Strain softening in stretched DNA

The microscopic mechanics of DNA stretching was characterized using extensive molecular dynamics simulations. By employing an anisotropic pressure-control method, realistic force-extension dependences of effectively infinite DNA molecules were obtained. A coexistence of B and S DNA domains was observed during the overstretching transition. The simulations revealed that strain softening may occur in the process of stretching torsionally constrained DNA. The latter observation was qualitatively reconciled with available experimental data using a random-field Ising model.     

A nonuniformly stretched vortex

A stretched vortex model is proposed which includes a nonuniform stretching in the radial direction that is clearly present in real flows, as well as a slow variation of velocity profiles along the vortex axis. Both features of this boundary layer approximation depart from the classical Burgers solution. This model is shown to be in very good agreement with experimental velocity measurements.     

Electron transport in stretched monoatomic gold wires

The conductance of monoatomic gold wires containing 3-7 gold atoms has been obtained from ab initio calculations. The transmission is found to vary significantly depending on the wire stretching and the number of incorporated atoms. Such oscillations are determined by the electronic structure of the one-dimensional (1D) part of the wire between the contacts. Our results indicate that the conductivity of 1D wires can be suppressed without breaking the contact.     

Denaturation transition of stretched DNA

We generalize the Poland-Scheraga model to consider DNA denaturation in the presence of an external stretching force. We demonstrate the existence of a force-induced DNA denaturation transition and obtain the temperature-force phase diagram. The transition is determined by the loop exponent c, for which we find the new value c = 4 nu-1/2 such that the transition is second order with c = 1.85 < 2 in d = 3. We show that a finite stretching force F destabilizes DNA, corresponding to a lower melting temperature T(F), in agreement with single-molecule DNA stretching experiments.     

Weakly stretched premixed flames in oscillating flows

The response of a premixed flame in stagnation-point flow with an imposed oscillating strain rate has been examined. This configuration is of fundamental interest and has potential application to turbulent combustion modelling. Of interest are flames which stand well clear of the front stagnation point of a bluff body. Under these conditions the flame can be treated as a surface of density discontinuity. A detailed solution is constructed in the burned and unburned gas regions and includes the flame response to the imposed fluctuations as well as the resulting displacement of the incident flow. Our analysis accounts for the full coupling between the flame and the underlying flow field and, unlike most previous studies, is not restricted to small-amplitude oscillations.     

Relaxation of stretched DNA in slitlike confinement

We experimentally observe two separate time scales governing the entropic recoil in the linear force-extension regime of single double-stranded DNA in slit confinement. We demonstrate the existence of two distinct relaxation regimes at different extensions during relaxation. Contrary to bulk measurements, the true longest relaxation time may only be probed very close to equilibrium. A simple model of the relaxation mechanism leads to a scaling analysis that correctly predicts the extension at the crossover between the two regimes.     

Detachment of stretched viscoelastic fibrils

New experimental results are presented about the final stage of failure of soft viscoelastic adhesives. A microscopic view of the detachment of the adhesive shows that after cavity growth and expansion, well adhered soft adhesives form a network of fibrils connected to expanded contacting feet which fail via a sliding mechanism, sensitive to interfacial shear stresses rather than by a fracture mechanism as sometimes suggested in earlier work. A mechanical model of this stretching and sliding failure phenomenon is presented which treats the fibril as a nonlinear elastic or viscoelastic rod and the foot as an elastic layer subject to a friction force proportional to the local displacement rate. The force on the stretched rod drives the sliding of the foot against the substrate. The main experimental parameter controlling the failure strain and stress during the sliding process is identified by the model as the normalized probe pull speed, which also depends on the magnitude of the friction and PSA modulus. In addition, the material properties, viscoelasticity and finite extensibility of the polymer chains, are shown to have an important effect on both the details of the sliding process and the ultimate failure strain and stress. Electronic supplementary material Appendix B is only available in electronic form at and are accessible for authorised users.     

Elasticity of strongly stretched ssDNA

We present a simple model which describes elastic response of single-stranded DNA (ssDNA) to stretching, including the regime of very high force (up to 1000 pN). ssDNA is modelled as a discrete persistent chain, whose ground state is a zigzag rather than a straight line configuration. This mimics the underlying molecular architecture and helps to explain the experimentally observed saturation of the stretching curve at very high force.     

Alignment of iodine species in stretched polyacetylene films

The¹²⁹I Mössbauer spectroscopy has been applied to iodine-doped highly conducting polyacetylene. For the unstretched polyacetylene films, the iodine species of I^?, I ₃ ^? and I ₅ ^? are observed. The polyiodides have symmetrical charge population. In addition to these anions, a small amount of iodines is covalently bonding to the carbon atoms, breaking the double bonds of the main chain. For the stretched polyacetylene films, the measurements were done under the condition that the stretch direction was parallel or perpendicular to the direction of incident ψ-rays. The relative change of the intensities of the Mössbauer quadrupole-split lines reveals the alignment of the polyiodine anions in the stretched film.     

Resonances of stretched states in open shell nuclei

-Form factors of 1?ω maximal spin resonances are obtained in the version “particle core coupling” of multiparticle shell model for the nuclei ²⁴Mg, ²⁶Mg, ²⁸Si, ⁴⁰Ca, ⁴²Ca, ⁴⁴Ca. Wave functions of excited states are calculated using information on the spectroscopy of direct reactions. Comparison with a cross section of (e, e′) reactions showed that the theoretical calculation is in good agreement with existing experimental data. Fragmentation of M6 resonance force in ⁴⁰Ca into 22 peaks is shown.     

