Tuning Network Elasticity by Controlling Constituent Filament Geometry

The rheological properties of a dense suspension of polymers are intrinsically tied to the microscopic properties of the constituent filaments. Filament shape, for example, is known to tune and control the rheological behavior of the bulk solution. To study the effect of filament shape on viscoelasticity, we introduce bacterial flagella, a biological colloid. Flagella are micron-sized protein superstructures which can adopt different helicities via point mutations of the constituent proteins. The geometries of these colloids range from rigid, straight rods to easily deformable, highly wound slinky-like coils, with suspensions of helical filaments forming stiff elastic networks due to entanglements. We find that greater helicity imbues filaments with a softness inherent to their shape, yet also allows for more entanglements and therefore further arrests diffusion in dense suspensions. Using rheology, we then relate the filament's shape and microscopic dynamics to the viscoelastic properties of dense suspensions. Filaments with lower helicities form stiffer networks than would coil-like filaments. Thus, while entanglements are essential for the formation of an entangled elastic network, the effective rigidity of the constituents determines the network mechanics.