arXiv subjects

Daniele Dini

Publications and source records attributed to Daniele Dini.

At least 19 recordsLinked to original sources

arXiv2026

Molecular dynamics simulation of high slip flow of water confined between graphene nanochannels at experimentally accessible strain rates

The transient time correlation function method (TTCF) has emerged as a powerful methodology for accurately probing systems at low shear rates. In the present study, TTCF was used to evaluate the shear rate dependence of the slip length in a high-slip system consisting of water confined between graphene walls at experimentally accessible shear rates, for which classical nonequilibrium molecular dynamics (NEMD) is unfeasible. The corresponding Navier friction coefficient was computed for all shear rates spanning six orders of magnitude and compared with the equilibrium limit. We report for the first time NEMD results obtained at experimentally accessible shear rates using the TTCF approach for a system that has attracted significant interest over the past decades. The slip length calculated with TTCF is in good agreement with previous equilibrium molecular dynamics simulations and experiments. Our aim here is to highlight the extraordinary power of TTCF, particularly for high-slip (low strain-rate) systems, and to verify that equilibrium methods directly match NEMD measurements at experimentally accessible strain rates.

physics.chem-ph↗

arXiv2025

Modelling Heterogeneous Interfaces using Element-based Finite Volumes

Accurately depicting multiphysics interactions in interfacial systems requires computational frameworks capable of reconciling geometric adaptability with strict conservation fidelity. However, traditional spatiotemporal discretisation methods often compromise between mesh flexibility and flow conservation enforcement, hence constraining their effectiveness in elucidating the underlying mechanisms. Here, we respond to these computational demands by developing a novel three-dimensional adaptation of the Element-based Finite Volume Method (EbFVM) -- a hybrid numerical strategy that merges the geometric flexibility of Finite Element Methods with the conservation-centric principles of Finite Volume Methods. The proposed framework introduces advanced discretisation techniques tailored to unstructured, irregular mesh entities, including detailed parametric shape functions, robust flux integration schemes and rigorous body-fitted curvilinear coordinate mappings. Through a series of lubrication-driven benchmark problems, we demonstrate the EbFVM's capacity to capture intricate transport phenomena, strong field couplings and scale disparities across geometrically complex domains. By enabling accurate modelling in geometrically and physically challenging interfacial systems, the three-dimensional EbFVM offers a versatile and generalisable tool for simulating transport phenomena in a plethora of multiphysics applications.

cs.CE↗

arXiv2025

Multiscale Dynamics of Roughness-Driven Flow in Soft Interfaces

Soft lubricated contacts exhibit complex interfacial behaviours governed by the coupled effects of multiscale surface roughness and non-linear fluid-solid interactions. Accurately capturing this interplay across thin-film flows is challenging due to the strong synergy between contact mechanics and hydrodynamic flow, spanning over various spatiotemporal scales. Here, we develop a rigorous computational framework to simulate the frictional behaviour of soft lubricated interfaces; its modularity and the use of optimal solvers provides solutions for realistic configurations in lubrication regimes ranging from direct solid contact to complete fluid separation. Surface roughness is described via Persson's statistical theory as well as a deterministic Conjugate Gradient with Fast Fourier Transform (CG-FFT) approach, while limitations associated with classical half-space models are addressed by developing the Reduced Stiffness Method (RSM) to rigorously model pressure-induced surface responses. The integrated framework captures the full evolution of frictional behaviour, validated against experiments on rough elastomer-glass interfaces, revealing how surface roughness and material compliance together drive the transition from solid contact to fluid-mediated sliding. The developed approach establishes a robust and versatile simulation tool for analysing a plethora of soft interfacial systems shaped by fluid-solid interactions, with potential applications including but not limited to biomechanics, soft robotics and microfluidic systems.

cond-mat.soft↗

arXiv2025

The influence of boundary conditions and interfacial slip on the time taken to achieve a nonequilibrium steady-state for highly confined flows

