We study nanoscopic friction forces between an atomic force microscope tip and a glass sample. We show how and why it is possible to tune friction forces in a predictable way by changing either the sample temperature, or the humidity in the experimental chamber. We relate the friction behavior to confined water phase transitions. We find that the water gas-liquid phase diagram is the same at the macroscopic scale as at the nanoscopic tip-sample contact.
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We assume that in our experiments temperature has no major effect besides modifying the relative humidity. This is confirmed by our unpublished results studying versus at and , 45, and . At all three temperatures, increases slightly with , in agreement with the curve at in Fig. 1(a). However, absolute values are lower at higher ; thus versus curves appear shifted towards lower values by increasing . We underline that this shift is only as compared to variations of about appearing in the measurements at [see Fig. 1(b)]. We would like also to remark that the formation of capillary bridges is a thermally activated phenomenon. However, in this case the thermally activation would play a role only if we had changed and not only . This topic is treated in detail in an upcoming publication.
In our study, we assume that the pressure is uniform in the experimental chamber; that is, also close to the heated sample. This assumption is based on the fact that at standard conditions (, ) the mean time between collisions in air is about . Since in our experiments we work in undersaturated conditions (maximum ) and the time during which the AFM tip stays in contact with the surface is always longer than , we conclude that during our measurements the pressure at the surface and the pressure in the buffer are at dynamic equilibrium and they have the same average value.
