Regensburg 2010 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 64: Solid/liquid interfaces I (focussed session)

O 64.5: Vortrag

Donnerstag, 25. März 2010, 12:15–12:30, H33

Temperature-dependence of molecular self-organization at solid-liquid interfaces — •Sissi de Beer, Peter Wennink, and Frieder Mugele — University of Twente, PCF, Enschede, the Netherlands

When a liquid is confined it behaves different from the bulk. A specifically interesting confinement-effect is called layering; close to the solid wall the liquid molecules structure and form layers. When the two surfaces are approached, this layering effect gives rise to an oscillatory disjoining pressure, which can be measured as the oscillatory forces. We present measurements of the effect of temperature on layering of an organic long-chained hydrocarbon (hexadecane) and a quasi-spherical liquid (octamethylcyclotetrasiloxane; OMCTS) confined upon approach of an atomic force microscope tip towards a solid Highly Ordered Pyrolytic Graphite (HOPG) surface. When the distance between the tip and the surface is reduced, both liquids rupture in discrete steps corresponding to the squeeze-out of individual liquid-layers. We measured the force required to squeeze-out the last liquid layers for various temperatures close to the melting-temperature. For OMCTS we found no change in the squeeze-out force. However, for hexadecane we found that the structuring at the wall strongly increases when the melting temperature is approached and surprisingly, the squeeze-out force is not maximum closest to the melting temperature, but around 3-5 degrees above. We argue that this is due to epitaxial effects between the alkanes and the HOPG surface; the hydrogen-atoms of hexadecane exactly occupy the centers of the graphite hexagons.