Thermal and directed motion in molecular friction processes investigated by atomic and photonic force microscopy

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Friction is a complex process of energy dissipation, which is important on most length scales, time scales and across disciplines. Several theories approach the molecular origin of friction, but a comprehensive understanding is still missing. Usually, friction is quantified by a friction coefficient. Two main routes to determine the friction coefficient, either from a directed motion or from thermal motion, can be addressed.Here we use a combination of the complementary approaches Atomic Force Microscopy(AFM) and Photonic Force Microscopy (PFM) to obtain a better understanding of molecular friction at soft (bio)interfaces. Towards that aim, we propose direction and frequency dependent measurements on different bead-surface model systems, which are decorated with well-defined binding partners. We are in particular interested, how the friction coefficients obtained from directed and thermal motion are related. In this context, we will address the question on how single molecule friction and adhesion are related. Finally, we will investigate cooperativity in molecular friction, which we believe is key in understanding how molecular friction properties determine macroscopic friction.Altogether our combined experimental approach will yield unprecedented insights into themolecular origins of frictions from various perspectives, which will help to better understand functionalized interfaces and hydrogels as well as biological transport processes.