A conveyor belt experimental setup to study the internal dynamics of granular avalanches

This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields’ interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche’s leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt’s results showed the potential for further theoretical developments in rheology and segregation-coupled models. Graphic Abstract

Refractive-index matching enhanced polarization-sensitive optical coherence tomography quantification in human brain tissue

The importance of polarization-sensitive optical coherence tomography (PS-OCT) has been increasingly recognized in human brain imaging. Despite the recent progress of PS-OCT in revealing white matter architecture and orientation, quantification of fine-scale fiber tracts in the human brain cortex has been a challenging problem, due to a low birefringence in the gray matter. In this study, we investigated the effect of refractive index matching by 2,2'-thiodiethanol (TDE) immersion on the improvement of PS-OCT measurements in ex vivo human brain tissue. We obtain the cortical fiber orientation maps in the gray matter, which reveals the radial fibers in the gyrus, the U-fibers along the sulcus, as well as distinct layers of fiber axes exhibiting laminar organization. Further analysis shows that index matching reduces the noise in axis orientation measurements by 56% and 39%, in white and gray matter, respectively. Index matching also enables precise measurements of apparent birefringence, which was underestimated in the white matter by 82% but overestimated in the gray matter by 16% prior to TDE immersion. Mathematical simulations show that the improvements are primarily attributed to the reduction in the tissue scattering coefficient, leading to an enhanced signal-to-noise ratio in deeper tissue regions, which could not be achieved by conventional noise reduction methods.

Two-photon excitation fluorescent spectral and decay properties of retrograde neuronal tracer Fluoro-Gold

Fluoro-Gold is a fluorescent neuronal tracer suitable for targeted deep imaging of the nervous system. Widefield fluorescence microscopy enables visualization of Fluoro-Gold, but lacks depth discrimination. Though scanning laser confocal microscopy yields volumetric data, imaging depth is limited, and optimal single-photon excitation of Fluoro-Gold requires an unconventional ultraviolet excitation line. Two-photon excitation microscopy employs ultrafast pulsed infrared lasers to image fluorophores at high-resolution at unparalleled depths in opaque tissue. Deep imaging of Fluoro-Gold-labeled neurons carries potential to advance understanding of the central and peripheral nervous systems, yet its two-photon spectral and temporal properties remain uncharacterized. Herein, we report the two-photon excitation spectrum of Fluoro-Gold between 720 and 990 nm, and its fluorescence decay rate in aqueous solution and murine brainstem tissue. We demonstrate unprecedented imaging depth of whole-mounted murine brainstem via two-photon excitation microscopy of Fluoro-Gold labeled facial motor nuclei. Optimal two-photon excitation of Fluoro-Gold within microscope tuning range occurred at 720 nm, while maximum lifetime contrast was observed at 760 nm with mean fluorescence lifetime of 1.4 ns. Whole-mount brainstem explants were readily imaged to depths in excess of 450 µm via immersion in refractive-index matching solution.

FACT For Fast And Three-Dimensional Imaging of Intact Tissues

Free of Acrylamide Sodium Fast Free-of-Acrylamide Clearing Tissue (FACT) is a developed technique using no acrylamide for clearing tissues. As the lipid removal normally is a harmful process and it causes loss of biological molecules such as proteins and on the other hand is crucial for transparency and efficient antibody staining throughout the whole tissue especially for microscopy and imaging, the FACT technique is suitable since it makes chemical bonding of membrane and intracellular proteins with the extracellular matrix creating a massive three-dimensional (3D) matrix and structural support to fortify the tissue during processing. Compared to other acrylamide-based techniques, FACT requires less labor, toxic, and harmful chemicals. Here we describe protocols encompassing every angle and dimension of the FACT protocol for antibody staining and imaging of whole-cleared tissues while preserving the structure and increasing the image quality. The entire protocol includes tissue perfusion, fixation, clearing, antibody staining, Refractive Index Matching (RIM), microscopy, and imaging; this timing varies due to the size, weight, different kind of tissues and the type of immunostaining. This technique has been favorably performed on different types of tissues for molecular interrogation analysis of large tissues.

Velocity measurements of dilute suspensions over and through various porous media models

This study is focused on the motion of a dilute suspension containing rigid, spherical, non-Brownian, noncolloidal particles flowing over and through porous media models. The flow is confined to very low Reynolds numbers. To examine the velocity distribution particle image velocimetry (PIV) was applied in conjunction with refractive index matching (RIM) techniques. This study is the first of its kind analyzing the interaction between two common engineering systems: suspension fluid and porous media.

Combining high-speed planar PIV and motion tracking of a flexible cylinder in cross-flow

Most modeling studies investigating the flow dynamics in vegetation canopies are limited to rigid models as proxies for vegetation elements. However, most canopies embody some degree of structural flexibility, resulting in aeroelastic mechanisms coupling the motion of the vegetation with the surrounding flow. Studies addressing flexible canopies typically quantify either the flow or the plant motion independently, thus missing the instantaneous coupling between turbulent stresses and structural deformations. Few experiments have been devoted to measuring both quantities simultaneously. Okamoto and Nezu (2009) utilized a combined PIV-PTV technique to capture both flow and canopy motion. However, only the motion of the stem tips was captured, as opposed to the deformation of the entire stem. Py et al. (2006) employed digital image correlation (DIC) to quantify the motion of crop canopies using in-field images. However, the wind itself was not measured across the domain. The present work presents an experimental technique that can be utilized to study the flow–structure interaction in flexible canopies, and that could be extended to other flexible and/or moving objects. High-speed PIV data of the flow surrounding an idealized canopy element, consisting of a flexible cylinder, together with the corresponding displacement field throughout the cylinder were simultaneously obtained combining fluorescent imaging and refractive index matching (RIM).