arXiv subjects

J. Harting

Publications and source records attributed to J. Harting.

At least 19 recordsLinked to original sources

arXiv2026

Gas-generating reactive flows in bicontinuous catalyst support structures

A major challenge in the field of heterogeneous catalysis is selecting an optimal catalyst support structure. Commercially available structures can be easily manufactured at scale, but their stochastic nature makes their chemical and transport properties suboptimal. This is particularly relevant for gas-generation reactions, where non-uniformity of a porous structure leads to bubble trapping. Such trapping impedes the flow of reactants to catalyst sites, leading to conversion inefficiencies. Previous experimental work demonstrated that spinodally-derived architectures, in particular bicontinuous interfacially jammed emulsion gels (bijels), can alleviate these issues and deliver superior performance. However, to the best of our knowledge, numerical studies to optimize the operating conditions for such a morphology have not been performed yet. In this work, we aim to close this gap using color-gradient lattice Boltzmann simulations of reactive flows with a novel central moments collision operator. We develop an analytical model to predict catalyst performance based on our simulation data. Our findings show that this type of morphology can achieve very high conversion efficiencies. Moreover, we demonstrate that its catalyst performance can be optimized using superhydrophilic surface coatings.

physics.chem-ph↗

arXiv2026

Bubbles in highly porous media: Clogging and unclogging at constrictions

Gas bubble transport through highly porous transport layers (PTLs) is a key process in electrochemical devices such as proton exchange membrane water electrolyzers, where bubbles generated at catalyst surfaces must migrate through complex porous networks. To understand this process, we focus on model systems, namely the motion of single, paired and multiple bubbles in capillaries and study these by combining analytical modeling, three-dimensional color-gradient lattice Boltzmann simulations, and X-ray radiography. For single bubbles, we derive an analytical expression for the critical Bond number separating passage from clogging and show that, in the low deformation regime, it accurately predicts this transition in circular capillaries. Extending the study to bubble pairs, we uncover additional clogging and unclogging pathways, including hydrodynamic unclogging driven by pressure buildup in the interbubble film, and coalescence-induced clogging and unclogging. By mapping our results as functions of confinement ratio and Bond number, we define distinct dynamical regimes that control bubble passage. Experiments on bubble chains rising through highly porous nickel foams confirm the predicted clogging and unclogging mechanisms.

cond-mat.soft↗

arXiv2026

Swirl flow in microchannels: patterned slip walls enhance heat transport

Microchannel heat sinks (MCHS) are widely used for thermal management in high-power electronics due to their ability to dissipate large heat fluxes with minimal coolant consumption. While numerous strategies - such as geometric modifications, surface disruptions, and enhanced coolant formulations - have been explored to improve heat transfer, many of these approaches increase hydraulic resistance and pumping power requirements. Recent studies have shown that slip/no-slip wall patterns can enhance flow rates and convective heat removal without additional energy input, and that patterned microstructures can induce secondary swirling motions known to promote mixing and heat transfer. Motivated by these findings, we investigate a slip/no-slip pattern specifically designed to generate swirl flow inside a straight microchannel. Building upon prior work on passive chaotic advection and boundary-condition engineering, we assess the hydrodynamic and thermal performance of this patterned configuration under conditions relevant to laminar microchannel cooling. Our results demonstrate that appropriately arranged slip/no-slip regions can induce swirl without geometric perturbations or increased pumping power, ultimately improving heat transfer efficiency at fixed volumetric flow rate. This study highlights the potential of boundary-condition patterning as a simple, energy-neutral strategy for enhancing the performance of microfluidic heat-transfer devices.

physics.flu-dyn↗

arXiv2025

Morphology Formation Pathways in Solution-Processed Perovskite Thin Films

The active layer in a perovskite solar cell is usually composed of a polycrystalline thin film. Fabrication of this layer by solution processing is a promising candidate for up-scaling to the mass market. However, the evolution of an evaporating and simultaneously crystallizing thin film is not yet fully understood. To contribute to the understanding of the formation of thin films, we develop a geometrical model that deals with the effect of the interplay between solvent evaporation and crystal growth on the dry film morphology. The possible film formation mechanisms are investigated, depending on the processing conditions. We find eleven formation pathways leading to four distinct morphologies. It is shown how these formation pathways can be utilized by adapting the process parameters to the material properties. Pinhole-free and flat films can be fabricated if the evaporation rate is high in comparison to the crystal growth rate. Alternatively, providing a high crystal number density on the substrate can lead to the desired film morphology at low drying rates. The generality of the model makes it applicable to any evaporating and simultaneously crystallizing thin film.

