Dresden 2006 – scientific programme

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CPP: Chemische Physik und Polymerphysik

CPP 20: SYMPOSIUM Microfluidics I: Boundary conditions, manipulation, and transport

CPP 20.3: Talk

Thursday, March 30, 2006, 10:15–10:30, ZEU 160

Single Molecule Diffusion in Confined Liquids under Shear — •Arne Schob and Frank Cichos — Photonics and optical materials, Institute of physics, TU Chemnitz, 09107 Chemnitz

The liquid-solid boundary is of fundamental importance for microfluidics, chromatography, lubrication and many other applications. Accessing the details and especially the molecular dynamics in this boundary region, however, involves a number of difficulties due to the fact that liquids dynamics varies on a length scale of a few nanometers. We present for the first time experiments that are based on the tracking of single dye molecules in liquid films confined in a surface forces apparatus. Our setup allows to follow molecular motion in liquid films of a few nanometer thickness and under applied shear flow. Employing a new method of single molecule diffusion analysis, we are able to separate the imposed shear velocity from the single molecule diffusion with an extremely high accuracy. Shear displacements of 8 nm within 15 ms can be detected even though diffusional displacements are at least by one order of magnitude larger. The studies on confined films of tetrakis(2-ethylhexoxy)silane demonstrate, that down to a film thickness of 10 nm no change in the effective viscosity of the liquid at the solid boundary occurs. Contrary to recent literature reports, no dependence of the diffusion constant on the shear rate is found.