Refractive index matched scanning and detection of soft particles

Abstract

We describe here how to apply the three-dimensional imaging technique of refractive index matched scanning to hydrogel spheres. Hydrogels are water based materials with a low refractive index, which allows for index matching with water-based solvent mixtures. We discuss here various experimental techniques required to handle specifically hydrogel spheres as opposed to other transparent materials. The deformability of hydrogel spheres makes their identification in three-dimensional images non-trivial. We will also discuss numerical techniques that can be used in general to detect contacting, non-spherical particles in a three-dimensional image. The experimental and numerical techniques presented here give experimental access to the stress tensor of a packing of deformed particles.

Silo outflow of soft frictionless spheres

“…Soft monodisperse hydrogel spheres [24,25] are incompressible, but deformable by moderate pressures so that they can adopt their shapes to local static conditions in the container. This changes the pressure profile and the discharge characteristic, as well as the flow field inside the container, and leads to a fill-height dependent outflow.…”

Preface: Focus on imaging methods in granular physics

“…Where the interstitial liquid can be chosen freely, index matching provides a means to acquire 3D data. 34 Alternatively, it is possible to embrace the multiple scattering effects and use coherent light to gather information from the resulting speckle pattern. 35 Finally, we can abandon the range of visible light entirely and instead use penetrating radiation to obtain the data from the bulk of the sample.…”

“…40 In addition, there are a number of post-processing methods to detect the location of a particle surface, for which the shape does not have to be known a priori. 34,51 After the extraction of detailed shape information about the location of the outer boundary of each particle, further analysis of the experimental data becomes possible. Common techniques include examining nearest neighbor distributions, 40 particle tracking and velocimetry (see Sec.…”

“…Refractive-index matching between fluid and particles allows the study of deeper layers in granular matter, a technique discussed in one of the articles of the Focus Issue. 34 Other techniques for tracking within the bulk include Positron Emission Particle Tracking (PEPT), 38 microwave radar tracking, 37 X-ray radiography, 54 or Magnetic Resonance Imaging (MRI). 39 Tracer particles (single radioactively labeled in the case of PEPT, high dielectric constant for radar tracking, steel spheres for X-ray radiography, or NMR-active nuclei for MRI) are then embedded inside the material (or spin-labeled in MRI) and the system is imaged while subjected to some excitation or loading.…”

“…66,78,79 In all cases, the central challenge is to measure the size of the contacts, which proceeds using similar techniques to those described in Section II. As described in a Focus Issue article, 34 such measurements are now possible while simultaneously performing mechanical tests under realistic strains. Due to the experimental and computational complexity of such studies, we present an overview of (partly published) experimental and computational methods used in the study of packings of hydrogel spheres.…”