Enhanced Friction on PDMS Surfaces Textured with “Film-Terminated Ridges” under the Elastohydrodynamic Lubrication

Enhancing the friction of soft materials, examples, rubber, gel, under lubricated conditions is a crucial prerequisite for designing and manufacturing functional devices like flexible electronics, skin sensors, and intelligent grasp-and-release systems. When exposed to a wet and humid environment or sliding under a lubricated case, these soft functional devices readily generate a soft elastohydrodynamic lubrication (EHL) layer on their surfaces, resulting in reduced friction force and exhibiting a low adhesion strength to the substrate. As a result, how to improve the surficial friction force of soft materials under a lubricated condition is a critical concern under the design of soft functional devices. The present study focuses on the fabrication of PDMS surfaces with “film-terminated ridges” textures, achieved through a combination of laser processing and multiple casting techniques. Subsequently, an investigation is conducted to explore their tribological characteristics at the contact interface under fluid lubrication conditions. Relative experiments were performed utilizing a home-made reciprocating friction tester. The flow characteristics of lubricating at the contact interface were also investigated in situ by an integrated digital electron microscope. The obtained results demonstrated that the sliding interface experienced an unstable behavior due to the periodic change of contact stress caused by the surfaces textured with “film-terminated ridges”. The corresponding microscopic image revealed a disruption in the lubricant flow within the contact region, accompanied by an elastic hysteresis at the interface between solid and liquid lubrication. Consequently, the energy of the contact system dissipated within the lubrication interface, which significantly enhanced the sliding friction performance of PDMS surfaces in their soft elastic lubrication state. Additionally, the enhancement in frictional force of the “film-terminated ridges” textures was highly dependent on the period spacing length of ridges. When the spacing between the “ridges” was below a threshold value of approximately 1 mm, the friction force of the textured surface closely resembled that of flat control samples with only marginal increase. However, as the spacing between the “ridges” continues to increase, there was a significant rise in friction force observed on the textured surface. Notably, when at the ridge spacing of 3 mm, the average friction on the textured surface reached 100.23 mN—49.6 times higher than that observed on flat control samples. It seemed that the large “ridge” spacing texture was more prone to inducing significant film deformation compared to the small “ridge” spacing texture, leading to increased dissipation of elastic hysteresis in energy conversion at the lubricating interface and consequently resulting in a higher friction effect at the interface. Furthermore, it was observed that while there was an increase in interfacial friction with increasing normal load for PDMS surfaces, no significant variation occurred across different sliding speeds. This was attributed to the increased sliding speed of the interface, which resulted in minimal bending deformation of the “ridge” under shear action. Consequently, the friction effect at the interface primarily arose from elastic deformation of the “thin film” structure when subjected to load. In contrast, surface friction of the “film-terminated ridges” texture was predominantly influenced by applied loads. Higher loads caused significant deformation in the “thin-film” structure, intensifying energy dissipation at the lubrication interface and leading to an elevated interface friction effect.