High Line Tension Research Articles

Rate of hemifusion diaphragm dissipation and ability to form three-junction bound HD determined by lipid composition.

Though the hemifusion diaphragm (HD) is widely accepted as an intermediate in bilayer membrane fusion, lipid contributions toward HD stability and dynamics are still not fully understood. In this paper, we study large, binary, protein-free HD systems at varying compositions of negative intrinsic curvature (NIC) lipids using molecular dynamics (MD) simulations of a solvent-free coarse-grained lipid model. Under MD, initially created HDs are found to relax to three major end states depending on the composition and lipid intrinsic curvature. Low compositions of NIC lipids or weak intrinsic curvature result in double-bilayer end states, and moderate compositions of moderate to strong NIC lipids result in metastable fusion pores. Importantly, high compositions of moderate NIC lipids result in a metastable HD that persists beyond μs time scales. NIC lipids stabilize the HD by filling the junction core around the HD. Sorting of NIC lipids toward the three-junction region occurs in fused-endpoint systems, but no significant sorting was seen in systems that end in a double bilayer indicating that high line tension at the triple junction drives HD dissipation faster than sorting can enrich that junction enough to lower that line tension. The appearance of three end states dependent on the NIC lipid composition highlights the necessity of NIC lipids for non-leaky fusion.

Atomic-scale investigation of triple junction role on defects binding energetics and structural stability in α-Fe

Nanocrystalline (NC) metals (mean grain sizes d ≤ 100 nm) have enhanced mechanical strength as compared to coarse-grained metals (d ≥ 1 μm), and thus, are a promising alternative as structural materials for future high energy nuclear reactors. However, during extreme conditions, the NC microstructure has been found to be thermodynamically unstable, thereby limiting its applicability. Further, for materials with average grain size <10 nm, the triple junctions (TJs) have been observed to have a significant contribution on the mechanical behavior and microstructural stability. Moreover, at the atomic-scale, the region surrounding the TJ demonstrates unique physical properties, such as rapid diffusion, non-equilibrium segregation and increased dislocation activity. Therefore, in this work, we systematically assess the role of TJs on the structural stability and the solute binding behavior in α-Fe. Using atomistic simulations, we show that the TJ resolved line tension is strongly correlated (inversely) with the mean activation energy for self-diffusion along the TJ, i.e., the thermodynamically unstable TJs demonstrated a lower activation energy barrier for self-diffusion along TJs with higher line tension. Next, we demonstrated that the strain energy evolution around the TJ can provide insights into the distinct binding behavior of point defects and solute atoms. In other words, the examination of solute binding behavior revealed a localized region of stable sites around the TJs which aids in accommodation of high solute concentration at high temperatures. In summary, our findings quantify the distinct role of TJs on the defect (vacancy, self-interstitial and solute atom) binding and migration behavior and these findings are necessary for designing future structural materials for extreme environments, including those needed in aerospace, naval, civilian and energy sector infrastructures.

Avoiding Lockout Instability for Towed Parafoil Systems

Many towed systems consist of a parent platform moving along the ground, air, or water surface connected to a towed vehicle via a tether line. Motion of a towed parafoil system can be complex and is driven by motion of the parent platform, canopy control inputs, and wind disturbances. A particularly problematic flight dynamic instability for this system is canopy lockout, in which the canopy attains a large lateral offset and bank angle resulting in high line tension. It is possible to use left and right brakes to return the system to its nominal position; however, if the lateral offset and bank angle are too large, it is not possible to restore the system to a nominal position, and the lateral offset and bank angle grows until impact with a ground surface. This paper explores the lockout phenomena using a multibody simulation and identifies passive means to avoid the instability by locating the tether connection point forward on the cradle in combination with sufficiently high cradle–canopy yaw stiffness. Moreover, it is shown that an active control system that regulates canopy roll angle with parafoil brake inputs can eliminate the instability for the tested configuration.

