Small External Force Research Articles

Ice adhesion on different microstructure superhydrophobic aluminum surfaces

The adhesion of ice to high -voltage overhead transmission lines should be small to ensure ease of ice shedding under small external forces. In this work, we studied the influence of the microstructure of superhydrophobic surfaces on the strength of ice adhesion at a working temperature of −6 °C. Compared to a bare aluminum surface, the microstructure superhydrophobic aluminum surfaces did decrease ice adhesion strength. The superhydrophobic aluminum surfaces with a larger number of micro-holes produced the lowest strength of ice adhesion; its ice adhesion strength was ∼163.8 times lower than that for the bare aluminum samples. Furthermore, such microstructure aluminum surfaces had water contact angles larger than 150° and water sliding angles of less than 8.2° even at a working temperature of −6 °C. The low values of the ice adhesion strength of the above samples were mainly attributed to the superhydrophobic property, which was obtained by creating a structure of micro-nanoscale holes on the aluminum surface after treatment with a low- surface-energy fluoroalkylsilane (FAS).

A New Large Scale Instability in Rotating Stratified Fluids Driven by Small Scale Forces

In this paper, we find a new large scale instability displayed by a stratified rotating flow in forced turbulence. The turbulence is generated by a small scale external force at low Reynolds number. The theory is built on the rigorous asymptotic method of multi-scale development. There is no other special constraint concerning the force. In previous papers, the force was either helical or violating parity invariance. The nonlinear equations for the instability are obtained at the third order of the perturbation theory. In this article, we explain a detailed study of the linear stage of the instability.

A note on the regularity of thermally coupled viscous flows with critical growth in nonsmooth domains

We consider a coupled model for steady flows of viscous incompressible heat‐conducting fluids with dissipative and adiabatic heating and temperature‐dependent material coefficients in a plane bounded domain. The existence of a strong solution is proven by a fixed‐point technique based on Schauder theorem for sufficiently small external forces. Copyright © 2012 John Wiley & Sons, Ltd.

Optimizing parametric oscillators with tunable boundary conditions

Parametric excitation or pumping is an effective method to create large oscillations by periodically altering a physical parameter of the governing dynamics. Precisely tuned pumping frequencies can lead to exponentially growing oscillations limited only by nonlinear effects like axial stretching of transversely vibrating string. It is demonstrated that a tuned passive dynamical system amplifies the otherwise limited transverse vibrations amplitudes of a nonlinear string considerably and thus increasing the selectivity of the system. This effect becomes more noticeable for shorter wavelengths where nonlinear stretching limits the obtainable vibration amplitudes severely. The present work analyses a passive dynamical system connected to one end of a taught string which parametrically couples its axial motion to transverse vibration. Analysis shows that a specific selection of parameters can reduce the limiting effect of nonlinear stretching thus allowing one to excite high-order modes with small external forces. The result can possibly affect other disciplines where effective parametric amplification is necessary.

Linear response theory in the Vlasov equation for homogeneous and for inhomogeneous quasistationary states

Response to small external forces is investigated in quasistationary states of Hamiltonian systems having long-range interactions. Quasistationary states are recognized as stable stationary solutions to the Vlasov equation, and, hence, the linear response theory to the Vlasov equation is proposed for spatially one-dimensional systems with periodic boundary condition. The proposed theory is applicable both to homogeneous and to inhomogeneous quasistationary states and is demonstrated in the Hamiltonian mean-field model. In the homogeneous case magnetic susceptibility is explicitly obtained, and the Curie-Weiss like law is suggested in a high-energy region. The linear response is also computed in the inhomogeneous case, and resonance absorption is investigated to extract nonequilibrium dynamics in the unforced system. Theoretical predictions are examined by direct numerical simulations of the Vlasov equation.

頸髄損傷が誘因となった偶発性低体温症の1 例

Accidental hypothermia is the state in which body temperature falls due for exposure to a chilly environment. In accidental hypothermia, the mortality rate is higher the lower the body temperature. We report a case of a consciousness disorder and severe hypothermia, with a body temperature below 28 degrees C, in which it later became clear that a cervical spinal cord injury had been caused by a small external force. A 70-year-old woman was transported to our hospital in an ambulance for consciousness disturbance and severe hypothermia. At the time of arrival, her rectal temperature was 26.2 degrees C. We promptly performed rewarming. Her consciousness level became clear, but paralysis and diminished sensation were observed below the C5 domain. We suspected cervical spinal cord injury and performed cervical magnetic resonance imaging. She was diagnosed as having C5 cervical spinal cord injury. When there is a consciousness disorder due to accidental hypothermia, it might not be possible to evaluate the neurological value of the cervical spinal cord injury correctly. The presence of cervical spinal cord injury should be considered when patients have a decreased consciousness level due to hypothermia.

