| Nanoconfined liquids have received increasing interest over the last two decades in view of their potential applications in biology and nanotribology. However, a complete understanding of their mechanical and dynamical behavior is still lacking, even after twenty years of vigorous research. In particular, the state and possible phase transition of a liquid confined to spaces of molecular dimensions has been controversial within the scientific community. While geometrically induced layering of confined liquids at an interface was observed by a variety of experimental methods, their behavior was ambiguously reported as ranging from crystallization, glass formation, to no transition at all. We developed a new Atomic Force Microscope (AFM) technique and performed high-sensitivity measurements on confined octamethylcyclotetrasiloxane (OMCTS) and tetrakis (2-ethylhexoxy) silane (TEHOS). In both cases, the liquid layering was induced by confining the liquid between the AFM tip and an atomically flat substrate. Our results agreed with previous findings that molecular close packing in a high density state is achieved when the separation between the confining surfaces is an integer multiple of the liquid molecules size. At all other separations the layering is geometrically frustrated resulting in a liquid low density state. While these results are common in nonpolar liquids of nearly spherical molecules such as OMCTS and TEHOS, we found significant differences in their dynamical behavior. We have shown that OMCTS can behave both as a Newtonian liquid with very little change in its dynamics or as a pseudo-solid depending on the rate of approach of the confining surfaces. Therefore, it appears that mere confinement cannot induce the solidification of OMCTS in thermodynamic equilibrium, but rather the solidification is induced kinetically when the confining surfaces are approached with a minimum critical rate of the order of 6 A/s. Although chemically similar and of comparable size, TEHOS revealed a significantly different behavior, becoming solid-like at a lower squeezing rate of only 3 A/s. These findings suggest that solidification may occur in TEHOS even in thermodynamic equilibrium, if not at a very slow rate of approach inaccessible through our experiments. |
