arXiv subjects

Paolo Malgaretti

Publications and source records attributed to Paolo Malgaretti.

At least 19 recordsLinked to original sources

arXiv2026

Closed-Form Solution for Oscillatory Flow and Wall Shear Stress in Axisymmetric Corrugated Tubes

Pulsatile flow in corrugated tubes arises in hemodynamics and microfluidics, where oscillatory forcing and geometric constrictions jointly determine transport and wall loading. We extend Womersley's classical solution for oscillatory flow in a rigid circular tube to rigid axisymmetric tubes of slowly varying radius. Within the lubrication approximation, we derive closed-form expressions for the axial velocity profile, volumetric flow rate, phase lag, and wall shear stress at an arbitrary Womersley number. Three-dimensional lattice Boltzmann simulations are used to assess the regime of validity of the theory. The analysis shows that the local velocity profile varies strongly along the tube, ranging from plug-like in wide sections to more parabolic near bottlenecks. The cycle-maximum flow rate decreases with increasing corrugation, but this reduction weakens as pulsatility increases, reflecting a crossover from the quasi-steady scaling $\langle R^{-4}\rangle^{-1}$ to the high-frequency scaling $\langle R^{-2}\rangle^{-1}$. The wall shear stress is maximal at the bottleneck and decreases with Womersley number. For sinusoidal corrugations, the bottleneck wall shear stress depends non-monotonically on the corrugation because of the competition between local shear amplification and global hydraulic resistance. Closed-form expressions for the time-averaged wall shear stress and oscillatory shear index further connect the theory to standard hemodynamic metrics.

physics.flu-dyn↗

arXiv2026

Free-Energy Analysis of Bubble Nucleation on Electrocatalytic Surfaces

Bubble nucleation at catalyst surfaces plays a critical role in the operation of electrolyzers. However, achieving controlled bubble nucleation remains challenging due to limited understanding of the underlying mechanisms. Here, we present a free-energy model that quantitatively predicts both the activation energy and critical nucleus size of bubbles at given supersaturation, temperature, pressure, and surface wettability. We find that the activation energy $\Delta G_{max}$ decreases with increasing supersaturation $\zeta$, following a power-law scaling of $\Delta G_{max} \sim \zeta^{-2}$, while the critical nucleus radius $R_c$ scales as $R_c\sim \zeta^{-1}$. Our theoretical predictions for the critical nucleus radius of hydrogen, oxygen and nitrogen bubbles are in quantitative agreement with experimental measurements. Finally, we present a simple model that couples gas diffusion and electrochemical reaction kinetics to determine the maximum gas supersaturation at a given current density. Our results advance the fundamental understanding of bubble nucleation at catalyst surfaces and provide practical guidelines for catalyst layer design to improve the performance of electrolyzers.

cond-mat.soft↗

arXiv2025

How back reaction, hydrogen transport, and capillarity control the performance of hydrogen release from liquid organic carriers

We derive a theoretical model to elucidate the inhibition of catalytic activity during the dehydrogenation of Liquid Organic Hydrogen Carriers (LOHC). Within our model, we account for the reversible nature of the hydrogenation-dehydrogenation reaction as well as the transport of both LOHC and produced hydrogen. Our analysis reveals that the main limiting factor for the performance of porous catalysts is the transport of dissolved hydrogen, which has been overlooked so far. In particular, we show that two distinct kinetic regimes can arise depending on whether hydrogen leaves the pellet in form of bubbles or via diffusion. Moreover, we derive the conditions for the onset of bubbling depending on hydrogen supersaturation and capillarity. Beyond LOHC systems, our findings are applicable to a broader class of reversible reactions, particularly those involving volatile products that can leave the liquid reaction medium in the form of bubbles.

