The friction peak that occurs in tire–road sliding when the contact changes from wet to dry was previously attributed to capillary cohesion, van der Waals attraction, and surface roughness, but the detailed mechanisms have yet to be revealed. In this study, friction and static contact experiments were conducted using a custom-built in situ optical microtribometer, which allowed us to investigate the evolution of the friction, normal load, and contact area between a polydimethylsiloxane (PDMS) film and a silicon nitride ball during water volatilization. The friction coefficient increased by 100%, and the normal force dropped by 30% relative to those in the dry condition during the wet-to-dry transition. In static contact experiments, the probe indentation depth increased, and the normal load decreased by ~60% as the water evaporated. Combining the friction and static contact results, we propose that the large friction peak that appeared in this study can be attributed to the combined effects of increased adhesive capillary force and increased plowing during the wet-to-dry transition.

Granular friction behaviors are crucial for understanding the ubiquitous packing and flow phenomena in nature and industrial production. In this study, a customized experimental apparatus that can simultaneously measure the time history of normal and tangential forces on the inside-shearing unit is employed to investigate the granular friction behaviors during a linear reciprocating sliding process. It is observed that the evolution behaviors of two normal forces distributed separately on the shearing unit can qualitatively reflect the effects of the force chain network. During the half-loop of the reciprocating sliding, the total normal force, which indicates the load-bearing capacity of the granular system, experiences the following typical stages: decreases abruptly and stabilizes momentarily, further decreases significantly to the minimum, gradually increases to the maximum, and then remains stable. These stages are associated closely with the relaxation, collapse, reconstruction, and stabilization of the force chain, respectively. Interestingly, the coefficient of friction (COF) can reach a stable value rapidly within the initial sliding stage and subsequently remain constant. The average COF within stable ranges decreases significantly with the external load G in the power-function form, G^-0.5. Meanwhile, the COF increases slightly with the sliding velocity. Finally, a complete illustration of the dependences of the granular COF on the external load and sliding velocity is provided. Our study contributes to granular friction research by providing an innovative experimental approach for directly measuring the COF and implicitly correlating the evolution of the force chain network.

The tribological characteristics of cotton fibers play an important role in engineering and materials science, and real contact behavior is a significant aspect in the friction behavior of cotton fibers. In this study, the tribological characteristics of cotton fibers and their relationship with the real contact behavior are investigated through reciprocating linear tribotesting and real contact analysis. Results show that the friction coefficient decreases with a general increase in load or velocity, and the load and velocity exhibit a co-influence on the friction coefficient. The dynamic change in the real contact area is recorded clearly during the experiments and corresponds to the fluctuations observed in the friction coefficient. Moreover, the friction coefficient is positively correlated with the real contact area based on a quantitative analysis of the evolution of friction behavior and the real contact area at different loads and velocities. This correlation is evident at low velocities and medium load.