We numerically study the dynamics of a passive fluid droplet confined within a microchannel whose walls are covered with a thin layer of active gel. The latter represents a fluid of extensile material modelling, for example, a suspension of cytoskeletal filaments and molecular motors. Our results show that the layer is capable of producing a spontaneous flow triggering a rectilinear motion of the passive droplet. For a hybrid design (a single wall covered by the active layer), at the steady state the droplet attains an elliptical shape, resulting from an asymmetric saw-toothed structure of the velocity field. In contrast, if the active gel covers both walls, the velocity field exhibits a fully symmetric pattern considerably mitigating morphological deformations. We further show that the structure of the spontaneous flow in the microchannel can be controlled by the anchoring conditions of the active gel at the wall. These findings are also confirmed by selected 3D simulations. Our results may stimulate further research addressed to design novel microfludic devices whose functioning relies on the collective properties of active gels.

Active fluid droplets are a class of soft materials exhibiting autonomous motion sustained by an energy supply. Such systems have been shown to capture motility regimes typical of biological cells and are ideal candidates as building-block for the fabrication of soft biomimetic materials of interest in pharmacology, tissue engineering and lab on chip devices. While their behavior is well established in unconstrained environments, much less is known about their dynamics under strong confinement. Here, we numerically study the physics of a droplet of active polar fluid migrating within a microchannel hosting a constriction with adhesive properties, and report evidence of a striking variety of dynamic regimes and morphological features, whose properties crucially depend upon droplet speed and elasticity, degree of confinement within the constriction and adhesiveness to the pore. Our results suggest that non-uniform adhesion forces are instrumental in enabling the crossing through narrow orifices, in contrast to larger gaps where a careful balance between speed and elasticity is sufficient to guarantee the transition. These observations may be useful for improving the design of artificial micro-swimmers, of interest in material science and pharmaceutics, and potentially for cell sorting in microfluidic devices.

A regularized version of the lattice Boltzmann method for efficient simulation of soft materials is introduced. Unlike standard approaches, this method reconstructs the distribution functions from available hydrodynamic variables (density, momentum, and pressure tensor) without storing the full set of discrete populations. This scheme shows significantly lower memory requirements and data access costs. A series of benchmark tests of relevance to soft matter, such as collisions of fluid droplets, is discussed to validate the method. The results can be of particular interest for high-performance simulations of soft matter systems on future exascale computers.

We present thread-safe, highly-optimized lattice Boltzmann implementations, specifically aimed at exploiting the high memory bandwidth of GPU-based architectures. At variance with standard approaches to LB coding, the proposed strategy, based on the reconstruction of the post-collision distribution via Hermite projection, enforces data locality and avoids the onset of memory dependencies, which may arise during the propagation step, with no need to resort to more complex streaming strategies. The thread-safe lattice Boltzmann achieves peak performances, both in two and three dimensions and it allows to reduce significantly the memory footprint (tens of GigaBytes for order billions lattice nodes simulations) by retaining the algorithmic simplicity of standard LB computing. Our findings open attractive prospects for high-performance simulations of complex flows on GPU-based architectures.

Clathrate hydrates are crystalline inclusion compounds wherein a water framework encages small guest atoms/molecules within its cavities. Among the others, methane clathrates are the largest fossil fuel resource still available. They can also be used to safely transport gases and can also form spontaneously under suitable conditions plugging pipelines. Understanding the crystallization mechanism is very important, and given the impossibility of experimentally identifying the atomistic path, simulations played an important role in this field. Given the large computational cost of these simulations, in addition to all-atom force fields, scientists considered coarse-grained water models. Here, we have investigated the effect of coarse-graining, as implemented in the water model mW, on the crystallization characteristics of methane clathrate in comparison with the all-atom TIP4P force field. Our analyses revealed that although the characteristics directly depending on the energetics of the water models are well reproduced, dynamical properties are off by the orders of magnitude. Being crystallization a non-equilibrium process, the altered kinetics of the process results in different characteristics of crystalline nuclei. Both TIP4P and mW water models produce methane clathrate nuclei with some amount of the less stable (in the given thermodynamic conditions) structure II phase and an excess of pentagonal dodecahedral cages over the tetrakaidecahedral ones regarding the ideal ratio in structure I. However, the dependence of this excess on the methane concentration in solution is higher with the former water model, whereas with the latter, the methane concentration in solution dependence is reduced and within the statistical error.

Physiological solvent flows surround biological structures triggering therein collective motions. Notable examples are virus/host-cell interactions and solvent-mediated allosteric regulation. The present work describes a multiscale approach joining the Lattice Boltzmann fluid dynamics (for solvent flows) with the all-atom atomistic molecular dynamics (for proteins) to model functional interactions between flows and molecules. We present, as an applicative scenario, the study of the SARS-CoV-2 virus spike glycoprotein protein interacting with the surrounding solvent, modeled as a mesoscopic fluid. The equilibrium properties of the wild-type spike and of the Alpha variant in implicit solvent are described by suitable observables. The mesoscopic solvent description is critically compared to the all-atom solvent model, to quantify the advantages and limitations of the mesoscopic fluid description.

