Experimental determination of order in non-equilibrium solids using colloidalgels

Abstract

The idea of quantifying order in disordered systems has been introduced recently byTorquato and co-workers (2000 Phys. Rev. E 62 993–1001). We are interested in theapplication of this idea to measure structure in non-equilibrium systems. Here we focus ongels, using as a model system colloidal gels formed from hard spheres with polymer addedto the systems to induce a controlled, weak attraction. To describe the structure of the gelswe use real space imaging via confocal microscopy to obtain the full three-dimensionalstructure. We measure experimentally both translational order and bond angle correlations,defining a new (refined) translational order parameter that is sensitive to long range orderin these non-random packings. This metric is also sensitive to anisotropy, which should beimportant in the many physical situations where an external force is present. The bondangle distribution shows coordinated organization. To give a clearer physical picture forgels, we compare the experimental data to computer generated hard sphere systems.