Decreasing System Size Research Articles

Searching for QGP droplets with high- pT hadrons and heavy flavor

The search for the smallest quark-gluon plasma (QGP) droplets in nature has motivated recent small collisions system programs at RHIC and LHC. Unambiguous identification of jet quenching due to final-state interactions is key to confirming QGP formation in these reactions. We compute the nuclear modification factors $R_{AA}$ and $R_{p(d)A}$ of charged hadrons and heavy flavor mesons in large (Au-Au, Xe-Xe, Pb-Pb) and small ($d$-Au, $p$-Pb, O-O) colliding systems, respectively. Our results include the Cronin effect and initial-state parton energy loss in cold nuclear matter. In the final state, hard partons undergo collisional energy loss and branching that was recently derived using Soft-Collinear-Effective-Theory with Glauber Gluon (SCET$_{\rm G}$). In large colliding systems, medium-modified QCD evolution of the fragmentation functions dominates the nuclear correction. As the system size decreases, we find that cold nuclear matter effects, collisional energy loss, and QGP-induced radiations can become equally important. A systematic scan over the medium size and mass/flavor dependence of $R_{AA}$ provides the opportunity to separate these individual contributions and identify QGP signatures in small systems. Predictions for $R_{AA}^{h}$, $R_{AA}^{D}$, $R_{AA}^{B}$ in O-O collisions at $\sqrt{s}=7$ TeV are presented with and without the formation of a QGP and contrasted with the corresponding $R_{p(d)A}$ calculations. Upcoming single-hadron measurements at the LHC will not only test the O-O predictions for both light and heavy flavor production, but will shed light on the possibly very different dynamics of $p$-A and A-A reactions at similar soft particle production multiplicities.

Multi-objective optimization of solar-assisted ground-source heat pumps for minimizing life-cycle cost and climate performance in heating-dominated regions

The use of solar-assisted ground-source heat pumps (SGSHPs) with serial and parallel configurations can reduce the energy consumption compared with conventional ground-source heat pumps (GSHPs). However, the environmental and economic issues associated with SGSHPs, which involve high initial costs and CO2 emissions, remain major barriers to their application. Therefore, SGSHPs should be carefully designed considering both the economic and environmental circumstances to guarantee their competitiveness over GSHPs. In this study, multi-objective optimization of SGSHPs with serial and parallel configurations was conducted to minimize the life-cycle cost (LCC) and climate performance (LCCP). Based on the estimated energy consumption using Transient System Simulation Tool (TRNSYS), the design parameters and refrigerants of the SGSHPs were optimized using an artificial neural network and multi-objective genetic algorithm in the four heating-dominated regions. The optimized SGSHP with a parallel configuration exhibited lower LCC and LCCP by up to 7% and 10%, respectively, compared to that with a serial configuration. Furthermore, as building insulation technology improves, the optimal system size decreases. In addition, R134a is the best option for minimizing LCC owing to its high performance, whereas R1234yf is the best option for minimizing LCCP in Helsinki and Stockholm, showing up to a 37% reduction in LCCP.

Using the nonhydrodynamic mode to study the onset of hydrodynamic behavior in ultraperipheral symmetric nuclear collisions

With the attempts of extending the hydrodynamic framework of heavy-ion collision to proton-proton and other small and low energy systems, we are confronted with the question of how small the system can get and still be safely modelled as a fluid. One of the transport coefficients required in the $2^{nd}$ order relativistic viscous hydrodynamics is the shear relaxation time, inclusion of which solves the causality violation problem in the Navier-Stokes equation. In phenomenological studies this coefficient has been taken as a constant and much attention has gone into finding and fixing the shear viscosity to entropy density ratio, $\eta/s$. This transport coefficient also happens to control the non-hydrodynamic mode of the out-of-equilibrium hydrodynamics theory. It has been predicted that for decreasing system size, observables become sensitive to variation in shear relaxation time as a result of increasing dominance of non-hydrodynamic mode, which could potentially indicate breakdown of hydrodynamics. In this study, we try to test this prediction in the peripheral Pb-Pb collisions at $2.76$ TeV and Au-Au collisions at $200$ GeV, with IPGlasma initial condition and $(2+1)-$Dimensional viscous hydrodynamics. We find that elliptic flow does show adequate sensitivity to variation in relaxation time for decreasing system size. The multiplicity rapidity density limit for applicability of hydrodynamics is found to be around $dN/dy\approx10$, with the possibility of refinement in this value given a way to improve the centrality resolution in experimental data for referencing in peripheral collisions.