physics.chem-ph↗

arXiv2025

How Spatially Modulated Activity Reshapes Active Polymer Conformations

Active polymers are driven out of equilibrium by internal forces and exhibit conformational properties that differ fundamentally from those of passive chains. Here we study how spatially modulated tangential activity reshapes the conformations of semiflexible polymers. Using a continuum Rouse model with bending rigidity, we develop a systematic expansion in the limit of weak activity and derive analytical expressions for mode correlations, gyration radius, and end-to-end distance under sinusoidally varying propulsion. We show that spatially structured activity breaks self-similar scaling and induces a mode-dependent transition between polymer shrinking and swelling. Uniform or low-mode forcing produces compact, globule-like conformations, whereas higher modes generate alternating stretched and compressed segments, leading to globally swollen chains. Different polymer sizes respond differently to activity, allowing for conformations that are compact in gyration radius yet extended in end-to-end distance. Langevin dynamics simulations quantitatively confirm the theoretical predictions. Our results demonstrate that even weak, patterned activity provides a powerful mechanism to control polymer conformations far from equilibrium.

cond-mat.soft↗

arXiv2025

Spontaneous Emergence of Solitary Waves in Active Flow Networks with Elastic Elements

Flow networks are fundamental for understanding systems such as animal and plant vasculature or power distribution grids. These networks can encode, transmit, and transform information embodied in the spatial and temporal distribution of their flows. In this work, we focus on a minimal yet physically grounded system that allows us to isolate the fundamental mechanisms by which active flow networks generate and regulate emergent dynamics capable of supporting information transmission. The system is composed of active units that pump fluid and elastic units that store volume. From first principles, we derive a discrete model -- an active flow network -- that enables the simulation of large systems with many interacting units. Numerically, we show that the pressure field can develop solitary waves, resulting in the spontaneous creation and transmission of localized packets of information stored in the physical properties of the flow. We characterize how these solitary waves emerge from disordered initial conditions in a one-dimensional network, and how their size and propagation speed depend on key system parameters. Finally, when the elastic units are coupled to their neighbors, the solitary waves exhibit even richer dynamics, with diverse shapes and finite lifetimes that display power-law behaviors that we can predict analytically. Together, these results show how simple fluidic elements can collectively create, shape and transport information, laying the foundations for understanding -- and ultimately engineering -- information processing in active flow systems.

physics.flu-dyn↗

arXiv2025

Effects of Hydrogen Transport on the Kinetic Regimes of 4-Nitrophenol Reduction by Sodium Borohydride

The reduction of 4-nitrophenol (4-NiP) with sodium borohydride is widely used to benchmark heterogeneous catalysts, yet its kinetics are commonly oversimplified as pseudo-first-order. In reality, borohydride hydrolysis and hydrogenation by dissolved hydrogen proceed concurrently, making hydrogen transport a decisive factor in shaping apparent activity. Re-examining data on Pt-SiO2 supraparticles with different pore structures, we attribute contrasting kinetic behavior to distinct regimes of hydrogen transport: diffusive transport sustains pseudo-first-order kinetics, while bubble-mediated escape causes hydrogen loss and incomplete conversion. We propose a kinetic model that captures this transition and enables consistent interpretation of experimental data. More broadly, our analysis shows that apparent differences in activity during 4-NiP benchmarking can arise from hydrogen transport rather than intrinsic properties of the catalyst, underscoring the need to account for transport effects when comparing catalyst performance.

physics.chem-ph↗

arXiv2025

Mechanical work extraction from an error-prone active dynamic Szilard engine

Isothermal information engines operate by extracting net work from a single heat bath through measurement and feedback control. In this work, we analyze a realistic active Szilard engine operating on a single active particle by means of steric interaction with an externally controlled mechanical element. In particular, we provide a comprehensive study of how finite measurement accuracy affects the engine's work and power output, as well as the cost of operation. Having established the existence of non-trivial optima for work and power output, we study the dependence of their loci on the measurement error parameters and identify conditions for their positivity under one-shot and cyclic engine operation. By computing a suitably defined information efficiency, we also demonstrate that this engine design allows for the violation of Landauer's bound on the efficiency of information-to-work conversion. Notably, the information efficiency for one-shot operation exhibits a discontinuous transition and a non-monotonic dependence on the measurement precision. Finally, we show that cyclic operation improves information efficiency by harvesting residual mutual information between successive measurements.