We investigate the equilibration time to attain steady-state for a system of liquid molecules under boundary-driven planar Couette flow via nonequilibrium molecular dynamics (NEMD) simulation. In particular, we examine the equilibration time for the two common types of boundary driven flow: one in which both walls slide with equal and opposite velocity, and the other in which one wall is fixed and the other moves with twice the velocity. Both flows give identical steady-state strain rates, and hence flow properties, but the transient behaviour is completely different. We find that in the case of no-slip boundary conditions, the equilibration times for the counter-sliding walls flow are exactly 4 times faster than those of the single sliding wall system, and this is independent of the atomistic nature of the fluid, i.e., it is an entirely hydrodynamic feature. We also find that systems that exhibit slip have longer equilibration times in general and the ratio of equilibration times for the two types of boundary-driven flow is even more pronounced. We analyse the problem by decomposing a generic planar Couette flow into a linear sum of purely symmetric and antisymmetric flows. We find that the no-slip equilibration time is dominated by the slowest decaying eigenvalue of the solution to the Navier-Stokes equation. In the case of slip, the longest relaxation time is now dominated by the transient slip velocity response, which is longer than the no-slip response time. In the case of a high-slip system of water confined to graphene channels, the enhancement is over two orders of magnitude.

physics.flu-dyn↗

arXiv2025

Dynamic film thickness measurement in a rolling bearing using numerical elastohydrodynamic-acoustic modelling to interpret reflected ultrasound data

The thickness of the lubricating film plays a vital role in the operational efficiency and reliability of rolling bearings. Ultrasonic reflection techniques offer a promising non-invasive approach for in situ evaluation of lubricant films.However, accurately identifying the central film thickness remains challenging due to several complex factors, including dynamic fluctuations, localized elastic deformation, cavitation effects, and variations in oil supply. This study presents a comprehensive theoretical and numerical framework to elucidate the influence of these factors on ultrasonic wave propagation in lubricated contacts. Numerical simulations considering elastohydrodynamic lubrication (EHL) regime and cavitation-induced effects are carried out to obtain the surface deformation profiles and the cavitation regions. Subsequently, high-fidelity acoustic simulations are conducted to interpret reflected ultrasound data.As main results, the EHL leads to a "double-peak with central valley" pattern in the reflection coefficient distribution. While the cavitation causes the central valley to shift toward the inlet region and increases the reflection coefficient.Accordingly, the central film thickness is extracted from the distribution of the reflection coefficient under different operating conditions. Experimental validation using both glass-oil-steel and steel-oil-steel bearing setups confirms the effectiveness of the proposed method. The high-resolution fluorescence measurement adopted in the glass-oil-steel configuration validates the simulation of the reflection coefficient distribution. Furthermore, the theoretical EHL calculations are employed with the steel-oil-steel configuration for validation of the measurement accuracy of central oil film thickness.

physics.ins-det↗

arXiv2024

Three-Dimensional, Multimodal Synchrotron Data for Machine Learning Applications

Machine learning techniques are being increasingly applied in medical and physical sciences across a variety of imaging modalities; however, an important issue when developing these tools is the availability of good quality training data. Here we present a unique, multimodal synchrotron dataset of a bespoke zinc-doped Zeolite 13X sample that can be used to develop advanced deep learning and data fusion pipelines. Multi-resolution micro X-ray computed tomography was performed on a zinc-doped Zeolite 13X fragment to characterise its pores and features, before spatially resolved X-ray diffraction computed tomography was carried out to characterise the homogeneous distribution of sodium and zinc phases. Zinc absorption was controlled to create a simple, spatially isolated, two-phase material. Both raw and processed data is available as a series of Zenodo entries. Altogether we present a spatially resolved, three-dimensional, multimodal, multi-resolution dataset that can be used for the development of machine learning techniques. Such techniques include development of super-resolution, multimodal data fusion, and 3D reconstruction algorithm development.

