Physics of Jammed Matter

Confocal studies of packing and stress

We use confocal microscopy to study the structure and mechanical properties of jammed colloids, such as emulsions and polymeric particles, in 3D. By refractive index matching and fluorescent labeling, this approach provides a window into the 3D microstructure and topology of particle packings presented in Figure [1]. The environment sensitive dye allows us to independently image the particles and the contact areas between them, from which we can also study the distribution of stresses in compressed emulsions [1]. Stress transmission through particulate matter under different packing conditions opens a wide range of questions on the viscoelastic properties of jammed matter. In a recent work, we showed that frictionless, spherical particles pack with an average of 6 contacting neighbours, in accordance with the theory of isostaticity, and devised a method for calculating the configurational entropy of the system [2]. Voronoi partitioning of our images probe the local packing density and the diversity of available configurations in randomly packed systems, shown in Figure 2. Using different particle assemblies, we can probe how the local and global properties are influenced by the particle size distribution, interaction potential between the particles and external pressure, to name but a few examples. Based on experimental evidence, our group aims to understand these complex structures using novel statistical models based on local geometric considerations. These analytical models are also tested using numerical simulations.

Figure 1: 3D Reconstruction of the confocal images of an emulsion consisting of silicone oil droplets in a water/glycerol solution.

Figure 2: `Navigation map' of the particulate packing partitions the image into first coordination shell volumes around each particle, thus probing the distribution of local volume fractions.

[1] J. Brujic, S. F. Edwards, I. Hopkinson, and H. A. Makse, Measuring the Distribution of Interdroplet Forces in a Compressed Emulsion System, Physica A. 2003 327, 201.

[2] J. Brujic, C. Song, P. Wang, C. Briscoe, G. Marty, and H. A. Makse, Measuring the coordination number and entropy of a 3D jammed emulsion packing by confocal microscopy, Phys. Rev. Lett. 2007, 98, 248001.