cond-mat.mtrl-sci↗

arXiv2025

Zigzagging Diffusion and Non-Standard Transport in Particle-laden Nanopores Under Extreme Confinement

Understanding transport subject to molecular-scale confinement is key to advancing nanofluidics, yet classical hydrodynamic laws often fail at these scales. Here, we study a model system: transport of toluene as a solvent and small fullerenes as model particles confined within alumina slit nanopores using molecular dynamics simulations. We find that toluene organizes into discrete layers whose commensurability with the pore width leads to a striking, non-monotonic, zig-zag dependence of transport coefficients on confinement. This layering drives oscillations not only in solvent diffusivity but also in flow velocity and permeability under pressure-driven conditions, breaking the expected scaling relations between diffusion, viscosity, and flow. Surprisingly, introducing a nanoparticle does not wash out these effects - although the fullerene perturbs local layering, the nanoparticle diffusivity retains a zig-zag dependence on pore width. Our results demonstrate how structural commensurability and interfacial effects dominate transport in nanoconfined liquids, and lead to important deviations from continuum expectations. These findings establish a microscopic basis for size-dependent transport in nanopores and highlight the need for beyond-hydrodynamic models in confined soft matter systems.

cond-mat.soft↗

arXiv2025

Parasitic hydrogen bubble evolution in vanadium redox flow batteries: A lattice Boltzmann study

Vanadium redox flow batteries (VRFBs) are a promising technology to capture and store energy from renewable sources, reducing the reliance on fossil fuels for energy generation. However, during the charging process, the parasitic hydrogen evolution reaction at the negative electrode affects the performance and durability of VFRBs. The evolution of hydrogen bubbles causes the loss of effective reaction area and blocks the transport of reactants. We employ the lattice Boltzmann method to investigate the two-phase flow transport in the negative electrode of VRFBs. Systematic parametric analyses reveal that increased gas production leads to uneven gas removal from the electrode, while an optimal flow rate can effectively remove bubbles and reduce external pumping energy. Additionally, increasing the compression ratio hinders gas removal but enhances electrode electrical conductivity. Overall, the present study provides valuable mechanistic insights into bubble generation at the negative electrode of VRFBs and offers a theoretical reference for designing and optimizing VRFBs.

physics.flu-dyn↗

arXiv2025

Transport of electrolytes across nanochannels: the role of slip

We characterize the electrokinetic flow due to the transport of electrolytes embedded in nanochannels of varying cross-section with inhomogeneous slip on their walls, modeled as an effective slip length on the channel wall. We show that, within linear response and Debye-Huckel regime, the transport coefficients, and so the fluxes, can be significantly improved by the presence of a hydrophobic surface coating located at the narrowest section of the nanochannel. Our model indicates that the enhancement is larger when considering electric conductive walls in comparison to dielectric microchannel walls, and it is produced by a synergy between the entropic effects due to the geometry and the presence of the slip boundary layer. Our results show that a tailored hydrophobic coating design can be an effective strategy to improve transport properties in the broad areas of lab-on-a-chip, biophysics, and blue energy harvesting and energy conversion technologies.

physics.flu-dyn↗

arXiv2024

The interplay of shape and catalyst distribution in the yield of compressible flow microreactors

We develop a semi-analytical model for transport in structured catalytic microreactors, where both reactant and product are compressible fluids. Making use of the lubrication and Fick-Jacobs approximations, we reduce the three-dimensional governing equations to an effective one-dimensional set of equations. Our model captures the effect of compressibility, of corrugations in the shape of the reactor, as well as of an inhomogeneous catalytic coating of the reactor walls. We show that in the weakly compressible limit (e.g., liquid-phase reactors), the distribution of catalyst does not influence the reactor yield, which we verify experimentally. Beyond this limit, we show that introducing inhomogeneities in the catalytic coating and corrugations to the reactor walls can improve the yield.

