Intertube Friction Research Articles

Modeling Tube Clearance and Bounding the Effect of Friction in Concentric Tube Robot Kinematics.

The shape of a concentric tube robot depends not only on the relative rotations and translations of its constituent tubes, but also on the history of relative tube displacements. Existing mechanics-based models neglect all history-dependent phenomena with the result that when calibrated on experimental data collected over a robot's workspace, the maximum tip position error can exceed 8 mm for a 200-mm-long robot. In this paper, we develop a model that computes the bounding kinematic solutions in which Coulomb friction is acting either to maximize or minimize the relative twisting between each pair of contacting tubes. The path histories associated with these limiting cases correspond to first performing all tube translations and then performing relative tube rotations of sufficient angle so that the maximum Coulomb friction force is obtained along the interface of each contacting tube pair. The robot tip configurations produced by these path histories are shown experimentally to bound position error with respect to the estimated frictionless model compared to path histories comprised of translation or mixed translation and rotation. Intertube friction forces and torques are computed as proportional to the intertube contact forces. To compute these contact forces, the standard zero-clearance assumption that constrains the concentrically combined tubes to possess the same centerline is relaxed. The effects of clearance and friction are explored through numerical and physical experiments and it is shown that friction can explain much of the prediction error observed in existing models. This model is not intended for real-time control, but rather for path planning-to provide error bounds and to inform how the ordering of tube rotations and translations can be used to reduce the effect of friction.

Torsional strain effects on intertube friction in carbon nanotube: strain engineering in friction

The possibility of strain engineering of friction in carbon nanotube is explored in this paper. The intertube friction in double-walled carbon nanotube under torsional strain is calculated by employing the molecular dynamics simulations. By studying the intertube friction in double-walled carbon nanotube subjected to torsional strain, it is found that the friction can be tuned by torsional strain. It shows that the temperature has great influence on torsional strain effects on intertube friction. Mechanism and physical insight of torsional strain effects are revealed by numerical and phonon spectra analysis. It is found that the torsional strain accelerates of the exchange from order kinetic energy to disorder thermal energy. The work in this paper implies the potential of strain engineering in nanoscale friction and brings new insight into fundamental understanding of strain-effect mechanism in nanoscale friction.

Sliding friction at poly(vinyl alcohol)-modified carbon nanotube interfaces

The sliding friction between adjacent carbon nanotubes (CNTs) determines greatly the mechanical property of CNT assembly materials. In order to enhance the intertube friction, polymer molecules are often introduced between CNTs. This paper reveals a new energy dissipation mechanism for the deformed CNT contacts by poly(vinyl alcohol) (PVA). When PVA is introduced into a CNT bundle, most segments of the polymer chain lay on the grooves of adjacent CNTs, while several short segments span over the contact CNTs by inducing a structural deformation on the tubular structure. During the tube sliding, the deformation is recovered and a new one is formed at the next position, contributing to new energy dissipation to prevent the tube sliding. As a result, the friction force can be enhanced by up to eight-fold. This study indicates that a network of transverse polymer chains and longitudinal CNTs is important towards high mechanical properties.

Concurrence of oscillatory and rotation of the rotors in a thermal nanotube motor

In studying dynamic response of a nanomotor made from triple-walled carbon nanotubes (TWCNTs), we find the concurrence of oscillation and rotation of the rotors in the fixed outer tubes (i.e., stators) when the system is in a canonical NVT ensemble with temperature higher than 100K. In the system, the mid tube, called mid-rotor, is driven to rotate by the collision between the end carbon atoms on mid-rotor and the inward radial deviated (IRD) atom(s) on stator(s). The collision depends on three factors, e.g., the inward radial deviation of end atom(s) on stator(s), the number of IRD atoms and temperature of environment. In present research, fixing the first factor with a constant, the acceleration of the mid-rotor needs lower time to approach the maximal rotational frequency when the stators have more IRD atoms. And the maximal rotational frequency of mid-rotor is greater at higher temperature. Due to intertube friction, the inner tube, called inner-rotor, is driven to rotate by the mid-rotor. The system is stable when the inner-rotor has a stable rotational frequency which can be different from that of mid-rotor. During rotating, the inner-rotor may have large-amplitude oscillation. Hence, the simple but interesting model suggests a potential application in a nano-electro-mechanical system (NEMS).

Effect of Defects on Oscillatory Behaviors of Double-Walled Carbon Nanotube Oscillators

Molecular dynamics simulation is performed on the inter-tube friction force and energy dissipation of double-walled carbon nanotube oscillators with vacancy defects. It is found that there are vacancy defect-size and temperature dependences of the friction force between the inner tube and the defective outer tube. The original distance between the “hole” formed by the vacancy carbon atoms and the inserted end of the core has a significant influence on the oscillation profile.

