Greens function molecular dynamics simulation of sliding, adhesive contacts

• Applicant: Professor Dr. Martin Müser
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: from 2010 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 192177457

• Final Report Year: 2023

Final Report Abstract

Adhesion is an everyday phenomenon, mainly based on what are known as Van der Waals forces. Although these forces are considered small, in principle, they could generate tensile stresses of about 400 MPa, which means they could distribute the weight of a 1.6-t car onto a 4 mm^2 area. However, for most macroscopic objects, the effects of adhesion are practically negligible. There are two significant reasons for this: Firstly, Van der Waals bonds typically do not break simultaneously but rather through crack propagation, as one might experience after stepping on freshly spat-out chewing gum. Secondly, microscopic surface roughness reduces the potential energy gained during contact formation. The central subject of our research was the question of how much surface roughness reduces adhesion on a microscopic scale. In this regard, various criteria have been controversially discussed in recent literature. Macroscopic and microscopic arguments opposed each other, with some attributing the absence of observable adhesion to a global energy balance, while others emphasized the relevance of individual contact points. Against this backdrop, it should be emphasized that adhesion forces in daily life are typically immeasurably small, much smaller than the product of 400 MPa and true contact areas, which, depending on the system, range from 0.01% to higher percentages of the apparent contact area. Our calculations did not reveal a convincing correlation between the adhesion force and the microscopic contact area or the total length of the contact lines. Instead, we found a surprisingly simple relationship: when the surface energy exceeds roughly half of the elastic energy required to form a (nearly) perfect contact, a relatively large maximum tensile stress of the contact occurs, which is at least 0.1% of the theoretical maximum but often greater. In the example provided above, this would be at least 1.6 kg per 4 mm^2 . This result proved to be consistent across various realizations of surface roughness and later for elastomers of finite thickness, such as thin films.

Publications

• Contact-Patch-Size Distribution and Limits of Self-Affinity in Contacts between Randomly Rough Surfaces. Lubricants, 6(4), 85.
  Müser, Martin H. & Wang, Anle
  
• On the usefulness of the height-difference-autocorrelation function for contact mechanics. Tribology International, 123(2018, 7), 224-233.
  Wang, Anle & Müser, Martin H.
  
• Modeling Adhesive Hysteresis. Lubricants, 9(2), 17.
  Wang, Anle; Zhou, Yunong & Müser, Martin H.
  
• Is there more than one stickiness criterion?. Friction, 11(6), 1027-1039.
  Wang, Anle & Müser, Martin H.
  
• Mechanochemical Ionization: Differentiating Pressure-, Shear-, and Temperature-Induced Reactions in a Model Phosphate. Tribology Letters, 70(4).
  Sukhomlinov, Sergey V.; Kickelbick, Guido & Müser, Martin H.
  
• On the viscous dissipation caused by randomly rough indenters in smooth sliding motion, Appl. Surf. Sci. 6, art.-no. 100182
  Sukhomlinov, Sergey V. & Müser, Martin H.