Quantum statistical fluctuation of energy and its novel pseudo-gauge dependence

We discuss the quantum statistical fluctuations of energy in subsystems of hot relativistic gas for both spin-zero and spin-half particles. We explicitly show the system size dependence of the quantum statistical fluctuation of energy. Our results show that with decreasing system size quantum statistical fluctuations increase substantially. As the consistency of the framework, we also argue that the quantum statistical fluctuations give rise to the known result for statistical fluctuation of energy in the canonical ensemble if we consider the size of the subsystem to be sufficiently large. For a spin-half particle, quantum fluctuations show some interesting novel features. We show that within a small sub-system quantum statistical fluctuation of energy for spin-half particles depends on the various pseudo-gauge choices of the energy-momentum tensor. Interestingly, for sufficiently large subsystems quantum fluctuations obtained for different pseudo-gauge choices converge and we recover the canonical-ensemble formula known for statistical fluctuations of energy. Our calculation is very general and can be applied to any branch of physics whenever one deals with a thermal system. As a practical application, we argue that our results can be used to determine a coarse-graining scale to introduce the concept of classical energy density or fluid element relevant for the strongly interacting matter, in particular for small systems produced in heavy-ion collisions.

Anisotropic frontal polymerization in a model resin-copper composite.

This work investigates experimentally and numerically frontal polymerization in a thermally anisotropic system with parallel copper strips embedded in 1,6-hexanediol diacrylate resin. Both experiments and multiphysics finite element analyses reveal that the front propagation in the thermally anisotropic system is orientation-dependent, leading to variations in the front shape and the front velocity due to the different front-metal strip interaction mechanisms along and across the metal strips. The parameters entering the cure kinetics model used in this work are chosen to capture the key characteristics of the polymerization front, i.e., the front temperature and velocity. Numerical parametric analyses demonstrate that the front velocity in the directions parallel and perpendicular to the metal strips increases as the system size decreases and approaches the analytical prediction for homogenized systems. A two-dimensional homogenized model for anisotropic frontal polymerization in the metal-resin system is proposed.

On the bandwidth of stable nonlinear stripe patterns in finite size systems.

Nonlinear stripe patterns occur in many different systems, from the small scales of biological cells to geological scales as cloud patterns. They all share the universal property of being stable at different wavenumbers q, i.e., they are multistable. The stable wavenumber range of the stripe patterns, which is limited by the Eckhaus- and zigzag instabilities even in finite systems for several boundary conditions, increases with decreasing system size. This enlargement comes about because suppressing degrees of freedom from the two instabilities goes along with the system reduction, and the enlargement depends on the boundary conditions, as we show analytically and numerically with the generic Swift-Hohenberg (SH) model and the universal Newell-Whitehead-Segel equation. We also describe how, in very small system sizes, any periodic pattern that emerges from the basic state is simultaneously stable in certain parameter ranges, which is especially important for the Turing pattern in cells. In addition, we explain why below a certain system width, stripe patterns behave quasi-one-dimensional in two-dimensional systems. Furthermore, we show with numerical simulations of the SH model in medium-sized rectangular domains how unstable stripe patterns evolve via the zigzag instability differently into stable patterns for different combinations of boundary conditions.

