A critical analysis of the CFD-DEM simulation of pharmaceutical aerosols deposition in upper intra-thoracic airways: Considerations on air flow.

A well-corroborated numerical methodology ensuring reproducibility in the modeling of pharmaceutical aerosols deposition in the respiratory system via CFD-DEM simulations within the RANS framework is currently missing. Often, inadequately clarified assumptions and approximations and the lack of evidences on their quantitative impact on the simulated deposition phenomenology, make a direct comparison among the different theoretical studies and the limited number of experiments a very challenging task. Here, with the ultimate goal of providing a critical analysis of some crucial computational aspects of aerosols deposition, we address the issues of velocity fluctuations propagation in the upper intra-thoracic airways and of the persistence of secondary flows using the SimInhale reference benchmark. We complement the investigation by describing how methodologies used to drive the flow through a truncated lung model may affect numerical results and how small discrepancies are observed in velocity profiles when comparing simulations based on different meshing strategies.

A critical analysis of the CFD-DEM simulation of pharmaceutical aerosols deposition in extra-thoracic airways.

Recent advances in the characterization of the human respiratory system and in multi-phase flow dynamics in complex geometries have led numerical simulations to play an expanding role for exploring aerosol deposition mechanisms in the lungs. However, the development of an efficient numerical and mathematical description is far from unique, and determining which aspects of the modelling are critical and which details are essentially irrelevant is indeed a difficult task. With the aim of addressing this lack of a rationalized framework, we propose a systematic analysis of pharmaceutical aerosols deposition in the extra-thoracic airways, focusing on several important modelling aspects whose related assumptions and approximations have not always been sufficiently discussed and clarified. We consider the importance of intrinsic time dependent fluctuations of the air flow, highlighting how their contribution in aerosol deposition is as important as the particle-turbulence interaction one. We show how sensitive the turbulence intensity can be to the meshing strategy and how aerosol deposition can be influenced by the latter choice. We demonstrate how a swirling air jet can enhance extra-thoracic deposition compared to a straight one, and how different the deposition patterns can be in case a realistic inhalation profile and aerosol plume are employed.

Modeling nanoribbon peeling.

The lifting, peeling and exfoliation of physisorbed ribbons (or flakes) of 2D material such as graphene off a solid surface are common and important manoeuvres in nanoscience. The feature that makes this case peculiar is the structural lubricity generally realized by stiff 2D material contacts. We model theoretically the mechanical peeling of a nanoribbon of graphene as realized by the tip-forced lifting of one of its extremes off a flat crystal surface. The evolution of shape, energy, local curvature and body advancement are ideally expected to follow a succession of regimes: (A) initial prying, (B) peeling with stretching but without sliding (stripping), (C) peeling with sliding, (D) liftoff. In the case where in addition the substrate surface corrugation is small or negligible, then (B) disappears, and we find that the (A)-(C) transition becomes universal, analytical and sharp, determined by the interplay between bending rigidity and adsorption energy. This general two-stage peeling transition is identified as a sharp crossover in published data of graphene nanoribbons pulled off an atomic-scale Au(111) substrate.

Lifted graphene nanoribbons on gold: from smooth sliding to multiple stick-slip regimes.

Graphene nanoribbons (GNRs) physisorbed on a Au(111) surface can be picked up, lifted at one end, and made to slide by means of the tip of an atomic-force microscope. The dynamic transition from smooth sliding to multiple stick-slip regimes, the pushing/pulling force asymmetry, the presence of pinning, and its origin are real frictional processes in a nutshell, in need of a theoretical description. To this purpose, we conduct classical simulations of frictional manipulations of a 30 nm-long GNR, one end of which is pushed or pulled horizontally while held at different heights above the Au surface. These simulations allow us to clarify theoretically the emergence of stick-slip originating from the short 1D edges rather than the 2D "bulk", the role of adhesion, of lifting, and of graphene bending elasticity in determining the GNR sliding friction. The understanding obtained in this simple context is of additional value for more general cases.

Friction and nonlinear dynamics.

The nonlinear dynamics associated with sliding friction forms a broad interdisciplinary research field that involves complex dynamical processes and patterns covering a broad range of time and length scales. Progress in experimental techniques and computational resources has stimulated the development of more refined and accurate mathematical and numerical models, capable of capturing many of the essentially nonlinear phenomena involved in friction.

The breakdown of superlubricity by driving-induced commensurate dislocations.

In the framework of a Frenkel-Kontorova-like model, we address the robustness of the superlubricity phenomenon in an edge-driven system at large scales, highlighting the dynamical mechanisms leading to its failure due to the slider elasticity. The results of the numerical simulations perfectly match the length critical size derived from a parameter-free analytical model. By considering different driving and commensurability interface configurations, we explore the distinctive nature of the transition from superlubric to high-friction sliding states which occurs above the critical size, discovering the occurrence of previously undetected multiple dissipative jumps in the friction force as a function of the slider length. These driving-induced commensurate dislocations in the slider are then characterized in relation to their spatial localization and width, depending on the system parameters. Setting the ground to scale superlubricity up, this investigation provides a novel perspective on friction and nanomanipulation experiments and can serve as a theoretical basis for designing high-tech devices with specific superlow frictional features.