1A1-V08 車輪倒立型搬送ロボットのパワーアシスト搬送制御 : インピーダンス制御を用いた動作検証(人間機械協調(1))

In this research, we are aiming to develop an inverted pendulum type transportation robot to effectively carrying a heavy object with a high center of gravity in a complicated space. Since an inverted pendulum type robot can be moved by very small external force to the back and forth direction maintaining its balance based on the principle of an inverted pendulum, it can act as a power-assist transportation robot. This paper proposes impedance transportation control for the robot to realize a power-assist motion and increase its stability. By adopting the proposed control, an operator can move the robot while taking the reaction force corresponding a virtual mass and viscosity from the robot. Experiments were conducted to confirm the effectiveness of the proposed method.

Nanoscale Poiseuille Flows of Liquid Argon

In nanoscale flow systems, the flow motion is affected by many parameters, some of which may play different roles under different conditions. In this work, we investigate the flux of liquid argon in nanoscale Poiseuille flows through molecular dynamics simulations. By illustrating the flux as a function of a dimensionless number, which represents the effective surface effect on the fluid, we show that the fluid motion in nanochannels under small external forces can be categorized into two regimes based on the role of the temperature. For large external forces, a bimodal behavior in the flux is observed as the fluid-wall interaction is varied. The underlying mechanisms that govern the flow fashions are discussed.

Gravitational Influence on an Oscillating Chemical Reaction

Pattern formation, oscillations and wave propagation as processes in excitable media can be controlled by small external forces including gravity. The Belousov–Zhabothinsky (BZ) reaction is possibly the best studied system and exhibits temporal as well as spatial patterns. Wave propagation in BZ systems already has been shown to depend on gravity, due to interactions with diffusion and convection. In a stirred bulk BZ system stable oscillations exist in the absence of diffusion, sedimentation, buoyancy and convection with a period in the minute range. In parabolic flight missions such a system can be investigated under gravity conditions changing between 1 g, 1.8 g and μ-g just on this timescale. Here we have found that the temporal pattern formation of an oscillating BZ reaction locks to the period of the gravity changes but is also destabilized due to the partially stochastic nature of the gravity changes. This points out to a gravity dependence of chemical rate constants as given in a formal description of the BZ-system. The BZ-reaction is the perfect system for such studies and serves as a model for self-organization and pattern formation, also in biological systems. The possibility to study the lack of gravity or changes in gravity destabilizing self-organization and pattern formation is of major interest to identify the underlying mechanisms.

Axial buckling of multi-walled carbon nanotubes and nanopeapods

In this paper, we investigate both pre- and post-buckling behaviors of multi-walled carbon nanotubes and multi-walled carbon nanopeapods by incorporating into the applied forces of a prescribed beam equation both van der Waals interactions between the adjacent walls of the nanotubes and the interactions between the fullerenes and the inner wall of the nanotube. Two beam theories are employed. First, we utilize Donnell’s equilibrium equation to derive an axial stability condition for the multi-walled carbon nanotubes and multi-walled carbon nanopeapods. We then determine analytically the critical forces for single-walled and double-walled nanotubes and nanopeapods. Given the outer nanotube of a fixed radius, we observe that the critical force and strain derived from the axial buckling stability criterion decrease as a result of the molecular interactions between the adjacent layers of the nanotubes and the molecular interactions between the embedded fullerenes and the inner carbon nanotube, which is in agreement with existing literature. Next, we utilize an Euler–Bernoulli beam equation incorporating the curvature effect to obtain the post-buckled axial bending displacement for the multi-walled nanotubes and nanopeapods. We find that the interactions between molecules generate an inward force, which tends to resist any applied forces. While the inward force induced by the fullerenes to the inner wall of the nanotube vanishes as we increase the applied force, the inward force induced by the layers increases as the applied force increases. The main contribution of this paper is the incorporation of both van der Waals interactions and the curvature effect into prescribed beam theories to accurately measure the critical forces and the buckled displacements of multi-walled nanotubes and nanopeapods subject to a small external force. Our analysis is relevant to future nano devices, such as biological sensors and measuring devices for small forces arising from electrical charges or Casimir forces.

