Critical Depinning Research Articles

The Droplet Creeping-Sliding Dynamic Wetting Mechanism on Bionic Self-Cleaning Surfaces.

The dynamic wetting behavior of droplets has been of wide concern due to the hazards of accretion/icing of supercooled droplets on engineering components/systems served in low temperature freezing rain environment; thus, it is urgent to establish the relationship between droplet depinning/removing behaviors and surface characteristics. In this article, the actual rotation conditions of moving components such as wind turbine blades are simulated. The self-cleaning hydrophobic coating surface(S1) and bionic superhydrophobic coating surface(S2) show outstanding droplet removal performance compared to hydrophilic bare steel surface(S0), and the average speed of the droplet removal is increased by 400-500%. The "creeping-sliding" behavior of droplets on self-cleaning coatings is investigated by the change of droplet displacement(ΔD). The effect of the energy storage caused by the droplet creeping process provides initial kinetic energy for the droplet removal. Combined with the experimental data and theoretical model, the critical depinning resistance is calculated. The difference of the wetting interface free energy(ΔEx) during the dynamic wetting process of the droplets on the bionic superhydrophobic self-cleaning surface is researched. And the influence mechanism of the droplet embedded depth(x) on the creeping/sliding behavior in the nanotexture is clarified. Thus, the mechanical criterion of droplet depinning is proposed (the error is about 10%). The results can provide a theoretical basis for the design principle of antifreezing rain coatings on moving components.

Dynamical commensuration effect in a two-dimensional Yukawa solid modulated by periodic substrates

Transverse depinning dynamics of a periodic-square-substrate modulated two-dimensional dusty plasma solid driven by a constant force in the longitudinal direction are investigated using Langevin dynamical simulations. When the commensuration ratio (the number ratio of particles to substrate potential wells) is increased, there is a nonmonotonic variation of the critical transverse depinning force, and the local maxima and minima of the critical transverse depinning force precisely correspond to the dynamical commensurate and incommensurate conditions, respectively. The dynamical commensuration effect is also clearly visible in the stable one-dimensional channel particle trajectories and the highly ordered structure, while both the particle trajectories and the structure are more disordered under the incommensurate conditions. The nonmonotonic variation of the critical transverse depinning force is attributed to the stability of the lattice structure at specific commensuration ratios.

Emergence of scale-free smectic rivers and critical depinning in emulsions driven through disorder

During the past 60 min, oil companies have extracted 6 trillion liters of oil from the ground, thereby giving a striking illustration of the impact of multiphase flows on the world economy. From a fundamental perspective, we largely understand the dynamics of interfaces separating immiscible fluids driven through heterogeneous environments. In stark contrast, the basic mechanisms ruling the transport of fragmented fluids, such as foams and emulsions, remain elusive with studies mostly limited to isolated droplets and bubbles. Here, we demonstrate that the mobilization of emulsion driven through model disordered media is a critical plastic depinning transition. To elucidate this collective dynamics, we track the trajectories of hundreds of thousands of microfluidic droplets advected through random lattices of pinning sites. Their dynamics reveals that macroscopic mobilization only requires the coordinated motion of small groups of particles and does not involve any large-scale avalanches. Criticality arises from the interplay between contact and hydrodynamic interaction, which channel seemingly erratic depinning events along smectic river networks correlated over system spanning scales. Beyond the specifics of emulsion transport, we close our article discussing the similarities and profound differences with the plastic depinning transitions of driven flux lines in high-T c superconductors, charged colloids, and grain transport in eroded sand beds.

Anisotropic avalanches and critical depinning of three-dimensional magnetic domain walls.

Simulations with more than 10^{12} spins are used to study the motion of a domain wall driven through a three-dimensional random-field Ising magnet (RFIM) by an external field H. The interface advances in a series of avalanches whose size diverges at a critical external field H_{c}. Finite-size scaling is applied to determine critical exponents and test scaling relations. Growth is intrinsically anisotropic with the height of an avalanche normal to the interface ℓ_{⊥} scaling as the width along the interface ℓ_{∥} to a power χ=0.85±0.01. The total interface roughness is consistent with self-affine scaling with a roughness exponent ζ≈χ that is much larger than values found previously for the RFIM and related models that explicitly break orientational symmetry by requiring the interface to be single-valued. Because the RFIM maintains orientational symmetry, the interface develops overhangs that may surround unfavorable regions to create uninvaded bubbles. Overhangs complicate measures of the roughness exponent but decrease in importance with increasing system size.

