Glasses exhibit liquid structure, but solid properties. They are formed from the liquid by rapid quench. The rapid quench prevents the equilibrium crystal phase to form, and freezes the liquid structure. The resulting arrested liquid has relaxation times many orders of magnitude larger than the liquid, and thus cannot rearrange easily, thereby appearing like a solid. Because of its non-equilibrium nature, the glass “ages”, i.e. its structure and properties change over time. While glasses have been used by mankind ever since, and have been studied extensively in the past decades, major questions remain: How can we understand the enormous rise of the viscosity at the glass transition, how can we understand aging? Is there a generic description of these out-of-equilibrium systems? How can we describe the mechanical response and relaxation under applied stress? To address these questions, we use colloidal glasses, composed of small micrometer-size particles suspended in a solvent. Unlike atoms, these particles can be imaged with conventional optical microscopy, but are still small enough to exhibit significant thermal motion. Instead of temperature for atomic systems, the relevant parameter for these particles is their volume fraction, i.e. the fraction of total volume occupied by the particles. When about 50% of the volume is occupied by particles, the particles crystallize spontaneously to increase their entropy, thereby reducing their free energy. Similar to atoms, this crystallization can be suppressed by rapid quench that arrests the disordered fluid state.
The unique advantage of this model system is that particle-scale imaging is possible using confocal microscopy, allowing the individual particles to be imaged in three dimensions, and their motion to be tracked precisely in time. This provides the key advantage over their molecular counterparts: the dynamics and structure of these colloidal glasses can be studied directly in three dimensions and real time.
We study how glasses form, age and respond to external stress. We use the individual particle trajectories to visualize particle rearrangements and the internal distribution of strain (see figures below) in a quiescent and flowing glasses. Dynamic correlation functions then allow us to explore how the particles move cooperatively in the dense packing. These dynamic correlations are central to understand hallmarks of glasses: as density increases, the particles move more cooperatively, resulting in long time scales of relaxation. We also analyze the vibration spectrum of the densely packed particles, allowing deep insight into their solid properties, and the relation between vibrations and liquid rearrangements. We have also measured the free energy directly: in hard-sphere systems, the free energy is determind by geometry alone, so that meaurement of the three-dimensonal structure provides a direct route to the free energy of the material..Doing this under applied shear allowed measurement of the nonequilibrium free energy under shear, which provided insight into the transient deformation of amorphous materials