Abstract
Laser Speckle Rheology (LSR) is an optical technique to evaluate the viscoelastic properties by analyzing the temporal fluctuations of backscattered speckle patterns. Variations of optical absorption and reduced scattering coefficients further modulate speckle fluctuations, posing a critical challenge for quantitative evaluation of viscoelasticity. We compare and contrast two different approaches applicable for correcting and isolating the collective influence of absorption and scattering, to accurately measure mechanical properties. Our results indicate that the numerical approach of Monte-Carlo ray tracing (MCRT) reliably compensates for any arbitrary optical variations. When scattering dominates absorption, yet absorption is non-negligible, diffusing wave spectroscopy (DWS) formalisms perform similar to MCRT, superseding other analytical compensation approaches such as Telegrapher equation. The computational convenience of DWS greatly simplifies the extraction of viscoelastic properties from LSR measurements in a number of chemical, industrial, and biomedical applications.
Highlights
Laser Speckle Rheology (LSR), an optical approach for the non-contact evaluation of the viscoelastic properties of the materials, has a number of industrial, chemical and biomedical applications related to material sciences, polymer engineering, food sciences, and clinical diagnosis [1,2,3,4,5,6,7,8,9,10,11]
Our goal in this study is to investigate the influence of absorption and scattering on the accuracy of LSR measurements
The inverse is not true, and the g2(t) curve cannot be directly inferred from speckle contrast measurements. Findings of these past studies are not directly applicable to the problem of assessing viscoelasticity from speckle fluctuations encountered in LSR
Summary
Laser Speckle Rheology (LSR), an optical approach for the non-contact evaluation of the viscoelastic properties of the materials, has a number of industrial, chemical and biomedical applications related to material sciences, polymer engineering, food sciences, and clinical diagnosis [1,2,3,4,5,6,7,8,9,10,11]. The capability of LSR for measuring this quantity in a non-destructive/non-contact manner from the speckle fluctuations is appealing, in applications where mechanical manipulation may alter the sample properties and in cases where only small sample volumes are available for measurement [1,2,3,4,5,6,7,8,9,10,11]. In LSR, a small volume of the sample is illuminated by a mono-chromatic laser source and a high-speed CMOS camera is used to capture the temporal fluctuations of back-scattered speckle patterns, induced by Brownian displacements of scattering particles [1, 6,7,8,9,10,11,12,13]. The generalized Stokes-Einstein relation (GSER) has been previously established to relate the MSD of scattering particles to the bulk viscoelastic modulus, G*(ω), of the medium [11, 14,15,16,17,18,19]
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