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 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 383534318

Transport in strongly confined fluid is of fundamental interest both from a theoretical point of view as well as for various applications ranging from microfluidic devices for suspensions of colloidal particles, tempering on the microscale, to friction and lubrication of thin films. While for confinement lengths much larger than the particle diameter, the structure remains essentially unperturbed and dynamics can be described by hydrodynamics with a possibly complicated constitutive equation, the structure and dynamics is drastically changed once the confining region becomes comparable to the interparticle distance. The most prominent effect is known as layering where the density profile displays an accumulation of particles close to a wall followed by typical oscillations due to close packing of particles. Furthermore two-point correlations such as the pair-distribution function are significantly affected by the interplay of the local short-range packing and the confining walls. For the dynamics this competition is expected to drastically influence transport coefficients, in particular, in the vicinity of structural arrest referred to as glass transition. The goal of this project is to provide a characterization of the structural and dynamical properties of dense fluids confined to narrow slits. The problem is addressed both from a simulational, a theoretical, as well as an experimental point of view. The quantities of interest are suitably adapted generalized intermediate scattering functions both for the coherent as well as the self dynamics. Our theoretical approach relies on a recently developed mode-coupling theory for the confinement problem and uses coherent intermediate scattering functions as primordial variables. A reliable numerical implementation of the theory is at the heart of the project. Then, a non-equilibrium-state diagram can be calculated and the suitably generalized nonergodicity parameter characterizing the frozen-in structure in the glassy state can be obtained. More generally, the theory should provide the complete time dependence of the intermediate scattering functions. Using extensions to the self dynamics the mean-square displacements and the diffusion coefficients can be predicted. Since the theory requires static properties as input we intend also to elucidate the behavior of an extremely confined fluid. The theoretical effort is to be complemented by a simulation for slightly polydisperse hard-sphere systems or mixtures in slit or wedge geometry as well a series of experiments on colloidal suspensions in confinement. The primary goal is to measure the relevant observables as introduced in the theory and make a careful comparison between theory, simulation, and experiments. The benefit of the project is to arrive at a first microscopic description for the dynamics in strong confinement that is thoroughly tested versus computer and laboratory experiments.

• DFG Programme: Research Grants
• International Connection: Austria
• Co-Investigator: Professor Dr. Thomas Franosch