(623a) Sedimentation Velocity Characterization of Rod-like Colloids

The sedimentation velocity of dispersed particles in the low Reynolds and rotational Peclet number regimes incorporates significant information about particle size and shape distributions as well as the particle density and hydration.  Using data analysis of sedimentation experiments we show that the hydrodynamic framework for several colloidal rod systems of single-wall carbon nanotubes (SWCNTs) can be evaluated.  For very stiff rods of known density (such as large diameter SWCNT populations covering a wide aspect ratio range, from <50 to >400), excellent agreement is found for the shape factor contributions to friction from experimental findings and the theoretical predictions for rigid rods from numerical calculations and multi-term analytical expansion approximations.  Moreover, using an independently measureable hydrated particle radius, the apparent distribution of nanotube lengths in each sorted population can be calculated directly from the sedimentation coefficient distribution without adjustable parameters, with excellent agreement to length distributions from atomic force microscopy.  For less stiff rods, such as small diameter SWCNTs, deviation from rigid rod hydrodynamics is observed, with the individual particles translating at increased velocities due to the lower frictional contribution of bent conformations.  The evaluation of this sedimentation velocity with respect to the hydrodynamic model  used, and the characterization of the apparent average conformation and persistence length of the rod is an exciting advance beyond simplified model application.  Applications and the physics of the detailed hydrodynamic and interfacial properties of these colloidal rod systems will be presented.