Watching Molecules Under Confinement

Ashis Mukhopadhyay and Steve Granick

Department of Materials Science and Engineering,, University of Illinois, Urbana, Illinois 61801

Direct experimental probes of molecules near surfaces and in confined geometries would be of intense interest but is difficult experimentally due to the small amount of material present. The Surface Forces Apparatus (SFA) has been used extensively with great success to study static and dynamic forces in molecularly thin films. However, a limitation is that molecular organization and dynamics must be inferred from the ensemble average of the rheological properties (e.g., viscosity). We have developed a new experimental platform where Fluorescence Correlation Spectroscopy (FCS) is combined with a modified SFA for direct experimental probe of the molecular dynamics in ultrathin liquid films. From the fluctuation autocorrelation curve, we measure the translational and rotational diffusion of the probe molecule under the application of both normal and shear forces. The practical significance is to contrast friction, an average number, with diffusion of individual molecules in a confined fluid.