Friction is involved in many different processes on different length scales, such as motions of joints, writing on a blackboard and the occurrence of earthquakes. Empirical theories have been developed in past centuries by Leonardo da Vinci, Amontons and Coulomb among others, founding the discipline of tribology. However, a general description of friction has not been achieved so far.
Fundamentally, friction is defined as the resistance acting against the motion of two bodies sliding past each other. In order to properly understand friction, we have to extend our knowledge to the molecular scale. In particular, the amount of energy dissipated in friction depends on many parameters such as the shape of the asperities, elasticity of interacting materials and type of medium between those materials sliding past each other.
AFM based methods offer the unique opportunity to detect frictional forces with piconewton resolution at a spatial scale from micrometers to Ångström. Wear can be quantified by imaging with AFM. Furthermore, for tribolelectric studies surface potentials can be obtained via Kelvin probe force microscopy with millivolt accuracy.
A combination of vertical and lateral pulling allows us to determine both the normal force (the adhesion force) and the friction force at the solid-liquid interface for a single polymer. Lateral pulling reveals three different friction motifs: slip, desorption stick and cooperative stick.
B.N. Balzer, M. Gallei, M. Hauf, M. Stallhofer, L. Wiegleb, A. Holleitner, M. Rehahn & T. Hugel
Nanoscale friction mechanisms at solid-liquid interfaces
Angew. Chem. Int. Ed., 52 (25), 6541 (2013). https://doi.org/10.1002/anie.2013012552
B.N. Balzer, S. Kienle, M. Gallei, R. v. Klitzing, M. Rehahn & T. Hugel
Stick-slip mechanisms at the nanoscale
Soft Materials, 12 (1), 106 (2014). https://doi.org/10.1080/1539445X.2014.945039
W. Cai, J.T. Bullerjahn, M. Lallemang, K. Kroy, B.N. Balzer & T. Hugel
Angle-dependent strength of a single chemical bond by stereographic force spectroscopy
Chemical Science, 13, 5734 (2022). https://doi.org/10.1039/D2SC01077A
W. Cai, J.L. Trefs, T. Hugel & B.N. Balzer
Anisotropy of π–π Stacking as Basis for Superlubricity
ACS Mater. Lett., 5 (1), 172 (2023). https://doi.org/10.1021/acsmaterialslett.2c00974
W. Cai, M. Jäger, J. T. Bullerjahn, T. Hugel, S. Wolf* & B. N. Balzer*
Anisotropic Friction in a Ligand-Protein Complex
Nano Lett., 23 (10), 4111 (2023). https://doi.org/10.1021/acs.nanolett.2c04632