Formation of micron-sized droplets on open surfaces continues to remain a challenge in microfluidics. The problem is even stiffer for highly spreading liquids. Here, we report the formation of microdroplets from a nanoscopically thick film of low surface tension and low-viscosity liquid following its spreading under high-frequency nanoscale acoustic wave excitation. Uniquely, these droplets are observed to “surf” on a thin air layer atop the liquid film along the direction of the acoustic wave. Using theoretical scaling and numerical simulations we explore this remarkable behavior and show that the droplet generates via shear-driven pinch off from spatially periodic fluid protrusions in the film, specified by the acoustic excitation wavelength in the solid (${\lambda }_{SAW}$). We also predict the drop size ($d_d$) and noncoalesced drop velocity ($U_d$) from theory and find an excellent match with experiments. Further, the drop “surfing” dynamics is found to be a consequence of the acoustic radiation pressure imposed on the droplets as the wave is transmitted through the film and into the overlying air phase.

The fundamental theoretical problem of the existence of plasma equilibrium in a nonsymmetric magnetic field with nested magnetic surfaces is resolved. The lack of examples of smooth solutions to the equilibrium equations with no symmetry of the configuration has for many years served as the argument supporting the hypothesis by Harold Grad that nondegenerated three-dimensional plasma equilibrium does not exist. This paper first presents explicit analytical counterexamples to Grad's hypothesis. Within the standard formulation of the plasma equilibrium problem, we obtain the family of smooth solutions to the equilibrium equations. These solutions describe the set of “true” nonsymmetric magnetic surfaces compatible with continuous profiles of plasma pressure and rotational transform. The convenient system of equilibrium equations is used in the form generalizing the Grad-Shafranov approach to the case of nonaxisymmetric magnetic configurations without involving the formalism of flux coordinates. The inapplicability of simple prototype models of slab or cylindrical topologies for the predictive conclusions about the equilibrium of toroidal plasma is also revealed and clearly demonstrated. The developed formalism proves the fallacy of Grad's hypothesis and opens opportunities for adequate modeling of three-dimensional equilibrium plasma configurations.

We have experimentally studied the formation of shock waves in a laser-induced annular configuration. By locating an aluminum target in the focal plane of a ${10}^9\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ laser, an annular plasma is formed that acts as a piston for a shock wave in a background gas composed by either argon or nitrogen. According to shock wave dynamics, the annular shock follows a two-dimensional propagation profile that is attributed to the boundary conditions of the piston. In addition, we observe the formation of a Mach stem at the center of the configuration, at a position and timing predictable by the adiabatic index of the background gas and shock wave profile. This controlled laboratory setting for producing Mach stems can be used as a benchmark platform for testing conjectures on the observations of clumpy features in astrophysical objects. Moreover, the observed Mach stem propagates as a one-dimensional shock wave given the boundary conditions imposed by the annular configuration.

Recently, fictitious identical particles have provided a promising way to overcome the fermion sign problem and have been used in path integral Monte Carlo to accurately simulate warm dense matter with up to 1000 electrons [T. Dornheim et al., J. Phys. Chem. Lett. 15, 1305 (2024)]. The inclusion of fictitious identical particles in path integral molecular dynamics (PIMD) can provide another way to simulate fermion systems. In a recent paper [Y. M. Y. Feldman et al., J. Chem. Phys. 159, 154107 (2023)], Feldman and Hirshberg improved the recursive formula for PIMD of identical bosons, significantly reducing the computational complexity. In this paper, we extend this latest recursive formula for bosons to PIMD of fictitious identical particles to improve the efficiency of simulating fermion systems. We also provide the virial estimator for calculating energy of fictitious identical particles by using the new recursive technique, which can help suppress fluctuations in energy calculation. As an example, we use the quadratic scaling PIMD for fictitious identical particles to study the simulation of hundreds and even thousands of fermions in a two-dimensional periodic potential, in the hope of providing a simulation tool for the large-scale Fermi-Hubbard model and other strongly correlated fermion systems, such as the simulation of ultracold fermionic gases in optical lattices and the extended Fermi-Hubbard model using quantum dots.