Avalanches and extreme value statistics in interfacial crackling dynamics.

We study the avalanche and extreme statistics of the global velocity of a crack front, propagating slowly along a weak heterogeneous interface of a transparent polymethyl methacrylate block. The different loading conditions used (imposed constant velocity or creep relaxation) lead to a broad range of average crack front velocities. Our high-resolution and large dataset allows one to characterize in detail the observed intermittent crackling dynamics. We specifically measure the size S, the duration D, as well as the maximum amplitude [Formula: see text] of the global avalanches, defined as bursts in the interfacial crack global velocity time series. Those quantities characterizing the crackling dynamics follow robust power-law distributions, with scaling exponents in agreement with the values predicted and obtained in numerical simulations of the critical depinning of a long-range elastic string, slowly driven in a random medium. Nevertheless, our experimental results also set the limit of such model which cannot reproduce the power-law distribution of the maximum amplitudes of avalanches of a given duration reminiscent of the underlying fat-tail statistics of the local crack front velocities.This article is part of the theme issue 'Statistical physics of fracture and earthquakes'.

The influence of annealing on domain wall propagation in bistable amorphous microwire with unidirectional effect

The effect of gradual annealing on the domain wall mobility (velocity), nucleation, critical depinning and propagation fields in amorphous FeSiB microwires has been studied. A new experimental set-up, presented in this paper, allows measurement of average domain wall velocity for four different conditions and detection of the presence of unidirectional effect in wall propagation without manipulation of the microwire. The proposed interpretation is that a domain wall is considered as a relatively long object which can change its axial dimension due to inhomogeneity of damping forces acting on the wall during its propagation. It is demonstrated that unidirectional effect in domain wall propagation can be strongly reduced by annealing the wire at temperatures higher than 350 °C.

Atomic-scale strengthening mechanism of dislocation-obstacle interaction in silicon carbide particle-reinforced copper matrix nanocomposites

Using molecular dynamics (MD) simulations, the strengthening mechanism of silicon carbide (SiC) particle acted as barrier for the motion of edge dislocation in copper (Cu) matrix nanocomposites under shear loading is investigated. The dislocation glide behavior and the dislocation-particle interaction accounting for the effect of the temperature and the particle size are discussed in terms of the stress-strain relationship, the crystal energy change, the stress distribution and the dislocation evaluation. The results show that the critical depinning stress and the critical depinning strain are found to vary significantly depending upon the temperature and the particle size, consistent with previous studies. Higher temperature or smaller particle results in lower critical depinning stress to allow dislocation to bypass particle. Moreover, the critical depinning stress from the current atomic study is lower than the theoretical value due to the presence of cross-slip and thermal activation. At low temperature the mechanical activation controls the depinning stress of the dislocation bypassing particle, while at high temperature the thermal activation dominates that. In addition, the trapping sites of dislocations for the dislocation multiplication are observed at high temperature or large particle, due to the lattice mismatch of particle taken as the dislocation source. Compared to the dislocation interacting with void or metal-particle, the interaction of dislocation and SiC-particle in Cu + SiC nanocomposite reveals much complex factors, such as the multiple glide planes, the nanosize effect, as well as the coupling effect of mechanical and thermal activation, on the strengthening mechanism. It maybe apply these results to a high throughput experimental design which provides the dense and targeted material data, to accelerate material discovery with the large-scale and low-cost fabrication strategies.

Critical depinning current of elastic vortex strings in superconductors with extended linear defects

The problem of the depinning of an Abrikosov elastic vortex string from an extended linear defect in the plate of a 3D anisotropic superconductor of thickness d > 2λ (where λ is the London penetration depth) under the action of an inhomogeneously distributed transport current that flows in the surface shielding layer has been solved using classical mechanics approach. Conditions for the appearance of the instability of the pinned state of a vortex have been investigated and calculations of the corresponding critical current density on the surface at which the depinning of the vortex string occurs have been carried out. The dependence of the average of the critical current density on the thickness of the plate has been determined.

