Rheology of complex fluids with vibrating fiber-optic sensors

We demonstrate a new optical rheometer where a fiber Bragg grating sensor (FBG) detects the periodical strain caused by the oscillations of a string tightened within a sample fluid. We show that the viscoelastic moduli of complex fluids can be obtained instantaneously from the FBG optical backreflection signal, without need of any current flow in the wire or magnetic field that could affect the sample chemistry. After a validation with known-viscosity solutions, the technique is employed to monitor the phase transition of a sol-gel compound. The results provide an insight in the early-stage gelation dynamics that cannot be obtained with traditional rheometers, and allow to clearly identify the gel point as the intersection of the viscoelastic moduli.