No abstract

To unravel molecular motion within confined liquids, we have combined a surface forces apparatus (SFA) with a highly sensitive fluorescence microscope. Details of our setup including important modifactions to enable the tracking of single dye molecules within nanometer thin confined liquid films are presented. The mechanical and optical performance of our setup is discussed in detail. For a load of 20 mN we observed a circular-shaped contact region (d approximately 300 microm), which results in a confining pressure of about 280 kPa. First experiments on liquid films of tetrakis(2-ethylhexoxy)silane (TEHOS) doped with rhodamine B demonstrated the ability to track single dye molecules within the confining gap of a SFA. The mean diffusion constant was independent of the liquid film thickness of approximately 3x10(-8) cm2/s and thus 10 times smaller than the diffusion constant of rhodamine B in bulk TEHOS. This points to the existence of a thin interface layer with slower molecular dynamics and an attractive potential parallel to the solid surface trapping molecules in this interface region.

No abstract

A method for analyzing single molecule tracking data is presented, which allows the measurement of flow profiles in shear flows of ultrathin liquid films. The results show that the velocity profile detected by single molecule tracking is in general different from the analytical solution of the Navier−Stokes equation due to the diffusion of the probe molecules. However, the theory presented allows the extraction of even slip boundary phenomena, which for the first time provides access to boundary conditions on a molecular scale. A verification of the theory and its capabilities is presented by means of tracking single polystyrene particles in a 500 nm thin water film confined in a surface forces apparatus.