Prof. Frank Scheffold "The fascination of randomly packed spheres: from dense emulsions to photonic materials design"

The random packing of spherical objects has fascinated researchers for centuries. When hard or soft spheres are quenched above a sphere volume fraction of about sixty per cent, a supercooled fluid of spheres enters an amorphous solid phase. The state of matter of this phase is peculiar since it has both fluid- and solid-like attributes: it has a liquid-like structure, yet mechanically responds to an applied stress as a solid. This called "jamming” phenomenon is not only of fundamental interest but also of prime importance in materials science since it governs the elasticity and flow of many soft materials such as emulsions, particle dispersions, polymeric thickeners as well as granular materials (sand, powders, etc.).
After a general introduction I will discuss experiments and concepts derived from jammed assemblies of microscale hard and soft spheres. In the first set of studies we investigate deformable particles suspended in water either consisting of oil droplets or microgel-particles that are driven into a jammed state [1,2]. We probe both translational and rotational motion using confocal microscopy, spatially resolved light scattering and optical birefringence. Our results show clear evidence of spatial and temporal heterogeneities, both thermally activated and driven by applying external fields. In a second set of experiments we used jammed assemblies of spheres as templates for photonic structures [3,4]. We follow a protocol based on mapping the hyperuniform point coordinates of the spheres into tessellations for disordered photonic materials design.  Using direct laser writing in a polymer photoresist we succeeded in fabricating such structures with features on the microscale.

[1] Magnetic orientation of soft particles in a jammed solid, C. Dagallier, F. Cardinaux, H. Dietsch,and F.Scheffold, Soft Matter 8, 4067-4071 (2012) – Cover article

[2] Linear and nonlinear rheology of dense emulsions across the glass and jamming regimes, Frank Scheffold, Fréderic Cardinaux and Thomas G. Mason, Journal of Physics: Condensed Matter, Fast Track Communication, to appear (2013)

[3] Fabrication of mesoscale polymeric templates for three-dimensional disordered photonic materials, Jakub Haberko and Frank Scheffold, Optics Express 21, Issue 1, pp. 1057-1065 (2013) 

[4] Silicon Hyperuniform Disordered Photonic Materials with a Pronounced Gap in the Shortwave Infrared. Nicolas Muller, Jakub Haberko, Catherine Marichy and Frank Scheffold, Advanced Optical Materials, to appear