Rheological measurements by AFM of the formation of polymer nanofibers

Polymer fiber can be formed by pulling a thread of polymeric liquid if the fiber solidifies before it breaks up by capillary thinning. Fiber diameter is well correlated with a processing parameter [1] that is a simple function of viscosity, surface tension and evaporation rate. Not only can the effect of the processing parameter be observed in the atomic force microscope (AFM), but the fundamental material parameters can also be determined with the same AFM setup. The usual problem with tapered AFM tips, of liquids wetting unstably up the tapered AFM tip and even onto the cantilever, is resolved by the use of long cylindrical tips of constant diameter. Previously, focused-ion-beam milled AFM tips had been used for this reason, but they are expensive and limited in length to a few microns. We recently demonstrated a straightforward method of growing Ag-Ga nanowires (100 nm diameter, 7-70 microns long) onto AFM tips at room temperature. These constant diameter nanowires are shown to give clearly measurable force-distance curves when inserted through the surface of a liquid, which provides clean measurements of surface tension, contact angle, and evaporation rate, while shear viscosity is determined through Q-damping as a function of insertion distance into the liquid.