Wed. talks

[CM]

JOINT ICTP/SISSA CONDENSED MATTER SEMINARS



Seminar Room - ICTP Leonardo Building  (first floor)



Wednesday, 16 September -     4:00 p.m.



Michael URBAKH     ( Tel Aviv University )



"Modeling friction: From the nano- to macro-scales"



Abstract



Frictional motion plays a central role in diverse systems and  
phenomena that span vast ranges of scales, from the nanometer contacts  
inherent in micro- and nanomachines and biological molecular motors to  
the geophysical scales characteristic for earthquakes. Despite the  
practical and fundamental importance of friction and the growing  
efforts in the field, many key aspects of dynamics of friction are  
still not well understood. One of the main difficulties in  
understanding and predicting frictional response is the complexity of  
highly non-equilibrium processes going on in any tribological contact  
which include detachment and re-attachment of multiple microscopic  
contacts (bonds) between the surfaces in relative motion while still  
in contact.

In this lecture I will discuss microscopic models which establish  
relationships between the dynamics of formation and rupture of  
individual contacts and frictional phenomena. First, I will focus on  
dynamics of nanoscale friction studied in friction force microscopy  
experiments. In this case we found that experimentally observed  
velocity and  temperature dependencies of friction can be rationalized  
by explicitly considering the influence of temperature on the  
formation and rupturing of microscopic contacts. Then,   dynamics of  
cracklike processes that occur at the interface between two  
macroscopic blocks prior to the onset of frictional motion will be  
discussed. In agreement with experimental observations, the proposed  
model demonstrates that the onset of sliding is preceded by well- 
defined detachment fronts initiated at the slider trailing edge and  
extended across the slider over limited lengths smaller than the  
overall length of the slider. We found that three different types of  
detachment fronts may play a role in the onset of sliding: (i)  
Rayleigh (surface sound) fronts, (ii) slow detachment fronts, and  
(iii) fast fronts. The important consequence of the precursor dynamics  
is that before the transition to overall sliding occurs, the initially  
uniform, unstressed slider is already transformed into a highly  
nonuniform, stressed state. Our model allows to explain experimental  
observations and predicts the effect of material properties on  
dynamics of transition to sliding.



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JOINT ICTP/SISSA CONDENSED MATTER SEMINARS





Seminar Room - ICTP Leonardo Building   (first floor)





Wednesday, 23 September -     4:00 p.m.





Oleg  BRAUN   ( Institute of Physics, Kiev )



"Master equation approach to  friction on a mesoscale"



Abstract



Kinetics of the earthquakelike model with a distribution of static  
thresholds is reduced to a master equation which may be considered  
analytically.  This approach naturally describes stick-slip and smooth  
sliding regimes of tribological systems within a framework which  
separates the calculation of the friction force from the studies of  
the properties of the contacts.