Abstract
The field of active matter seeks to understand how the collective behavior of many animate constituents leads to large-scale organization and dynamics. In this thesis, I studied an active matter system composed of purified cytoskeletal components to examine the statistical properties of the material dynamics and their microscopic origins.
First, I quantified the two-dimensional flows of a microtubule-based active nematic liquid crystal and extracted their statistical properties. In agreement with hydrodynamic theory, I found that the vortex areas comprising the turbulent-like flows were exponentially distributed above the `active length scale.'
Then, I quantified the dynamics of individual filaments in the active nematic. The extension speed of an isolated microtubule pair was comparable to the molecular motor stepping speed. In contrast, the net extension in dense 2D active nematics was significantly slower; the interfilament sliding speeds were widely distributed about the average and the filaments exhibited both contractile and extensile relative motion. These measurements highlight the challenge of connecting the extension rate of isolated bundles to the multi-motor and multi-filament interactions present in a dense 2D active nematic. They also provide quantitative data that is essential for building multiscale models.
Finally, I developed an optogenetic material with photoactivatable control over extensile active stress. Using the optogenetic domain iLID with kinesin-1, I demonstrated a system in which a modified opto-kinesin construct can be used to program extensile active stresses in microtubule networks. The engineered opto-motors provided robust and reversible control over the magnitude of the active stress. Through spatial patterning, I probed the onset of turbulence and found confinement suppressed the bend instability, in agreement with theory. In addition, using this opto-kinesin I created a contracting material in which photoactivation induces a change from contraction to extensile behavior.