Ballistic thermophoresis of adsorbates on free-standing graphene

The textbook thermophoretic force which acts on a body in a fluid is proportional to the local temperature gradient. The same is expected to hold for the macroscopic drift behavior of a diffusive cluster or molecule physisorbed on a solid surface. The question we explore here is whether that is still valid on a 2D membrane such as graphene at short sheet length. By means of a non-equilibrium molecular dynamics study of a test system -- a gold nanocluster adsorbed on free-standing graphene clamped between two temperatures $\Delta T$ apart -- we find a phoretic force which for submicron sheet lengths is parallel to, but basically independent of, the local gradient magnitude. This identifies a thermophoretic regime that is ballistic rather than diffusive, persisting up to and beyond a hundred nanometer sheet length. Analysis shows that the phoretic force is due to the flexural phonons, whose flow is known to be ballistic and distance-independent up to relatively long mean-free paths. Yet, ordinary harmonic phonons should only carry crystal momentum and, while impinging on the cluster, should not be able to impress real momentum. We show that graphene, and other membrane-like monolayers, support a specific anharmonic connection between the flexural corrugation and longitudinal phonons whose fast escape leaves behind a 2D-projected mass density increase endowing the flexural phonons, as they move with their group velocity, with real momentum, part of which is transmitted to the adsorbate through scattering. The resulting distance-independent ballistic thermophoretic force is not unlikely to possess practical applications

Friction anomalies at first-order transition spinodals: 1T-TaS2

Revealing phase transitions of solids through mechanical anomalies in the friction of nanotips sliding on their surfaces, a successful approach for continuous transitions, is still an unexplored tool for first-order ones. Owing to slow nucleation, first-order structural transformations occur with hysteresis, comprised between two spinodal temperatures where, on both sides of the thermodynamic transition, one or the other metastable free energy branches terminates. The spinodal transformation, a collective one-shot event without heat capacity anomaly, is easy to trigger by a weak external perturbation. Here we show that even the gossamer mechanical action of an AFM-tip can locally act as a trigger, narrowly preempting the spontaneous spinodal transformation, and making it observable as a nanofrictional anomaly. Confirming this expectation, the CCDW-NCCDW first-order transition of the important layer compound 1T-TaS2 is shown to provide a demonstration of this effect

Study of structures and thermodynamics of CuNi nanoalloys using a new DFT-fitted atomistic potential

Shape, stability and chemical ordering patterns of CuNi nanoalloys are studied as a function of size, composition and temperature. A new parametrization of an atomistic potential for CuNi is developed on the basis of ab initio calculations. The potential is validated against experimental bulk properties, and ab initio results for nanoalloys of sizes up to 147 atoms and for surface alloys. The potential is used to determine the chemical ordering patterns of nanoparticles with diameters of up to 3 nm and different structural motifs (decahedra, truncated octahedra and icosahedra), both in the ground state and in a wide range of temperatures. The results show that the two elements do not intermix in the ground state, but there is a disordering towards solid-solution patterns in the core starting from room temperature. This order-disorder transition presents different characteristics in the icosahedral, decahedral and fcc nanoalloys

Solid-solid transitions in Pd-Pt nanoalloys

Solid-solid transformations in Pd-Pt nanoalloys in the size range 32-38 atoms and for different compositions are computationally studied by the superposition approximation to the partition function, and by molecular dynamics simulations. A broad spectrum of transition types is shown to take place. These transition types are: (i) one-to-one type, in which the global minimum, which is dominant at low temperatures, transforms into another single isomer with increasing temperature; (ii) one-to-many type, in which the transition is from a single isomer to a family of other isomers; (iii) many-to-many type, in which the transition is between two different families of isomers; (iv) many-to-one type, in which the effect of vibrational entropy is to greatly reduce the number of relevant structures with increasing temperatures. We provide a rationale for these behaviors, which stem from the interplay between energetics and vibrational entropy effects. The vibrational entropy is explained by analyzing the vibrational density of states and the specific features of the normal modes. Quantum effects on the structural transitions are also discussed

