A Case Study of Sedimentation of Charged Colloids: The Primitive Model and the Effective One-Component Approach

Sedimentation-diffusion equilibrium density profiles of suspensions of charge-stabilized colloids are calculated theoretically and by Monte Carlo simulation, both for a one-component model of colloidal particles interacting through pairwise screened-Coulomb repulsions and for a three-component model of colloids, cations, and anions with unscreened-Coulomb interactions. We focus on a state point for which experimental measurements are available [C.P. Royall et al., J. Phys.: Cond. Matt. {\bf 17}, 2315 (2005)]. Despite the apparently different picture that emerges from the one- and three-component model (repelling colloids pushing each other to high altitude in the former, versus a self-generated electric field that pushes the colloids up in the latter), we find similar colloidal density profiles for both models from theory as well as simulation, thereby suggesting that these pictures represent different view points of the same phenomenon. The sedimentation profiles obtained from an effective one-component model by MC simulations and theory, together with MC simulations of the multi-component primitive model are consistent among themselves, but differ quantitatively from the results of a theoretical multi-component description at the Poisson-Boltzmann level. We find that for small and moderate colloid charge the Poisson-Boltzmann theory gives profiles in excellent agreement with the effective one-component theory if a smaller effective charge is used. We attribute this discrepancy to the poor treatment of correlations in the Poisson-Boltzmann theory.Comment: 9 pages, 7 figure

Dynamic Monte Carlo Simulations of Inhomogeneous Colloidal Suspensions

The Dynamic Monte Carlo (DMC) method is an established molecular simulation technique for the analysis of the dynamics in colloidal suspensions. An excellent alternative to Brownian Dynamics or Molecular Dynamics simulation, DMC is applicable to systems of spherical and/or anisotropic particles and to equilibrium or out-of-equilibrium processes. In this work, we present a theoretical and methodological framework to extend DMC to the study of heterogeneous systems, where the presence of an interface between coexisting phases introduces an additional element of complexity in determining the dynamic properties. In particular, we simulate a Lennard-Jones fluid at the liquid-vapor equilibrium and determine the diffusion coefficients in the bulk of each phase and across the interface. To test the validity of our DMC results, we also perform Brownian Dynamics simulations and unveil an excellent quantitative agreement between the two simulation techniques.Comment: 14 pages with appendi

One-year survey of a single Micronesian reef reveals extraordinarily rich diversity of Symbiodinium types in soritid foraminifera

Recent molecular studies of symbiotic dinoflagellates (genus Symbiodinium) from a wide array of invertebrate hosts have revealed exceptional fine-scale symbiont diversity whose distribution among hosts, regions and environments exhibits significant biogeographic, ecological and evolutionary patterns. Here, similar molecular approaches using the internal transcribed spacer-2 (ITS-2) region were applied to investigate cryptic diversity in Symbiodinium inhabiting soritid foraminifera. Approximately 1,000 soritid specimens were collected and examined during a 12-month period over a 40m depth gradient from a single reef in Guam, Micronesia. Out of 61 ITS-2 types distinguished, 46 were novel. Most types found are specific for soritid hosts, except for three types (C1, C15 and C19) that are common in metazoan hosts. The distribution of these symbionts was compared with the phylotype of their foraminiferal hosts, based on soritid small subunit ribosomal DNA sequences, and three new phylotypes of soritid hosts were identified based on these sequences. Phylogenetic analyses of 645 host-symbiont pairings revealed that most Symbiodinium types associated specifically with a particular foraminiferal host genus or species, and that the genetic diversity of these symbiont types was positively correlated with the genetic diversity found within each of the three host genera. Compared to previous molecular studies of Symbiodinium from other locations worldwide, the diversity reported here is exceptional and suggests that Micronesian coral reefs are home to a remarkably large Symbiodinium assemblag

Microrheology of isotropic and liquid-crystalline phases of hard rods by dynamic Monte Carlo simulations

