Stability and Elasticity of Ultrathin Sphere-Patterned Block Copolymer Films.

Sphere-patterned ultrathin block copolymer films are potentially interesting for a variety of applications in nanotechnology. We use self-consistent field theory to investigate the elastic response of sphere monolayer films with respect to in-plane shear, in-plane extension, compression deformations, and bending. The relations between the in-plane elastic moduli are roughly compatible with the expectations for two-dimensional elastic systems with hexagonal symmetry, with one notable exception: The pure shear and the simple shear moduli differ from each other by roughly 20%.

Shock compression of semiflexible polymers.

We employ molecular dynamics simulations to investigate the shock compression of linear semiflexible polymers. While the propagation velocity of a shock primarily depends on density, both chain rigidity and chain orientation significantly influence the shock width and the final temperature of the system. In general, the shock wave triggers molecular buckling in chains oriented perpendicular to the compression front.

Environmentally friendly starch/alginate aerogels for copper adsorption from aqueous media. A microstructural and kinetic study.

This work investigated the synthesis and characterization of alginate/starch porous materials and their application as copper ions adsorbents from aqueous media. Initially, pregel aqueous solutions with different biopolymer concentrations (1, 3, and 5% w/w) and alginate contents (25, 50, and 75% w/w) were prepared. Hydrogel formation was performed by internal and external gelation methods.

Shock melting of lamellae-forming block copolymers.

While the propagation of shocks through monoatomic liquids and solids is now well understood, the response of macromolecular systems to shock compression remains far less studied. Here we use molecular dynamics simulations to study the shock compression of diblock copolymers assembled in a lamellae morphology, which may display outstanding ballistic performance. For the first time, we show that the morphologies observed after the passage of the shock resemble those observed at equilibrium, at a temperature dictated by the compression velocity.

Velocity distributions in a gas-gun microparticle accelerator.

Here, we build and characterize a single-stage gas-gun microparticle accelerator, where a pressurized gas expands and launches particles on a target. The microparticles in the range of 60-250 μm are accelerated by the expansion of pressurized nitrogen. By using a high-speed camera, we study how the velocity distribution of accelerated particles is modified by particle size, pressure in the gas reservoir, valve's opening time, and diaphragm's thickness and composition.

Two-dimensional crystalization on spheres: Crystals grow cracked.

Here we study how curvature affects the structure of two-dimensional crystals growing on spheres. The mechanism of crystal growth is described by means of a Landau model in curved space that accounts for the excess of strain on crystal bonds caused by the substrate's curvature (packing frustration). In curved space elastic energy penalization strongly dictates the geometry of growing crystals.

Anomalous Slowdown of Polymer Detachment Dynamics on Carbon Nanotubes.

The "wrapping" of polymer chains on the surface of carbon nanotubes allows one to obtain multifunctional hybrid materials with unique properties for a wide range of applications in biomedicine, electronics, nanocomposites, biosensors, and solar cell technologies. We study by means of molecular dynamics simulations the force-assisted desorption kinetics of a polymer from the surface of a carbon nanotube. We find that, due to the geometric coupling between the adsorbing surface and the conformation of the macromolecule, the process of desorption slows down dramatically upon increasing the windings around the nanotube.

Curvature as a Guiding Field for Patterns in Thin Block Copolymer Films.

Experimental data on thin films of cylinder-forming block copolymers (BC)-free-standing BC membranes as well as supported BC films-strongly suggest that the local orientation of the BC patterns is coupled to the geometry in which the patterns are embedded. We analyze this phenomenon using general symmetry considerations and numerical self-consistent field studies of curved BC films in cylindrical geometry. The stability of the films against curvature-induced dewetting is also analyzed.

Thermal properties of vortices on curved surfaces.

We use Monte Carlo simulations to study the finite temperature behavior of vortices in the XY model for tangent vector order on curved backgrounds. Contrary to naive expectations, we show that the underlying geometry does not affect the proliferation of vortices with temperature respect to what is observed on a flat surface. Long-range order in these systems is analyzed by using two-point correlation functions.

Semiflexible Polymers in Spherical Confinement: Bipolar Orientational Order Versus Tennis Ball States.

Densely packed semiflexible polymers with contour length L confined in spheres with radius R of the same order as L cannot exhibit uniform nematic order. Depending on the chain stiffness (which we vary over a wide range), highly distorted structures form with topological defects on the sphere surface. These structures are completely different from previously observed ones of very long chains winding around the inner surface of spheres and from nematic droplets.

Guillain-Barré syndrome: causes, immunopathogenic mechanisms and treatment.

Guillain-Barré syndrome is a rare disease representing the most frequent cause of acute flaccid symmetrical weakness of the limbs and areflexia usually reaching its peak within a month. The etiology and pathogenesis remain largely enigmatic and the syndrome results in death or severe disability in 9-17% of cases despite immunotherapy. Areas covered: In terms of etiology, Guillain-Barré syndrome is linked to Campylobacter infection but less than 0.

Guillain-Barré syndrome in Colombia: where do we stand now?

