| Various mechanisms govern pattern formation in chemical and biological reaction systems, giving rise to structures with distinct morphologies and physical properties. The self-organization of polymerizing microtubules (MTs) is of particular interest because of its implications for biological function. We report a study of the microscopic structure and properties of the striped patterns that spontaneously form in polymerizing tubulin solutions and propose a mechanism driving this assembly. We further present a mechanical model to explain the process and mechanism of buckling, and to infer properties of MT bundles such as packing geometry and size.;Using oscillatory optical tweezers based active microrheology, the frequency-dependent viscoelasticity of MT bundled networks were measured. This method implements forced oscillation of a 1.5mum silica bead embedded in a MT bundled network. Both the storage modulus and the loss modulus depend on the direction of the oscillatory motion of the bead relative to the alignment direction of the bundles. Furthermore, we have built an infrared laser tweezers setup for passive microrheology and to study the biophysics of bacterial motility and adhesion. We determined the force profile both near the center of the trap and at the far edge. Using the newly-built laser tweezers and position detection system, we studied microrheological properties of F-actin in both isotropic and nematic phases. This method records the displacement of thermally driven micron-sized beads, based on which shear moduli of the underlying network were calculated. The result is consistent with that obtained by video particle tracking.;In addition, we use the calibrated optical trap to determine the trapping force on a swimming Caulobacter crescentus swarmer cell during its escape from the trap center. We further apply laser tweezers to study the rotation of a trapped Caulobacter swarmer cell and the process by which the cell struggles to escape from the trap center. Aided by the laser trap, we also show that bringing the holdfast of a stalked cell close to a glass surface facilitates adhesion of the cell to the surface, consistent with the model that cells must overcome a repulsive barrier at the surface to adhere efficiently. |