We have studied the fluctuation (noise) in the position of sliding blocks under constant driving forces on different substrate surfaces. The experimental data are complemented by simulations using a simple spring-block model where the asperity contact regions are modeled by miniblocks connected to the big block by viscoelastic springs. The miniblocks experience forces that fluctuate randomly with the lateral position, simulating the interaction between asperities on the block and the substrate. The theoretical model provides displacement power spectra that agree well with the experimental results.

The problem of estimating entropy production from incomplete information in stochastic thermodynamics is essential for theory and experiments. Whereas a considerable amount of work has been done on this topic, arguably, most of it is restricted to the case of nonequilibrium steady states driven by a fixed thermodynamic force. Based on a recent method that has been proposed for nonequilibrium steady states, we obtain an estimate of the entropy production based on the statistics of visible transitions and their waiting times for the case of periodically driven systems. The time dependence of transition rates in periodically driven systems produces several differences in relation to steady states, which is reflected in the entropy production estimation. More specifically, we propose an estimate that does depend on the time between transitions but is independent of the specific time of the first transition, thus it does not require tracking the protocol. Formally, this elimination of the timedependence of the first transition leads to an extra term in the inequality that involves the rate of entropy production and its estimate. We analyze a simple model of a molecular pump to understand the relation between the performance of the method and physical quantities such as energies, energy barriers, and thermodynamic affinity. Our results with this model indicate that the emergence of net motion in the form of a probability current in the space of states is a necessary condition for a relevant estimate of the rate of entropy production.

We introduce an aggregation process that is based on templating, where a specified number of constituent clusters must assemble on a larger scaffold aggregate for a reaction to occur. A simple example is a dimer scaffold, upon which two monomers meet and create another dimer, while dimers and larger clusters irreversibly join at mass-independent rates. In the mean-field approximation, templating aggregation has unusual kinetics in which the monomer density $m\left(t\right)$ and the density $c\left(t\right)$ of all clusters heavier than monomers decay with time as $c\sim m^2\sim t^{-2/3}$. These strongly contrast with the corresponding behaviors in conventional aggregation, where $c\sim m^{1/2}\sim t^{-1}$. We also treat three natural extensions of templating: (a) the reaction in which $L$ monomers meet and react on an $L$-mer scaffold to create two $L$-mers, (b) multistage scaffold reactions, and (c) templated ligation, in which clusters of all masses serve as scaffolds and binary aggregation does not occur.

Green areas are a crucial element in a city's evolution, improving citizens' lives, reducing the effects of climate change, and making possible the survival of other species in urban areas. Unfortunately, these effects are difficult to assess quantitatively for regulators, stakeholders, and experts, making the planning of city development. Here we present a method to estimate the impact of these areas on city life based on the network topology of the city itself and on a simple model of the dynamics of this structure. Movements between various areas of the city are simulated using an agent-based biased-diffusion process where citizens try to reach the nearest public green area (PGA) from their position, and the model is fed with real data about the density of populations in the cases of study. First, we define a centrality measure of city blocks based on average farness measured on the city network; this approach outperforms information based on the simple topology. We then improve this quantity by considering the occupation of PGAs, thereby providing a quantitative measure of PGA usage for regulators.

We investigate the dynamics of phase oscillators in the fully disordered Kuramoto model with couplings of defined asymmetry. The mean-field dynamics is reduced to a self-consistent stochastic single-oscillator problem which we analyze perturbatively and by numerical simulations. We elucidate the influence of the asymmetry on the correlation and response function of the system as well as on the distribution of the order parameter. The so-called volcano transition is shown to be robust with respect to a small degree of coupling asymmetry but to disappear when the antisymmetry in the couplings outweighs the symmetry.

We study the effect of network topology on the collective dynamics of an oscillator ensemble. Specifically, we explore explosive synchronization in a system of interacting star networks. Explosive synchronization is characterized by an abrupt transition from an incoherent state to a coherent state. In this paper, we couple multiple star networks through their hubs and study the emergent dynamics as a function of coupling strength. The dynamics of each node satisfies the equation of a Kuramoto oscillator. We observe that for a small interstar coupling strength, the hysteresis width between the forward and backward transition point is minimal, which increases with an increase in the interstar coupling strength. This observation is independent of the size of the network. Further, we find that the backward transition point is independent of the number of stars coupled together and the interstar coupling strength, which is also verified using the Watanabe and Strogatz theory.

