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SUMMARY:Calculation of adhesion between elastic spheres from molecular for
 ces - Dr Jim Greenwood\, CUED
DTSTART:20120203T140000Z
DTEND:20120203T150000Z
UID:TALK35726@talks.cam.ac.uk
CONTACT:Ms Helen Gardner
DESCRIPTION:The Johnson\, Kendall\,& Roberts (JKR) theory of adhesion in e
 ffect uses fracture mechanics to show that the adhesive force between two 
 elastic spheres will be. But earlier Bradley\, by integration of the force
 s between every pair of “molecules” of the two spheres\, calculated th
 e force to be: and since the JKR value is independent of the elastic modul
 us\, it too applies to rigid spheres\, and we have a conflict.  Tabor offe
 red a resolution of the conflict\, later confirmed when Derjaguin and his 
 collaborators introduced a method of analysing the problem by assuming tha
 t the forces due to the molecular interactions could be replaced by a surf
 ace force law acting across the gap between the two bodies\, and that the 
 surface force law could be treated as applying a surface traction to the b
 odies. The deformation is then calculated by the usual methods of elastic 
 contact mechanics.\nThe unexpected results of this analysis will be descri
 bed\; and are generally accepted when the spheres are large and the region
  of interaction small..... in effect for a “Hertzian” geometry. But re
 cent advances in MEMS and in studying biological adhesion\, as well as in 
 powder technology\, have drawn attention to small spheres with large areas
  of ‘contact’: and here the basic equivalence between the molecular fo
 rces and a surface force law is suspect. Various ‘improvements’ will b
 e described: particularly the author’s preferred option with its absurd 
 conclusion that the radius of the ‘contact’ area can exceed the radius
  of the sphere!
LOCATION:Oatley Seminar Room\, Department of Engineering
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