Tuning of frictional properties in torsional contact by means of disk grading

The contact of two surfaces in relative rotating motion occurs in many practical applications, from mechanical devices to human joints, displaying an intriguing interplay of effects at the onset of sliding due to the axisymmetric stress distribution. Theoretical and numerical models have been developed for some typical configurations, but work remains to be done to understand how to modify the emergent friction properties in this configuration. In this paper, we extend the two-dimensional (2D) spring-block model to investigate friction between surfaces in torsional contact. We investigate how the model describes the behavior of an elastic surface slowly rotating over a rigid substrate, comparing results with analytical calculations based on energy conservation. We show that an appropriate grading of the tribological properties of the surface can be used to avoid a non-uniform transition to sliding due to the axisymmetric configuration.

A Recent Approach towards Fluidic Microstrip Devices and Gas Sensors: A Review

This paper aims to review some of the available tunable devices with emphasis on the techniques employed, fabrications, merits, and demerits of each technique. In the era of fluidic microstrip communication devices, versatility and stability have become key features of microfluidic devices. These fluidic devices allow advanced fabrication techniques such as 3D printing, spraying, or injecting the conductive fluid on the flexible/rigid substrate. Fluidic techniques are used either in the form of loading components, switching, or as the radiating/conducting path of a microwave component such as liquid metals. The major benefits and drawbacks of each technology are also emphasized. In this review, there is a brief discussion of the most widely used microfluidic materials, their novel fabrication/patterning methods.

A Simple Solution to the Cylindrical Indentation of an Elastic Compressible Thin Layer Resting on a Rigid Substrate

In this paper, an analytical model is presented to study the contact that recedes between an elastic thin film that could be compressed and a substrate of rigidity. The surface of rigidity was formed due to cylindrical indentation. The substrate was assumed to be a rough surface without any friction. Further, the contact width of the substrate was derived, and the relationship between the compression force, compression depth, and the compression width was determined using the energy method. Finally, the obtained results were validated using finite element analysis.

The interface competitive debonding of a bilayer elastic film on a rigid substrate

Different from the system of a single-layer elastic film on a rigid substrate, it is difficult to determine which interface will debond in a bilayer or multilayer film-substrate system. A peeling model of a bilayer elastic film on a rigid substrate is established in the present paper, in order to predict which interface debonding occurs first. The interfacial competitive debonding mechanism is theoretically analyzed with the help of the beam bending theory. A criterion of which interface debonding occurs first is proposed. It is found that the interfacial debonding path is mainly controlled by five dimensionless parameters, i.e., the strength ratio and the critical separation distance ratio of the upper and lower interfaces, the Young's modulus ratio and the thickness ratio of the upper and lower films, and the possible initial cantilever length for ease of loading. The corresponding competitive debonding map is well obtained. From the map, which interface debonds first can be easily predicted. It is interesting to find that the interfacial debonding path can be well tuned by any one of the five parameters. The results of the finite element calculation further confirm the theoretical predictions. The present work can not only provide a theoretical method to determine the interfacial debonding path but also be helpful for the optimal design of multilayer film-substrate systems in practical applications.

Dynamically induced friction reduction in micro-structured interfaces

We investigate the dynamic behavior of a regular array of in-plane elastic supports interposed between a sliding rigid body and a rigid substrate. Each support is modelled as a mass connected to a fixed pivot by means of radial and tangential elastic elements. Frictional interactions are considered at the interface between the supports and the sliding body. Depending on the specific elastic properties of the supports, different dynamic regimes can be achieved, which, in turn, affect the system frictional behavior. Specifically, due to transverse microscopic vibration of the supports, a lower friction force opposing the macroscopic motion of the rigid body can be achieved compared to the case where no supports are present and rubbing occurs with the substrate. Furthermore, we found that the supports static orientation plays a key role in determining the frictional interactions, thus offering the chance to specifically design the array aiming at controlling the resulting interfacial friction force.

Multiphysics modeling and experiments of grayscale photopolymerization with application to microlens fabrication

A phenomenological model of a single-shot grayscale photopolymerization process is developed and used within a virtual process planning framework for microlens fabrication. Along with previous research, the kinetic relations describing the solidification of UV curable resin are derived based on the underlying chemical reactions involved in free radical photopolymerization. As enhancements to the state-of-the-art, our multiphysics model includes a recently proposed super-Gaussian description of the light field, as well as the photobleaching effect due to the live reduction in photoinitiator concentration during UV illumination. In addition, heat generation and thermal strains due to the exothermic chemical reactions, and chemical shrinkage due to polymerization and cross-linking of monomers are considered. The model is numerically implemented via finite element method in COMSOL Multiphysics software. Using a simulation-based virtual process planning framework, customized microlenses are fabricated with an in-house grayscale lithography experimental setup for digital micromirror device (DMD)-based volumetric additive manufacturing. Simulation and experimental results show that after the end of exposure, the temperature quickly rises by the advancement of exothermic chemical reactions and reaches a maximum rise of 100 K in a few seconds, followed by a slow cooling and recovery of thermal strains. It is observed that chemical and thermal shrinkages can compromise the dimensional accuracy of the final part near the resin-substrate interface due to the strong adhesion of the solidified part to the rigid substrate that prevents material shrinkage in the vicinity of the rigid substrate.