physics.flu-dyn↗

arXiv2024

Simulation of perovskite thin layer crystallization with varying evaporation rates

Perovskite solar cells (PSC) are promising potential competitors to established photovoltaic technologies due to their superior efficiency and low-cost solution processability. However, the limited understanding of the crystallization behaviour hinders the technological transition from lab-scale cells to modules. In this work, we perform Phase Field (PF) simulations of the doctor-bladed film formation to obtain mechanistic and morphological information that is experimentally challenging to access. PF simulations are validated extensively using in- and ex-situ experiments for different solvent evaporation rates. The well-known transition from a film with many pinholes, for a low evaporation rate, to a smooth film, for high evaporation rates, is recovered in simulation and experiment. From the simulation, the transition can be assigned to the change in the ratio of evaporation to crystallization rate because of two distinct mechanisms. Firstly, for larger evaporation rates, nuclei appear at higher concentrations, which favors nucleation as compared to growth. Secondly, the growth of the crystals is confined in a thinner film, which limits their vertical size. Both effects are expected to be valid independent of the specific chemistry of the chosen experimental system, as long as the evaporation time of the solvent is comparable to the crystallization time.

cond-mat.mtrl-sci↗

arXiv2024

A lattice Boltzmann approach for acoustic manipulation

We employ a lattice Boltzmann method to compute the acoustic radiation force produced by standing waves on a compressible object. Instead of simulating the fluid mechanics equations directly, the proposed method uses a lattice Boltzmann model that reproduces the wave equation, together with a kernel interpolation scheme, to compute the first order perturbations of the pressure and velocity fields on the object's surface and, from them, the acoustic radiation force. The procedure reproduces with excellent accuracy the theoretical expressions by Gor'kov and Wei for the sphere and the disk, respectively, even with a modest number of lattice Boltzmann cells. The proposed method shows to be a promising tool for simulating phenomena where the acoustic radiation force plays a relevant role, like acoustic tweezers or the acoustic manipulation of microswimmers, with applications in medicine and engineering.

physics.flu-dyn↗

arXiv2023

Electroneutrality breakdown for electrolytes embedded in varying-section nanopores

We determine the local charge dynamics of a $z-z$ electrolyte embedded in a varying-section channel. By means of an expansion based on the length scale separation between the axial and transverse direction of the channel, we derive closed formulas for the local excess charge for both, dielectric and conducting walls, in $2D$ (planar geometry) as well as in $3D$ (cylindrical geometry). Our results show that, even at equilibrium, the local charge electroneutrality is broken whenever the section of the channel is not homogeneous for both dielectric and conducting walls as well as for $2D$ and $3D$ channels. Interestingly, even within our expansion, the local excess charge in the fluid can be comparable to the net charge on the walls. We critically discuss the onset of such local electroneutrality breakdown in particular with respect to the correction that it induces on the effective free energy profile experienced by tracer ions.

cond-mat.soft↗

arXiv2023

Turning catalytically active pores into active pumps

We develop a semi-analytical model of self-diffusioosmotic transport in active pores, which includes advective transport and the inverse chemical reaction which consumes solute. In previous work (Phys. Rev. Lett. 129, 188003, 2022), we have demonstrated the existence of a spontaneous symmetry breaking in fore-aft symmetric pores that enables them to function as a micropump. We now show that this pumping transition is controlled by three timescales. Two timescales characterize advective and diffusive transport. The third timescale corresponds to how long a solute molecule resides in the pore before being consumed. Introducing asymmetry to the pore (either via the shape or the catalytic coating) reveals a second type of advection-enabled transitions. In asymmetric pores, the flow rate exhibits discontinuous jumps and hysteresis loops upon tuning the parameters that control the asymmetry. This work demonstrates the interconnected roles of shape and catalytic patterning in the dynamics of active pores, and shows how to design a pump for optimum performance.

cond-mat.soft↗

arXiv2023

Closed formula for the transport of micro-nano-particle across model porous media