Architecture of Hierarchical Stellar Systems and Their Formation

Accumulation of new data on stellar hierarchical systems and the progress in numerical simulations of their formation open the door to genetic classification of these systems, where properties of a certain group (family) of objects are tentatively related to their formation mechanisms and early evolution. A short review of the structure and statistical trends of known stellar hierarchies is given. Like binaries, they can be formed by the disk and core fragmentation events happening sequentially or simultaneously and followed by the evolution of masses and orbits driven by continuing accretion of gas and dynamical interactions between stars. Several basic formation scenarios are proposed and associated qualitatively with the architecture of real systems, although quantitative predictions for these scenarios are still pending. The general trend of increasing orbit alignment with decreasing system size points to the critical role of the accretion-driven orbit migration, which also explains the typically comparable masses of stars belonging to the same system. The architecture of some hierarchies bears imprints of chaotic dynamical interactions. Characteristic features of each family are illustrated by several real systems.

Rapid frontal polymerization achieved with thermally conductive metal strips.

Frontal polymerization, which involves a self-propagating polymerizing reaction front, has been considered as a rapid, energy-efficient, and environmentally friendly methodology to manufacture lightweight, high-performance thermoset polymers, and composites. Previous work has reported that the introduction of thermally conductive elements can enhance the front velocity. As follow-up research, the present work investigates this problem more systemically using both numerical and experimental approaches by investigating the front shape, front width, and heat exchange when aluminum and cooper metal strips are embedded in the resin. The study reveals that the enhancement in the front velocity is mainly due to a preheating effect associated with the conductive element. Moreover, the numerical parametric study for the system size shows that the front speed increases as the system size decreases, ultimately approaching a prediction provided by a homogenized model for polymer-metal composites.

Non-Fourier heat transport across 1D nano film between thermal reservoirs with different boundary resistances

We describe the nonlocal heat transport across a thin film in contact with two thermal baths using discrete variable model. The effective thermal conductivities , the boundary temperature jumps, the steady-state heat flux, and the internal temperature gradient are calculated analytically as functions of the film size and the boundary resistances. As the system size decreases, all the parameters demonstrate the transition from diffusive (Fourier) heat conduction when the scattering occurs mainly inside the film to ballistic transport when scattering occurs at the boundaries, while inside the film there is no scattering and the temperature profile is flat. We show that when the boundary resistance increases, the temperature profile inside the film also tends to be flat, but in this case the scattering, though small, occurs inside the film. The predicted effective thermal conductivity is consistent with classical continuous theories and available experimental data. • We consider heat transport across 1D nano film. • Discrete variable model has been used. • Effective thermal conductivity is size and resistance dependent. • Transition to ballistic regime is described.

Model of Magnetic Damping and Anisotropy at Elevated Temperatures: Application to Granular FePt Films

Understanding the damping mechanism in finite size systems and its dependence on temperature is a critical step in the development of magnetic nanotechnologies. In this work, nano-sized materials are modeled via atomistic spin dynamics, the damping parameter being extracted from Ferromagnetic Resonance (FMR) simulations applied for FePt systems, generally used for heat-assisted magnetic recording media (HAMR). We find that the damping increases rapidly close to Tc and the effect is enhanced with decreasing system size, which is ascribed to scattering at the grain boundaries. Additionally, FMR methods provide the temperature dependence of both damping and the anisotropy, important for the development of HAMR. Semi-analytical calculations show that, in the presence of a grain size distribution, the FMR linewidth can decrease close to the Curie temperature due to a loss of inhomogeneous line broadening. Although FePt has been used in this study, the results presented in the current work are general and valid for any ferromagnetic material.

The Effect of KcsA Channel on Lipid Bilayer Electroporation Induced by Picosecond Pulse Trains.

Membrane proteins are the major component of plasma membranes, and they play crucial roles in all organisms. To understand the influence of the presence of KcsA channel on cell membrane electroporation induced by picosecond pulse trains (psPT), in this paper, the electroporation of KcsA membrane protein system and bare lipid bilayer system (POPC) with the applied psPT are simulated using molecular dynamics (MD) method. First, we find that the average pore formation time of the KcsA system is longer than the bare system with the applied psPT. In the KcsA system, water protrusions appear more slowly. Then, the system size effects of psPT in the MD simulations are investigated. When the system size decreases, the average pore formation time of small KcsA membrane protein system is shorter than the bare system with the applied psPT. It is found that the psPT makes the protein fluctuation of small system increase greatly; meanwhile the instability of protein disturbs the water and then affects the water protrusion appearance time. Furthermore, it shows that the protein fluctuation of constant electric field is smaller than that of psPT and no field, and protein fluctuation increases with the psPT repetition frequency increasing.

