Colloids are common in everyday life. Some examples include whipped cream, mayonnaise, milk, butter, gelatine, jelly, muddy water, plaster, colored glass, and paper. From these examples, one comes to a conclusion about colloids. Colloids are a mixture in which one substance of microscopically dispersed insoluble particles is suspended throughout another substance.
Since the dynamics of colloidal particles have a significant role in many places, several studies have been done with the same. The vast majority of studies of condensed matter phenomena only focus on translational degrees of freedom. These include crystallization, melting, gelation and glass transition. In addition to the translational degrees of freedom, the colloidal particles also feature rotational Brownian motion which includes spherical particles that do not have an easily visualized orientation.
The Rotational Brownian motion of a suspension of spherical particles is governed by two physical effects: hydrodynamics and contact forces, including friction. Even though extensive experiments and theoretical works are made on this subject, the direct access to the rotational degrees of freedom of spherical particles, that allows access to hydrodynamic and mechanical interactions at the particle level, has been challenging to achieve. There are exceedingly few studies of rotational fluctuations in dense suspensions of spheres because of the lack of a colloidal model system that allows both the position and orientation of all the spheres in a field of view to be imaged up to arbitrarily high-volume fractions. The creation of a viable system was a great challenge to the scientific world.
A recent study by an international team including researchers from The University of Tokyo Institute of Industrial Science has created particles that can be tracked using microscopy. These particles are monodispersing and compositionally homogeneous colloidal spheres with a core-shell structure. The core and shell are both 3-trimethoxysilyl propyl methacrylate (TPM) but labeled with different dyes. These particles are mentioned as “off-center core under laser illumination” (OCULI) particles. The core is referred to as the “eye” and to the whole particle as the “body”. Then these particles are observed under full three-dimensional (3D) characterization of rotational fluctuations in monodisperse spheres using confocal laser scanning microscopy (CLSM) in arbitrarily dense systems.
With the help of this system, they were able to directly visualize hydrodynamic and contact interactions in dense suspensions of newly developed colloidal spheres. They could measure transient rotation-rotation coupling in charged colloidal crystals and observe the stick-slip rotational motion which indicates the contact friction in dense particulate materials. Thus, this method allows effective detection of solid-like contact friction at the single-particle level, induced by either mechanical forces or interparticle attraction.
The researchers believe that this system now offers a more direct route, mapping microscopic frictional response to bulk behavior, an approach that might be applied to any number of complex rheological phenomena at the interface between colloidal and granular matter.
References:
o aps.org
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