Guillain-Barré syndrome (GBS) is a rapid-onset muscle weakness disease caused by the immune-mediated damage of the peripheral nervous system. Since there is an increase incidence of GBS cases in Latin America, particularly in Colombia, and most of them are currently preceded by Zika virus (ZIKV) infection, we aimed to assess the available evidence of the disease in Colombia through a systematic literature review. Out of 51 screened abstracts, only 16 corresponded to articles that met inclusion criteria, of which 15 were case reports or case series.

Wrinkles and splay conspire to give positive disclinations negative curvature.

Recently, there has been renewed interest in the coupling between geometry and topological defects in crystalline and striped systems. Standard lore dictates that positive disclinations are associated with positive Gaussian curvature, whereas negative disclinations give rise to negative curvature. Here, we present a diblock copolymer system exhibiting a striped columnar phase that preferentially forms wrinkles perpendicular to the underlying stripes.

Phase nucleation in curved space.

Nucleation and growth is the dominant relaxation mechanism driving first-order phase transitions. In two-dimensional flat systems, nucleation has been applied to a wide range of problems in physics, chemistry and biology. Here we study nucleation and growth of two-dimensional phases lying on curved surfaces and show that curvature modifies both critical sizes of nuclei and paths towards the equilibrium phase.

Smectic block copolymer thin films on corrugated substrates.

In this work we study equilibrium and non-equilibrium structures of smectic block copolymer thin films deposited on a topographically patterned substrate. A Brazovskii free energy model is employed to analyze the coupling between the smectic texture and the local mean curvature of the substrate. The substrate's curvature produces out-of-plane deformations of the block copolymer such that equilibrium textures are modified and dictated by the underlying geometry.

Defect formation and coarsening in hexagonal 2D curved crystals.

In this work we study the processes of defect formation and coarsening of two-dimensional (2D) curved crystal structures. These processes are found to strongly deviate from their counterparts in flat systems. In curved backgrounds the process of defect formation is deeply affected by the curvature, and at the onset of a phase transition the early density of defects becomes highly inhomogeneous.

Crystallization dynamics on curved surfaces.

We study the evolution from a liquid to a crystal phase in two-dimensional curved space. At early times, while crystal seeds grow preferentially in regions of low curvature, the lattice frustration produced in regions with high curvature is rapidly relaxed through isolated defects. Further relaxation involves a mechanism of crystal growth and defect annihilation where regions with high curvature act as sinks for the diffusion of domain walls.

Defect dynamics in crystalline buckled membranes.

We study the dynamics of defect annihilation in flexible crystalline membranes suffering a symmetry-breaking phase transition. The kinetic process leading the system toward equilibrium is described through a Brazovskii-Helfrich-Canham Hamiltonian. In membranes, a negative disclination has a larger energy than a positive disclination.

Amorphous precursors of crystallization during spinodal decomposition.

A general Landau's free energy functional is used to study the dynamics of crystallization during liquid-solid spinodal decomposition (SD). The strong length scale selectivity imposed during the early stage of SD induces the appearance of small precursors for crystallization with icosahedral order. These precursors grow in densely packed clusters of tetrahedra through the addition of new particles.

Dynamics of dislocations in a two-dimensional block copolymer system with hexagonal symmetry.

Block copolymer thin films have attracted considerable attention for their ability to self-assemble into nanometre-scale architectures. Recent advances in the use of block copolymer thin films as nano-lithographic masks have driven research efforts in order to have better control of long-range ordering in the plane of the film. Irrespective of the method of sample preparation, different quasi-two-dimensional systems with hexagonal symmetry unavoidably contain translational defects, called dislocations.

Spinodal-assisted nucleation during symmetry-breaking phase transitions.

The kinetics of spinodal-assisted crystallization in a region of the phase diagram where the dynamics is controlled by the critical slow down was studied by means of a Cahn-Hilliard model. The length-scale selectivity conducted by the spinodal process led to the formation of a filamentary network of density fluctuations that resemble the scarred states found in quantum-chaos systems. The present work reveals that the early structure of density fluctuations acts such as a precursor for crystallization and deeply affects the orientational and translational correlation between growing crystals.

Relaxational dynamics of smectic phases on a curved substrate.

We study the dynamics of pattern formation of two-dimensional smectic systems constrained to lie on a substrate with sinusoidal topography. We observe a coupling between defects and geometry that induces the preferential location of positive (negative) defects onto regions with positive (negative) Gaussian curvature. For the curvatures studied here we observe unbinding and self-organization of disclination pairs.

Lifshitz-Safran coarsening dynamics in a 2D hexagonal system.

The coarsening process in a two-dimensional hexagonal system in the region close to both spinodal and order-order transitions was investigated through the Cahn-Hilliard model. We found a distinctive region of the phase diagram where the pinning of dislocations plays only a minor role and the dynamics is led by the triple points. In this region, we found configurations of domains with the same features as those proposed by Lifshitz.

Arm retraction potential of branched polymers in the absence of dynamic dilution.

We study the stress relaxation of model polymer networks containing low contents of star shaped and linear dangling polymers. As compared with their melts, the behavior of star and dangling polymers leads to a dynamic response with unprecedented large relaxation times. By comparing data of star melts with those corresponding to stars and dangling chains residing in polymer networks, we were able to identify the effects of dynamic dilution clearly.