I’ve never seen the Icarus story as a lesson about the limitations of humans. I see it as a lesson about the limitations of wax as an adhesive.
The properties of materials, like metals or plastics, are hard to explain when considering only their constituent particles (atoms and molecules): a purely reductionist approach fails. Instead, we must consider the complex interplay between all components: properties emerge from the collective behaviour of the system. Such emergent behaviour is ubiquitous in the natural world: from consciousness to protein folding, these behaviours are hard to explain even with a near-perfect understanding of the underlying principles and components. The process of self-assembly is a particularly interesting element of emergence, and the main topic of this thesis; relatively simple building blocks can assemble into complex, counterintuitive structures. Colloidal particles are a prime example of self-assembling building blocks: they are simple, well-understood spherical blobs, but together, a very wide range of unexpected behaviours emerge, assembling into a wide range of materials, from small, self-limiting clusters to larger superstructures, and from crystalline lattices to disordered gels.
In the past few years, significant advancements in colloidal synthesis have given us more synthetic control than ever over the composition and geometry of colloidal particles. Simulations and theory have shown that particles with specific shape and valency can be used to assemble highly controllable functional architectures that have specific, tunable material properties. A particularly exciting particle design is the so-called patchy particle, a particle decorated with patches of specific surface chemistry, with well-defined symmetry, allowing the reproduction of the specificity and geometry of atom-like valence bonds. Despite promising in silica studies, experimental realization of complex structures using patchy particles has remained illusive, and promises remain largely unfulfilled. In this thesis, we take steps into this largely unexplored territory using a system of well-defined patchy particles with highly specific patch-to-patch attraction using the critical Casimir force, a solvent-mediated force that can easily be tuned with temperature. We assemble this system into a wide variety of colloidal structures, varying from small clusters to phase-spanning networks. Using optical microscopy, we study the assembly, dynamics, and other properties of the structures by tracking particles and their bonds.
We start small: in Chapter 3, we assemble small colloidal clusters with the same bonding geometry as alkanes. These colloidal molecules, assembled from di- and tetrameric patchy particles undergo the same chemical transformations as their atomic counterparts. Specifically colloidal cyclopentane shows interesting behaviour. By direct observation via optical microscopy, we reveal that the small colloidal cluster undergoes transitions between chair and twist conformations, just like in atomic cyclopentane. We elucidate the interplay of bond bending strain and entropy in the molecular transition states and ring-opening reactions. These results open the door to investigate complex molecular kinetics and molecular reactions in the high-temperature classical limit, in which the colloidal analogue becomes a good model.
We describe how we grow a similar patchy particle system into larger architectures in Chapter 4. One of the patches of tetravalent particles is bonded to the sample wall, leaving the three other patches free to bind other particles in-plane. This leads to the assembly of colloidal graphene, a honeycomb lattice of patchy particles. Direct observations of the growth, defects, and healing of the lattice grants us insight into what may occur during the growth process of atomic graphene, normally hidden due to the extreme conditions in which it takes place. These direct observations reveal that the origins of the most common defects lie in the early stages of graphene assembly, where pentagons are kinetically favoured over the equilibrium hexagons of the honeycomb lattice, subsequently stabilized during further growth.
In Chapter 5, we dive deeper into the system of pseudo-trivalent patchy particles confined to a plane and map the full phase diagram of the system. Apart from the colloidal graphene described comprehensively in Chapter 4, under certain conditions we observe the spontaneous formation of a triangular lattice and amorphous network. We investigate these unexpected condensed phases, revealing their shared structural motifs. Combining results from simulations and experiment, we elucidate the origin of the three condensed phases and construct the phase diagram of the system. This chapter illustrates the rich phase behaviour a relatively simple patchy particle system can display.
Finally, Chapter 6 treats an unordered patchy particle superstructure: networks constructed from a mix of di- and pseudo-trivalent patchy particles. This network is a so-called equilibrium gel, and has some very counterintuitive properties: while the properties of a ‘regular’ colloidal gel strongly depend on the conditions of its formation, the history of an equilibrium network does not influence its eventual properties. On top of that, we can use a corrected Flory-Stockmayer theory to accurately describe and predict the behaviour of the system as a function of the normalized number of bonds.
The simple bonding geometry of carbon atoms is at the basis of materials ranging from wood to diamonds. In this thesis, we have taken the first steps on a similar path: we have experimentally explored the assembly of patchy colloids, and revealed how these precisely controllable building blocks can be used to assemble into a range of different structures. The continuous improvement in synthesis of patchy particles and their increasingly complex assembly shows that our control over microscopic assembly is ever-increasing. This exploratory work should be seen as the basis for the design of more advanced future smart materials, with precisely tunable mechanical, electronic and optical properties. Future ‘colloidal architects’ may have abilities approaching that of nature: with excellent control over the building blocks of a material comes excellent control over the macroscopic material properties.
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