cs.LG↗

arXiv2024

Nanoscale Surfactant Transport: Bridging Molecular and Continuum Models

Surfactant transport is central to a diverse range of natural phenomena, and for many practical applications in physics and engineering. Surprisingly, this process remains relatively poorly understood at the molecular scale. This study investigates the mechanism behind the transport of surfactant monolayers on flat and curved liquid vapor interfaces using nonequilibrium molecular dynamics simulations, which are compared with the continuum transport model. This approach not only provides fresh molecular level insight into surfactant dynamics, but also confirms the nanoscale mechanism of the lateral migration of surfactant molecules along a thin film that continuously deforms as surfactants spread. By connecting the continuum model where the long wave approximations prevail, to the molecular details where such approximations break down, we establish that the transport equation preserves substantial accuracy in capturing the underlying physics. Moreover, the relative importance of the different mechanisms of the transport process are identified. Consequently, we derive a novel, exact molecular equation for surfactant transport along a deforming surface. Finally, our findings demonstrate that the spreading of surfactants at the molecular scale adheres to expected scaling laws and aligns well with experimental observations.

cond-mat.soft↗

arXiv2024

LMI-based robust model predictive control for a quarter car with series active variable geometry suspension

This paper proposes a robust model predictive control-based solution for the recently introduced series active variable geometry suspension (SAVGS) to improve the ride comfort and road holding of a quarter car. In order to close the gap between the nonlinear multi-body SAVGS model and its linear equivalent, a new uncertain system characterization is proposed that captures unmodeled dynamics, parameter variation, and external disturbances. Based on the newly proposed linear uncertain model for the quarter car SAVGS system, a constrained optimal control problem (OCP) is presented in the form of a linear matrix inequality (LMI) optimization. More specifically, utilizing semidefinite relaxation techniques a state-feedback robust model predictive control (RMPC) scheme is presented and integrated with the nonlinear multi-body SAVGS model, where state-feedback gain and control perturbation are computed online to optimise performance, while physical and design constraints are preserved. Numerical simulation results with different ISO-defined road events demonstrate the robustness and significant performance improvement in terms of ride comfort and road holding of the proposed approach, as compared to the conventional passive suspension, as well as, to actively controlled SAVGS by a previously developed conventional H-infinity control scheme.

eess.SY↗

arXiv2023

TTCF4LAMMPS: A toolkit for simulation of the non-equilibrium behaviour of molecular fluids at experimentally accessible shear rates

We present TTCF4LAMMPS, a toolkit for performing non-equilibrium molecular dynamics (NEMD) simulations to study fluid behaviour at low shear rates using the LAMMPS software. By combining direct NEMD simulations and the transient-time correlation function (TTCF) technique, we study the behaviour of fluids over shear rates spanning $15$ orders of magnitude. We present two example systems consisting of simple monatomic systems: one containing a bulk liquid and another with a liquid layer confined between two solid walls. The small bulk system is suitable for testing on personal computers, while the larger confined system requires high-performance computing (HPC) resources. We demonstrate that the TTCF formalism can successfully detect the system response for arbitrarily weak external fields. We provide a brief mathematical explanation for this feature. Although we showcase the method for simple monatomic systems, TTCF can be readily extended to study more complex molecular fluids. Moreover, in addition to shear flows, the method can be extended to investigate elongational or mixed flows as well as thermal or electric fields. The reasonably high computational cost needed for the method is offset by the two following benefits: i) the cost is independent of the magnitude of the external field, and ii) the simulations can be made highly efficient on HPC architectures by exploiting the parallel design of LAMMPS. We expect the toolkit to be useful for computational researchers striving to study the nonequilibrium behaviour of fluids under experimentally-accessible conditions.