Understanding computational capabilities of simple biological circuits, such as the regulatory circuits of single-cell organisms, remains an active area of research. Recent theoretical work has shown that a simple cross-talk architecture based on end-product inhibition can exhibit predictive behavior by learning fluctuation statistics of one or two environmental parameters. Here we extend this analysis to higher dimensions, i.e., a large number of fluctuating inputs. We show that a generalized version of the cross-talk architecture can learn not only the dominant direction of fluctuations, as shown previously, but also the subdominant modes, orienting its responsiveness spectrum to the fluctuation eigenmodes. We comment on the relevance of our results to living systems at other scales of organization, such as ecosystems of species competing for fluctuating resources.

We explore the impact of hydrodynamic interactions on the conformational and dynamical properties of externally driven active polymers with tangential forces using multiparticle collision dynamics simulations. This model applies to non-force-neutral motile polymers, such as biofilaments in motility assays with substrate-fixed molecular motors or colloidal chains driven externally by forces tangential to the backbone exerted through dynamic tweezing. By analyzing active filaments with varying degrees of flexibility, we find that fluid-mediated interactions significantly influence both their conformation and dynamics. These interactions cause polymer conformations to shrink relative to their dry counterparts, especially for semiflexible polymers at high activity levels, where the average size of wet chains becomes nearly three times smaller, due to local buckling of wet polymers. This hydrodynamic-induced shrinkage is a hallmark of active polymers, as fluid-mediated interactions do not affect conformational properties of passive polymers. Furthermore, for tangentially driven polymers where activity and conformation are coupled, hydrodynamic interactions significantly enhance the orientational and translational dynamics compared to their dry counterparts.

Underwater acoustic propagation has emerged as a focal point of research across various fields due to its low attenuation characteristics, particularly in applications such as underwater detection, monitoring, and imaging. Based on the orbital angular momentum (OAM) degree of freedom, an intriguing technique known as the acoustic vortex (AV) based communication is developed to enhance underwater communication capacity. At present, precise directional information transfer and concentration of signal intensity are crucial to guarantee the performance of underwater AV communication. Considering that the vortex beams with curved or inclined trajectories could exhibit unique advantages in resolving the scenarios like the obstacles obstructing the propagation path, this paper intends to offer a general scheme to actively tune the propagation of the AV beam with a reconfigurable directional signal transmission device, which could effectively improve the performance of OAM-based underwater communication systems. Here, we combine two gradient lenses and a circular sawtooth lens on top of an active circular phased array, where the center of emitted quasi-Bessel AV beams can be transferred to any position in a two-dimensional plane. In comparison to traditional off-axis AV beam construction methods, our work could actively adjust the transmission direction and deliver the highly concentrated signals to the target receiver's location, which exhibits lower costs and greater flexibility, holding promising prospects for widespread applications in the field of acoustic communication.

We introduce a discrete numerical method based on the diffusion-limited aggregation (DLA) approach to simulate two-fluid Hele-Shaw flow subject to the Saffman-Taylor interfacial instability, in the case where the displaced fluid is non-Newtonian. Focusing on fluids for which the most relevant non-Newtonian aspect of the thin-gap flow is shear thinning, we introduce a history-dependent aspect into the algorithm, modeling shear-rate-dependent fluid viscosity. The main finding is that the morphology of the emerging patterns, characterized by the fractal dimension, is modified in a nontrivial manner by the shear-thinning nature of the displaced fluid. In particular, we consistently find that shear thinning leads to the formation of patterns characterized by a smaller fractal dimension, compared to the corresponding Newtonian fluid.

