Topological barriers to defect nucleation generate large mechanical forces in an ordered fluid

Common fluids cannot sustain static mechanical stresses at the macroscopic scale because they lack molecular order. Conversely, crystalline solids exhibit long-range order and mechanical strength at the macroscopic scale. Combining the properties of fluids and solids, liquid crystal films respond to mechanical confinement by both flowing and generating static forces. The elastic response, however, is very weak for film thicknesses exceeding 10 nm. In this study, the mechanical strength of a fluid film was enhanced by introducing topological defects in a cholesteric liquid crystal, producing unique viscoelastic and optomechanical properties. The cholesteric was confined under strong planar anchoring conditions between two curved surfaces with sphere–sphere contact geometry similar to that of large colloidal particles, creating concentric dislocation loops. During surface retraction, the loops shrank and periodically disappeared at the surface contact point, where the cholesteric helix underwent discontinuous twist transitions, producing weak oscillatory surface forces. On the other hand, new loop nucleation was frustrated by a topological barrier during fluid compression, creating a metastable state. This generated exceptionally large forces with a range exceeding 100 nm as well as extended blueshifts of the photonic bandgap. The metastable cholesteric helix eventually collapsed under a high compressive load, triggering a stick-slip–like cascade of defect nucleation and twist reconstruction events. These findings were explained using a simple theoretical model and suggest a general approach to enhance the mechanical strength of one-dimensional periodic materials, particularly cholesteric colloid mixtures.

Membrane pore energetics and the pathways to membrane rupture

Biological membranes owe their strength and low permeability to the phospholipid bilayers at their core. Membrane strength is determined by the energetics and dynamics of membrane pores, whose tension-dependent nucleation and growth leads to rupture. Creation of nanoscale membrane pores is central to exocytosis, trafficking and other processes fundamental to life that require breaching of secure plasma or organelle membranes, and is the basis for biotechnologies using drug delivery, delivery of genetic material for gene editing and antimicrobial peptides. A prevailing view from seminal electroporation and membrane rupture studies is that pore growth and bilayer rupture are controlled by macroscopically long-lived metastable defect states that precede fully developed pores. It was argued that defect nucleation becomes rate-limiting at high tensions, explaining the exponential tension-dependence of rupture times [E. Evans et al., Biophys. J. 85, 2342-2350 (2003)]. Here we measured membrane pore free energies and bilayer rupture using highly coarse-grained simulations that probe very long time scales. We find no evidence of metastable pore states. At lower tensions, small hydrophobic pores mature into large hydrophilic pores on the pathway to rupture, with classical tension dependence of rupture times. Above a critical tension membranes rupture directly from a small hydrophobic pore, and rupture times depend exponentially on tension. Thus, we recover the experimentally reported regimes, but the origin of the high tension exponential regime is unrelated to macroscopically long-lived pre-pore defects. It arises because hydrophilic pores cannot exist above a critical tension, leading to radically altered pore dynamics and rupture kinetics.

Analysis of the initial stage of fatigue wear in heterostructure materials under contact cyclic loading

Introduction. The process of formation of fatigue defects in metal alloys with different structural morphology is considered. The work objective is to develop a computational tool for determining the moment of the defect nucleation under cyclic loading.Materials and Methods. A physical model is built, calculation expressions are presented. The physical model is based on the theory of dislocations. It is shown that a structure factor is particularly important in the process of fracture nucleus origination under dynamic cyclic loading. Depending on the structure and properties of the material, as well as on the nature of the loads, the critical fatigue defect develops in the form of cracks, pores or micro-crater wear.Research Results. A numerical experiment was performed to determine the moment of nucleation of the critical-size defect in iron-base alloys under the drop hypervelocity impacts. Comparative data of calculations and bench tests for droplet impingement erosion of steels and alloys with the structure of ferrite, austenite, sorbitol and martensite are presented. The efficiency of the nucleation stage during the incubation period of erosive wear of the materials studied was evaluated.Discussion and Conclusions. There are no strict instrumental methods for determining the duration of the nucleation stage; therefore, it is recommended to use the proposed analytical model. In addition, the work performed gave a significant application result, i.e. it showed that the focused design of the material structure can significantly increase the wear resistance.

Growth of curved crystals: competition between topological defect nucleation and boundary branching

Sketch of competing topological defect nucleation and boundary branching in curved crystal growth driven by curvature induced stress.

Etching gas-sieving nanopores in single-layer graphene with an angstrom precision for high-performance gas mixture separation

One of the bottlenecks in realizing the potential of atom-thick graphene membrane for gas sieving is the difficulty in incorporating nanopores in an otherwise impermeable graphene lattice, with an angstrom precision at a high-enough pore density. We realize this design by developing a synergistic, partially decoupled defect nucleation and pore expansion strategy using O2 plasma and O3 treatment. A high density (ca. 2.1 × 1012 cm−2) of H2-sieving pores was achieved while limiting the percentage of CH4-permeating pores to 13 to 22 parts per million. As a result, a record-high gas mixture separation performance was achieved (H2 permeance, 1340 to 6045 gas permeation units; H2/CH4 separation factor, 15.6 to 25.1; H2/C3H8 separation factor, 38.0 to 57.8). This highly scalable pore etching strategy will accelerate the development of single-layer graphene-based energy-efficient membranes.