Electrical charging effects on the sliding friction of a model nano-confined ionic liquid.

Recent measurements suggest the possibility to exploit ionic liquids (ILs) as smart lubricants for nano-contacts, tuning their tribological and rheological properties by charging the sliding interfaces. Following our earlier theoretical study of charging effects on nanoscale confinement and squeezout of a model IL, we present here molecular dynamics simulations of the frictional and lubrication properties of that model under charging conditions. First, we describe the case when two equally charged plates slide while being held together to a confinement distance of a few molecular layers. The shear sliding stress is found to rise strongly and discontinuously as the number of IL layers decreases stepwise. However, the shear stress shows, within each given number of layers, only a weak dependence upon the precise value of the normal load, a result in agreement with data extracted from recent experiments. We subsequently describe the case of opposite charging of the sliding plates and follow the shear stress when the charging is slowly and adiabatically reversed in the course of time, under fixed load. Despite the fixed load, the number and structure of the confined IL layers change with changing charge, and that in turn drives strong friction variations. The latter involves first of all charging-induced freezing of the IL film, followed by a discharging-induced melting, both made possible by the nanoscale confinement. Another mechanism for charging-induced frictional changes is a shift of the plane of maximum shear from mid-film to the plate-film interface, and vice versa. While these occurrences and results invariably depend upon the parameters of the model IL and upon its specific interaction with the plates, the present study helps identifying a variety of possible behavior, obtained under very simple assumptions, while connecting it to an underlying equilibrium thermodynamics picture.

Squeezout phenomena and boundary layer formation of a model ionic liquid under confinement and charging.

Electrical charging of parallel plates confining a model ionic liquid down to nanoscale distances yields a variety of charge-induced changes in the structural features of the confined film. That includes even-odd switching of the structural layering and charging-induced solidification and melting, with important changes of local ordering between and within layers, and of squeezout behavior. By means of molecular dynamics simulations, we explore this variety of phenomena in the simplest charged Lennard-Jones coarse-grained model including or excluding the effect a neutral tail giving an anisotropic shape to one of the model ions. Using these models and open conditions permitting the flow of ions in and out of the interplate gap, we simulate the liquid squeezout to obtain the distance dependent structure and forces between the plates during their adiabatic approach under load. Simulations at fixed applied force illustrate an effective electrical pumping of the ionic liquid, from a thick nearly solid film that withstands the interplate pressure for high plate charge to complete squeezout following melting near zero charge. Effective enthalpy curves obtained by integration of interplate forces versus distance show the local minima that correspond to layering and predict the switching between one minimum and another under squeezing and charging.

Sliding over a phase transition.

The effects of a displacive structural phase transition on sliding friction are in principle accessible to nanoscale tools such as atomic force microscopy, yet they are still surprisingly unexplored. We present model simulations demonstrating and clarifying the mechanism and potential impact of these effects. A structural order parameter inside the material will yield a contribution to stick-slip friction that is nonmonotonic as temperature crosses the phase transition, peaking at the critical T(c) where critical fluctuations are strongest, and the sliding-induced order-parameter local flips from one value to another more numerous. Accordingly, the friction below T(c) is larger when the order-parameter orientation is such that flips are more effectively triggered by the slider. The observability of these effects and their use for friction control are discussed, for future application to sliding on the surface of and ferro- or antiferrodistortive materials.

Nanofriction in cold ion traps.

Sliding friction between crystal lattices and the physics of cold ion traps are so far non-overlapping fields. Two sliding lattices may either stick and show static friction or slip with dynamic friction; cold ions are known to form static chains, helices or clusters, depending on the trapping conditions. Here we show, based on simulations, that much could be learnt about friction by sliding, through, for example, an electric field, the trapped ion chains over a corrugated potential. Unlike infinite chains, in which the theoretically predicted Aubry transition to free sliding may take place, trapped chains are always pinned. Yet, a properly defined static friction still vanishes Aubry-like at a symmetric-asymmetric structural transition, found for decreasing corrugation in both straight and zig-zag trapped chains. Dynamic friction is also accessible in ringdown oscillations of the ion trap. Long theorized static and dynamic one-dimensional friction phenomena could thus become accessible in future cold ion tribology.

Controlling microscopic friction through mechanical oscillations.

We study in detail the recent suggestions by Tshiprut [Phys. Rev. Lett. 95, 016101 (2005)] to tune tribological properties at the nanoscale by subjecting a substrate to periodic mechanical oscillations. We show that both in stick-slip and sliding regimes of motion friction can be tuned and reduced by controlling the frequency and amplitude of the imposed substrate lateral excitations. We demonstrate that the mechanisms of oscillation-induced reduction of friction are different for stick-slip and sliding dynamics. In the first regime the effect results from a giant enhancement of surface diffusion, while in the second regime it is due to the interplay between washboard and oscillation frequencies that leads to the occurrence of parametric resonances. Moreover, we show that for a particular set of parameters it is possible to sustain the motion with only the oscillations.

