Molecular-Dynamics Simulations of the Emergence of Surface Roughness in a Polymer under Compression

Roughness of surfaces is both surprisingly ubiquitous on all length scales and extremely relevant practically. The appearance of multi-scale roughness has been linked to avalanches and plastic deformation in metals. However, other, more-complex materials have mechanisms of plasticity that are significantly different from those of metals. We investigated the emergence of roughness in a polymer under compression. We performed molecular-dynamics simulations of a slab of solid polyvinyl alcohol that was compressed bi-axially, and we characterised the evolution of the surface roughness. We found significantly different behaviour than what was previously observed in similar simulations of metals. We investigated the differences and argue that the visco-elasticity of the material plays a crucial role.

The Influence of the Angstrom-scale Roughness on the Laser-induced Damage Threshold of the ZnGeP2 Single Crystal

Magnetorheological processing was applied to polish the working surfaces of the ZnGeP2 single crystal, in which a non-aqueous liquid with magnetic particles of carbonyl iron with the addition of nanodiamonds was used. Samples of a single crystal ZnGeP2 with an angstrom level of surface roughness were received. the use of MRP has allowed more accurately characterizing possible structural defects that have emerged on the surface of a single crystal and have a size of ~ 0.5-1.5 μm. the LIDT value at the indicated or-ders of magnitude of the surface roughness parameters is determined not by the quality of polishing, but by the number of point depressions caused by physical limitations of the structural configuration of the crystal volume. These results are in good agreement with the assumption made about a significant effect of the concentration of dislocations in a ZnGeP2 crystal on LIDT.

Mechanically Switchable Wetting Petal Effect in Self-Patterned Nanocolumnar Films on Poly(dimethylsiloxane)

Switchable mechanically induced changes in the wetting behavior of surfaces are of paramount importance for advanced microfluidic, self-cleaning and biomedical applications. In this work we show that the well-known polydimethylsiloxane (PDMS) elastomer develops self-patterning when it is coated with nanostructured TiO2 films prepared by physical vapor deposition at glancing angles and subsequently subjected to a mechanical deformation. Thus, unlike the disordered wrinkled surfaces typically created by deformation of the bare elastomer, well-ordered and aligned micro-scaled grooves form on TiO2/PDMS after the first post-deposition bending or stretching event. These regularly patterned surfaces can be reversibly modified by mechanical deformation, thereby inducing a switchable and reversible wetting petal effect and the sliding of liquid droplets. When performed in a dynamic way, this mechanical actuation produces a unique capacity of liquid droplets (water and diiodomethane) transport and tweezing, this latter through their selective capture and release depending on their volume and chemical characteristics. Scanning electron and atomic force microscopy studies of the strained samples showed that a dual-scale roughness, a parallel alignment of patterned grooves and their reversible widening upon deformation, are critical factors controlling this singular sliding behavior and the possibility to tailor their response by the appropriate manufacturing of surface structures.

Recent advances in the mechanical durability of superamphiphobic surfaces: A review

Superamphiphobic (water and oil repellent) surfaces have distinct application areas such as anti-corrosion, anti-friction, anti-icing, self-cleaning, etc. To fabricate these types of surfaces, the base material must have low surface energy and dual-scale roughness (micro and nano levels). The durability of such surfaces is indicated by the duration up to which they remain stable and effective. When mechanical forces interact with these surfaces, it deteriorates the surface integrity and ultimately degrades its superamphiphobic property. Hence, these surfaces have very limited mechanical wear sturdiness and long-term durability. Therefore, in this article, efforts have been made to review the different types of mechanical durability tests performed on superamphiphobic metallic and non-metallic surfaces. Moreover, various kinds of surface texturing techniques in context with durability of such surfaces have been discussed. Finally, from the reported literature, critical conclusions, challenges, and scopes for future work have also been presented.

Comparative Evaluation of Primary Stability between Different Diameters Multi-Scale Roughness Dental Implant by Solid Rigid Polyurethane Simulation

Background: Implant primary stability is determined by screw characteristics and surgical procedure. The aim of the present study was to evaluate, on a polyurethane model, the insertion torque (IT), removal torque (RT), and resonance frequency analysis (RFA) of multi-scale roughness dental implants of different diameters. Methods: Two implant sizes were tested on two polyurethane blocks (20 pounds per cubic foot (PCF) and 30 PCF): 3.0 diameter and 13 mm length and 5.0 diameter and 13 mm length. The IT, RT, and RFA were assessed. Results: A significant difference of IT and RT was present in favor of wider implants at both polyurethane densities. No statistical difference was present between the 5.0 diameter and 3.0 diameter implants at both polyurethane densities. A statistically increased RFA was reported for 5.0 implant 30 PCF polyurethane blocks. Conclusions: Multi-scale roughness dental implants of both diameters showed high insertion torque and primary stability on polyurethane blocks, which is valuable for implant loading protocols.

Size, shape, and orientation of macro-sized substrate protrusions affect the toe and foot adhesion of geckos

Geckos are excellent climbers using compliant, hierarchically-arranged adhesive toes to negotiate diverse terrains varying in roughness at multiple size scales. Here, we complement advancements at smaller size scales with measurements at the macro-scale. We studied the attachment of a single toe and whole foot of geckos on macro-scale rough substrates by pulling them along, across, and off smooth rods and spheres mimicking different geometric protrusions of substrates. When we pulled a single toe along rods, the force increased with the rod diameter. Whereas, the attachment force of dragging toes across rods increased from about 60% on small diameter rods relative to a flat surface to approximately 100% on larger diameter rods, but showed no further increase as rod diameter doubled. Toe force also increased as the pulling changed from along-rod loading to across-rod loading. When pulled off from spheres, toe force increased continuously with sphere diameter as observed in along-rod pulling. For feet with separated toes, attachment on spheres was stronger than that on rods with the same diameter. Attachment force of a foot decreased as rod and sphere size increased, but remained sufficient to support the body weight of geckos. These results provide a bridge to the macro-scale roughness seen in nature by revealing the importance of the dimension, shape, and orientation of macro-sized substrate features for compliant toe and foot function of geckos. Our data not only enhances our understanding of geckos’ environmental adaptive adhesion, but can also provide inspiration for novel robot feet in development.