Fluctuating interfaces in colloidal crystals

In an ideal crystal all atoms are located at perfectly periodically positioned lattice sites. However, in practice, crystals always contain various types of defects and typically consist of smaller crystallites, each with a different orientation. This is of considerable interest since crystal imperfections and interfaces between different crystallites in particular have important implications for the mechanical and optical properties of crystals. Understanding the nature of interfaces in crystals, their fluctuations, structure and evolution, is therefore of fundamental importance to a wide range of scientific fields and technological applications. Here, we propose to use colloidal crystals to study interfaces in crystals.Colloidal suspensions, in which particles with a diameter between roughly one nanometer and a few micrometers are dispersed in a molecular solvent, offer a unique model system to study interfaces in crystals for a number of reasons. The phase behaviour of colloidal suspensions is thermodynamically equivalent to atomic and molecular systems and includes colloidal gas, liquid and crystal phases. Furthermore, the interaction potential is tunable, which makes it possible to closely mimic atomic interaction potentials and colloidal systems are easily manipulated using external fields. Because the typical colloidal length and time scales are of the order of micrometers and seconds respectively, the structure and dynamics can be analyzed at the particle level using optical microscopy. The combination of colloidal crystals and optical microscopy thus opens up a wide range of exciting possibilities to investigate the intimate relation between interface fluctuations in crystals and phenomena like recrystallization, grain growth and crystal nucleation at the `model atomic' level. We will focus on the fluctuations of the interfaces between different crystallites. Analyzing these fluctuations directly leads to the key quantities that control interface migration, the interfacial stiffness and interface mobility. Interface migration is central to processes like grain growth, phase transformations and recrystallization. These processes are sensitively affected by the presence of impurities, which are almost always present in crystals in nature. Therefore, we will also address the effect of impurities on interface fluctuations. Finally, we will study interfaces in confinement. This is nowadays very relevant due to the proceeding miniaturization of many systems and devices in science and technology.The proposed work will lead to a better understanding of interfaces in crystals, which is essential for many applications including ceramic and photonic materials.

The proposed research will shed detailed light on the role of interface fluctuations in important phenomena like crystal growth and recrystallization using colloids as a model system. The results of this research are relevant to a wide range of industrial and technological beneficiaries, because colloids are used in many applications ranging from dairy products and pottery to advanced coatings and photonic materials. Hence, many companies are direct beneficiaries of this research. Just a few examples include Unilever (foods), Beiersdorf (personal care products), Philips (photonic materials), ICI/Akzo Nobel (coatings) and the ceramic industry. The improvement of the products and applications of these companies will have an indirect beneficial effect on the users of these products in the UK and abroad. The beneficiaries will profit in various ways from this research. The understanding of colloidal systems is generally relevant to the development of a host of products like personal care products, paints and foods. More specifically, colloidal crystals have special optical properties which make them attractive for photonic applications, such as switchable mirrors and optical filters. In addition, colloidal processing is very successful in improving the quality of ceramic products. For both photonic and ceramic applications it is of vital importance to carefully control the structure of colloidal crystals. Because interfaces play a pivotal role in crystallization, recrystallization and crystal growth, the results of this research will be of direct technological benefit to companies that develop photonic or ceramic materials. The role of impurities in colloidal crystals is also of direct interest to ceramics, as heterogeneities in ceramic materials are usually caused by impurities. This sensitively changes the electrical properties of functional ceramics. The fundamental insights into impurity drag and crystallization are therefore very beneficial for the improvement of colloidal processing of ceramic materials. The effect of confinement is also becoming increasingly important in many technological applications due to the continuing miniaturization of devices. Understanding the impact of confinement on crystallization is hugely relevant for the successful development of advanced optical materials, whose performance is extremely sensitive to the quality of the crystal. In addition to the developed technological expertise, we train highly skilled researchers in this specific area. Companies will benefit from the availability of these specialists, which will have a positive impact on their customers and on the UK economy. To establish the engagement of companies, the PI has a number of contacts with industry. An active collaboration with Beiersdorf is ongoing and MatOx has expressed its interest in the application of colloidal crystal as photonic materials. Also, his active involvement in SoftComp, a European Network of Excellence, gives the PI direct access to important multinationals like BASF and Rhodia. Dissemination to a wider public will be achieved using the active public engagement program of the University of Oxford, which for instance includes a science blog. The `Pathway to impact' described further details on what will be done to ensure that the beneficiaries have the opportunity to benefit from this research.