Using time-dependent interactions to control self-assembly of soft matter

In self-assembly processes, novel materials or even functional devices can spontaneously build themselves up from simple components. For example, ordered structures of tiny particles and nano-wires can interact with light and electricity in unusual ways: self-assembly of these structures offers a new route to cheap and efficient organic solar cells, or other photonic devices. If the spontaneous assembly of functional materials seems too good to be true, one should bear in mind that many biological structures are formed by self-assembly, and that highly-ordered crystals like ice also assemble themselves during freezing. Building on these examples, artificial self-assembling systems are now becoming possible in the laboratory.

However, there are many challenges in designing and developing man-made systems that self-assemble. In particular, one often finds that interactions between particles must be tuned very accurately in order to achieve high-quality assembly. Loosely speaking, the most common results of experiments and computer simulations of self-assembly are either that the particles fail to assemble at all, or that they aggregate into a disordered clump (this is known as kinetic trapping). Effective self-assembly sits on a knife-edge between these regimes: particles must interact strongly enough to make assembly possible, but interactions must also be weak enough that disordered aggregates do not dominate the system.

Here, we propose to consider how this problem can be avoided if interactions between particles can be varied with time. This is increasingly becoming possible in experimental colloidal systems, which are ideal for self-assembly because they involve microscopic particles whose interactions are well-understood and can therefore be controlled and designed. However, we do not yet know time-dependent interactions should be exploited in these systems, in order to arrive at any specific result.

We will address this question using theory and computer simulation. We will investigate how time-dependent interactions can be exploited in self-assembly of different kinds of crystal, using colloidal particles. We will also consider colloidal gels, which sometimes interfere with crystallisation, but are also industrially-relevant materials in their own right. By considering several experimentally-relevant systems and a range of protocols for controlling interactions, we aim to arrive at general "design rules" for these self-assembly processes, showing how time-dependent interactions may be used to control and optimise the production of different kinds of ordered state.

Self-assembly processes offer an exciting opportunity for simple and efficient synthesis of materials and devices with nano-scale precision. These might be photonically-active materials for use in optical devices, or closed molecular capsules that deliver pharmaceutical drug molecules to targeted tissues in the body. However, before self-assembly can be harnessed in this way, we require clearer and more applicable theoretical guidelines as to how the assembly processes take place, and how their efficiency can be maximised.

The aim of this proposal is to develop such guidelines, aiming to exploit time-dependent particle interactions during assembly. We expect quite generally that time-dependent interactions should offer a route to improving assembly rates and the quality of the assembled product. Here we aim to achieve this for a small number of model systems, for which our theoretical picture can be tested directly in experiments. We emphasise that for the colloidal systems that we will consider, experiments can be used to observe and characterise non-equilibrium processes as they proceed, and direct comparison between theory and experiment is possible. This has been observed in the past for phase transitions and interfacial phenomena: here we will exploit and extend these methods to study self-assembly with time-dependent interactions.

The main route towards application of these results is to proceed via experiments within academia. If the usefulness of our results can be proven in these experimental model systems then we can seek to apply them in related systems with possible industrial relevance. Hence, our contacts with leading experimental and theoretical researchers will be valuable in identifying future applications of the insights we will gain from this proposal. For example, the Molecular Foundry in Berkeley (USA), is at the forefront of developing nano-scale self-assembled systems, and our ongoing collaboration with Dr Whitelam there continues to stimulate new ideas, both for developing theories of assembly and for application to practical systems. In this proposal, we also discuss the controlled assembly of disordered systems such as colloidal gels, which are already of enormous value in industry. For example, our collaborator Dr Royall (Bristol) has an ongoing collaboration with Bayer CropScience as to the controlled synthesis of gel states in colloidal systems - the results of the research proposed here would have potential relevance for that industrial collaboration.

Finally, we emphasise that the postdoc employed by this project will develop their expertise, including the mathematical, computational and logical reasoning skills required for research in statistical mechanics. These skills are extremely valuable outside academia. For example, recent PhD students and postdocs in the Condensed Matter Theory group at Bath have gone on to work in climate modelling (Met Office), analysis of financial risk (Barclays and Barclays Capital), effectiveness of bibliometric analyses (Higher Education Funding Council for England), and statistical modelling of traffic and public transport networks (Transport Research Laboratory). These career paths all exploit the postgraduate and postdoctoral training and skills development that come from studies of theoretical physics.