Complex Fluids

Assembly and mechanical properties of chiral ribbons in rod/polymer mixtures

Under certain conditions rod-like colloids in a rod/polymer mixture will assemble into chiral ribbons. The ribbons are a colloidal membrane one rod length thick and twisted along one direction. Ribbons grown in this manner can reach lengths of several hundred microns. We are trying to understand the formation of ribbons in terms of competition between line tension and intrinsic chirality of the molecule. In addition the theory predicts that twisted ribbons have novel mechanical properties. We are planning to test these predictions by measuring elastic properties using optical tweezers.

The top image is a twisted ribbon with a very large pitch. In the bright regions the rods lie in the plane of the image and therefore the ribbons appear birefringent under polarization microscope. In the middle of the image the rods point into the image plane and ribbon appears isotropic between crossed polorizers. On the bottom is the image of a ribbon with a much smaller pitch.

Movie: Collapsing ribbon to disk fast. [MOV - 2mb] Movie: collapsing ribbon to ribbon short. [MOV - 2mb]

Using shear flow it is possible to stretch and untwist colloidal ribbons. After cessation of the flow we observe how the twisted ribbons relax towards their equilibrium state.