Knotting up a Network: Controlling Particle Shape to Tune Macroscopic Rheology*

The macroscopic rheological behavior of a dense suspension is intrinsically linked to the microscopic interactions of its constituent particles. We construct a novel colloidal system using bacterial flagella, a rigid biological filament, that allows control over these interactions from the colloid's shape. Flagella are polymorphic, capable of transforming between a number of discrete shapes in response to external chemical conditions, temperature, and stress. For instance, after mild heating in a particular biocompatible solvent, straight flagella filaments twist into helices with uniform pitch and diameter. This morphological transformation is directly imaged using fluorescent microscopy. In a dense suspension, the abrupt change in shape results in intricate collective motion as each coiling particle rapidly entangles with its neighbors. Macroscopically, these entanglements manifest as a dramatic increase in the system's bulk viscosity, which we characterize using a rheometer. Many such shape changes are reversed upon restoration of the initial sample conditions, offering a unique way to switch the system's rheological properties between markedly different states. Our findings pave the way for a new category of complex fluids whose viscous properties can be toggled by manipulating their constituent particles' shape.