The effect of finite electrical conductivity of small-scale beam resonators on their vibrational response under electrostatic fields

Electrostatic actuation is one of the most commonly used methods for excitation and measurement in micro and nanoscale resonators. In the dynamical behavior analyses of such systems, the resonating beam is often assumed to be a perfect conductor. In this paper, the effect of electrical resistivity on the vibrational response of these systems, including the natural frequency and damping, is investigated. The governing coupled nonlinear partial differential equations of motion are derived and a new finite element method formulation is presented by developing a new electromechanical element. The numerical natural frequencies are compared with experimental measurements and the achieved correlation is better than that in the prior studies. Results indicate that there is a jump in the frequency and damping of the system at a critical resistivity. As the system size decreases and the applied voltage approaches the pull-in voltage, the electrical resistivity completely dominates the response nature of the system. An experiment is also conducted, and good agreement with the theory is observed regarding the effect of electrical resistivity.

Relation between the convective field and the stationary probability distribution of chemical reaction networks

We investigate the relation between the stationary probability distribution of chemical reaction systems and the convective field derived from the chemical Fokker–Planck equation (CFPE) by comparing predictions of the convective field to the results of stochastic simulations based on Gillespie’s algorithm. The convective field takes into account the drift term of the CFPE and the reaction bias introduced by the diffusion term. For one-dimensional systems, fixed points and bifurcations of the convective field correspond to extrema and phenomenological bifurcations of the stationary probability distribution whenever the CFPE is a good approximation to the stochastic dynamics. This provides an efficient way to calculate the effect of system size on the number and location of probability maxima and their phenomenological bifurcations in parameter space. For two-dimensional systems, we study models that have saddle-node and Hopf bifurcations in the macroscopic limit. Here, the existence of two stable fixed points of the convective field correlates either with two peaks of the stationary probability distribution, or with a peak and a shoulder. In contrast, a Hopf bifurcation that occurs in the convective field for decreasing system size is not accompanied by the onset of a crater-shaped probability distribution; decreasing system size rather destroys craters and replaces them by local maxima.

Dimiristoylphosphatidylcholine/genistein molecular interactions: A physico-chemical approach to anti-glioma drug delivery systems

Regarding free genistein small delivery to the central nervous system, physico-chemical parameters of dimiristoylphosphatidylcholine liposome-loaded genistein were investigated, as well as its in vitro activity against the DPPH radical and glioma cells. Data obtained by UV–vis spectroscopy, Fourier Transform Infrared Spectroscopy, Nuclear Magnetic Resonance, Differential Scanning Calorimetry and Dynamic Light Scattering were used to characterize the liposomal system with respect to motion restriction, hydration degree, trans-gauche isomerization and phase state. In vitro antitumoral effects were monitored through conting and viability assays. Genistein hydroxyl group and lipid hydrogen bonds may have important role in dimiristoylphosphatidylcholine phosphate and choline motion restriction. Genistein-induced choline restriction may be also related to a decrease in the group rotation rate. Genistein: dimiristoylphosphatidylcholine system showed higher molecular package at the acyl chains region compaired to empty liposomes, and it may be related to a decrease in gauche bonds quantity and system size. Lipid acyl chain length seems to influence different genistein effects on membranes, due to the presence of gauche conformers. Genistein: dimiristoylphosphatidylcholine liposome was more efficient as DPPH reducting system than the free-Gen. Liposomal system, at genistein 100 μM, was so efficient as the correspondent free-form genistein, probably showing higher stability to cross the blood-brain barrier. Genistein and the lipid did not show an additive activity against glioma cells. Antioxidant and anti-glioma genistein-loaded liposome potential may be related to the isoflavone location and its restriction effect in the lipid molecular motion. Anti-glioma activity may also be related to a decrease of system size and trans-gauche isomerization.

