. To obtain these compositions, we chose P6006AD grade polyethylene and C-1 colloidal graphite. Samples of polyethylene compositions were obtained in two stages: 1) obtaining a strand by extrusion; 2) additional mixing of the composition on a disc mixer and pressing the obtained compositions into a film.Findings. The software has been developed to assess the uniformity of the distribution of the filler in the polyethylene matrix. The data were established on the dependence of the coefficient of heterogeneity of polyethylene compositions on the content of colloidal graphite with use of the developed software. The increase in the content of the filler leads to a decrease in its heterogeneity. It is shown that this effect can be explained by the structuring of the filler in the polyethylene matrix. Despite the formation of aggregates in polyethylene compositions, a significant amount of small colloidal particles of graphite is located between the aggregate space. This leads to a certain leveling of the concentration in the film and reduces its inhomogeneity.Scientific novelty. The influence of the content of colloidal graphite on the homogeneity of polyethylene compositions is determined. It is shown that with an increase in the graphite content from 0 to 20% vol. the coefficient of heterogeneity of the composition decreases from 5.3% to 3.9%, which is due to the structuring of the filler in the polyethylene matrix.Practical value. Software that makes it possible to evaluate the uniformity of the distribution of filler particles in a polymer matrix, and can be used to study the quality of mixing of polymer composite materials has been developed.

High Internal-Phase Pickering Emulsions Stabilized by Xanthan Gum/Lysozyme Nanoparticles: Rheological and Microstructural Perspective

Food-grade high internal-phase Pickering emulsions (HIPPEs) stabilized by solid or colloidal particles with different advantages have attracted extensive attention nowadays. However, looking for new appropriate particle stabilizers is the common practice for HIPPEs preparation. It is crucial to find a new strategy for the development of functional HIPPEs with controllable properties. In this study, a high concentration of xanthan gum/lysozyme nanoparticles (XG/Ly NPs) was used for the preparation of HIPPEs for the first time. Visual observations, creaming index (CI), microstructure, and rheology tests were carried out to investigate the potential of XG/Ly NPs as HIPPEs stabilizers. Results indicated that XG/Ly NPs could stabilize oil droplets in the concentration range of 0.5–4% (w/v). The HIPPEs with a minimal particle concentration of 1% exhibited remarkable physical stability. Rheological measurements showed that a high stability of solid-like HIPPEs was successfully obtained with a higher concentration of XG/Ly NPs. Overall, the HIPPEs stabilized by different concentrations of XG/Ly NPs exhibited excellent rheological and structural properties, which might provide a feasible strategy for the development of functional emulsion systems with controllable structures.

Tiling a tubule: How increasing complexity improves the yield of self-limited assembly

The ability to design and synthesize ever more complicated colloidal particles opens the possibility of self-assembling a zoo of complex structures, including those with one or more self-limited length scales. An undesirable feature of systems with self-limited length scales is that thermal fluctuations can lead to the assembly of nearby, off-target states. We investigate strategies for limiting off-target assembly by using multiple types of subunits. Using simulations and energetics calculations, we explore this concept by considering the assembly of tubules built from triangular subunits that bind edge to edge. While in principle, a single type of triangle can assemble into tubules with a monodisperse width distribution, in practice, the finite bending rigidity of the binding sites leads to the formation of off-target structures. To increase the assembly specificity, we introduce tiling rules for assembling tubules from multiple species of triangles. We show that the selectivity of the target structure can be dramatically improved by using multiple species of subunits, and provide a prescription for choosing the minimum number of subunit species required for near-perfect yield. Our approach of increasing the system’s complexity to reduce the accessibility of neighboring structures should be generalizable to other systems beyond the self-assembly of tubules.

Влияние ионной подсистемы на термоэлектрический эффект в коллоидных растворах

The thermoelectric effect is investigated in mixtures of colloidal solutions with ionic electrolytes in the initial state, when the formation of concentration gradients under the influence of an inhomogeneous temperature field can be neglected. Based on experimental measurements in mixtures with different concentrations of colloidal particles and ions, the conditions under with the coefficient of thermoelectric EMF is determined by the ion subsystem and under which the main contribution to the value of the thermoelectric force is made by colloidal particles are determined.

Magnetically driven in-plane modulation of the 3D orientation of vertical ferromagnetic flakes.

External magnetic fields are known to attract and orient magnetically responsive colloidal particles. In the case of 2D microplatelets, rotating magnetic fields are typically used to orient them parallel to...