Ultrasonic investigation of artificial solid materials based on agar gel and marble powder

Gel mixtures are widely used in practical applications such as food products or mimicking soft tissues. Furthermore, current research on medical and industrial fields include the development of bio-compatible materials to produce artificial bones or engineered stones. In this work, we focus on restoration of rocks in cultural heritage applications. Therefore, we hereby investigate ultrasonic properties of agar gels incorporating a mineral filler in order to understand how elastic and viscous properties are tuned in soft solids composites. The samples were prepared from agar gels of 3% (w/w) concentration subjected to different proportions of marble powder used as filler (0 to 20%). The mixtures were prepared setting a homogeneous distribution of filler in the matrix. Two ultrasonic techniques were used to characterize and monitor the viscoelastic properties of the samples at different investigation scales. The mixtures were studied firstly using a 1 MHz contact transducer in pulse echo mode to measure the compression wave velocity (millimeter scale). Secondly, using Thickness Shear Mode (TSM) transducer allowing high frequency ultrasonic rheology and thus the estimation of viscoelastic parameters in the 15 – 35 MHz range (micrometer scale). The ultrasonic measurements were performed using thermostated cells at 25°C. Thus, the viscoelastic properties were monitored as function of time and mass fraction of the filler. For agar gels samples without filler (used as reference), ultrasonic properties were found in agreement with literature values and viscoelastic properties at high frequencies are reported. The composite samples were then compared to the reference in order to evaluate how are modified the elasticity (related to the compression wave velocity and shear storage modulus G’) and the viscosity (related to the shear loss modulus G”). The combination of ultrasonic techniques and multi-scale investigation approach may allow to determine the mineral filler proportion in agar gel matrix needed to adjust desired properties of the final soft solid composite.