Comparison of phase behavior between water soluble and insoluble surfactants at the air–water interface

The surface phase behavior of 2-hydroxyethyl myristate (2-HEM) has been studied in Langmuir monolayers by measuring surface pressure ( π)–area ( A) isotherms with a film balance and observing monolayer morphology with a Brewster angle microscope (BAM). These results are compared with the phase behavior of 2-hydroxyethyl laurate (2-HEL) in Gibbs monolayers studied by measuring π–time ( t) curves and observing monolayer morphology. The π– A isotherms of 2-HEM show a first-order phase transition from a liquid expanded (LE) phase to a liquid condensed (LC) phase in the temperature range between 5 and 35 °C whereas the π– t curves of 2-HEL represent a similar phase transition in the temperature range between 2 and 25 °C. The critical surface pressure, π c necessary for the phase transitions increases with increasing temperature in both the cases. The LC domains formed in 2-HEM show circular shapes, which are independent of the temperature. In contrast, the circular domains having stripe texture formed at lower temperatures show a shape transition to fingering domains with uniform brightness at 15 °C. The amphiphile, 2-HEM having 13-carbon chain has higher line tension than 2-HEL that has 11-carbon chain as tail. Thus, for 2-HEM, this high line tension always dominates over other factors giving rise to circular domains at the all studied temperatures.

Reversible Collapse of Insoluble Monolayers: New Insights on the Influence of the Anisotropic Line Tension of the Domain

In this paper, we study the collapse of a mixed insoluble monolayer formed by a cationic matrix, dioctadecyl-dimethylammonium bromide (DOMA), and a tetra-anionic porphyrin, tetrakis(4-sulfonatophenyl)porphyrin (TSPP), in a molar ratio TSPP/DOMA = 1:4. During the collapse of this system, we visualized the formation of circular domains consisting exclusively of trilayer, although the domains coalescence was not observed. The coexistence of trilayer and monolayer at the final step of the collapse cannot be interpreted exclusively in terms of a thermodynamic phase equilibrium, intervening as an additional factor the anisotropic line tension of the domain. A high line tension implies a high resistance to the domain deformation, and the anisotropy of the line tension implies the lack of coalescence between these domains, which has been experimentally observed by Brewster angle microscopy for us. Under these circumstances, the domains of collapsed material could enclose monolayer regions where the local surface pressure drops thus stopping the collapse process. The collapse of the TSPP/DOMA system is reversible, that is, the return of the three-dimensional material to the monolayer fits into a simple kinetics according to the nucleation-growth-collision theory. As for the collapse, the reverse process is also affected by the line tension of the domains. This paper relates the high line tension and the anisotropic line tension of a given domains with the reversible nature of the collapse process.

Flicker Spectroscopy of Thermal Lipid Bilayer Domain Boundary Fluctuations

This contribution describes measurements of lipid bilayer domain line tension based on two-dimensional thermal undulations of membranes with liquid ordered/liquid disordered phase coexistence and near-critical composition at room temperature. Lateral inhomogeneity of lipid and protein composition is currently a subject of avid research aimed at determining both fundamental properties and biological relevance of membrane domains. Line tension at fluid lipid bilayer membrane domain boundaries controls the kinetics of domain growth and therefore regulates the size of compositional heterogeneities. High line tension promotes membrane domain budding and fission. Line tension could therefore be an important control parameter regulating functional aspects of biological membranes. Here the established method of fluid domain flicker spectroscopy is applied to examine thermal domain wall fluctuations of phase-separated bilayer membranes. We find a Gaussian probability distribution for the first few excited mode amplitudes, which permits an analysis by means of appropriately specialized capillary wave theory. Time autocorrelation functions are found to decay exponentially, and relaxation times are fitted by means of a hydrodynamic theory relating line tensions and excited mode relaxation kinetics. Line tensions below 1 pN are obtained, with these two approaches yielding similar results. We examine experimental artifacts that perturb the Fourier spectrum of domain traces and discuss ways to identify the number of modes that yield reliable line tension information.

Simulations of edge behavior in a mixed-lipid bilayer: Fluctuation analysis

Coarse-grained molecular dynamics simulations of lipid bilayer ribbons consisting of a mixture of lipids of different tail lengths have been performed to gain insight into bicelle mixtures. The line tension of the bilayer edge decreases as the mole fraction of short-chain lipids in the system is increased, dropping below zero between 30% and 35%. The mole fraction of short-chain lipids in the ribbon interior is lower than the total mole fraction, as the short-chain lipids segregate towards the edge, but continues to rise even after the line tension vanishes, in contrast to predictions of a two-component two-phase model. The fluctuations of the bilayer edge in both high and low line tension regimes have been analyzed to extract information about the factors that influence the length and shape of the edge. At high line tension the wavelength-dependent in-plane fluctuations of the edge are predicted quantitatively using a simple analytical model using only the line tension as input. Where line tension is vanishing, the fluctuations can be modeled as arising from a combination of harmonic fluctuations around a minimum energy contour length and an in-plane bending elasticity. The estimated value of the in-plane bending modulus is of order 10(-29) J m, placing the intrinsic persistence length for the edge near the bilayer thickness of 4 nm.

