Piezoresistance in liquid suspensions: graphite in silicone elastomer

Piezoresistance is defined as a change in the conductivity or resistivity of a solid material with deformations. Phenomenological formulation of this effect involves either stresses or strains to describe the deformations. Each description introduces two piezoresistance coefficients, characterizing the response of an isotropic material. This study extends the concept of piezoresistance to liquid suspensions. Deformations are limited to small changes in the material microstructure and, therefore, induce small variations in specific resistance. As a working hypothesis, it is assumed the piezoresistance effect is determined by fluctuations in the local electric field caused by displacement of the inclusions. An experimental study is conducted with graphite micro-particles dispersed in silicone rubber. Small amplitude oscillatory shear deformations are produced by a rheometer in cone-and-plate configuration. A specially designed sensor rosette, attached to the fixed plate of a rheometer, extracts two piezoresistance coefficients. Both the stress and strain descriptions are verified against experimental data. Typically, the stress description shows better linearity of the piezoresistance response in liquid systems; the strain-based approach seems more suitable for rigorously defining experimental conditions and microscopic modelling.