arXiv subjects

Graham L. W. Cross

Publications and source records attributed to Graham L. W. Cross.

4 recordsLinked to original sources

arXiv2026

Directional commensurability stabilizes structural superlubricity in patterned mesoscale interfaces

Structural superlubricity, arising from lattice incommensurability, offers a promising route to eliminate friction and associated energy losses in mechanical systems. In real-world systems, roughness and wear currently pose severe limitations on its robustness and especially the contact size. Here, we consider patterned surfaces as a possible route to overcome some of these limitations. We show that the simplest choice of patterning, contacts made up of two incommensurate triangular-triangular patterns, fails at elevated loads because of the small number of load-bearing contacts, causing the maximum local contact pressure to exceed the strength of the superlubric coating. We introduce a square-triangular patterned interface that increases the number of load-bearing contacts and organizes them into continuous contact lines. When sliding along specific directions relative to these lines, superlubricity is maintained at significantly higher loads by reducing pressure-induced coating failure while also remaining somewhat tolerant to surface imperfections. These findings establish a mechanism for stabilizing structural superlubricity against coating failure and a design principle for engineering low-friction interfaces with enhanced load-bearing capacity and defect tolerance.

cond-mat.mtrl-sci↗

arXiv2026

Mechanical Manipulation of Graphene Auto-Kirigami with an AFM tip

Graphene auto-kirigami describes the thermodynamically self-driven tearing, sliding and folding of graphene sheets to form micrometre-scale, folded ribbons. However, this process typically requires specialised multi-axial nanoindentation systems or highly laborious AFM-based scratching methods. We here introduce a novel, scalable, wholly AFM-based method to nucleate high yields of ribbons in comparable timeframes to previous multi-axial indentation methods, by AFM-based indentation and "hard tapping", whereby high setpoint AFM imaging can nucleate, manipulate and dynamically image the auto-kirigami ribbons. This can be performed with any conventional AFM, enabling extensional growth, rotation and reversal of ribbons towards potential applications as NEMS devices.

cond-mat.mtrl-sci↗

arXiv2024

Kinetic theory applied to pressure-controlled shear flows of frictionless spheres between rigid, bumpy planes

We numerically investigate, through discrete element simulations, the steady flow of identical, frictionless spheres sheared between two parallel, bumpy planes in the absence of gravity and under a fixed normal load. We measure the spatial distributions of solid volume fraction, mean velocity, intensity of agitation and stresses, and confirm previous results on the validity of the equation of state and the viscosity predicted by the kinetic theory of granular gases. In a first, we also directly measure the spatial distributions of the diffusivity and the rate of collisional dissipation of the fluctuation kinetic energy, and successfully test the associated constitutive relations of the kinetic theory. We then phrase and numerically integrate a system of differential equations governing the flow, with suitable boundary conditions, and show a remarkable qualitative and quantitative agreement with the results of the discrete simulations in terms of the dependence of the profiles of the hydrodynamic fields, the ratio of shear stress-to-pressure and the gap between the bumpy planes on the coefficient of collisional restitution, the imposed load and the bumpiness of the planes. Finally, we propose a criterion to predict, on the basis of the solution of the boundary-valued problem, the critical value of the imposed load above which crystallization may occur. This notably reproduces what we observe in the discrete simulations.

cond-mat.soft↗

arXiv2021

Extrinsic plastic hardening of polymer thin films in flat punch indentation

Confined geometries offer useful and experimentally amenable mechanical testing arrangements in which to study the molecular and micro-structural processes which govern plastic yield in stress environments dominated by hydrostatic pressure over shear. However, the changes to macroscopic stress strain behaviour that result from switching from an unconfined mode such as uniaxial compression to a confined one are often overlooked and display a surprising level of complexity, even for simple elastic plastic constitutive models. Here we report a confinement induced strain hardening effect in polystyrene thin films achieved through repeated plastic loading with a cylindrical flat punch whose diameter is many times the initial film thickness. This high aspect ratio combines with constraint provided by film material surrounding the contact to generate a state of confined uniaxial strain in the indented region, rendering the deformation one dimensional. By repeated loading into the plastic domain, we achieve a 66% increase in the confined yield stress, from 0.3 GPa to 0.5 GPa. Through finite element simulation and analytic modelling of the principal stresses and strains, we show that this effect arises not from intrinsic changes to the structure of the material, but rather residual stresses imparted during plastic loading. We contrast this effect with intrinsic changes to glassy thin films such as physical ageing and thermal cross-linking.

cond-mat.mtrl-sci↗