ENGINEERING AND CFD
An Extended Immersed Boundary Lattice Boltzmann Method for Wetting Dynamics of Ga2O3 Droplets and Dynamics of Viscoelastic Dense Suspensions
Principal Investigator:
Francesca Pelusi, Fabio Guglietta
Affiliation:
Helmholtz Institute Erlangen-Nürnberg for Renewable Energy, Forschungszentrum Jülich, Germany
Local Project ID:
POLPS
HPC Platform used:
JUWELS Cluster at JSC
Date published:
Abstract
Liquid metals like Gallium (Ga) are a promising platform for catalytic devices such as SCALMS (Supported Catalytically Active Liquid Metal Solutions). Ga develops an oxidized surface layer (Ga₂O₃), which is known to have a major impact on droplet dynamics and technological performance.
We simulate droplets via a coupled Immersed Boundary Lattice-Boltzmann (IBLB) method, for which we introduce a generalized model for elastic properties of the membrane, to cover properties of oxidized droplets and beyond [1]. When we add viscoelastic effects into the surface properties of spherical capsules in dense suspensions, we observe an increase in deformation resistance, while dynamics (loading time) surprisingly restores behavior of capsules with no surface viscosity [2].
Report
Modeling Droplet Surfaces
How do droplets of liquid metal behave? For “classical” (simple) interfaces, we can rely on Young's equation to predict geometry. However, for the dynamics of real droplets, surfaces play a much more active role than just the division between bulk phases. Liquid metals like gallium (Ga), when in contact to oxygen, cover in an oxide layer, that acts like a “skin” with vastly different wetting behavior than the metal. Another example are “liquid marbles”, that is, droplets covered in a layer of a poorly wetting powder.
To catch the properties such complex droplets, we need constitutive laws of interface elasticity, that are complex enough to catch the dynamics of realistic systems. This is the strength of the Immersed Boundary-Lattice Boltzmann (IBLB) method. It couples a lattice-based fluid solver with a triangulated model of interfaces, for which many properties (stiffness, surface viscosity, etc.) can be analyzed for their influence on the geometry and dynamics.
High-performance fluid dynamics simulations
Mesoscopic simulations that involve complex geometries, are computationally costly, due to the resolution of the involved length and time scales (long-range hydrodynamic interactions, slow transient times). They become particularly intricate when particles are interacting with each other or with complex geometries, beyond the assumptions of simple analytical models.
To extend the horizon of what has been achievable so far, highly scalable simulation techniques are state of the art. Our choice is IBLB, that is Lattice Boltzmann, coupled with the Immersed Boundary method, which simulates membranes or boundaries between immiscible fluids.
The excellent scaling behavior of IBLB allows this method to be applied on supercomputers. Then, the resources of massively parallel simulation campaigns are also necessary, to reach scientifically relevant scales of simulation box sizes, mesh resolution, and simulated time.
Generalized Membrane Elasticity and Wetting Dynamics
In its general formulation, the elastic IBLB interface model includes three free parameters: prestress, resistance against area dilation, and shear modulus. With only a subset of these elastic constants present (the others set to zero), this reduces to special cases that have been discussed in literature since the 1970s. If, e. g., only prestress is applied, the properties are equivalent to a free droplet.
Our work [1] explores the combinations of these elastic constants more systematically, to explore intermediate membrane types like what we call “softly coated droplets” (surface tension like a droplet, but also strain modulus present), or “rigidly coated droplets” (with a finite dilatational term).
How does the presence of an oxide layer on liquid metal droplets impact wetting? For this, we model droplets with generalized surface properties, and conduct a systematic series of spreading experiments (IBLB simulations): a droplet is placed close to a surface with an attractive force (Lennard-Jones). Initially spherical, the droplet deforms, and the contact area to the substrate follows a power-law scaling. We analyze this transient, as well as the equilibrium contact angle. We demonstrate the interdependence between the choice of elastic constants (type of particle) and their respective values. Our generalized model covers a wide range of practically relevant cases of membrane properties.
Simulation using Immersed-Boundary Lattice-Boltzmann: A droplet is undergoing wetting on a flat substrate, with its interface resolved as a mesh. Forces on the faces of the triangle consist of interface elasticity (rigidity), volume conservation, and the attractive force from the substrate. Figure from [1].
Surface Viscosity and Dense Suspensions
Another long disregarded extension of membrane model is viscoelasticity on the surface. The introduction of surface viscosity decreases deformation, induces oscillations in the shape, and reduces the time required to reach the steady state (loading time). Surprisingly, we find that placement of the particles in a dense suspension, offsets some of these effects (e. g., deformation) while loading times are quite insensitive [2].
Our simulation campaign on the GCS supercomputer JUWELS Cluster grounded our extended theory of the dynamics of droplets and capsules with a solid foundation of data, and allowed us to connect phenomenology with theory. We improved the predictions for the dynamics of droplets with intricate surface properties. From this insight, we can contribute to the design and durability of liquid-metal based catalytic devices, but beyond this, the insights on the dynamics of droplets and capsules improve the understanding of suspensions of soft particles and droplets in general.
References
[1] A sharp interface approach for wetting dynamics of coated droplets and soft particles, F. Pelusi, F. Guglietta, M. Sega, O. Aouane, J. Harting, Physics of Fluids 2023, 35(8), 082126, https://doi.org/10.1063/5.0160096
[2] Suspensions of viscoelastic capsules: effect of membrane viscosity on transient dynamics, F. Guglietta, F. Pelusi, M. Sega, O. Aouane, J. Harting, Journal of Fluid Mechanics 2023, 971A13, https://doi.org/10.1017/jfm.2023.694