In the last decade the Fick-Jacobs approximation has been exploited to capture the transport across constrictions. Here, we review the derivation of the Fick-Jacobs equation with particular emphasis on its linear response regime. We show that for fore-aft symmetric channels the flux of non-interacting systems is fully captured by its linear response regime. For this case we derive a very simple formula that captures the correct trends and that can be exploited as a simple tool to design experiments or simulations. Finally, we show that higher order corrections in the flux may appear for non-symmetric channels.

cond-mat.soft↗

arXiv2022

Pumping and mixing in active pores

We show both numerically and analytically that a chemically patterned active pore can act as a micro/nano-pump for fluids, even if it is fore-aft symmetric. This is possible due to a spontaneous symmetry breaking which occurs when advection rather than diffusion is the dominant mechanism of solute transport. We further demonstrate that, for pumping and tuning the flow rate, a combination of geometrical and chemical inhomogeneities is required. For certain parameter values, the flow is unsteady, and persistent oscillations with a tunable frequency appear. Finally, we find that the flow may lose its axial symmetry and hence promotes mixing in the low Reynolds number regime.

cond-mat.soft↗

arXiv2022

Lattice Boltzmann simulations of two linear microswimmers using the immersed boundary method

The performance of a single or the collection of microswimmers strongly depends on the hydrodynamic coupling among their constituents and themselves. We present a numerical study for a single and a pair of microswimmers based on lattice Boltzmann method (LBM) simulations. Our numerical algorithm consists of two separable parts. Lagrange polynomials provide a discretization of the microswimmers and the lattice Boltzmann method captures the dynamics of the surrounding fluid. The two components couple via an immersed boundary method. We present data for a single swimmer system and our data also show the onset of collective effects and, in particular, an overall velocity increment of clusters of swimmers.

cond-mat.soft↗

arXiv2019

Numerical simulations of self-diffusiophoretic colloids at fluid interfaces

The dynamics of active colloids is very sensitive to the presence of boundaries and interfaces which therefore can be used to control their motion. Here we analyze the dynamics of active colloids adsorbed at a fluid-fluid interface. By using a mesoscopic numerical approach which relies on an approximated numerical solution of the Navier-Stokes equation, we show that when adsorbed at a fluid interface, an active colloid experiences a net torque even in the absence of a viscosity contrast between the two adjacent fluids. In particular, we study the dependence of this torque on the contact angle of the colloid with the fluid-fluid interface and on its surface properties. We rationalize our results via an approximate approach which accounts for the appearance of a local friction coefficient. By providing insight into the dynamics of active colloids adsorbed at fluid interfaces, our results are relevant for two-dimensional self assembly and emulsion stabilization by means of active colloids.

cond-mat.soft↗

arXiv2015

Mesoscopic simulation of diffusive contaminant spreading in gas flows at low pressure

Many modern production and measurement facilities incorporate multiphase systems at low pressures. In this region of flows at small, non-zero Knudsen- and low Mach numbers the classical mesoscopic Monte Carlo methods become increasingly numerically costly. To increase the numerical efficiency of simulations hybrid models are promising. In this contribution, we propose a novel efficient simulation approach for the simulation of two phase flows with a large concentration imbalance in a low pressure environment in the low intermediate Knudsen regime. Our hybrid model comprises a lattice-Boltzmann method corrected for the lower intermediate Kn regime proposed by Zhang et al. for the simulation of an ambient flow field. A coupled event-driven Monte-Carlo-style Boltzmann solver is employed to describe particles of a second species of low concentration. In order to evaluate the model, standard diffusivity and diffusion advection systems are considered.

physics.flu-dyn↗

arXiv2013

Hydrodynamic interactions induce anomalous diffusion under partial confinement

Under partial confinement, the motion of colloidal particles is restricted to a plane but their dynamics is influenced by hydrodynamic interactions mediated by the unconfined, three--dimensional flow of the embedding fluid. We demonstrate that this dimensionality mismatch induces a characteristic divergence in the two--dimensional collective diffusion coefficient of the colloidal monolayer. This result, independent from the specific interparticle forces in the monolayer, is solely due to the kinematical constraint on the colloidal particles, and it is different from the known divergence of transport coefficients in purely two--dimensional fluids.

cond-mat.soft↗