Structure and dynamics of liquids in confinement

• Applicant: Professor Dr. Stefan U. Egelhaaf (†)
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: from 2017 to 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 383534318

• Final Report Year: 2022

Final Report Abstract

Transport in strongly confined fluids is of fundamental interest both from a basic scientific point of view as well as for various applications ranging from microfluidic devices for suspensions of colloidal particles and tempering on the microscale to friction and lubrication of thin films. For confinement lengths much larger than the particle diameter, the structure and dynamics remain essentially unperturbed. However, they drastically change once the confinement length becomes comparable to the interparticle distance. The most prominent effect is known as layering where the density profile displays strong modulations with peaks close to the walls, indicating accumulation of particles close to the walls, as well as characteristic oscillations further from the walls, indicating the close packing of particles. Furthermore, two-point correlations, such as the pair-distribution function, are significantly affected by the interplay between the local short-range packing of the particles, which in bulk is isotropic, and the confining walls, which introduce anisotropy. This competition also drastically influences the dynamics and transport coefficients, especially in dense suspensions close to dynamic arrest, referred to as the glass transition. The goal of this project was to characterize, on the single-particle level, the structural and dynamical properties of colloidal fluids confined by two walls. This question was addressed from a theoretical and simulational perspective as well as from an experimental point of view. This (German) part of the project was concerned with the experiments while the theoretical work and the computer simulations were performed by the group of the co-PI in Austria. Suspensions containing colloidal hard spheres served as model system. They were confined between two walls and observed by confocal microscopy. This provides information on a single-particle level, in particular the particle locations and trajectories. Based on the particle locations and trajectories, we obtained information on the dependencies of the structure and dynamics on the confinement length, quantified by, e.g., the density profile, the radial distribution function, the structure functions, the mean-square displacement and the intermediate scattering functions. These parameters allowed for a systematic and quantitative comparison with theoretical and simulational results. This resulted in a consistent microscopic description of the structure and dynamics of colloidal fluids in confinement on a single-particle level.

Publications

• Two-dimensional Brownian motion of anisotropic dimers. Physical Review E, 104(1).
  Mayer, Daniel B.; Sarmiento-Gómez, Erick; Escobedo-Sánchez, Manuel A.; Segovia-Gutiérrez, Juan Pablo; Kurzthaler, Christina; Egelhaaf, Stefan U. & Franosch, Thomas
  
• Layering and packing in confined colloidal suspensions. Soft Matter, 18(25), 4699-4714.
  Villada-Balbuena, Alejandro; Jung, Gerhard; Zuccolotto-Bernez, Angel B.; Franosch, Thomas & Egelhaaf, Stefan U.