cond-mat.stat-mech↗

arXiv2025

Quantum tunneling and anti-tunneling across entropic barriers

We study the dynamics of a quantum particle in a constricted two-dimensional channel and analyze how the onset of quantum corrections impacts the (semi-)classical high-temperature behaviour, as temperature is lowered. We characterize both equilibrium and non-equilibrium (transport) properties of the system, considering the case of a narrow and disorder-free channel. Counterintuitively, we find that quantum corrections do not monotonically enhance the particle current as the temperature is lowered, as naively expected from the activation of coherent tunnelling, but they rather inhibit transport at intermediate temperatures, increasing the effective free-energy barrier. We illustrate this ``anti-tunnelling'' effect numerically by computing the non-equilibrium steady-state of a quantum master equation describing the system, and confirm it analytically by adopting the Quantum Smoluchowski limit.

quant-ph↗

arXiv2025

Modelling of the dewetting of ultra-thin liquid films on chemically patterned substrates: linear spectrum and deposition patterns

Liquid films of nanometric thickness are prone to spinodal dewetting driven by disjoining pressure, meaning that a non-wetting liquid film of homogeneous thickness in the range of tens of nanometers will spontaneously break into droplets. The surface energy of the underlying solid substrate heavily influences the dynamics and resulting droplet configurations. Here, we study the dewetting of thin liquid films on physically flat but chemically heterogeneous substrates using the thin film equation. We use linear stability analysis (LSA) to describe and predict the system's behavior until the film ruptures and compare it to numerical simulations. The good agreement between the numerical solutions and the LSA allows us to propose a method for measuring surface energy patterns from early time-step film height profiles with good precision. Furthermore, we study the non-linear dynamics and the eventually formed droplet pattern by numerical simulations. This offers insights into the dependency of the resultant droplet arrays on shape, feature size, and magnitude of the chemical patterning of the underlying substrate.

physics.flu-dyn↗

arXiv2024

Active Polymer Behavior in Two Dimensions: A Comparative Analysis of Tangential and Push-Pull Models

In this work, we compare the structural and dynamic behavior of active filaments in two dimensions using tangential and push-pull models, including a variant with passive end monomers. These models serve as valuable frameworks for understanding self-organization in biological polymers and synthetic materials. At low activity, all models exhibit similar behaviors. Differences emerge in the intermediate range as activity increases, though at higher activity levels, their behaviors converge. Importantly, adjusting for differences in mean active force reveals nearly identical behavior across models. Our results highlight the importance of force definitions in active polymer simulations and provide insights into phase transitions across varying filament configurations.

cond-mat.soft↗

arXiv2024

Modeling of thin liquid films with arbitrary many layers

We propose the generalization of the thin film equation (TFE) to arbitrarily many immiscible liquid layers. Then, we provide different pathways for deriving the hydrodynamic pressure within the individual layers, showing how to understand the equation as a Cahn-Hilliard-type conservation equation and providing an algorithm to derive the associated Onsager Matrix. Furthermore, we employ a numerical solver based on the multilayer shallow water-lattice Boltzmann method (LBM) for two and three liquid layers in pseudo two and three dimensions to gain insights into the dynamics of the system and to validate the model. We perform a linear stability analysis and assess droplet equilibrium shapes. Furthermore, we compare the dynamics of the proposed thin film equation to full Navier-Stokes simulations and show the possible equilibrium states of the multilayer liquid thin film system.

