Non-Invasive Full Rheological Characterization via Combined Speckle and Brillouin Microscopy

Rheology serves to measure the deformation and flow of materials. Its associated quantities, for example, the Young's modulus, shear modulus, bulk modulus, or longitudinal modulus, are important in the biomedical field, in particular for soft materials, to characterize the response of materials to external force. Usually, mechanical probes, in particular rheometers or atomic force microscopy, are used to characterize these quantities. In the last decade, optical measurements have been derived to obtain these quantities even in small sample volumes. However, usually only one quantity is evaluated using optical techniques, such as Brillouin microscopy, which does not allow a full rheological characterization. The latter requires measuring at least two quantities, which allows for calculating all further rheological properties. In this paper, we aim to close this gap by combining two optical rheology methods, Brillouin microscopy to measure the longitudinal modulus and Laser Speckle Rheology for the shear modulus. We built an optical setup that allows the non-contact and hence non-destructive and non-invasive measurement of both quantities simultaneously in the same sample using a 780 nm, narrow linewidth (similar to 50 kHz) laser system. We evaluate our approach using defined samples of glycerol and polydimethylsiloxane and we demonstrate image acquisition using the combined setup. We also investigate porcine corneae, as biological samples, and demonstrate direct measurement of longitudinal modulus and shear modulus and calculation of Young's modulus, bulk modulus, and the Poission ratio, which are all in good agreement with published quantities. In the future, our approach allows for full characterization of the rheology of biological specimens.