cond-mat.soft↗

arXiv2023

Life and Death of a Thin Liquid Film

Thin films, bubbles and membranes are central to numerous natural and engineering processes, i.e., in thin-film solar cells, coatings, biosensors, electrowetting displays, foams, and emulsions. Yet, the characterization and an adequate understanding of their rupture is limited by the scarcity of atomic detail. We present here the complete life-cycle of freely suspended films using non-equilibrium molecular dynamics simulations of a simple atomic fluid free of surfactants and surface impurities, thus isolating the fundamental rupture mechanisms. Counter to the conventional notion that rupture occurs randomly, we discovered a short-term 'memory' by rewinding in time from a rupture event, extracting deterministic behaviors from apparent stochasticity. A comprehensive investigation of the key rupture-stages including both unrestrained and frustrated propagation is made - characterization of the latter leads to a first-order correction to the classical film-retraction theory. Furthermore, the highly resolved time window reveals that the different modes of the morphological development, typically characterized as heterogeneous nucleation and spinodal decomposition, continuously evolve seamlessly with time from one into the other.

physics.flu-dyn↗

arXiv2023

Advanced Modeling of Lubricated Interfaces in General Curvilinear Grids

Tackling fluid-flow problems involving intricate surface geometries has been the catalyst for a plethora of numerical investigations aimed at accommodating curved complex boundaries. An example is the application of body-fitted curvilinear coordinate transformation, where the one-to-one correspondence of grid points from the physical to the computational domain is achieved. In lubricated interfaces, such conversion is challenging due to the complex governing equations in the mapped-grid, the numerical instabilities exhibited by their non-linearities and the severity of operating conditions. The present contribution proposes a Reynolds-based, finite volume fluid-structure interaction (FSI) framework for solving thermal elastohydrodynamic lubrication (TEHL) problems mapped onto non-orthogonal curvilinear grids in the computational domain. We demonstrate how the strong conservation form of the pertinent governing equations can be expressed in three-dimensional curvilinear grids and discretised using finite volume method to ensure fluid-flow conservation and enforce mass-conserving cavitation conditions. Numerical and experimental benchmarks showcase the robustness and versatility of the proposed framework to simulate a diverse range of lubrication problems, hence achieving a predictive computational tool that would enable a shift towards tribology-aware design.

physics.flu-dyn↗

arXiv2022

Thin Film Rupture from the Atomic Scale

The retraction of thin films, as described by the Taylor-Culick (TC) theory, is subject to widespread debate, particularly for films at the nanoscale. We use non-equilibrium molecular dynamics simulations to explore the validity of the assumptions used in continuum models, by tracking the evolution of holes in a film. By deriving a new mathematical form for the surface shape and considering a locally varying surface tension at the front of the retracting film, we reconcile the original theory with our simulation data to recover a corrected TC speed valid at the nanoscale.

physics.flu-dyn↗

arXiv2022

Quantifying Wetting Dynamics with Triboelectrification

Wetting is often perceived as an intrinsic surface property of materials, but determining its evolution is complicated by its complex dependence on roughness across the scales. The Wenzel state, where liquids have intimate contact with the rough substrate, and the Cassie-Baxter state, where liquids sit onto air pockets formed between asperities, are only two states among the plethora of wetting behaviors. Furthermore, transitions from the Cassie-Baxter to the Wenzel state dictate completely different surface performance, such as anti-contamination, anti-icing, drag reduction etc.; however, little is known about how transition occurs during time between the several wetting modes. In this paper, we show that wetting dynamics can be accurately quantified and tracked using solid-liquid triboelectrification. Theoretical underpinning reveals how surface micro-/nano-geometries regulate stability/infiltration, also demonstrating the generality of our theoretical approach in understanding wetting transitions.

cond-mat.soft↗

arXiv2022

A new finite element paradigm to solve contact problems with roughness

This article's main scope is the presentation of a computational method for the simulation of contact problems within the finite element method involving complex and rough surfaces. The approach relies on the MPJR (eMbedded Profile for Joint Roughness) interface finite element proposed in [arXiv:1805.07207], which is nominally flat but can embed at the nodal level any arbitrary height to reconstruct the displacement field due to contact in the presence of roughness. Here, the formulation is generalized to handle 3D surface height fields and any arbitrary nonlinear interface constitutive relation, including friction and adhesion. The methodology is herein validated with BEM solutions for linear elastic contact problems. Then, a selection of nonlinear contact problems prohibitive to be simulated by BEM and by standard contact algorithms in FEM are detailed, to highlight the promising aspects of the proposed method for tribology.

