Complex Fluids

Fluctuations of the semi-flexible polymers dissolved in a nematic phase

We study the fluctuations of semi-flexible polymers dissolved in a nematic liquid crystal. A coil to rod transition of polymers with persistence length larger then 0.2 µm is observed by changing the background phase from isotropic to nematic. Using image analysis it is possible to measure the orientational distribution function.

Movie: Actin 3 [MOV - 2mb]

Fluorescently labeled actin filaments fluctuating in a “invisible” nematic background. By analyzing these fluctuations we can show that at long wavelengths, the fluctuations of the polymers are driven by tight coupling to the nematic filed that is itself fluctuating. This has been predicted theoretically by Randy Kamien and David Nelson. In addition we directly observe the existence of hairpin defects where the direction of the polymer rapidly changes by 180 degrees. Our recent results on this subject are summarized in the following paper.

Images of fluoresecently labeled biopolymers in the isotropic and nematic phase of fd virus in the left and right column respectively. The persistence length (L_p) of these polymers varies from 0.05 µm to 16 µm. Figures a to d are images of DNA (L_p=0.05 µm), neurofilaments (L_p=0.2 µm), wormlike micelles (L_p=0.5 µm) and actin (L_p= 16 µm) respectively. Part e shows a sequence of images illustrating an escape of an actin filament from a hairpin defect. Scale bar is 5 µm.

Z. Dogic, J. Zhang, A. W. C. Lau, P. Dalhaimer, H. Aranda-Espinoza, D. Disher, P. Janmey, T. C Lubensky and A. G. Yodh, “Elongation and fluctuations of semi-flexibile polymers in a nematic solvent”, submitted to Phys. Rev. Lett.