Spatial organization of chromosomes leads to heterogeneous chromatin motion and drives the liquid- or gel-like dynamical behavior of chromatin

Chromosome organization and dynamics are involved in regulating many fundamental processes such as gene transcription and DNA repair. Experiments unveiled that chromatin motion is highly heterogeneous inside cell nuclei, ranging from a liquid-like, mobile state to a gel-like, rigid regime. Using polymer modeling, we investigate how these different physical states and dynamical heterogeneities may emerge from the same structural mechanisms. We found that the formation of topologically associating domains (TADs) is a key driver of chromatin motion heterogeneity. In particular, we showed that the local degree of compaction of the TAD regulates the transition from a weakly compact, fluid state of chromatin to a more compact, gel state exhibiting anomalous diffusion and coherent motion. Our work provides a comprehensive study of chromosome dynamics and a unified view of chromatin motion enabling interpretation of the wide variety of dynamical behaviors observed experimentally across different biological conditions, suggesting that the “liquid” or “solid” state of chromatin are in fact two sides of the same coin.

Equilibrium measures on trees

We give a characterization of equilibrium measures for p-capacities on the boundary of an infinite tree of arbitrary (finite) local degree. For $$p=2$$ p = 2 , this provides, in the special case of trees, a converse to a theorem of Benjamini and Schramm, which interpretes the equilibrium measure of a planar graph’s boundary in terms of square tilings of cylinders.

Spatial organization of chromosomes leads to heterogeneous chromatin motion and drives the liquid- or gel-like behavior of chromatin

Chromosome organization and dynamics are involved in the regulation of many fundamental processes such as gene transcription and DNA repair. Experiments unveiled that inside cell nuclei chromatin motion is highly heterogeneous ranging from a liquid-like, mobile state to a gel-like, rigid state. Using polymer modeling, we investigated how these different physical states and dynamical heterogeneities may emerge from the same structural mechanisms. We found that the formation of topologically-associating domains (TADs) is a key driver of chromatin motion heterogeneity. In particular, we demonstrated that the local degree of compaction of the TAD regulates the transition from a weakly compact, fluid state of chromatin to a more compact, gel state exhibiting anomalous diffusion and coherent motion. Our work provides a comprehensive study of chromosome dynamics and offers a unified view of chromatin motion allowing to interpret the wide variety of dynamical behaviors observed experimentally across different biological conditions.

Orientation Averaging of Optical Chirality Near Nanoparticles and Aggregates

Artificial nanostructures enable fine control of electromagnetic fields at the nanoscale, a possibility that has recently been extended to the interaction between polarised light and chiral matter. The theoretical description of such interactions, and its application to the design of optimised structures for chiroptical spectroscopies, brings new challenges to the common set of tools used in nano-optics. In particular, chiroptical effects often depend crucially on the relative orientation of the scatterer and the incident light, but many experiments are performed with randomly-oriented scatterers, dispersed in a solution. We derive new expressions for the orientation-averaged local degree of optical chirality of the electromagnetic field in the presence of a nanoparticle aggregate. This is achieved using the superposition T -matrix framework, ideally suited for the derivation of efficient orientation-averaging formulas in light scattering problems. Our results are applied to a few model examples, and illustrate several non-intuitive aspects in the distribution of orientation-averaged degree of chirality around nanostructures. The results will be of significant interest for the study of nanoparticle assemblies designed to enhance chiroptical spectroscopies, and where the numerically-efficient computation of the averaged degree of optical chirality enables a more comprehensive exploration of the many possible nanostructures.

Orientation Averaging of Optical Chirality Near Nanoparticles and Aggregates

Artificial nanostructures enable fine control of electromagnetic fields at the nanoscale, a possibility that has recently been extended to the interaction between polarised light and chiral matter. The theoretical description of such interactions, and its application to the design of optimised structures for chiroptical spectroscopies, brings new challenges to the common set of tools used in nano-optics. In particular, chiroptical effects often depend crucially on the relative orientation of the scatterer and the incident light, but many experiments are performed with randomly-oriented scatterers, dispersed in a solution. We derive new expressions for the orientation-averaged local degree of optical chirality of the electromagnetic field in the presence of a nanoparticle aggregate. This is achieved using the superposition T -matrix framework, ideally suited for the derivation of efficient orientation-averaging formulas in light scattering problems. Our results are applied to a few model examples, and illustrate several non-intuitive aspects in the distribution of orientation-averaged degree of chirality around nanostructures. The results will be of significant interest for the study of nanoparticle assemblies designed to enhance chiroptical spectroscopies, and where the numerically-efficient computation of the averaged degree of optical chirality enables a more comprehensive exploration of the many possible nanostructures.

Colombian Language Teachers Abroad: An Overview of Their Professional Experience

This article presents the preliminary results of an enquiry on the work experience of Colombian language teachers who have entered the workforce in foreign countries. The study aims at unveiling aspects of the personal and professional experience of those graduates that might be relevant for pre-service teachers in local degree programs ahead of their potential move abroad in a growing international mobility context. The study includes data from surveys and personal interviews carried out with a group of 26 participants whose narratives of experience invite one to consider the variables that may either facilitate or hinder one’s teaching practice in a foreign workplace and the intercultural communication issues that language teachers may need to deal with. A reflection is brought forward on the needs that teacher training programs at university level in Colombia should address to broaden the scope of their graduates’ likely move abroad.

Dispersion of Graphite Nanoplates in Polypropylene by Melt Mixing: The Effects of Hydrodynamic Stresses and Residence Time

This work combines experimental and numerical (computational fluid dynamics) data to better understand the kinetics of the dispersion of graphite nanoplates in a polypropylene melt, using a mixing device that consists of a series of stacked rings with an equal outer diameter and alternating larger and smaller inner diameters, thereby creating a series of converging/diverging flows. Numerical simulation of the flow assuming both inelastic and viscoelastic responses predicted the velocity, streamlines, flow type and shear and normal stress fields for the mixer. Experimental and computed data were combined to determine the trade-off between the local degree of dispersion of the PP/GnP nanocomposite, measured as area ratio, and the absolute average value of the hydrodynamic stresses multiplied by the local cumulative residence time. A strong quasi-linear relationship between the evolution of dispersion measured experimentally and the computational data was obtained. Theory was used to interpret experimental data, and the results obtained confirmed the hypotheses previously put forward by various authors that the dispersion of solid agglomerates requires not only sufficiently high hydrodynamic stresses, but also that these act during sufficient time. Based on these considerations, it was estimated that the cohesive strength of the GnP agglomerates is in the range of 5–50 kPa.

The Spatial Diffusion of Social Conformity and its Effect on Voter Turnout

Social conformity can spread social norms and behaviors through a society. This research examines such a process geographically for conformity with the norm that citizens should vote and consequent voter turnout. A mathematical model for this process is developed based on the Laplace equation, and predictions are tested with qualitative and quantitative spatial analyses of state-level voter turnout in American presidential elections. Results show that the diffusion of conformist behavior affects the local degree of turnout and produces highly specific and predictable voting behavior patterns across the United States, confirming the model.