Theoretical study of conductance in stretched monatomic nanowires

Recent experiments showed that the last, single channel conductance step in monatomic gold contacts exhibits significant fluctuations as a function of stretching. From simulations of a stretched gold nanowire linked to deformable tips, we determine the distribution of the bond lengths between atoms forming the nanocontact and analyze its influence on the electronic conductance within a simplified single channel approach. We show that the inhomogeneous distribution of bond lengths can explain the occurrence and the 5% magnitude of conductance fluctuations below the quantum conductance unit g₀=2e²/h.     

Pattern formations in chaotic spatio-temporal systems

Pattern formations in chaotic spatio-temporal systems modelled by coupled chaotic oscillators are investigated. We focus on various symmetry breakings and different kinds of chaos synchronization-desynchronization transitions, which lead to certain types of spontaneous spatial orderings and the emergence of some typical ordered patterns, such as rotating wave patterns with splay phase ordering (orientational symmetry breaking) and partially synchronous standing wave patterns with in-phase ordering (translational symmetry breaking). General pictures of the global behaviors of pattern formations and transitions in coupled chaotic oscillators are provided.     

Analysis of the Wrinkle of an Automobile Weather-Strip Seal in Bending

Weather-strip seals in automotive door systems play a major role in determining the door closing effort, isolating the passenger compartment from water and reducing the wind noise inside the vehicle. The phenomenon of a wrinkle often occurs in weather-strips when they are assembled on the door surfaces which have bent or twisted profiles. Wrinkles in automobile weather-strip seals will affect the closing effort and noise isolation characteristics of the automotive door system. The wrinkle of anethylene-propylene-diene (EPDM) automobile weather-strip seals when bent and the critical conditions for a wrinkle to develop were characterized in this study. The hyperelastic material behavior of the EPDM sponge and EPDM dense rubber in the seal were modeled using the Foam model and Mooney–Rivlin model, respectively. Then a finite element analysis (FEA) model of the automobile weather-strip seal in assembly was developed and analyzed using then on linear FEA software Marc. The deformation of the seal and the thickness decrease in the critical region where it was prone to wrinkle were obtained by this FEA method. Finally the critical criterion of wrinkling was determined by analysis of the reaction forces in the critical region. The proposed analysis method can shorten the product design cycle and decrease the design and trial-product cost considerably.     

On the stability of stretched flames

It is shown that the well known difficulty concerning the unconditional stability of a planar flame sustained in a stagnation-point flow is due to the infinite aspect length scale of the system. In finite geometries occurring in numerical and laboratory experiments the flame may well become unstable provided the flow-induced stretch is weak enough.     

Realistic searches on stretched exponential networks

We consider navigation or search schemes on networks which have a degree distribution of the form P(k) ∝ exp(−k ^γ). In addition, the linking probability is taken to be dependent on social distances and is governed by a parameter λ. The searches are realistic in the sense that not all search chains can be completed. An estimate of μ = ρ/s _d, where ρ is the success rate and s _d the dynamic path length, shows that for a network of N nodes, μ ∝ N ^−δ in general. Dynamic small world effect, i.e., δ ≃ 0 is shown to exist in a restricted region of the λ−γ plane.      

Biaxially stretched nematic liquid crystalline elastomers

The biaxial stretching of sheets of liquid crystalline neo-Hookean elastomer has been studied in the isotropic case. The results suggest two types of laminate structures in the process of quasiconvexification of the free energy, a fact that implies the appearance of several shear terms in the deformation gradient matrix. More that one decomposition of the deformation gradient is possible, which is consistent with a bifurcation in the undeformed configuration (l \lambda = 1) . This situation is similar to the well-known Rivlin’s problem of the triaxial symmetric traction of a neo-Hookean cube. The problem can easily be generalized for an anisotropic material by introducing a semisoft term in the free-energy expression. In this case, the horizontal plateau corresponding to the minimal energy, characteristic of the soft elasticity, disappears, and only an equilibrium condition is obtained.     

Anisotropic dewetting on stretched elastomeric substrates

We study the instability of a very thin liquid film resting on a uniformly stretched soft elastomeric substrate driven by van der Waals forces. A linear stability analysis shows that the critical fluctuation wavelength in the tensile direction is larger than those in the other directions. The magnitudes of the critical wavelengths are adjustable in the sense that they depend on the principal stretch of the substrate. For example, when the principal stretch of the substrate varies from 1.0 (unstretched) to 3.0, the range of the critical wavelength in the tensile direction increases by 7.0% while that normal to the tensile direction decreases by 8.7%. Therefore, the phenomenon may find potential applications in creating tunable topographically patterned surfaces with nano-to microscale features.     

Solenoids and plectonemes in stretched and twisted elastomeric filaments

We study the behavior of a naturally straight highly extensible elastic filament subjected to large extensional and twisting strains. We find that two different phases can coexist for a range of parameter values: the plectoneme and the solenoid. A simple theory based on a neo-Hookean model for the material of the filament and accounting for the slender geometry suffices to explain these observations, and leads to a phase diagram that is consistent with observations. Extension and relaxation experiments on these phases show the presence of large hysteresis loops and sawtooth-like force-displacement curves which are different for the plectoneme and the solenoid.