It might be tempting to regard colloidal crystals as one of the simplest states of matter – in the cartoon picture of a colloidal crystal, a set of classically interacting spheres are beautifully arranged on an ordered lattice and simply fluctuate around their respective sites. Reality, however, is in no way bound to this idealized picture, and colloidal crystals exhibit an impressively rich variety of intriguing structural and dynamical phenomena, depending on the constituent particles and the crystallization conditions.

Structurally, for example, (hard) polyhedra with different shapes [1 - 4], asymmetrically charged particles [5, 6], and polydisperse mixtures of hard or charged spheres [7, 8] all can be persuaded to form a stunning variety of different crystal lattices as their microscopic features are tuned. This includes not only traditional crystals, but also plastic crystals [4], substitutional and interstitial solid solutions [9, 10], and “cluster crystals” [11, 12], where multiple particles occupy the same lattice site. Additionally, several of these have even been shown to form quasicrystals: exotic aperiodic structures with symmetries forbidden by the traditional rules of crystallography [13 - 20]. These structures can be further tuned by causing them to self-assemble in complex environments, e.g. under the influence of confinement or at a curved interface [21 - 23]. To further complicate this picture, all equilibrium crystals have a finite defect concentration of point defects, and while in some cases defects in colloidal crystals are fairly standard and localized, such as in hard spheres [24, 25], colloidal crystal have also been found to exhibit extremely high defect concentrations – to the point where one in twenty lattice sites is empty [26], or in colloid-polymer crystals, small polymers accumulate at interstitial sites [27].

On the dynamical side, despite their solid nature, some colloidal crystals permit almost fluid-like self-diffusion, due to e.g. high concentrations of exotically structured defects [26, 28], or the presence of highly mobile interstitial dopants [9, 29, 30]. Alternatively, mobility in crystals can be achieved via the fluctuations of grain boundaries [31- 33], the inclusion of active dopants [34 - 36], or perturbations from external fields. Intriguingly, topologically constrained transport, known from electronic transport, can be obtained using magnetic fields [37 - 39]. Varying quenching speeds, polydispersity and other parameters, the competition between mechanisms leading to crystallization versus vitrification can be followed in situ [7,40-46].

Underlying the rich crystal behaviour of colloidal systems is the important role played by entropy in systems where the interactions are weak compared to the thermal energy scale, implying that colloidal phases are highly susceptible to deformations and defects. Unravelling the physics of these complex colloidal crystals requires a coordinated research effort combining experiments, simulations, and theory. This workshop will act as a platform to bring together scientists from all interested communities, and will give them the opportunity to exchange ideas, knowledge, and methodologies. The main aims of this workshop include:

• To compare experimental, numerical and theoretical insights into the rules that govern the stability of exotic colloidal crystals, with a focus on the emergence of quasicrystals, defect-rich phases, and cluster crystals.
• To compare experimental, numerical and theoretical insights into the crystallization process of complex colloidal building blocks, with both a focus on how crystallization competes with glass formation and how grain boundaries evolve.
• To discuss from experimental, numerical and theoretical perspectives the interplay between geometry, defects, and complex colloidal interactions, with a focus on topological phenomena in crystals caused e.g. by curved interfaces, confinement, or edge states.