Electromagnetic showers developing from the collision of an ultraintense laser pulse with a beam of high-energy electrons or photons are investigated under conditions relevant to future experiments on multipetawatt laser facilities. A semianalytical model is derived that predicts the shower multiplicity, i.e., the number of pairs produced per incident seed particle (electron or $\gamma$ photon). The model is benchmarked against particle-in-cell simulations and shown to be accurate over a wide range of seed particle energies (from 100 MeV to 40 GeV), laser relativistic field strengths ($10<a_0<1000$), and quantum parameter ${\chi }_0$ (ranging from 1 to 40). It is shown that, for experiments expected in the next decade, only the first generations of pairs contribute to the shower while multiplicities larger than unity are predicted. Guidelines for forthcoming experiments are discussed, considering laser facilities such as Apollon and ELI Beamlines. The difference between electron and photon seeding and the influence of the laser pulse duration are investigated.

A quantum walk (QW) utilizes its internal quantum states to decide the displacement, thereby introducing single-particle entanglement between the internal and positional degrees of freedom. By simulating three variants of QWs with the conventional, symmetric, and split-step translation operators with or without classical randomness in the coin operator, we show the entanglement is robust against both time- and spatially dependent randomness, which can cause localization transitions of QWs. We propose a classical quantity called overlap, which literally measures the overlap between the probability distributions of the internal states as a proxy of entanglement. The overlap is associated with the off-diagonal terms of the reduced density matrix in the internal space, which then reflects its purity. Therefore, the overlap captures the inverse behavior of the entanglement entropy in most cases. We test the limitation of the classical proxy by constructing a special case with high population imbalance between the internal states to blind the overlap. Possible implications and experimental measurements are also discussed.

The author, in a previous work, solved the generalized Langevin equation of a Brownian particle in a thermal bath whose constituents were composed of noninteracting harmonic oscillators interacting with a parabolic potential. This approach acceptably describes the memory kernel and the frequency-dependent friction coefficient when compared with the molecular dynamic simulation at a constant temperature of methane immersed in water modeled as a Lennard-Jones fluid. In this work, we determine properties, for a field frequency greater than that of the simulation, such as the susceptibility, the timescales of the colored noise correlation function, the average position of the tagged particle, the standard deviation of the position probability density, the time-dependent diffusion coefficient, the system's entropy and production, and the mechanical work generated by an optimum external protocol. The calculations show the system would undergo an atypical-anomalous diffusion because a solvent aggregation process around the particle occurs before it reaches the steady state. This leads to momentary negative entropy production, which vanishes at longer times and is explained in terms of Maxwell's demons and the fulfillment of the second law. Likewise, the optimum driving is no longer linear, and work can be extracted. Furthermore, an alternate method to determine the fluctuation-dissipation theorem is derived. The procedure hasn't appeared in the literature and doesn't appeal to its probability distribution but to simple rules.

The Gauss map (or continued fraction map) is an important dissipative one-dimensional discrete-time dynamical system that exhibits chaotic behavior, and it generates a symbolic dynamics consisting of infinitely many different symbols. Here we introduce a generalization of the Gauss map, which is given by $x_{t+1}=\frac{1}{x_t^{\alpha }}-\left[\frac{1}{x_t^{\alpha }}\right]$ where $\alpha \ge 0$ is a parameter and $x_t\in [0,1]$ ($t=0,1,2,3,...$). The symbol $[\cdots ]$ denotes the integer part. This map reduces to the ordinary Gauss map for $\alpha =1$. The system exhibits a sudden “jump into chaos” at the critical parameter value $\alpha ={\alpha }_c\equiv 0.241485141808811\cdots$ which we analyze in detail in this paper. Several analytical and numerical results are established for this new map as a function of the parameter $\alpha$. In particular, we show that, at the critical point, the invariant density approaches a $q$-Gaussian with $q=2$ (i.e., the Cauchy distribution), which becomes infinitely narrow as $\alpha \to {\alpha }_c^{+}$. Moreover, in the chaotic region for large values of the parameter $\alpha$ we analytically derive approximate formulas for the invariant density, by solving the corresponding Perron-Frobenius equation. For $\alpha \to \infty$ the uniform density is approached. We provide arguments that some features of this transition scenario are universal and are relevant for other, more general systems as well.