cond-mat.soft↗

arXiv2022

The Intrinsic Fragility of the Liquid-Vapor Interface: A Stress Network Perspective

The evolution of the liquid-vapour interface of a Lennard-Jones fluid is examined with molecular dynamics simulations using the intrinsic sampling method. Results suggest, in agreement with capillary wave theory, clear damping of the density profiles as the temperature is increased. We identify a linear variation of the space-filling nature (fractal dimension) of the stress-clusters at the intrinsic surface with increasing surface tension, or equivalently, with decreasing temperature. A percolation analysis of these stress networks indicates that the stress field is more disjointed at higher temperatures. This leads to more fragile interfaces that result in a reduction in surface tension at higher temperature.

physics.flu-dyn↗

arXiv2022

On the Origin of Plasticity-Induced Microstructure Change under Sliding Contacts

Discrete dislocation plasticity (DDP) calculations are carried out to investigate the response of a single crystal contacted by a rigid sinusoidal asperity under sliding loading conditions to look for causes of microstructure change in the dislocation structure. The mechanistic driver is identified as the development of lattice rotations and stored energy in the subsurface, which can be quantitatively correlated to recent tribological experimental observations. Maps of surface slip initiation and substrate permanent deformation obtained from DDP calculations for varying contact size and normal load suggest ways of optimally tailoring the interface and microstructural material properties for various frictional loads.

cond-mat.mtrl-sci↗

arXiv2022

Mu-synthesis PID Control of Full-Car with Parallel Active Link Suspension Under Variable Payload

This paper presents a combined mu-synthesis PID control scheme, employing a frequency separation paradigm, for a recently proposed novel active suspension, the Parallel Active Link Suspension (PALS). The developed mu-synthesis control scheme is superior to the conventional H-infinity control, previously designed for the PALS, in terms of ride comfort and road holding (higher frequency dynamics), with important realistic uncertainties, such as in vehicle payload, taken into account. The developed PID control method is applied to guarantee good chassis attitude control capabilities and minimization of pitch and roll motions (low frequency dynamics). A multi-objective control method, which merges the aforementioned PID and mu-synthesis-based controls is further introduced to achieve simultaneously the low frequency mitigation of attitude motions and the high frequency vibration suppression of the vehicle. A seven-degree-of-freedom Sport Utility Vehicle (SUV) full car model with PALS, is employed in this work to test the synthesized controller by nonlinear simulations with different ISO-defined road events and variable vehicle payload. The results demonstrate the control scheme's significant robustness and performance, as compared to the conventional passive suspension as well as the actively controlled PALS by conventional H-infinity control, achieved for a wide range of vehicle payload considered in the investigation.

eess.SY↗

arXiv2022

Feedforward PID Control of Full-Car with Parallel Active Link Suspension for Improved Chassis Attitude Stabilization

PID control is commonly utilized in an active suspension system to achieve desirable chassis attitude, where, due to delays, feedback information has much difficulty regulating the roll and pitch behavior, and stabilizing the chassis attitude, which may result in roll over when the vehicle steers at a large longitudinal velocity. To address the problem of the feedback delays in chassis attitude stabilization, in this paper, a feedforward control strategy is proposed to combine with a previously developed PID control scheme in the recently introduced Parallel Active Link Suspension (PALS). Numerical simulations with a nonlinear multi-body vehicle model are performed, where a set of ISO driving maneuvers are tested. Results demonstrate the feedforward-based control scheme has improved suspension performance as compared to the conventional PID control, with faster speed of convergence in brake in a turn and step steer maneuvers, and surviving the fishhook maneuver (although displaying two-wheel lift-off) with 50 mph maneuver entrance speed at which conventional PID control rolls over.

eess.SY↗