Peter Hoffmann
Dept. of Physics & Astronomy
Wayne State University
Detroit, Michigan
Monday, November 7, 2011
4:30 p.m.
Cardwell 102
That solid materials show new phenomena at the nanoscale is well known, but the behavior of nanoscale liquids is less well understood. Liquids confined to the nanoscale are of great importance in biology, where the average spacing between macromolecules in a cell is of order nanometers, and in nanotribology, where friction is due to the interaction between surfaces entrapping nanoscale liquid films. Atomic force microscopy (AFM) and surface force apparatus (SFA) have been used to confine liquids to nanoscale spaces and to measure their structural and mechanical properties. Unfortunately, these studies have produced contradictory results, ranging from a dramatic spontaneous solidification to no change at all compared to bulk behavior. While the molecular short-range order (layering) along flat surfaces is well established, the mechanical response of the liquid is still highly controversial with sometimes diametrically opposed results reported for the same liquid under identical conditions. However, it seems clear that drastic changes in structure and mechanical behavior do not emerge until the liquid is squeezed down to less than 5-6 molecular layers. It also seems clear that some of the controversy can be resolved if we realize that the mechanical response of the liquid depends on its dynamic state and the used measurement technique (area, stiffness, frequency, amplitude). In this talk, I will review the history of the field, and then discuss some of the most recent results, including AFM results on water and the silicone oils OMCTS and TEHOS obtained in our nanomechanics lab at Wayne State University in Detroit, USA. I will argue that nanoconfined liquids behave in ways that do not have a counterpart in bulk liquids, even considering the complicated behavior of some non-Newtonian fluids.