1,315 research outputs found
Structural relaxation in amorphous materials under cyclic tension-compression loading
The process of structural relaxation in disordered solids subjected to
repeated tension-compression loading is studied using molecular dynamics
simulations. The binary glass is prepared by rapid cooling well below the glass
transition temperature and then periodically strained at constant volume. We
find that the amorphous system is relocated to progressively lower potential
energy states during hundreds of cycles, and the energy levels become deeper
upon approaching critical strain amplitude from below. The decrease in
potential energy is associated with collective nonaffine rearrangements of
atoms, and their rescaled probability distribution becomes independent of the
cycle number at sufficiently large time intervals. It is also shown that
yielding during startup shear deformation occurs at larger values of the stress
overshoot in samples that were cyclically loaded at higher strain amplitudes.
These results might be useful for mechanical processing of amorphous alloys in
order to reduce their energy and increase chemical resistivity and resistance
to crystallization.Comment: 22 pages, 10 figure
Steric interactions between mobile ligands facilitate complete wrapping in passive endocytosis
Receptor-mediated endocytosis is an ubiquitous process through which cells
internalize biological or synthetic nanoscale objects, including viruses,
unicellular parasites, and nanomedical vectors for drug or gene delivery. In
passive endocytosis the cell plasma membrane wraps around the "invader"
particle driven by ligand-receptor complexation. By means of theory and
numerical simulations, here we demonstrate how particles decorated by freely
diffusing and non-mutually-interacting (ideal) ligands are significantly more
difficult to wrap than those where ligands are either immobile or interact
sterically with each other. Our model rationalizes the relationship between
uptake mechanism and structural details of the invader, such as ligand size,
mobility and ligand/receptor affinity, providing a comprehensive picture of
pathogen endocytosis and helping the rational design of efficient drug delivery
vectors.Comment: Updated version of the manuscript. Accepted for publication in PR
Relaxation dynamics in amorphous alloys under asymmetric cyclic shear deformation
The influence of cyclic loading and glass stability on structural relaxation
and yielding transition in amorphous alloys was investigated using molecular
dynamics simulations. We considered a binary mixture cooled deep into the glass
phase and subjected to cyclic shear deformation where strain varies
periodically but remains positive. We found that rapidly cooled glasses under
asymmetric cyclic shear gradually evolve towards states with lower potential
energy and finite stress at zero strain. At the strain amplitude just below a
critical value, the rescaled distributions of nonaffine displacements converge
to a power-law decay with an exponent of about -2 upon increasing number of
cycles. By contrast, more stable glasses yield at lower strain amplitudes, and
the yielding transition can be delayed for hundreds of cycles when the strain
amplitude is near a critical value. These results can be useful for the design
of novel thermo-mechanical processing methods to improve mechanical and
physical properties of metallic glasses.Comment: 22 pages, 8 figure
Anomalous approach to thermodynamic equilibrium:structure formation of molecules after vapor deposition
We describe experiments and computer simulations of molecular deposition on a substrate in which the molecules (substituted adenine derivatives) self-assemble into ordered structures. The resulting structures depend strongly on the deposition rate (flux). In particular, there are two competing surface morphologies (α and β), which differ by their topology (interdigitated vs lamellar structure). Experimentally, the α phase dominates at both low and high flux, with the β phase being most important in the intermediate regime. A similar nonmonotonic behavior is observed on varying the substrate temperature. To understand these effects from a theoretical perspective, a lattice model is devised which reproduces qualitatively the topological features of both phases. Via extensive Monte Carlo studies we can, on the one hand, reproduce the experimental results and, on the other hand, obtain a microscopic understanding of the mechanisms behind this anomalous behavior. The results are discussed in terms of an interplay between kinetic trapping and temporal exploration of configuration space.</p
Kinetics of Nanoparticle-Membrane Adhesion Mediated by Multivalent Interactions.
Multivalent adhesive interactions mediated by a large number of ligands and receptors underpin many biological processes, including cell adhesion and the uptake of particles, viruses, parasites, and nanomedical vectors. In materials science, multivalent interactions between colloidal particles have enabled unprecedented control over the phase behavior of self-assembled materials. Theoretical and experimental studies have pinpointed the relationship between equilibrium states and microscopic system parameters such as the ligand-receptor binding strength and their density. In regimes of strong interactions, however, kinetic factors are expected to slow down equilibration and lead to the emergence of long-lived out-of-equilibrium states that may significantly influence the outcome of self-assembly experiments and the adhesion of particles to biological membranes. Here we experimentally investigate the kinetics of adhesion of nanoparticles to biomimetic lipid membranes. Multivalent interactions are reproduced by strongly interacting DNA constructs, playing the role of both ligands and receptors. The rate of nanoparticle adhesion is investigated as a function of the surface density of membrane-anchored receptors and the bulk concentration of nanoparticles and is observed to decrease substantially in regimes where the number of available receptors is limited compared to the overall number of ligands. We attribute such peculiar behavior to the rapid sequestration of available receptors after initial nanoparticle adsorption. The experimental trends and the proposed interpretation are supported by numerical simulations
Anomalous approach to thermodynamic equilibrium:structure formation of molecules after vapor deposition
We describe experiments and computer simulations of molecular deposition on a substrate in which the molecules (substituted adenine derivatives) self-assemble into ordered structures. The resulting structures depend strongly on the deposition rate (flux). In particular, there are two competing surface morphologies (α and β), which differ by their topology (interdigitated vs lamellar structure). Experimentally, the α phase dominates at both low and high flux, with the β phase being most important in the intermediate regime. A similar nonmonotonic behavior is observed on varying the substrate temperature. To understand these effects from a theoretical perspective, a lattice model is devised which reproduces qualitatively the topological features of both phases. Via extensive Monte Carlo studies we can, on the one hand, reproduce the experimental results and, on the other hand, obtain a microscopic understanding of the mechanisms behind this anomalous behavior. The results are discussed in terms of an interplay between kinetic trapping and temporal exploration of configuration space.</p
Surface-triggered cascade reactions between DNA linkers direct the self-assembly of colloidal crystals of controllable thickness
Functionalizing colloids with reactive DNA linkers is a versatile way of programming self-assembly. DNA selectivity provides direct control over colloid-colloid interactions allowing the engineering of structures such as complex crystals or gels. However, the self-assembly of localized and finite structures remains an open problem with many potential applications. In this work, we present a system in which functionalized surfaces initiate a cascade reaction between linkers leading to the self-assembly of crystals with a controllable number of layers. Specifically, we consider colloidal particles functionalized by two families of complementary DNA linkers with mobile anchoring points, as found in experiments using emulsions or lipid bilayers. In bulk, intra-particle linkages formed by pairs of complementary linkers prevent the formation of inter-particle bridges and therefore colloid-colloid aggregation. However, colloids interact strongly with the surface given that the latter can destabilize intra-particle linkages. When in direct contact with the surface, colloids are activated, meaning that they feature more unpaired DNA linkers ready to react. Activated colloids can then capture and activate other colloids from the bulk through the formation of inter-particle linkages. Using simulations and theory, validated by existing experiments, we clarify the thermodynamics of the activation and binding process and explain how particle-particle interactions, within the adsorbed phase, weaken as a function of the distance from the surface. The latter observation underlies the possibility of self-assembling finite aggregates with controllable thickness and flat solid-gas interfaces. Our design suggests a new avenue to fabricate heterogeneous and finite structures.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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