Any macroscopic deformation of a filamentous bundle is
necessarily accompanied by local sliding and/or stretching
of the constituent filaments1,2. Yet the nature of the sliding
friction between two aligned filaments interacting through
multiple contacts remains largely unexplored. Here, by directly
measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly
large, scale logarithmically with sliding velocity as in solid-like
friction, and exhibit complex dependence on the filaments’
overlap length. We also show that a reduction of the frictional
force by orders of magnitude, associated with a transition from
solid-like friction to Stokes’s drag, can be induced by coating
F-actin with polymeric brushes. Furthermore, we observe
similar transitions in filamentous microtubules and bacterial
flagella. Our findings demonstrate how altering a filament’s
elasticity, structure and interactions can be used to engineer
interfilament friction and thus tune the properties of fibrous
composite materials.