Particle tracking in soft materials allows one to characterise the material’s local viscoelastic response, a technique referred to as microrheology (MR). In particular, MR can be especially powerful to ponder the impact of structural ordering on the tracer’s transport mechanism and thus disclose intriguing elements that cannot be observed in isotropic fluids. In this work, we perform Dynamic Monte Carlo simulations of isotropic and liquid-crystalline phases of rod-like particles and employ MR to characterise their linear viscoelastic response. By incorporating tracers of different diameters, we can assess the combined effect of size and ordering across the relevant time and length scales of the systems’ relaxation. While the dynamics of small tracers is dramatically determined by the background ordering, sufficiently large tracers have a reduced perception of the medium nanostructure and this difference directly influences the observed MR. Our results agree very well with the picture of a microviscosity increasing with the relevant system length scales, but also suggest the crucial relevance of long-ranged order as a key element governing the system’s viscoelastic response.The Royal Society (IES\R1\191066); Leverhulme Trust (RPG-2018-415); Spanish Ministerio de Ciencia, Innovación y Universidades (PGC2018-101555-B- I00); UAL/CECEU/FEDER (UAL18-FQM- B038-A); Consejería de Transformación Económica, Industria, Conocimiento y Universidades de la Junta de Andalucía/FEDER (P20-00816

Establishing the relationship between cortical atrophy and semantic deficits in Alzheimer's disease and Mild Cognitive Impairment patients through Voxel-Based Morphometry

The aim of this study was to determine the brain areas responsible for the semantic impairment observed in Alzheimer's disease (AD) and Mild Cognitive Impairment (MCI) patients. Thirteen AD, 14 MCI patients, and 13 matched healthy older adults were assessed with a test battery aimed to study their semantic competence. Different subtasks were designed to study their semantic knowledge related to objects and faces in the context of semantic retrieval- and semantic association-dependent tasks. Aggregate scores obtained in the different tests were entered into voxel-based regression analyses with grey matter volume values obtained from three-dimensional brain MRI scans. Areas of significant correlation between volume loss and poor semantic scores were restricted to the temporal lobe in the AD group, while in the MCI and control groups significant associations were found with lower grey matter volume values in a widely distributed network of bilateral fronto-temporo-parietal regions. Our results suggest that degradation of partially overlapping and widely distributed neural networks, mainly including temporal regions, subserve semantic deficits related to objects and faces in AD and MCI patients

Neural correlates of phonological, orthographic and semantic reading processing in dyslexia

Developmental dyslexia is one of the most prevalent learning disabilities, thought to be associated with dysfunction in the neural systems underlying typical reading acquisition. Neuroimaging research has shown that readers with dyslexia exhibit regional hypoactivation in left hemisphere reading nodes, relative to control counterparts. This evidence, however, comes from studies that have focused only on isolated aspects of reading. The present study aims to characterize left hemisphere regional hypoactivation in readers with dyslexia for the main processes involved in successful reading: phonological, orthographic and semantic. Forty-one participants performed a demanding reading task during MRI scanning. Results showed that readers with dyslexia exhibited hypoactivation associated with phonological processing in parietal regions; with orthographic processing in parietal regions, Broca's area, ventral occipitotemporal cortex and thalamus; and with semantic processing in angular gyrus and hippocampus. Stronger functional connectivity was observed for readers with dyslexia than for control readers 1) between the thalamus and the inferior parietal cortex/ventral occipitotemporal cortex during pseudoword reading; and, 2) between the hippocampus and the pars opercularis during word reading. These findings constitute the strongest evidence to date for the interplay between regional hypoactivation and functional connectivity in the main processes supporting reading in dyslexia. Keywords: Dyslexia, Reading, Hypoactivation, Functional connectivity, Thalamus, Hippocampu

Increased Functional Connectivity in the Default Mode Network in Mild Cognitive Impairment: A Maladaptive Compensatory Mechanism Associated with Poor Semantic Memory Performance

