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 h...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.展开更多
Adhesive forces commonly exhibit a monotonic increase or a maximum with increasing relative humidity. However, anomalous behavior has been reported. Here, a numerical model of adhesive forces, comprised mainly of capi...Adhesive forces commonly exhibit a monotonic increase or a maximum with increasing relative humidity. However, anomalous behavior has been reported. Here, a numerical model of adhesive forces, comprised mainly of capillary and van der Waals forces, between a tip and a surface is established. It is described by a power law that considers the geometry, the liquid bridge wetting radius, the contact angle, and the separation distance. Capillary forces （sum of surface tension and Laplace pressure） and van der Waals forces are calculated. The latter cannot be neglected in the adhesion even at high humidity. Decrease in adhesion with increasing relative humidity can be attributed to a blunt tip shape, which is validated by experimental data. Specifically, the decrease in adhesion is attributed primarily to a transition from a rounded to a blunt tip shape. Structuring objects at the micro- or nanoscale can either increase or decrease adhesion as a function of relative humidity. This has a wide range of applications in robotic manipulation and can provide a better understanding of adhesion mechanisms in atomic force microscopy in ambient air.展开更多
基金The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China(Nos.51875152,51875153,and 51975174).
文摘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.
文摘Adhesive forces commonly exhibit a monotonic increase or a maximum with increasing relative humidity. However, anomalous behavior has been reported. Here, a numerical model of adhesive forces, comprised mainly of capillary and van der Waals forces, between a tip and a surface is established. It is described by a power law that considers the geometry, the liquid bridge wetting radius, the contact angle, and the separation distance. Capillary forces （sum of surface tension and Laplace pressure） and van der Waals forces are calculated. The latter cannot be neglected in the adhesion even at high humidity. Decrease in adhesion with increasing relative humidity can be attributed to a blunt tip shape, which is validated by experimental data. Specifically, the decrease in adhesion is attributed primarily to a transition from a rounded to a blunt tip shape. Structuring objects at the micro- or nanoscale can either increase or decrease adhesion as a function of relative humidity. This has a wide range of applications in robotic manipulation and can provide a better understanding of adhesion mechanisms in atomic force microscopy in ambient air.
