Refractive index matching (RIM) of liquid and semi-solid materials to acrylic glass for optically measuring the mechanics in soft granular matter

Abstract

The main context of this research is the fluid mechanical analysis of stirred chunky fruit preparations, which are typically highly loaded suspensions (ca. 50%w/w) with particles susceptible to mechanical damage. Knowledge about the transport of such particles in fluid matrices is important in natural and technical processes and can be obtained using optical measurement techniques, e.g. Particle Image Velocimetry. Matching the refractive indices of the relevant material components, a way to ensure signal reliability, is difficult for highly concentrated dispersed systems. Material properties such as plasticity and elasticity of the solid phase and the rheological behaviour of the fluid must be met simultaneously. Fluid motion across the full range of the stirred volume and the immediate surroundings of the stirrer could not be observed without successful refractive index matching of acrylic glass, stirred liquid, and suspended particles. Using the presented materials, the mechanical firmness (but not the resistance against breaking) of soft granular matter can be mimicked. The movement of gel particles in suspensions, their resulting deformation and ultimately, the inflicted damage can be observed with optical methods. The rigidity of the gels may be varied to some extent with the concentrations of the respective hydrocolloids, which, at low concentrations, have no apparent effect on the refractive index. Introducing ethanol, thickeners or other components may yield more degrees of freedom in modelling their flow behaviour.Graphic abstract