Semantic memory decline and changes of default mode network (DMN) connectivity have been reported in mild cognitive impairment (MCI). Only a few studies, however, have investigated the role of changes of activity in the DMN on semantic memory in this clinical condition. The present study aimed to investigate more extensively the relationship between semantic memory impairment and DMN intrinsic connectivity in MCI. Twenty-one MCI patients and 21 healthy elderly controls matched for demographic variables took part in this study. All participants underwent a comprehensive semantic battery including tasks of category fluency, visual naming and naming from definition for objects, actions and famous people, word-association for early and late acquired words and reading. A subgroup of the original sample (16 MCI patients and 20 healthy elderly controls) was also scanned with resting state functional magnetic resonance imaging and DMN connectivity was estimated using a seed-based approach. Compared with healthy elderly, patients showed an extensive semantic memory decline in category fluency, visual naming, naming from definition, words-association, and reading tasks. Patients presented increased DMN connectivity between the medial prefrontal regions and the posterior cingulate and between the posterior cingulate and the parahippocampus and anterior hippocampus. MCI patients also showed a significant negative correlation of medial prefrontal gyrus connectivity with parahippocampus and posterior hippocampus and visual naming performance. Our findings suggest that increasing DMN connectivity may contribute to semantic memory deficits in MCI, specifically in visual naming. Increased DMN connectivity with posterior cingulate and medio-temporal regions seems to represent a maladaptive reorganization of brain functions in MCI, which detrimentally contributes to cognitive impairment in this clinical population

Enseñar fluidez lectora en el aula: de la investigación a la práctica

This paper describes a reading fluency intervention focused on empowering teachers with resources about reading fluency learning for making decisions in classrooms routines using accuracy, speed and prosody. 36 second-grade-students participated in fluency training. School teachers formed a school working group which count on two professor´s contribution to carry out knowledge and tasks on reading fluency. The group is designed to enhance collaboration between teachers to carry out evaluating fluency tasks to identified risk, emerging, and level group. An adapted fluency reading program based on repeated readings was designed and applied in collaboration for 7 months.Results have been much better in all tasks. Teacher perceptions of their own teaching skills also improved.  Fluency classroom programs based in repetitive readings impact and how to help teachers in a collaborative approach to develop fluency programs tailored to the size of their class group are discussed.Se presenta una propuesta colaborativa entre universidad y escuela para diseñar e implementar en el aula un programa de fluidez lectora basado en evidencias. Participaron 36 alumnos de 2º de Educación Primaria, sus maestras y dos profesores de universidad que actuaron como asesores, constituyendo todos un grupo de trabajo. Se formó a las maestras en contenidos de fluidez y se realizó una evaluación inicial para identificar los grupos de riesgo, emergente y establecido en fluidez lectora. Se diseñó e implementó durante 7 meses  un entrenamiento ajustado a cada nivel basado en la lectura repetida de palabras y frases. Los resultados reflejan una mejoría en la competencia de las maestras por la integración del entrenamiento de la fluidez en su sistema de respuesta, adaptándolo a distintos niveles de enseñanza. En el alumnado se constató una mejoría en la fluidez. Se plantea la necesidad de acercar a las escuelas las aportaciones de la investigación educativa y de ayudar a los docentes a desarrollar intervenciones basadas en la misma y ajustadas al nivel del grupo de clase

Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?

We study sedimentation equilibrium of low-salt suspensions of binary mixtures of charged colloids, both by Monte Carlo simulations of an effective colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture. We show that the theoretically predicted lifting and layering effect, which involves the entropy of the screening ions and a spontaneous macroscopic electric field [J. Zwanikken and R. van Roij, Europhys. Lett. {\bf 71}, 480 (2005)], can also be understood on the basis of an effective colloid-only system with pairwise screened-Coulomb interactions. We consider, by theory and by simulation, both repelling like-charged colloids and attracting oppositely charged colloids, and we find a re-entrant lifting and layering phenomenon when the charge ratio of the colloids varies from large positive through zero to large negative values