Translational dynamic friction analysis of double-walled carbon nanotubes

Conflicting results on the effects of commensurability, overlap area, helicity and end configuration of double-walled carbon nanotubes (DWCNTs) on translational intertube friction have been reported. We perform molecular dynamics simulations on DWCNTs with different commensurabilities, overlap areas, helicities and end configurations to analyse the intertube friction behaviour and clarify these results. It is found that commensurability and overlap area play an insignificant role, while the atomic configurations of nanotube ends play a dominant role: armchair, normal and reconstructed zigzag ends contribute little to intertube friction; while the irregular ends with dangling atoms greatly increase the friction force. This end effect may also explain the role of helicity in the intertube friction. Implications of the end effect on experimental observations are also discussed.

Motion control in double-walled carbon nanotube systems using a Stone–Thrower–Wales defect cluster

The ability to control the motion of a single molecule will have an important impact in nano-mechanical systems. Multi-walled carbon nanotube systems, which have extremely low intertube friction and strong motion confinement, can form the basis for mechanically based motion control. We devise two molecular motion control units based on double-walled carbon nanotubes embedded with a Stone–Thrower–Wales defect cluster, and perform molecular dynamics simulations to determine the characteristics of these two control units. We show that one of the molecular control units is able to perform a logic operation on one logic input and produce three logic outputs, while the other is able to produce two logic outputs. Potential applications of the motion control units include molecular switches, shuttles and mechanically based logic devices.

Energy dissipation of high-speed nanobearings from double-walled carbon nanotubes

The behavior of nanobearings constructed from double-walled carbon nanotubes(DWCNTs) is investigated with molecular dynamics simulations. The results show that the(5, 5)/(10, 10) DWCNTs can work as stable and reliable nanobearings to a speed as high as∼2.65 r ps−1 with an inner tube as rotator. When the speed is lower than∼0.75 r ps−1, the nanobearings remain in an ultrasmooth state, beyond which the intertube frictionincreases and fluctuates sharply. The rotational friction is sensitive to many factors such asrotation speed, radial size, and flexibility of CNTs. Increase in rotation speed and theradial sizes of CNTs leads to increase of centrifugal force and decrease of intertubedistance, thus, increases the intertube friction. As a result, both the critical speed forultrasmooth rotation and the ultimate speed decrease with increasing radius of the innertube with constant intertube distance. The centrifugal force and thermal motion of atomswill stimulate flexile deformation of CNTs, namely waving tube axis and distortingcross-section, which will lead to an increase in rotational friction. When the outer tubeserves as the rotator, the DWCNT nanobearing becomes more easily damaged.

Directly Synthesized Strong, Highly Conducting, Transparent Single-Walled Carbon Nanotube Films

We report the direct synthesis of strong, highly conducting, and transparent single-walled carbon nanotube (SWNT) films. Systematically, tests reveal that the directly synthesized films have superior electrical and mechanical properties compared with the films made from a solution-based filtration process: the electrical conductivity is over 2000 S/cm and the strength can reach 360 MPa. These values are both enhanced by more than 1 order. We attribute these intriguing properties to the good and long interbundle connections. Moreover, by the help of an extrapolated Weibull theory, we verify the feasibility of reducing the interbundle slip by utilizing the long-range intertube friction and estimate the ultimate strength of macroscale SWNTs without binding agent.

Internanotube friction

Field induced displacement of carbon nanotubes is detected within a bundle and intertube friction is calculated via cohesive energy and electromagnetic formula. Tube-tube friction is found to be 1.4×10−4N which is five orders of magnitude greater than value obtained between adjacent layers within a nanotube.

A tribological study of double-walled and triple-walled carbon nanotube oscillators

We reported in a previous study (Zhao et al 2003 Phys. Rev. Lett. 91 175504) that energytransfer from the orderly intertube translational oscillation to intratube vibrational modesfor an isolated system of two coaxial carbon nanotubes at low temperatures takes placeprimarily via two distinct types of collective motion of the carbon nanotubes, i.e., off-axialrocking motion of the inner tube and radial wavy motion of the outer tube, andthat these types of motion may or may not occur for such a system, dependingupon the amount of the initial extrusion of the inner tube out of the outer tube.Our present study, using micro-canonical molecular dynamics (MD), indicatesthe existence of an energy threshold, largely independent of system sizes andconfigurations, for a double-walled nano-oscillator to deviate from the intertubetranslational oscillation and thus to encounter significant intertube friction. Thefrictional forces associated with several distinct dissipative mechanisms are all foundto exhibit no proportional dependence upon the normal force between the twosurfaces in relative sliding, contrary to the conventional understanding resultingfrom tribological studies of macroscopic systems. Furthermore, simulation hasbeen performed at different initial temperatures, revealing a strong temperaturedependence of friction in the early phase of oscillation. Finally, our studies ofthree-walled nano-oscillators show that an initial extrusion of the middle tubecan cause inner-tube off-axial instabilities, leading to strong frictional effects.