COLLOIDAL MONOLAYERS
In analogy with atomic materials, spherical colloidal particles with repulsive inter-particle potential self-assemble into complex polycrystals. These structure are usually littered with structural defects (e.g., vacancies or dislocation) which, in the extreme case of highly polydisperse suspensions (e.g. binary mixtures), lead to the formation of disordered glassy structures. Bulk measurements provide only a partial understanding of the mechanical properties of these heterogeneous systems. On the other hand, two-dimensional structures (colloidal monolayers) offer the possibility to image the behavior of individual particles.
(Left) Polycrystalline colloidal monolayer. (Right) Glassy colloidal monolayer.
We prepare colloidal monolayers by forcing the confinement of particles onto a single plane (e.g. by sedimentation on a substrate or by adsorption to an interface between two fluid phases) and study their mechanical response to external and internal stresses at the single-particle level. Local deformations can be for instance achieved by compressing the system, applying shear flows (steady or oscillatory), generating local shocks, displacing individual particles (optically or magnetically) or doping the structure with self-propelling colloids.
Grain boundaries in a colloidal polycrystal under oscillatory shear. The colors indicate the orientation of the crystalline domains.
Annihilation of point defects under oscillatory shear.
Microscopic elastic and plastic response to an oscillatory deformation.
Strain wave propagation along a colloidal crystal, after pulsed laser ablation of a gold-coated particle
Light scattered by a monolayer of core-shell microparticles during compression