A Finite Element Modeling for Dilatometric Nonisotropy in Steel

It is well known that nonisotropic volume changes in dilatometry were observed during the phase transformation in steel. In this study, a finite element (FE) model incorporating the transformation plasticity was adopted to describe the nonisotropic dilatometric behavior during the phase transformation in steel. An implicit numerical solution procedure to calculate the deformation during the dilatometric experiment was incorporated into the general purpose implicit FE program. The nonisotropic dilatometric behavior could be successfully reproduced by using the FE simulation considering the transformation plasticity. The transformation plasticity was caused by the small amount of stress that naturally developed in the specimen during the dilatometric experiment. In conventional low carbon steel, the stress in the specimen mainly forms due to the very small external force supplied to support it during the dilatometric experiment. As regards ultralow carbon steel, whose phase transformation occurs within an extraordinarily narrow temperature range, the inhomogeneous phase transformation due to the temperature deviation in the specimen was mainly responsible for the stress field in the specimen during the dilatometric experiment.

Nonlinear viscoelasticity of adherent cells is controlled by cytoskeletal tension

The viscoelastic response of living cells to small external forces and deformations is characterized by a weak power law in time. The elastic modulus of cells and the power law exponent with which the elastic stresses decay depend on the active contractile prestress in the cytoskeleton. It is unknown whether this also holds in the physiologically relevant regime of large external forces and deformations. We used magnetic tweezers to apply stepwise increasing forces to magnetic beads bound to the cytoskeleton of different cell lines, and recorded the resulting cell deformation (creep response). The creep response followed a weak power law at all force levels. Stiffness and power law exponent increased with force in all cells, indicating simultaneous stress stiffening and fluidization of the cytoskeleton. The amount of stress stiffening and fluidization differed greatly between cell types but scaled with the contractile prestress as the only free parameter. Our results demonstrate that by modulating the internal mechanical tension, cells can actively control their mechanical properties over an exceedingly large range. This behavior is of fundamental importance for protection against damage caused by large external forces, and allows the cells to adapt to the highly variable and nonlinear mechanical properties of the extracellular matrix.

Full Reconstruction of a Vectorial Protein Folding Pathway by Atomic Force Microscopy and Molecular Dynamics Simulations

During co-translational folding, the nascent polypeptide chain is extruded sequentially from the ribosome exit tunnel and is [corrected] under severe conformational constraints [corrected] dictated by the one-dimensional geometry of the tunnel. [corrected] How do such vectorial constraints impact the folding pathway? Here, we combine single-molecule atomic force spectroscopy and steered molecular dynamics simulations to examine protein folding in the presence of one-dimensional constraints that are similar to those imposed on the nascent polypeptide chain. The simulations exquisitely reproduced the experimental unfolding and refolding force extension relationships and led to the full reconstruction of the vectorial folding pathway of a large polypeptide, the 253-residue consensus ankyrin repeat protein, NI6C. We show that fully stretched and then relaxed NI6C starts folding by the formation of local secondary structures, followed by the nucleation of three N-terminal repeats. This rate-limiting step is then followed by the vectorial and sequential folding of the remaining repeats. However, after partial unfolding, when allowed to refold, the C-terminal repeats successively regain structures without any nucleation step by using the intact N-terminal repeats as a template. These results suggest a pathway for the co-translational folding of repeat proteins and have implications for mechanotransduction.

The rich phenomenology of Brownian particles in nonlinear potential landscapes

Non-interacting Brownian particles obey Langevin equations fulfilling a fluctuation–dissipation relation between friction and thermal noise. Under a linear potential (constant force) Einstein found a relation between diffusion and transport through mobility. In nonlinear potentials this prediction is only satisfied within the limits of very small and large constant external forces. Moreover, other more interesting behaviors do appear, such as: dispersionless transport, sorting, giant diffusion, subdiffusion, superdiffusion, subtransport, etc. All these phenomena depend on the characteristics of the nonlinear potential landscape: periodic or random, the symmetries and boundary conditions. Moreover, the presence of transport is the keystone of most of this phenomenology. In this review, we present numerical simulations illustrating these facts and theoretical analysis when possible.

