Structural relaxation in the wave-vector dependence of the longitudinal rigidity modulus.

Mikolaj Pochylski

doi:10.1364/boe.10.001957

Abstract

Brillouin microscopy recently attracted much attention for being a promising tool for all-optical label-free determination of mechanical properties of biological samples. Before its widespread utilization for biomedical applications, numbers of nuances related with this technique need to be recognized. In this article, we discuss the process of structural relaxation, the phenomena not commonly addressed by the emerging bio-Brillouin community, and its effect on longitudinal rigidity modulus. Using a model aqueous polymer mixture, we show how scattering measurements performed on the same specimen using different experimental geometries can lead to different results.

Highlights

Spontaneous Brillouin scattering is a process by which light is inelasticaly scattered on propagating excitations thermally induced within the material [1,2]
The well recognized difficulties concern: 1) the actual design of the Brillouin spectrometer appropriate for fast mechanical mapping of biological material; 2) translation of the parameter directly measured in experiment to the value of mechanical parameter of interest; 3) the interrelation between the parameters being determined with Brillouin spectroscopy and the mechanical quantities obtained with other experimental techniques
In this article we discussed the influence of the structural relaxation on the value of longitudinal rigidity modulus M’ determined in Brillouin scattering experiment

Summary

Introduction

Spontaneous Brillouin scattering is a process by which light is inelasticaly scattered on propagating excitations thermally induced within the material [1,2]. The usual interpretation of the longitudinal rigidity modulus, M’, presented in many recent articles devoted to biomedical applications of Brillouin microscopy, is based on the assumption of “simple liquid”, meaning that all dynamical processes of the liquid proceed on a timescale much faster than the period of acoustic oscillation. In this scenario, the sound velocity (and so the rigidity modulus) is just a single-value parameter. We show how the existence of structural relaxation makes any attempt of using Brillouin spectroscopy/microscopy for determination of sample refractive index, basing on the measurement on different wave-vectors, generally unsafe

Brillouin scattering and relaxation in liquids

Experimental

Results and discussion

Conclusion