Friction with self assembled monolayers: hydrodynamic slip and activated solid/solid friction

Seminar by Josh McGraw

The friction between a solid and a liquid was historically assumed to be so large that fluid molecules in contact with a solid boundary were stuck. Thus, the classical no-slip boundary condition was applied. With the advent of micro- and nano-fluidics in the last decades, however, this boundary condition has been observed to fail in many instances. Particularly, as will be discussed here, a slip boundary condition is observed when unentangled polystyrene (PS) dewets from a hydrophobic, alkylsilane self-assembled monolayer (SAM). This boundary condition can furthermore be tuned over an order of magnitude by subtly changing the SAM [1], indicating that the friction between solid and liquid is highly sensitive to atomic level details. We have also investigated the friction between these same SAMs and a solid metal tip of nanoscopic dimensions. Remarkably, this apparent solid/solid friction is velocity dependent and exhibits a crossover from a linear to logarithmic scaling in the velocity [2]. The results can be described in terms of a distribution of nanocontacts with very weak stiffness compared to the expected one for metal/solid contacts. The work presented was completed in collaboration with the authors of refs. [1, 2]. 
[1] J.D. McGraw, M. Klos, A. Bridet, H. Hähl, M. Paulus, J.M. Castillo, M. Horsch, K. Jacobs, J. Chem. Phys. 146 203326 (2017).
[2] J.D. McGraw, A. Niguès, A. Chennevière, A. Siria, Scaling crossover of the velocity dependence for solid/solid friction at the nanoscale, Nano Lett. 17 6335 (2017).
Published Nov. 23, 2018 3:42 PM - Last modified Dec. 3, 2018 2:36 PM