Random walk in a two-dimensional self-affine random potential: Properties of the anomalous diffusion phase at small external force

We study the dynamical response to an external force F for a particle performing a random walk in a two-dimensional quenched random potential of Hurst exponent H=1/2 . We present numerical results on the statistics of first-passage times that satisfy closed backward master equations. We find that there exists a zero-velocity phase in a finite region of the external force 0<F<F{c} , where the dynamics follows the anomalous diffusion law x(t)∼ξ(F)t^{μ(F)} . The anomalous exponent 0<μ(F)<1 and the correlation length ξ(F) vary continuously with F . In the limit of vanishing force F→0 , we measure the following power laws: the anomalous exponent vanishes as μ(F)∝F^{a} with a≃0.6 (instead of a=1 in dimension d=1 ), and the correlation length diverges as ξ(F)∝F^{-ν} with ν≃1.29 (instead of ν=2 in dimension d=1 ). Our main conclusion is thus that the dynamics renormalizes onto an effective directed trap model, where the traps are characterized by a typical length ξ(F) along the direction of the force, and by a typical barrier 1/μ(F) . The fact that these traps are "smaller" in linear size and in depth than in dimension d=1 , means that the particle uses the transverse direction to find lower barriers.

L p stability of solutions of Boltzmann equation with external force in soft potentials

In this paper, we present a uniform Lp stability theory for the solutions to Boltzmann equation with small or large external forces in vacuum in soft potentials. We assume that the initial data are sufficiently small and a fast decay in space and velocity. Our stability analysis is divided into two parts. For small external forces, we first consider the time derivative of the pth power of Lp norm of the distance between two mild solutions f and g, then define nonlinear functionals, and apply the Grönwall’s inequality to obtain the Lp stability. For large external forces, we make use of the constructive assumptions on external force, Minkowski’s inequality, and Grönwall’s inequality to derive the Lp stability estimate.

Nanomechanics of individual amyloid fibrils using atomic force microscopy

Compression elasticity of glucagon amyloid fibrils in the transverse direction was investigated by a nanoindentation approach based on atomic force microscopy (AFM). With force-volume mapping, we obtained the correlations between radially applied force and compression of amyloid fibrils, from which the radial compressive elasticity can be deduced. The estimated elastic modulus at three typical locations of fibrils varied from (0.72±0.80) GPa to (1.26±0.62) GPa under small external forces, implying the structural heterogeneity of different fibrils.

Onset of frictional slip by domain nucleation in adsorbed monolayers.

It has been known for centuries that a body in contact with a substrate will start to slide when the lateral force exceeds the static friction force. Yet the microscopic mechanisms ruling the crossover from static to dynamic friction are still the object of active research. Here, we analyze the onset of slip of a xenon (Xe) monolayer sliding on a copper (Cu) substrate. We consider thermal-activated creep under a small external lateral force, and observe that slip proceeds by the nucleation and growth of domains in the commensurate interface between the film and the substrate. We measure the activation energy for the nucleation process considering its dependence on the external force, the substrate corrugation, and particle interactions in the film. To understand the results, we use the classical theory of nucleation and compute analytically the activation energy which turns out to be in excellent agreement with numerical results. We discuss the relevance of our results to understand experiments on the sliding of adsorbed monolayers.

Ratchetlike Properties of In Vitro Microtubule Translocation by a Chlamydomonas Inner-Arm Dynein Species c in the Presence of Flow

To investigate the force generation properties of Chlamydomonas axonemal inner-arm dyneins in response to external force, we analyzed microtubule gliding on dynein-coated surfaces under shear flow. When inner-arm dynein c was used, microtubule translocation in the downstream direction accelerated with increasing flow speed in a manner that depended on the dynein density and ATP concentration. In contrast, the microtubule translocation velocity in the upstream direction was unaffected by the flow speed. The number of microtubules on the glass surface was almost constant with and without flow, suggesting that gliding acceleration was not simply caused by weakened dynein-microtubule binding. With other inner-arm dynein species, the microtubule gliding velocity was unaffected by the flow regardless of the flow direction or nucleotide concentration. The flow-generated force acting on a single dynein was estimated to be as small as ∼0.03 pN/dynein. These results indicate that dynein c possesses a ratchetlike property that allows acceleration only in one direction by a very small external force. This property should be important for slow- and fast-moving dyneins to function simultaneously within the axoneme.

Effect of mechanical load on the shuttling operation of molecular muscles

We use molecular dynamics simulations to investigate the effect of mechanical force on stimulus-induced deformation of rotaxane-based artificial molecular muscles. The study shows that a small external force slows down the shuttling motion and leads to longer actuation time for a muscle to reach its full extension. Further increase in the force can significantly reduce the traveling distance of the ring, leading to reduced strain output. A force larger than 28 pN can completely suppress the shuttling motion, suggesting a limit of force output of molecular muscles.