AC Quantum Interference Effects in Nanopatterned Nb Microstrips

The mixed-state dc voltage response of nanopatterned Nb microstrips is investigated by combined microwave (0.5 MHz—1 GHz) and dc electrical resistance measurements. The nanopatterns are arrays of symmetric grooves on the film surface fabricated by focused ion beam milling. They provide a pinning potential of the washboard type for the vortex motion. When subject to a microwave stimulus, the dc current–dc voltage curves of the microstrips exhibit Shapiro steps. The steps occur at voltages V=n V 0=n N f Φ 0, where n is an integer, N is the number of vortex rows between the voltage leads, f is the microwave frequency, and Φ 0 is the magnetic flux quantum. These steps arise as an interference effect when one or a multiple of the hopping period of the coherently moving Abrikosov vortices coincides with the period of the ac drive. When tuning the field value away from the matching field, the steps disappear due to lacking coherence in the motion of the Abrikosov vortex lattice. Dependencies of the dc critical (depinning) current on the microwave power and frequency are reported.

Fractional vortex in asymmetric 0-πlong Josephson junctions

We consider an infinitely long 0-$\pi$ Josephson junction consisting of 0 and $\pi$ regions having different critical current densities $j_{c,0}$ and $j_{c,\pi}$. The ground state of such a junction corresponds to a spontaneosly formed asymmetric semifluxon with tails decaying on different length scales. We calculate the depinning current of such a fractional vortex and show that it is different for positive and negative bias polarity. We also show that upon application of a bias current, the fractional flux (topological charge) associated with the vortex changes. We calculate the range of fractional flux associated with the vortex when the bias changes from negative to positive critical (depinning) values.

Order–disorder transition and flow noise in an [formula omitted] film

We measure voltage noise spectra S V ( f ) generated by current-driven vortices as a function of field B for an amorphous Mo x Ge 1 - x film with Corbino-disk geometry. For high voltage V in the flux-flow state, S V at low frequency ( f = 100 Hz ) exhibits a sharp peak just below a peak field B p of the critical (depinning) current. For low V in the plastic-flow state, an additional broad peak of S V appears at low B in addition to the sharp peak at around B p . Origin of large noise just below B p is due to metastable vortex states associated with the order–disorder transition, while that in low B is attributed to pinning-induced incoherent vortex flow.

Transverse commensurability effect for vortices in periodic pinning arrays

Using computer simulations, we demonstrate a type of commensurability that occurs for vortices moving longitudinally through periodic pinning arrays in the presence of an additional transverse driving force. As a function of vortex density, there is a series of broad maxima in the transverse critical depinning force that do not fall at the matching fields where the number of vortices equals an integer multiple of the number of pinning sites. The commensurability effects are associated with dynamical states in which evenly spaced structures consisting of one or more moving rows of vortices form between rows of pinning sites. Remarkably, the critical transverse depinning force can be more than an order of magnitude larger than the longitudinal depinning force.

Transverse depinning in weakly pinned vortices driven by crossed ac and dc currents

We present experiments on weakly pinned vortices, which exhibit a large critical transverse depinning force. These results are obtained in the superconducting metallic glasses ${\text{Fe}}_{x}{\text{Ni}}_{1\ensuremath{-}x}{\text{Zr}}_{2}$ using crossed ac and dc driving currents. We study the vortex depinning force due to the transverse ac drive as a function of a longitudinal dc drive; the ac and dc combination permits the separation of the transverse drive from the longitudinal one. We show that the force required for depinning in the transverse direction is greatly enhanced by the longitudinal drive, which demonstrates the existence of a large transverse critical force. The measurements are performed as a function of magnetic field and temperature and show that the transverse critical force exists in a large portion of the phase diagram. Hysteresis observed at the transverse depinning threshold is consistent with a first-order transverse depinning transition.

Pinning mechanism anisotropy and dimensionality in Nb/Cu superlattices

Abstract Nb/Cu superconducting superlattices have been fabricated by dc magnetron sputtering. The structural characterization has been made by X-ray diffraction techniques. We have measured upper critical fields both parallel and perpendicular to the layered structure, H c2∥ and H c2⊥ . Also, the critical depinning currents j c have been measured as a function of the angle between the applied magnetic field and Nb/Cu interfaces. The superlattice induced anisotropy of this system has been analyzed. It turns out that, for superlattices with Nb layers decoupled by the Cu spacer, a much higher anisotropy is observed in critical currents than in critical fields.

