Friction is everywhere – it keeps your car on the road, your coffee cup in your hand, and your feet beneath you. If you took physics in high school, it’s likely you know about static (non-moving) and kinetic (moving) friction. Static friction is stronger than kinetic friction, both resist movement and are constant regardless of time elapsed or speed of sliding. This model, the one most commonly known, is called Amonton-Coulomb Friction, and it is anything but the full picture.
Aging of Frictional Interfaces
One fascinating feature of static friction is that it is not, in fact, static. If two surfaces are placed in contact with a force pushing them together, over time the resistance to sliding will increase logarithmically in time. This is a well known phenomenon and has been observed in materials as diverse as paper, steel, and plastic. The increase in resistance is understood to be due to an increase in the real area of contact between the surfaces, which are actually only touching at small points of contact distributed over their apparent contact.
Using various techniques we aim to investigate this aging of frictional contact surfaces. Mapping of the (real) contact surface over time gives us insight into the evolution of contact points, their interactions – if any exist – and their distribution.
The above image is a zoomed in view of the real area of contact for two plastic samples on glass. The sample on the right has thinner and taller peaks and valleys on the surface, and thus the points of contact are more dispersed and circular.
Friction and Tabletop Earthquakes
A significant fraction of the industrialized world GDP is wasted on the unwanted effects of friction and understanding it better will allow us to reduce the energy loss. Frictional stick-slip in tectonic plate motion gives earthquakes. In addition it has proven a highly nontrivial statistical problem with plenty of open questions.
The main parameter of interest is the real area of contact which is roughly a few percent of the nominal area of contact. It exhibits logarithmic aging with time and logarithmic weakening with velocity. Still this is a description that has not been connected to the microstate of the interface. My experiment allows to dynamically observe the displacement at the frictional interface due to the interacting micro asperities during stick-slip motion. Using modern additive manufacturing 3D printing techniques I prepare rough surfaces with prescribed height roughness and spatial correlations. Employing fast camera video photography I directly observe the slipping of the interfacial material. My goal is to extract its dynamical statistical correlations in order to ultimately understand how they arise from the quenched statistical properties of the surface.
Movie of one slip event that occurs at the frictional interface between two rough PDMS blocks under relative motion. The green vectors represent the in-plane instantaneous displacement field. They are produced from particle image velocimetry (PIVlab) performed on the tracer particles seen as black-and-white patches in the background. The particle are embedded near the interface, at the rough surface of one of the blocks.
Third Body Friction
Friction is most commonly viewed as response due to material properties occurring in a two-body system. Usually two approximately flat sliding solids are considered, and friction is usually characterized by a single relevant quantity, the ‘coefficient of friction’. However, realistically, third body or phases will frequently play a role in the response of frictional system and therefore affect the frictional dynamics. In systems containing a 3rd body/ 3rd bodies, the coefficient of friction is not merely a material property but is an outcome response of the, potentially complex, interaction between three bodies or phases.
By performing a systematic study of the frictional resistances between two rough surfaces with introducing discrete particle into the interface, we show that even one particle can qualitatively affect the sliding dynamics. The frictional dissipation increases as more and more particles are added to the interface. Each friction event, which is reflected in shear force signal as a force spike, is caused by particle/particles motion in interface. The rate of increase in the friction of dissipation depends on the ratio between the size of the particles and the typical feature size of the interface.
Left: Shear is performed and the torque is measured by a commercial rheometer. Even one particle can qualitatively affect the sliding dynamics. The frictional dissipation increases as more and more particles are added to the interface. Interfacial dynamics is simultaneously observed through a 45 degree mirror under the bottom surface