In the last decades kinetic theory has developed into a very elegant and effective framework to handle a broad spectrum of problems involving complex states of flowing matter, far beyond the original realm of rarefied gas dynamics. In this paper, we present recent applications of the lattice Boltzmann method to the computational design of soft mesoscale materials, including soft flowing crystals, dense multicore emulsions, as well as Petascale simulations of deep-sea glassy sponges. This manuscript is a tribute to the groundbreaking work of Carlo Cercignani and his undiminished impact on modern non-equilibrium statistical physics.

Preface to DSFD 2021, the 30th edition of the discrete simulation of fluid dynamics at the University of Tuscia, Viterbo, Italy, on September 13-17, 2021.

Active droplets are artificial microswimmers built from a liquid dispersion by microfluidic tools and showing self-propelled motion. These systems hold particular interest for mimicking biological phenomena, such as some aspects of cell locomotion and collective behaviors of bacterial colonies, as well as for the design of droplet-based biologically inspired materials, such as engineered tissues. Growing evidence suggests that geometrical confinement crucially affects their morphology and motility, but the driving physical mechanisms are still poorly understood. Here, we study the effect of activity on a droplet containing a contractile polar fluid confined within microfluidic channels of various sizes. We find a surprising wealth of shapes and dynamic regimes, whose mechanics is regulated by a subtle interplay between contractile stress, droplet elasticity, and microchannel width. They range from worm-like and cell-like shaped droplets displaying an oscillating behavior within wider channels to bullet-shaped droplets exhibiting rectilinear motion in narrower slits. Our findings support the view that geometrical confinement can provide a viable strategy to control and predict the propulsion direction of active droplets. It would be of interest to look for analogs of these motility modes in biological cells or in synthetic active matter.

By combining experiments and numerical simulations which model the dynamics of shaken atomic Bose-Einstein condensates, we reveal the surprising nature of quantum turbulence in these systems. Unlike the tangles of vortex lines described in the superfluid helium literature, we find that our turbulent atomic condensate contains a mixture of strong fragmented density fluctuations and small random vortex loops which are not homogeneously distributed. This unusual form of turbulence, with its own properties and scaling behavior, which we call strong quantum turbulence, is significantly different from the turbulence which is observed in either classical or other quantum systems, thus posing a new challenge in turbulence research.

Drain vortices are among the most common vortices observed in everyday life, yet their physics is complex due to the competition of vorticity's transport and diffusion, and the presence of viscous layers and a free surface. Recently, it has become possible to study experimentally drain vortices in liquid helium II, a quantum fluid whose physics is characterised by the absence of viscosity and the quantisation of the circulation in the superfluid component. Using the Gross-Pitaevskii equation, we make a simple model of the problem which captures the essential physics ingredients, showing that the drain vortex of a pure superfluid consists of a bundle of vortex lines which, in the presence of a radial drain, twist, thus strengthening the axial flow into the drain.

We collect and describe the observed geometrical and dynamical properties of turbulence in quantum fluids, particularly superfluid helium and atomic condensates for which more information about turbulence is available. Considering the spectral features, the temporal decay, and the comparison with relevant turbulent classical flows, we identify three main limiting types of quantum turbulence: Kolmogorov quantum turbulence, Vinen quantum turbulence, and strong quantum turbulence. This classification will be useful to analyze and interpret new results in these and other quantum fluids.

When the intensity of turbulence is increased (by increasing the Reynolds number, e.g., by reducing the viscosity of the fluid), the rate of the dissipation of kinetic energy decreases but does not tend asymptotically to zero: it levels off to a nonzero constant as smaller and smaller vortical flow structures are generated. This fundamental property, called the dissipation anomaly, is sometimes referred to as the zeroth law of turbulence. The question of what happens in the limit of vanishing viscosity (purely hypothetical in classical fluids) acquires a particular physical significance in the context of liquid helium, a quantum fluid which becomes effectively inviscid at low temperatures achievable in the laboratory. By performing numerical simulations and identifying the superfluid Reynolds number, here we show evidence for a superfluid analog to the classical dissipation anomaly. Our numerics indeed show that as the superfluid Reynolds number increases, smaller and smaller structures are generated on the quantized vortex lines on which the superfluid vorticity is confined, balancing the effect of weaker and weaker dissipation.

We show that a toroidal bundle of quantized vortex rings in superfluid helium generates a large-scale wake in the normal fluid which reduces the overall friction experienced by the bundle, thus greatly enhancing its lifetime, as observed in experiments. This collective effect is similar to the drag reduction observed in systems of active, hydrodynamically cooperative agents such as bacteria in aqueous suspensions, fungal spores in the atmosphere, and cyclists in pelotons.