Displacement profile of charge density waves and domain walls at critical depinning.

The influence of a strong surface potential on the critical depinning of an elastic system driven in a random medium is considered. If the surface potential prevents depinning completely the curvature C of the displacement profile exhibits at zero temperature a pronounced rhombic hysteresis curve of width 2f(c) with the bulk depinning threshold f(c). The hysteresis disappears at nonzero temperatures if the driving force is changed adiabatically. If the surface depins by the applied force or thermal creep, C is reduced with increasing velocity. The results apply, e.g., to driven magnetic domain walls, fluxline lattices, and charge-density waves.

Switching current noise and relaxation of ferroelectric domains

We simulate field-induced nucleation and switching of domains in a three-dimensional model of ferroelectrics with quenched disorder and varying domain sizes. We study (1) bursts of the switching current at slow driving along the hysteresis loop (electrical Barkhausen noise) and (2) the polarization reversal when a strong electric field was applied and back-switching after the field was removed. We show how these processes are related to the underlying structure of domain walls, which in turn is controlled by the pinning at quenched local electric fields. When the depolarization fields of bound charges are properly screened we find that the fractal switching current noise may appear with two distinct universal behaviors. The critical depinning of plane domain walls determines the universality class in the case of weak random fields, whereas for large randomness the massive nucleation of domains in the bulk leads to different scaling properties. In both cases the scaling exponents decay logarithmically when the driving frequency is increased. The polarization reverses in the applied field as a power-law, while its relaxation in zero field is a stretch exponential function of time. The stretching exponent depends on the strength of pinning. The results may be applicable for uniaxial relaxor ferroelectrics, such as doped SBN:Ce.

Critical depinning force and vortex lattice order in disordered superconductors

We simulate the ordering of vortices and its effects on the critical current in superconductors with varied vortex-vortex interaction strength and varied pinning strengths for a two-dimensional system. For strong pinning the vortex lattice is always disordered and the critical depinning force only weakly increases with decreasing vortex-vortex interactions. For weak pinning the vortex lattice is defect-free until the vortex-vortex interactions have been reduced to a low value, when defects begin to appear with a simultaneous rapid increase in the critical depinning force. In each case the depinning force shows a maximum for noninteracting vortices. The relative height of the peak increases and the peak width decreases for decreasing pinning strength in agreement with experimental trends associated with the peak effect. We show that scaling relations exist between the distance between defects in the vortex lattice and the critical depinning force.

Interface Dynamics at the Depinning Transition

We study the local scaling properties of driven interfaces in disordered media modeled by the Edwards-Wilkinson equation with quenched noise. We find that, due to the super-rough character of the interface close to the depinning transition, the local width exhibits an anomalous temporal scaling. It does not saturate at a characteristic time $t_s(l) \sim l^z$ as expected, but suffers an anomalous temporal crossover to a different time regime $t^{\beta_*}$, where $\beta_* \simeq 0.21 $. This is associated with the existence of a local roughness exponent $\alpha_{loc} \simeq 1$ that differs from the global one $\alpha \simeq 1.2$. The relevance of the typical size of pinned sites regions near the critical point is investigated and the definition of the critical depinning is dicussed.

Incomplete symmetry breaking and anomolous crystallization kinetics at close-packed crystal-liquid interfaces

Stacking faults in close-packed planes are commonly observed during crystallization. In this paper we demonstrate that this behavior can be described in terms of a simple theoretical description of crystallization kinetics that couples two structural order parameters. We establish that the packing degeneracy that permits such faults also gives rise to a striking kinetic effect. The close-packed surface can freeze only when the symmetry of the two possible in-layer packings is broken, either by fluctuations or longer-range interactions. In the absence of long-range interactions, growth can take place only once the supercooling exceeds a critical depinning value, associated with a second-order phase transition in a single surface layer.

Driven interfaces in quenched disorder at critical depinning.

Driven interfaces in quenched disorder at critical depinning.