Colloids with a Twist: Tuning Bulk Mechanical Behavior with Shape-Shifting Helical Colloids

Biological systems achieve complex reconfiguration and motion through the collective action of countless shape-changing internal components. Mimicking this behavior is challenging for colloidal systems, as conventional particles have fixed shapes and lack dynamic reconfigurability. We experimentally overcome this limitation by building a novel colloidal system from polymorphic bacterial flagella — rigid, micron-sized filaments that switch between discrete shapes in response to stimuli like temperature. For example, straight flagella filaments twist into uniform helices upon mild heating. In a dense suspension, this abrupt shape change causes filaments to rapidly coil into their neighbors, with each colloid's individual motion accumulating into a system-wide mechanical wave. Macroscopically, these newly-formed entanglements manifest as a dramatic increase in the system's bulk viscosity. We also observe a contractile transition where a native helical filament coils into a much tighter structure, reducing its axial length by >3-fold. By bundling hundreds of flagella with a depletant, this transition becomes a sharp, discrete snapping event, creating a microscopic actuator. When entangled, these snapping events percolate through the material, creating a smoothly contracting elastic network. Our work demonstrates that actively changing particle geometry enables us to toggle macroscopic properties and generate bulk mechanical motion, allowing a single component to produce fundamentally different mechanical states.