Dynamic hysteresis of a confined lubricant under shear.

A nonlinear model inspired by the tribological problem of a thin solid lubricant layer between two sliding periodic surfaces is used to analyze the novel phenomenon of hysteresis at pinning or depinning around a moving state rather than around a statically pinned state. The cycling of an external driving force F_{ext} is used as a simple means to destroy and then to recover the dynamically pinned state previously discovered for the lubricant center-of-mass velocity. Depinning to a freely sliding state occurs either directly, with a single jump, or through a sequence of discontinuous transitions. The intermediate sliding steps are reminiscent of phase-locked states and stick-slip motion in static friction, and can be interpreted in terms of the appearance of traveling density defects in an otherwise regular arrangement of kinks. Repinning occurs more smoothly, through the successive disappearance of different traveling defects. The resulting bistability and multistability regions may also be accessed by varying mechanical parameters other than F_{ext} . The hysteretic phenomena are confined to the underdamped dynamics, and the overdamped dynamics of the same model is generally not hysteretic, much like in static friction.

Static friction on the fly: velocity depinning transitions of lubricants in motion.

The dragging velocity of a model solid lubricant confined between sliding periodic substrates exhibits a phase transition between two regimes, respectively, with quantized and with continuous lubricant center-of-mass velocity. The transition, occurring for increasing external driving force F ext acting on the lubricant, displays a large hysteresis, and has the features of depinning transitions in static friction, only taking place on the fly. Although different in nature, this phenomenon appears isomorphic to a static Aubry depinning transition in a Frenkel-Kontorova model, the role of particles now taken by the moving kinks of the lubricant-substrate interface. We suggest a possible realization in 2D optical lattice experiments.

Transition from smooth sliding to stick-slip motion in a single frictional contact.

We show that the transition from smooth sliding to stick-slip motion in a single planar frictional junction always takes place at an atomic-scale relative velocity of the substrates.

Incommensurability of a confined system under shear.

We study a chain of harmonically interacting atoms confined between two sinusoidal substrate potentials, when the top substrate is driven through an attached spring with a constant velocity. This system is characterized by three inherent length scales and closely related to physical situations with confined lubricant films. We show that, contrary to the standard Frenkel-Kontorova model, the most favorable sliding regime is achieved by choosing chain-substrate incommensurabilities belonging to the class of cubic irrational numbers (e.g., the spiral mean). At large chain stiffness, the well known golden mean incommensurability reveals a very regular time-periodic dynamics with always higher kinetic friction values with respect to the spiral mean case.

Underdamped commensurate dynamics in a driven Frenkel-Kontorova-type model.

The dynamics of a Frenkel-Kontorova chain subject to a substrate potential with a multiple-well structure and driven by an external dc force is studied in the underdamped regime. Making a rational choice among the three inherent length scales characterizing the system allows us to consider the possible formation of commensurate structures during sliding over the complex on-site potential. We comment both on the nature of the particle dynamics in the vicinity of the pinning-depinning transition point, and on the dynamical states displayed during the chain motion at different strengths of the dc driving. Varying the number of particles in the simulations allows us to consider, on a multiple-well substrate, the role played by the coverage variable on the depinning mechanism. The dependence of the minimal force required to initiate the chain motion (static friction) on the ratio of the model interaction strengths is analyzed and compared to the well-known case of the standard Frenkel-Kontorova model, which has only two inherent lengths.

Driven, underdamped Frenkel-Kontorova model on a quasiperiodic substrate.

We consider the underdamped dynamics of a chain of atoms subject to a dc driving force and a quasiperiodic substrate potential. The system has three inherent length scales which we take to be mutually incommensurate. We find that when the length scales are related by the spiral mean (a cubic irrational) there exists a value of the interparticle interaction strength above which the static friction is zero. When the length scales are related by the golden mean (a quadratic irrational) the static friction is always nonzero. From considerations based on the connection of this problem to standard map theory, we postulate that zero static friction is generally possible for incommensurate ratios of the length scales involved. However, when the length scales are quadratic irrationals, or have some commensurability with each other, the static friction will be nonzero for all choices of interaction parameters. We also comment on the nature of the depinning mechanisms and the steady states achieved by the moving chain.

Spontaneous pattern formation in driven nonlinear lattices

We demonstrate the spontaneous formation of spatial patterns in a damped, ac-driven cubic Klein-Gordon lattice. These patterns are composed of arrays of intrinsic localized modes characteristic for nonlinear lattices. We analyze the modulation instability leading to this spontaneous pattern formation. Our calculation of the modulational instability is applicable in one- and two-dimensional lattices; however, in the analyses of the emerging patterns we concentrate particularly on the two-dimensional case.