Project B8 (N): Hydrodynamic Simulation of Passive and Active Janus Particles

Janus particles are colloidal particles whose surface has been modified differently in different locations, creating so-called patches. The patches are designed in a way to generate directional interactions between the Janus particles. Janus particles, therefore, often self-assemble into ordered structures, commonly referred to as lattices or crystal structures, even though the system still is a colloidal solution. By variation of the chemical nature, size and location of the patches, a rich set of lattice structures is accessible. In our work so far, we have focused on triblock Janus particles, which carry attractive van-der-Waals patches on the poles and repulsive electrostatic charges around the equator. We developed a detailed dissipative-particle dynamics model for them, which includes surface chemistry and explicit solvent molecules. With this model and our newly devised adaptive metadynamics method, we could clarify their self-assembly into two-dimensional ordered lattices, when they deposit onto a solid substrate, as well as determine the free-energy barriers and molecular mechanisms for transitions between all phases. All this work has, so far, been carried out outside the SFB TRR146 in the framework of DFG Research Grant MU1412/25.

We would like this project to join the SFB TRR146 as project B8. Not only does it fall into the scope of the SFB TRR146, since the dominating component is the development of multiscale methods for Janus colloids. It has also recently become apparent that there are connections to two other projects (A9 and C7), which can be usefully exploited. The scientific scope of B8 is, therefore, widened compared with the original MU1412/25 proposal. On the one hand, in FP3, the research is going to be extended toward the formation of three-dimensional structures for which there are much fewer experimental reference points. This was planned, and is already taking place. In addition, B8 will also encompass active colloids. These are Janus particle, which have some means of propulsion, usually by carrying one catalytic patch on their surface, where suitable fuel molecules react and release energy. As a result, they undergo diffusiophoretic motion. They are currently intensively investigated, including by molecular simulation. Most simulations have so far been using only minimal models: single-site models for the particles, no solvent molecules, no hydrodynamic interactions, and often no solid substrate. One advantage of our level of modelling over minimal models is to include an explicit solvent and thus allow the treatment of hydrodynamics. Taking into account the surface chemistry of Janus particles (patches as well as surface charges), and the chemical reaction of solute particles at the surface of the catalytic patch, is another advantage of our model. It allows us to target the effect of activity on the self-assembly into ordered structures and to compare with the wealth of results which we have gathered for the ordering of passive particles. This includes the nucleation and growth mechanisms, and the dynamics of self-assembly in three dimensions. Explicit inclusion of a solid wall in the model is yet another advantage. It accounts for the perturbation by the wall of the hydrodynamic flow and the distribution of fuel and product species in the solution. Gradients of the composition may induce fluid flow parallel to the wall (diffusioosmosis). Both hydrodynamics and diffusioosmosis may couple to the diffusiophoretic motion of the active particles. There is ample experience with such processes in projects C7 and A9. We will cooperate to join their expertise on the processes with our more detailed and more realistic models for describing Janus colloids. This will be used to study the self-assembly mechanisms of active Janus particles in three dimensions in the presence of solven (with C7), as well as the coupling of hydrodynamics and diffusioosmosis with the diffusiophoretic motion of active Janus particles near walls (with A9.). All three lines of research share the need for multiscale models with correct dynamics and explicit hydrodynamic interactions.

Water Uptake by Gecko beta-Keratin and the Influence of the Relative Humidity on its Mechanical and Volumetric Properties
M. Khani, T. Materzok, H. Eslami, S. Gorb, and F. Müller-Plathe
J. R. Soc. Interface 19, 20220372 (2022)
see publication

How Ethanolic Disinfectants Disintegrate Coronavirus Model Membranes: A Dissipative-Particle-Dynamics Simulation Study
T. Zhou, Z. Wu, S. Das, H. Eslami, and F. Müller-Plathe
J. Chem. Theor. Comput. 18, 2597–2615 (2022)
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Self-Assembly of Model Triblock Janus Colloidal Particles in Two Dimensions
K. Bahri, H. Eslami, and Florian Müller-Plathe
J. Chem. Theor. Comput. 18, 1870–1882 (2022)
see publication