Colloidal Monolayers in Periodic Laser Fields

• Applicant: Professor Dr. Stefan U. Egelhaaf (†)
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 459399860

Soft matter systems are frequently encountered in everyday life. Many cleaning and cosmetic products, glue, nappies, food stuff as well as natural and biological materials are considered soft matter. The main characteristic of soft matter is its susceptibility to external perturbations, such as mechanical, electrical or magnetic forces. Thus, soft matter systems are soft, as suggested by their name.Within this project we investigate the response of a colloidal system to an external potential. Colloidal systems are part of soft matter and consist of particles with a size between a few nanometres and a few micrometres that are dispersed in a medium. We use spherical particles that are arranged in one layer and thus represent a quasi-two-dimensional system. They are exposed to a sinusoidal light pattern that imposes a periodic potential on the particles, where the wavelength of the potential is of the order of the particle size. Exposing colloidal particles to a sinusoidal potential appears as a very fundamental situation. Surprisingly, however, theoretical models describing this situation are rare. Even very basic questions, like the particle arrangement and dynamics in such a potential, have been virtually untouched. To make progress, we combine experiments, simulations and theory. A theoretical model will be developed and its predictions compared with simulation and experimental results. The experiments furthermore are used to guide and inspire the development of the theoretical description.Optical microscopy will be used to follow the particles. This will yield systematic and quantitative information down to the level of individual particles and hence allow us to determine any parameter that is derived theoretically. Different theoretical approaches will be applied, including density-functional theory, integral-equation theory and mode-coupling theory. We aim at a determination of the arrangement and dynamics of particles exposed to a sinusoidal potential. The arrangement will be quantified in terms of density profiles and structure factors, while the dynamics will be characterized by the mean-square displacement, diffusion coefficient and intermediate scattering function. This information will also be used to construct the glass-transition lines in the state diagram. We anticipate a non-monotonic behaviour of the particle arrangement and dynamics as the periodicity and amplitude of the potential is varied.By studying colloidal particles exposed to a modulated potential, we push matter to extreme states. This allows us on one hand to probe material properties and gain a deeper understanding of the underlying processes and on the other hand to prepare and characterize materials with new properties.

• DFG Programme: Research Grants
• International Connection: Austria
• Cooperation Partner: Professor Dr. Thomas Franosch