Colloids with a Twist: Entanglement's Effect on Diffusion of Helical Filaments

Connecting the thermal motion of a dense ensemble of microscopic particles to the system's macroscopic properties remains a challenge. We examine this relationship using a model colloidal system built from bacterial flagella, a biological colloid whose overall shape can be engineered and controlled. Point mutations of the flagellar constituent protein result in shapes ranging from a rigid straight rod to a helix with a well-defined pitch and wavelength. The 3D motion of individual filaments in a dense suspension is characterized using confocal microscopy. Helicity changes particle dynamics: A dense suspension of helices exhibits entanglements, which constrain the motion of individual helices. This results in a corkscrew diffusive motion that is absent for straight rods. These entangled helices have a significantly higher viscosity than a system consisting of straight rods. Utilizing a diffusion-informed model provides a framework to understand how viscosity can emerge from the confined motion of individual particles. Together, these results show how small changes in a colloid's shape can lead to a multi-scale change in a system's behavior.