Self-Assembly of Janus Patchy Particles: A Dissipative Particle Dynamics Simulation Study

• Applicant: Professor Dr. Florian Müller-Plathe
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Experimental and Theoretical Physics of Polymers
• Term: from 2014 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 257844953

There is a growing interest in the self-assembly of colloidal particles toward the fabrication of nanocrystals that offer unique material properties for magnetic, photovoltaic, sensing, biomedical, and catalytic applications. Particles with orientation-dependent interactions (patchy particles, Janus particles) are of special interest, as they allow a wider variety of self-assembled structures. One major stumbling block for their use is the limited understanding of the relation between the architecture of the individual particles and the structures, into which they self-assemble. Molecular simulations are a powerful tool for elucidating the mechanisms of self-assembly. While most existing simulation studies of the self-assembly of colloidal patchy particles use simplified, implicit-solvent models, we have developed in funding period 1 a much more detailed model for triblock Janus particles in solution. The Janus particles were modeled as crosslinked polystyrene spheres, capped at the poles with alkyl patches (attractive) and in the equatorial region with negative charges (repulsive). Explicit solvent, counterions and a substrate, on whose surface the two-dimensional crystallization takes place, were included in the model. Hydrodynamic and many-body interactions were accounted for by the many-body dissipative particle dynamics method. To sample nucleation and phase transitions, a new variant of metadynamics has been developed and applied to the formation of Kagome and hexagonal lattices. In funding period 2, we will use our realistic model to further study the phase equilibria and assembly processes for triblock Janus particles. With the basic phase diagram known, we will search for alternative assembly pathways by allowing the simulations successively more freedom: In step 1, transition-path sampling will be used to find (or rule out) other assembly routes between the known phases. In step 2, bias-free simulations will explore the possible existence of other ordered structures. In both steps, we will investigate the influence of the particle architecture (size and shape of attractive and repulsive regions) on the assembled structures and the pathways between them. Simultaneously, we will explore the evolution of crystal nuclei by free, unbiased simulations. To this end, we will seed particle solutions with crystal nuclei of different sizes and shapes and observe, whether and by which mechanism they grow or shrink. Moreover, we will extend our investigations to three-dimensional self-assembly. We will follow the route established for two-dimensional assembly and proceed from biased simulations (metadynamics) to transition-path sampling and free simulations. A strong focus will be on open three-dimensional structures, i.e. any structure that is not close-packed, such as perovskite or pyrochlore lattices. We will investigate which particle architecture favours ordered open structures and what are the pathways for their assembly.

• DFG Programme: Research Grants
• International Connection: Iran
• Cooperation Partner: Professor Dr. Hossein Eslami