Parallel and perpendicular drag coefficients of a slender filament
= Parallel and perpendicular drag coefficients of a slender filament
{title2=$\xi_\parallel,\ \xi_\perp$}
The local <resistive-force theory> coefficients $\xi_\parallel$ and $\xi_\perp$ describe drag along and across a filament tangent. For very slender bodies their leading logarithmic values have ratio $\xi_\perp/\xi_\parallel\simeq2$. Geometry, ends and nonlocal fluid interactions affect the corrections. Anisotropy enables propulsion by rotating <helices>.