Line tension of Langmuir monolayer phase boundaries determined with optical tweezers

The line tension λ of the liquid expanded (LE)/gas (G)-phase boundary of a methyl octadecanoate Langmuir monolayer at the air/water interface is measured using fluorescence microscopy combined with optical tweezers. Silica spheres, immersed into the monolayer and manipulated by the tweezers, deform the phase boundary. After switching off the tweezers, the relaxation of the deformed region is dominated by the competition between line tension and hydrodynamic resistance while dipole–dipole forces between the molecules can be neglected. A linear relation between the deformation length and time is found, from which a line tension of λ=7.5 pN is deduced. The linearity gives an upper bound for the surface potential differences of both phases. It is shown that viscous forces from the two-dimensional LE surroundings dominate the subphase friction. The optical tweezers enable one to observe relaxations on a shorter time scale, extending the range of measurement of previous techniques toward higher line tension or lower viscosities of the monolayer and of the subphase.

Precipitation on low-angle grain boundaries in Al-Zn-Mg

Weak beam microscopy was used to examine the precipitation of the 17 phase (MgZn2) on low angle grain boundaries which had formed in an Al-2.84 wt pct Zn-1.95 wt pct Mg alloy. The low angle boundaries observed in the partially recovered structure can be separated into two categories, planar and nonplanar. The planar boundaries form either lozenge or hexagonal configurations. The nonplanar boundary dislocation intersections form a “chair”-shaped figure. The geometry of the dislocation strain fields in the boundaries controls the precipitate growth and the size, shape, and position of the precipitates on each boundary type. The lozenge boundary can have both equiaxed and lath shaped precipitates growing upon it. The equiaxed precipitates form only at four dislocation junctions because at this position the strain field distribution is symmetric. The lath precipitates are restricted to grow along dislocations in the boundary whose line direction is coincident with the long axis of the precipitate since in these pinned dislocation segments there are high line tension forces present to resist dislocation bowing. The “chair” boundary contains only lath precipitates, which begin to grow from alternate three dislocation intersections and continue growth along the dislocation line. The contrast observed bordering the lath on each side is produced by lattice dislocations. A proposed explanation for the above observations involves assuming that the dislocations in the chair boundary dissociate into Lomer-Cottrell locks which have constricted-extended node pairs. A lath can begin growth from the constricted node forcing the stair rod dislocation to dissociate. The reaction products of the stair rod can combine with the partials in the Lomer-Cottrell lock to form the observed lattice dislocations.

Elastic line energies and line tensions of dislocations in polyethylene crystals

Abstract The elastic line energies and tensions of dislocations in the {100} planes of a polyethylene crystal are rigorously calculated taking the anisotropy of the crystal fully into account and allowing for uncertainties in the values of the elastic constants. The results are used to substantiate previous qualitative conclusions concerning the behaviour of dislocations in polyethylene crystals (Shadrake and Guiu 1976). In particular it is found that the [001] screw dislocation has even higher line tension than previously estimated, and it is concluded that homogeneous nucleation by thermal fluctuations is the most likely source of such dislocations. As a consequence the stress required to produce chain-axis slip is not only a function of deformation temperature but must also be strongly dependent upon the crystal thickness.

Dislocations in polyethylene crystals: line energies and deformation modes

Abstract Explicit expressions are obtained for the line energy of straight dislocations in orthorhombic polyethylene crystals. It is found that the ⟨0, 0, c⟩ screw dislocation is the perfect dislocation with the lowest line energy whilst the ⟨0, 0, c⟩ edge has negative line tension and is therefore unstable. The possible dissociation of perfect dislocations into partials is discussed, and it is shown that both (110) and (310) twin dislocations can arise from such dissociations. Numerical calculations also show that homogeneous nucleation of ⟨0, 0, c⟩ dislocations, and twin dislocations, has a very high probability of occurrence under a stress of the order of the yield stress of polyethylene crystals. These results help to explain the readiness of polyethylene crystals to deform by c axis slip, twinning, and monoclinic transformation modes. The supression of c axis slip by irradiation and cross-linking is also explained by the very high line tension of the ⟨0, 0, c⟩ screw dislocations.