physics.flu-dyn↗

arXiv2024

Tangentially Active Polymers in Cylindrical Channels

We present an analytical and computational study characterizing the structural and dynamical properties of an active filament confined in cylindrical channels. We first outline the effects of the interplay between confinement and polar self-propulsion on the conformation of the chains. We observe that the scaling of the polymer size in the channel, quantified by the end-to-end distance, shows different anomalous behaviours at different confinement and activity conditions. Interestingly, we show that the universal relation, describing the ratio between the end-to-end distance of passive polymer chains in cylindrical channels and in bulk is broken by activity. Finally, we show that the long-time diffusion coefficient under confinement can be rationalised by an analytical model, that takes into account the presence of the channel and the elongated nature of the polymer.

cond-mat.soft↗

arXiv2024

Enhancement of bubble transport in porous electrodes and catalysts

We investigate the formation and transport of gas bubbles across a model porous catalyst/electrode using lattice Boltzmann simulations. This approach enables us to systematically examine the influence of a wide range of morphologies, flow velocities, and reaction rates on the efficiency of gas production. By exploring these parameters, we identify critical parameter combinations that significantly contribute to an enhanced yield of gas output. Our simulations reveal the existence of an optimal pore geometry for which the product output is maximized. Intriguingly, we also observe that lower flow velocities improve gas production by leveraging on coalescence-induced bubble detachment from the catalyst.

cond-mat.soft↗

arXiv2024

Chemically reactive thin films: dynamics and stability

Catalyst particles or complexes suspended in liquid films can trigger chemical reactions leading to inhomogeneous concentrations of reactants and products in the film. We demonstrate that the sensitivity of the liquid film's gas-liquid surface tension to these inhomogeneous concentrations strongly impacts the film stability. Using linear stability analysis, we identify novel scenarios in which the film can be either stabilized or destabilized by the reactions. Furthermore, we find so far unrevealed rupture mechanisms which are absent in the chemically inactive case. The linear stability predictions are confirmed by numerical simulations, which also demonstrate that the shape of chemically active droplets can depart from the spherical cap and that unsteady states such as traveling and standing waves might appear. Finally, we critically discuss the relevance of our predictions by showing that the range of our selected parameters is well accessible by typical experiments.

cond-mat.soft↗

arXiv2023

Non-monotonous translocation time of polymers across pores

Polymers confined in corrugated channels, i.e. channels of varying amplitude, display {multiple local maxima and minima of the diffusion coefficient upon increasing their degree of polymerization $N$}. We propose a theoretical effective free energy for linear polymers based on a Fick-Jacobs approach. We validate the predictions against numerical data, obtaining quantitative agreement for {the effective free energy, the diffusion coefficient and the Mean First Passage Time}. Finally, we employ the effective free energy to compute the polymer lengths $N_{min}$ at which the diffusion coefficient presents a minimum: we find a scaling expression that we rationalize with a blob model. Our results could be useful to design porous adsorbers, that separate polymers of different sizes without the action of an external flow.

cond-mat.soft↗

arXiv2023

Surface viscosity in simple liquids

The response of Newtonian liquids to small perturbations is usually considered to be fully described by homogeneous transport coefficients like shear and dilatational viscosity. However, the presence of strong density gradients at the liquid/vapor boundary of fluids hints at the possible existence of an inhomogeneous viscosity. Here, we show that a surface viscosity emerges from the collective dynamics of interfacial layers in molecular simulations of simple liquids. We estimate the surface viscosity to be 8-16 times smaller than that of the bulk fluid at the thermodynamic point considered. This result can have important implications for reactions at liquid surfaces in atmospheric chemistry and catalysis.

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↗

arXiv2022

Resolving the microscopic hydrodynamics at the moving contact line

The molecular structure of moving contact lines (MCLs) and the emergence of a corresponding macroscopic dissipation have made the MCL a paradigm of fluid dynamics. Through novel averaging techniques that remove capillary waves smearing we achieve an unprecedented resolution in molecular dynamics (MD) simulations and find that they match with the continuum description obtained by finite element method (FEM) down to molecular scales. This allows us to distinguish dissipation at the liquid-solid interface (Navier-slip) and at the contact line, the latter being negligible for the rather smooth substrate considered.

cond-mat.soft↗