Experimental analysis of single particle deformations and rotations in colloidal and granular systems

Abstract

Confocal microscopy of fluorescent labeled particles has been used to study the dynamical and structural properties of colloidal and granular matter in real space. Localization algorithms allow for a fully automatized determination of the three dimensional positions and translational motions of all constituent (spherical) particles in the observed volume. Though, particle deformations or rotational motions were hardly addressed. Here we present preparation and image processing techniques to also extract the deformation and rotational state of the particles. The deformation analysis is worked out for particles with a hollow sphere-like fluorescence intensity distribution that are more sensitive to small deformations. In the second case of rotations we utilize the angle dependence of the light absorption of the incorporated dye molecules to prepare optically anisotropic particles in analogy to polarized fluorescence after photobleaching. Rotations of these particles are expressed as intensity fluctuations in the confocal images. In contrast to existing methods our techniques do not reduce the quality of the actual particle localization. They can help understanding complex reorganization processes in arrested states of colloidal and granular materials during aging or under external stimuli such as shear or compression.

Buckling of paramagnetic chains in soft gels

“…A home-built LSCM setup was used to observe the chain structure in the gel [43,44]. We were able to analyze a sample of thickness of about 150 µm.…”

“…Compared to normal optical microscopy, LSCM is able to observe 3D structures and it has a better resolution [42]. Compared to X-ray micro-tomography, LSCM is faster in obtaining a 3D image and easier to combine with other techniques for real-time investigation [43,44].…”

Versatile high-speed confocal microscopy using a single laser beam

“…The short dwell times are known to reduce bleaching and photo toxicity 29 . Despite the high intensity in the focus of the microscope objective (≈ 11.04mW / µm 2 ), we can image standard fluorescent samples (silica particles labelled with rhodamine, see next section) 55,56 more than 8000 times (xy-scan) without a significant amount of beaching. In earlier experiments 55,56 with the described confocal microscope and the used intensities (≈ 0.09 µW/µm 2 ) we were only able to scan the same spot around 600 times due to a dwell time of at least ≈ 5 µs.…”

“…The colloids are labelled silica particles with a mean diameter of 10 µm and standard deviation of the size distribution of 1.7 µm. Further details about the sample preparation are given in 55,56 . The colloids are dispersed in an index matching solution of NaSCN in water.…”

“…Therefore, only single beam confocal microscopes are suitable to measure dense probes. In earlier studies 55,56 the shearing of dense colloidal probes was studied. However, due to the small available scanning speeds only slow processes could be imaged.…”