Phase Separation in AgCu and AgNi Core\u2013Shell Icosahedral Nanoparticles: A Harmonic Thermodynamics Study

The temperature dependence of equilibrium chemical ordering in Ag-rich AgCu and AgNi icosahedral nanoalloys is numerically studied within the harmonic superposition approximation to the partition function. The nanoalloys are modeled by an atomistic force field derived from the second-moment approximation to the tight-binding model. It is found that the equilibrium chemical ordering consists of phase-separated arrangements of the core\u2013shell type, at least up to room temperature for AgCu and up to 600 K for AgNi. This result is rationalized in terms of the concept of preferred nucleation sites for phase separation, such as the central site in the icosahedron

Collective foraging of active particles trained by reinforcement learning

Abstract Collective self-organization of animal groups is a recurring phenomenon in nature which has attracted a lot of attention in natural and social sciences. To understand how collective motion can be achieved without the presence of an external control, social interactions have been considered which regulate the motion and orientation of neighbors relative to each other. Here, we want to understand the motivation and possible reasons behind the emergence of such interaction rules using an experimental model system of light-responsive active colloidal particles (APs). Via reinforcement learning (RL), the motion of particles is optimized regarding their foraging behavior in presence of randomly appearing food sources. Although RL maximizes the rewards of single APs, we observe the emergence of collective behaviors within the particle group. The advantage of such collective strategy in context of foraging is to compensate lack of local information which strongly increases the robustness of the resulting policy. Our results demonstrate that collective behavior may not only result on the optimization of behaviors on the group level but may also arise from maximizing the benefit of individuals. Apart from a better understanding of collective behaviors in natural systems, these results may also be useful in context of the design of autonomous robotic systems

Stable cooperation emerges in stochastic multiplicative growth

Understanding the evolutionary stability of cooperation is a central problem in biology, sociology, and economics. There exist only a few known mechanisms that guarantee the existence of cooperation and its robustness to cheating. Here, we introduce a new mechanism for the emergence of cooperation in the presence of fluctuations. We consider agents whose wealth change stochastically in a multiplicative fashion. Each agent can share part of her wealth as public good, which is equally distributed among all the agents. We show that, when agents operate with long time-horizons, cooperation produce an advantage at the individual level, as it effectively screens agents from the deleterious effect of environmental fluctuations.Comment: 7 pages, 3 figure

MARTINI Coarse-Grained Models of Polyethylene and Polypropylene

The understanding of the interaction of nanoplastics with living organisms is crucial both to assess the health hazards of degraded plastics and to design functional polymer nanoparticles with biomedical applications. In this paper, we develop two coarse-grained models of everyday use polymers, polyethylene (PE) and polypropylene (PP), aimed at the study of the interaction of hydrophobic plastics with lipid membranes. The models are compatible with the popular MARTINI force field for lipids, and they are developed using both structural and thermodynamic properties as targets in the parametrization. The models are then validated by showing their reliability at reproducing structural properties of the polymers, both linear and branched, in dilute conditions, in the melt, and in a PE-PP blend. PE and PP radius of gyration is correctly reproduced in all conditions, while PE-PP interactions in the blend are slightly overestimated. Partitioning of PP and PE oligomers in phosphatidylcholine membranes as obtained at CG level reproduces well atomistic data

Nanoscale Effects on Phase Separation

Classical nucleation theory predicts that a binary system which is immiscible in the bulk should become miscible at the nanoscale when lowering its size below a critical size. Here we tackle the problem of miscibility in nanoalloys with a combination of ab initio and atomistic calculations, developing a statistical-mechanics approach for the free energy cost of forming phase-separated aggregates. We apply it to the controversial case of AuCo nanoalloys. AuCo is immiscible in the bulk, but a rich variety of nanoparticle configurations, both phase-separated and intermixed, have been obtained experimentally. Our calculations strongly point to the permanence of an equilibrium miscibility gap down to the nanoscale and to the nonexistence of a critical size below which phase separation is impossible. We show that this is due to nanoscale effects of general character, caused by the existence of preferred nucleation sites in nanoparticles, which lower the free-energy cost for phase separation with respect to bulk systems