A flexible rheometer design to measure the visco-elastic response of soft solids over a wide range of frequency

We present a flexible setup for determining the rheology of visco-elastic materials which is based on the mechanical response of a magnet deposited at the surface of a slab of material and excited electromagnetically. An interferometric measurement of the magnet displacement allows one to reach an excellent accuracy over a wide range of frequency. Except for the magnet, there is no contact between the material under investigation and the apparatus. At low frequency, inertial effects are negligible so that the mechanical response, obtained through a lock-in amplifier, directly gives the material complex modulus. At high frequency, damped waves are emitted and the rheology must be extracted numerically from a theoretical model. To validate the design, the instrument was used to measure the rheology of a test polydimethylsiloxane gel which presents an almost perfect scale free response at high frequency.