Riding the Wave: Switchable Complex Fluids from Coupled Shape-Shifting Colloids

The macroscopic rheological behavior of a dense suspension is intrinsically linked to the microscopic interactions of its constituent particles. Thus, a colloid whose shape can be changed in solution offers new possibilities for designing switchable complex fluids. Here, we use bacterial flagella—a rigid, micron-sized biological filament—to develop a novel colloidal system that provides this shape-control. Flagella can switch between discrete shapes in response to external stimuli like chemical conditions, temperature, and stress. For example, straight flagella filaments twist into helices with uniform pitch and diameter upon mild heating. This out-of-equilibrium morphological transformation is directly observed using fluorescence microscopy. In a dense suspension, the spacing between particles is small, and the abrupt change in shape results in each filament rapidly coiling into its neighbors. Aligning the rods introduces a strong coupling between neighbors, allowing the individual motions of single particles to accumulate into a system-wide mechanical wave. Macroscopically, these entanglements manifest as a dramatic increase in the system's bulk viscosity. Our findings reveal a new class of switchable metafluids, where the viscous properties can be dynamically tuned by controlling the shape transformations of their constituent particles.