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<\/a><\/p>On the atomic scale, the surfaces are composed of either crystalline or disordered material, and the materials may also have loose molecules adsorbed to their surfaces. \u00a0In the simplest case, the two surfaces are crystalline and clean. \u00a0Furthermore, it is simplest if the crystalline surfaces are aligned (ie<\/em> commensurate) and if the surfaces do not stick together (ie<\/em> are non-adhesive). \u00a0The other situations are more complicated, though actually more common in every-day situations, and we analyze them in other work [2].\u00a0 Here, we describe the friction in the simplest, cleanest case.<\/p> Most models of friction predict a friction coefficient, The effect of atomic geometry for this system had been considered before [4,5,6].\u00a0 However, those previous studies used interactions characteristic of adhesive surfaces (tested in [2]), and did not simulate with atomic geometry.\u00a0 In our large-scale simulations of non-adhesive crystalline sphere sliding, we found unexpected behavior.<\/p> Fundamentally, to understand static friction, one must find the ways in which energy barriers arise in the system that oppose sliding. \u00a0To understand kinetic friction, one must find the ways in which energy leaves the system.\u00a0 The energy is determined by the small scale atomic geometry at the surface and the long-range elastic interactions of the two materials.<\/p> For the sphere sliding on a flat surface, elastic interactions distort the material during sliding in different ways depending on the amount of area in contact, leading to different values of the static friction coefficient.\u00a0 Under a sphere, the contact area is a circle, and In regime I, the only elastic distortion comes from the sphere dragging the elastic substrate along the direction of motion.\u00a0 The stress, In regime II, stress builds primarily at the edges of the contact until a lattice dislocation develops and glides through the contact.\u00a0 The nucleation instability undercuts the ability of the friction to raise to This surprising result of non-monotonic friction in the simplest manifestation of the sliding asperity model shows that friction still hides many behaviors waiting to be unraveled.<\/p> [1] Sharp, Tristan A. et al. “Scale-and load-dependent friction in commensurate sphere-on-flat contacts.”\u00a0Physical Review B<\/i>\u00a096.15 (2017): 155436.\u00a0https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.96.155436<\/a><\/p> [2] Sharp, Tristan A. et al. “Elasticity limits structural superlubricity in large contacts.”\u00a0Physical Review B<\/em> 93 (2016):121402(R)\u00a0 https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.93.121402<\/a><\/p> [3] Johnson, Kenneth.\u00a0Contact mechanics<\/i>. Cambridge university press, 1987.<\/p> Others’ work that helped build up this picture: [5] Juan A. Hurtado and Kyung\u2013Suk Kim.\u00a0 “Scale effects in friction of single\u2013asperity contacts. II. Multiple\u2013dislocation\u2013cooperated slip”\u00a0Proceedings of the Royal Society A\u00a0<\/em>(1999):\u00a0455 1989\u00a0http:\/\/rspa.royalsocietypublishing.org\/content\/455\/1989\/3363.short<\/a><\/p><\/a><\/p><\/a><\/p>
\n[4] Juan A. Hurtado and Kyung\u2013Suk Kim.\u00a0 “Scale effects in friction of single\u2013asperity contacts. I. From concurrent slip to single\u2013dislocation\u2013assisted slip.”\u00a0Proceedings of the Royal Society A\u00a0<\/em>(1999):\u00a0455 1989\u00a0http:\/\/rspa.royalsocietypublishing.org\/content\/455\/1989\/3363.short<\/a><\/p>