This Letter proposes a solution of the Vacuum Energy and the Cosmological Constant (CC)paradox based on the Zel'dovich's ansatz, which states that the observable contribution to thevacuum energy density is given by the gravitational energy of virtual particle-antiparticle pairs,continually generated and annihilated in the vacuum state. The novelty of this work is the use of anultraviolet cut-off length based on the Holographic Principle, which is shown to yield current valuesof the CC in good agreement with experimental observations.

We compute to high post-Newtonian accuracy the 4-momentum (linear momentum and energy), radiatedas gravitational waves in a two-body system undergoing gravitational scattering. We include, for the firsttime, all the relevant time-asymmetric effects that arise when consistently going three post-Newtonianorders beyond the leading post-Newtonian order. We find that the inclusion of time-asymmetric radiativeeffects (both in tails and in the radiation-reacted hyperbolic motion) is crucial to ensure the masspolynomiality of the post-Minkowskian expansion (G expansion) of the radiated 4-momentum. Imposingthe mass polynomiality of the corresponding individual impulses determines the conservativelikeradiative contributions at the fourth post-Minkowskian order and strongly constrains them at the fifthpost-Minkowskian order.

We compute the leading order contribution to radiative losses in the case of spinning binaries with alignedspins due to their spin-orbit interaction. The orbital average along hyperboliclike orbits is taken through anappropriate spin-orbit modification to the quasi-Keplerian parametrization for nonspinning bodies, whichmaintains the same functional form, but with spin-dependent orbital elements. We perform consistencychecks with existing post-Newtonian-based and post-Minkowskian (PM)-based results. In the former case,we compare our expressions for both radiated energy and angular momentum with those obtained in [G. Choet al., From boundary data to bound states. Part III. Radiative effects,J. High Energy Phys. 04 (2022) 154] byapplying the boundary-to-bound correspondence to known results for ellipticlike orbits, finding agreement.The linear momentum loss is instead newly computed here. In the latter case, we also find agreement with thelow-velocity limit of recent calculations of the total radiated energy, angular momentum and linearmomentum in the framework of an extension of the worldline quantum field theory approach to the classicalscattering of spinning bodies at the leading PM order [G. U. Jakobsen et al., Gravitational Bremsstrahlungand Hidden Supersymmetry of Spinning Bodies, Phys. Rev. Lett. 128, 011101 (2022), M. M. Riva et al.,Gravitational bremsstrahlung from spinning binaries in the post-Minkowskian expansion, Phys. Rev. D 106,044013 (2022)]. We get exact expressions of the radiative losses in terms of the orbital elements, even if theyare at the leading post-Newtonian order, so that their expansion for large values of the eccentricity parameter(or equivalently of the impact parameter) provides higher-order terms in the corresponding PM expansion,which can be useful for future crosschecks of other approaches.

We compare recent one-loop-level, scattering-amplitude-based, computations of the classical partof the gravitational bremsstrahlung waveform to the frequency-domain version of the corresponding Multipolar-Post-Minkowskianwaveform result. When referring the one-loop result to the classical averaged momenta $\bar p_a = \frac12 (p_a+p'_a)$,the two waveforms are found to agree at the Newtonian and first post-Newtonian levels,as well as at the first-and-a-half post-Newtonian level, i.e. for the leading-order quadrupolar tail.However, we find that there are significant differences at the second-and-a-half post-Newtonian level,$O\left( \frac{G^2}{c^5} \right)$, i.e.when reaching: (i) the first post-Newtonian correction to the linear quadrupole tail; (ii) Newtonian-level linear tailsof higher multipolarity (odd octupole and even hexadecapole); (iii) radiation-reaction effects on the worldlines;and (iv) various contributions of cubically nonlinear origin (notably linked tothe quadrupole$\times$ quadrupole$\times$ quadrupole coupling in the wavezone).These differences are reflected at the sub-sub-sub-leading level in the soft expansion, $ \sim \om \ln \om $, i.e. $O\left(\frac{1}{t^2} \right)$in the time domain.Finally, we computed the first four terms of the low-frequency expansion of the Multipolar-Post-Minkowskian waveform and checkedthat they agree with the corresponding existing classical soft graviton results.

Based on a previous ansatz by Zel'dovich for the gravitational energy of virtualparticle-antiparticle pairs, supplemented with the Holographic Principle, we estimate the vacuumenergy in a fairly reasonable agreement with the experimental values of the Cosmological Constant.We further highlight a connection between Wheeler's quantum foam and graviton condensation,as contemplated in the quantum N-portrait paradigm, and show that such connection also leads toa satisfactory prediction of the value of the cosmological constant. The above results suggest thatthe "unnaturally" small value of the cosmological constant may find a quite "natural" explanationonce the nonlocal perspective of the large N-portrait gravitational condensation is endorsed.

The class of Petrov type I curvature tensors is further divided into those for whichthe span of the set of distinct principal null directions has dimension four (maximallyspanning type I) or dimension three (nonmaximally spanning type I). Explicit examplesare provided for both vacuum and nonvacuum spacetimes.