Effect of pattern topology on the self-cleaning properties of textured surfaces.

The water contact angle and self-cleaning property of microfabricated surface textures possessing different topologies are compared. In one kind of surface textures, the protruded regions form a connected square network. In the other kind of surface textures, the protruded regions form a regular array of square posts. We find that the water apparent contact angle of the connected textures agrees with the Cassie equation [Discuss. Faraday Soc. 3, 11 (1948)], but that of the disconnected textures is much larger. Nevertheless, the disconnected textures exhibit inferior self-cleaning property, contrary to conventional conception. We discuss the possible reasons for these observations.

Effect of C60 molecular rotation on nanotribology.

The effect of C60 molecular rotation on the nanotribological properties of C60 single crystal surfaces has been studied by atomic/frictional force microscopy. The orientational order-disorder phase transition, in which the high temperature C60 free rotation is reduced to a low temperature hindered rotation, is shown to give rise to an abrupt change in friction and adhesion. This change in frictional force is quantitatively consistent with the observed change in adhesion. The similar slopes of the friction versus load curves in both phases indicate that the friction coefficient in the two phases remains about the same. Hence the C60 rotation does not provide an additional energy dissipation channel in the friction process.

Temporal evolution of relaxation in rubbed polystyrene thin films.

Temporal relaxation of rubbed polystyrene (PS) thin films (50 nm thick) was studied by probing the decay of the optical anisotropy in the sample. Our results show that the relaxation process is characterized by two single-exponential decays plus a temperature-dependent constant. Both relaxations are orders of magnitude faster than the main chain (alpha) relaxation at temperatures well below (>10 degrees C)T(g), with activation energies 3.0 kcal/mol and 5.1 kcal/mol, respectively, either of which are much smaller than that of the alpha relaxation ( approximately 50 kcal/mol). The decay time constants are found to be independent of the sample molecular weights M(w) (=13.7 K to 550 K Daltons, M(w)/M(n)

Statistical analysis of X-ray speckle at the NSLS.

A statistical analysis of the static speckle produced by illuminating a disordered aerogel sample by a nominally coherent X-ray beam at wiggler beamline X25 at the National Synchrotron Light Source is reported. The results of the analysis show that the coherence delivered to the X25 hutch is within 35% of what is expected. The rate of coherent photons is approximately two times smaller than expected on the basis of the X25 wiggler source brilliance.