arXiv subjects

Andreas Baer

Publications and source records attributed to Andreas Baer.

6 recordsLinked to original sources

arXiv2023

The Stokes-Einstein-Sutherland equation at the nanoscale revisited

The Stokes-Einstein-Sutherland (SES) equation is at the foundation of statistical physics, relating a particle's diffusion coefficient and size with the fluid viscosity, temperature and the boundary condition for the particle-solvent interface. It is assumed that it relies on the separation of scales between the particle and the solvent, hence it is expected to break down for diffusive transport on the molecular scale. This assumption is however challenged by a number of experimental studies showing a remarkably small, if any, violation, while simulations systematically report the opposite. To understand these discrepancies, analytical ultracentrifugation experiments are combined with molecular simulations, both performed at unprecedented accuracies, to study the transport of buckminsterfullerene C60 in toluene at infinite dilution. This system is demonstrated to clearly violate the conditions of slow momentum relaxation. Yet, through a linear response to a constant force, the SES equation can be recovered in the long time limit with no more than 4% uncertainty both in experiments and in simulations. This nonetheless requires partial slip on the particle interface, extracted consistently from all the data. These results, thus, resolve a long-standing discussion on the validity and limits of the SES equation at the molecular scale.

cond-mat.mes-hall↗

arXiv2023

Anisotropic molecular diffusion in confinement I: Transport of small particles in potential and density gradients

Hypothesis: Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue. In the vicinity of interfaces, density fluctuations as a consequence of layering locally impose statistical drift, which impedes the analysis of spatially dependent diffusion coefficients even further. We hypothesise, that we can derive a model to spatially resolve interface-perpendicular diffusion coefficients based on local lifetime statistics with an extension to explicitly account for the effect of local drift using the Smoluchowski equation, that allows us to resolve anisotropic and spatially dependent diffusivity landscapes at interfaces. Methods and simulations: An analytic relation between local crossing times in system slices and diffusivity as well as an explicit term for calculating drift-induced systematic errors is presented. The method is validated on Molecular Dynamics simulations of bulk water and applied to simulations of water in slit pores. Findings: After validation on bulk liquids, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Significant spatial variations in the diffusivities correlate with interface-induced structuring but cannot be solely attributed to the drift induced by local density fluctuations.

cond-mat.soft↗

arXiv2023

Anisotropic molecular diffusion in confinement II: A model for structurally complex particles applied to transport in thin ionic liquid films

Hypothesis:Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue and for objects of nanometric size, which structurally fluctuate on a similar time scale as they diffuse, no methodology has been established so far. We hypothesise that the complex, coupled dynamics can be captured and analysed by using a model built on the $2$-dimensional Smoluchowski equation and systematic coarse-graining. Methods and simulations: For large, flexible species, a universal approach is offered that does not make any assumptions about the separation of time scales between translation and other degrees of freedom. The method is validated on Molecular Dynamics simulations of bulk systems of a family of ionic liquids with increasing cation sizes where internal degrees of freedom have little to major effects. Findings: After validation on bulk liquids, where we provide an interpretation of two diffusion constants for each species found experimentally, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Spatial variations in the diffusivities relate to interface-induced structuring of the ionic liquids. Notably, the length scales in strongly confined ionic liquids vary consistently but differently at the solid-liquid and liquid-vapour interfaces.

cond-mat.soft↗

arXiv2022

Modelling diffusive transport of particles interacting with slit nanopore walls: The case of fullerenes in toluene filled alumina pores

Accurate modeling of diffusive transport of nanoparticles across nanopores is a particularly challenging problem. The reason is that for such narrow pores the large surface-to-volume ratio amplifies the relevance of the nanoscopic details and of the effective interactions at the interface with pore walls. Close to the pore wall, there is no clear separation between the length scales associated with molecular interactions, layering of the solvent at the interface with the pore and the particle size. Therefore, the standard hydrodynamic arguments may not apply and alternative solutions to determining average transport coefficients need to be developed. We here address this problem by offering a multiscale ansatz that uses effective potentials determined from molecular dynamics simulations to parametrise a four state stochastic model for the positional configuration of the particle in the pore. This is in turn combined with diffusivities in the centre of the pore and at the pore wall to calculate the average diffusion constant. We apply this model to the diffusion of fullerenes in a toluene filled slit nanopore and calculate the mean diffusion coefficient as a function of the pore size. We show that the accuracy of our model is affected by the partial slip of the toluene on the pore wall.

cond-mat.mes-hall↗

arXiv2019

Water in an electric field does not dance alone: The relation between equilibrium structure, time dependent viscosity and molecular motions

Dynamic structuring of water is a key player in a large class of processes underlying biochemical and technological developments today, the latter often involving electric fields. However, the anisotropic coupling between the water structure and the field has not been understood on a molecular level so far. Here we perform extensive molecular dynamics simulations to explore the influence of an externally imposed electric field on liquid water under ambient conditions. Using self-developed analysis tools and rigorous statistical analysis, we unambiguously show that water hydration shells break into subcompartments, which were hitherto not observed due to radial averaging. The shape of subcompartments is sensitive to the field magnitude, and affects excitations of the hydrogen bond network including the femtosecond stretching and the sub-picosecond restructuring of hydrogen bonds. Furthermore, by analysing the reorientational dynamics of water molecules, we ascertain the existence of cooperative excitations of small water clusters. Enabled by the interplay between hydrogen bonding, and the coupling of water dipoles to the field, these coordinated motions, occurring on the picosecond time scale, are associated with fluctuations between torque-free states of water dipoles. We show that unlike the coupling between translation and reorientation of water molecules, which takes place on even longer time scales, these coordinated motions are the key for understanding the emergent anisotropy of diffusion and viscosity of water. Particular effort is invested to provide an analysis that allows for future experimental validation.

cond-mat.soft↗

arXiv2019

Insights from Molecular Dynamics Simulations on Structural Organization and Diffusive Dynamics of an Ionic Liquid at Solid and Vacuum Interfaces

Hypothesis A prototypical modelling approach is required for a full characterisation of the static and equilibrium dynamical properties of confined ionic liquids (ILs), in order to gain predictive power of properties that are difficult to extract from experiments. Such a protocol needs to be constructed by benchmarking molecular dynamics simulations against available experiments. Simulations We perform an in-depth study of [C2Mim][NTf2] in bulk, at the vacuum and at hydroxylated alumina surface. Using the charge methods CHelpG, RESP-HF and RESP-B3LYP with charge scaling factors 1.0, 0.9 and 0.85, we search for an optimum non-polarizable force field by benchmarking against self-diffusion coefficients, surface tension, X-ray reflectivity data, and structural data. Findings Benchmarking, which relies on establishing the significance of an appropriate size of the model systems and the length of the simulations, yields RESP-HF/0.9 as the best suited force field for this IL overall. A complete and accurate characterisation of the spatially-dependent internal configurational space and orientation of IL molecules relative to the solid and vacuum interfaces is obtained. Furthermore, the density and mobility of IL ions in the plane parallel and normal to the interfaces is evaluated and the correlation between the stratification and dynamics in the interfacial layers is detectable deep into the films.

cond-mat.mtrl-sci↗