Viscoelastic properties of semiflexible filamentous bacteriophage fd.

The cytoskeletal protein filament F-actin has been treated in a number of recent studies as a model physical system for semiflexible filaments. In this work, we studied the viscoelastic properties of entangled solutions of the filamentous bacteriophage fd as an alternative to F-actin with similar physical parameters. We present both microrheometric and macrorheometric measurements of the viscoelastic storage and loss moduli, G'(f ) and G"(f ), respectively, in a frequency range 0.01<f<4 Hz, for fd solutions in the concentration range 5<c<15 mg/ml. The onset of a narrow and slanted plateaulike region of G'(f ) is located at around 2 Hz. The variation of the plateau modulus with concentration obeys a power law G(')(N) approximately c(1.4+/-0.3), similar to that found for entangled solutions of F-actin. In the low-frequency regime, the frequency dependence of the viscoelastic moduli can be described by power laws G'(f ) approximately f(0.9-1.2) and G"(f ) approximately f(0.7-0.9), which deviate significantly from the simple theoretical predictions of G'(f ) approximately f(2) and G"(f ) approximately f(1). The latter behavior cannot yet be understood within the framework of current theories of semiflexible filament networks. For the dynamic viscosity at the low shear rate limit, a concentration dependence of eta(0) approximately c(2.6) was found. Finally, a linear scaling of the terminal relaxation time with concentration, tau(d) approximately c, was observed.