Transition temperature enhancement of nano-structured superconductors: Shape dependence and effects of Cooper pair distribution

Enhancement of superconducting critical temperature Tc for nano-structured square and rectangular plates is investigated, using the Gor’kov equations and the finite element method. Tc increases with decreasing system size and shows sawtooth behavior as the function of system size. We find the enhancement of Tc becomes larger for narrower rectangular plate. We discuss mechanisms of these behaviors of Tc, investigating quasi-particle eigen-energies and spatial order parameter distributions.

Role of electronic thermal transport in amorphous metal recrystallization: A molecular dynamics study.

Recrystallization of glasses is important in a wide range of applications including electronics and reactive materials. Molecular dynamics (MD) has been used to provide an atomic picture of this process, but prior work has neglected the thermal transport role of electrons, the dominant thermal carrier in metallic systems. We characterize the role of electronic thermal conductivity on the velocity of recrystallization in Ni using MD coupled to a continuum description of electronic thermal transport via a two-temperature model. Our simulations show that for strong enough coupling between electrons and ions, the increased thermal conductivity removes the heat from the exothermic recrystallization process more efficiently, leading to a lower effective temperature at the recrystallization front and, consequently, lower propagation velocity. We characterize how electron-phonon coupling strength and system size affect front propagation velocity. Interestingly, we find that initial recrystallization velocity increases with decreasing system size due to higher overall temperatures. Overall, we show that a more accurate description of thermal transport due to the incorporation of electrons results in better agreement with experiments.

Transport coefficients in a finite volume Polyakov–Nambu–Jona-Lasinio model

Finite size consideration of matter significantly affects transport coefficients like shear viscosity, bulk viscosity, electrical conductivity, which we have investigated here in the framework of the Polyakov-Nambu-Jona-Lasinio model. Owing to the basic quantum mechanics, a non-zero lower momentum cut-off is implemented in momentum integrations, used in the expressions of constituent quark masses and transport coefficients. When the system size decreases, the values of these trans- port coefficients are enhanced in low temperature range. At high temperature domain, shear viscos- ity and electrical conductivity become independent of system sizes. Whereas, bulk viscosity, which is associated with scale violating quantities of the system, faces some non-trivial size dependence in this regime. In the phenomenological direction, our microscopic estimations can also be linked with the macroscopic outcome, based on dissipative hydrodynamical simulation.

Size-Tuned Plastic Flow Localization in Irradiated Materials at the Submicron Scale.

Three-dimensional discrete dislocation dynamics (3D-DDD) simulations reveal that, with reduction of sample size in the submicron regime, the mechanism of plastic flow localization in irradiated materials transitions from irradiation-controlled to an intrinsic dislocation source controlled. Furthermore, the spatial correlation of plastic deformation decreases due to weaker dislocation interactions and less frequent cross slip as the system size decreases, thus manifesting itself in thinner dislocation channels. A simple model of discrete dislocation source activation coupled with cross slip channel widening is developed to reproduce and physically explain this transition. In order to quantify the phenomenon of plastic flow localization, we introduce a "deformation localization index," with implications to the design of radiation-resistant materials.

Destructive impact of molecular noise on nanoscale electrochemical oscillators

We study the loss of coherence of electrochemical oscillations on meso- and nanosized electrodes with numeric simulations of the electrochemical master equation for a prototypical electrochemical oscillator, the hydrogen peroxide reduction on Pt electrodes in the presence of halides. On nanoelectrodes, the electrode potential changes whenever a stochastic electron-transfer event takes place. Electrochemical reaction rate coefficients depend exponentially on the electrode potential and become thus fluctuating quantities as well. Therefore, also the transition rates between system states become time-dependent which constitutes a fundamental difference to purely chemical nanoscale oscillators. Three implications are demonstrated: (a) oscillations and steady states shift in phase space with decreasing system size, thereby also decreasing considerably the oscillating parameter regions; (b) the minimal number of molecules necessary to support correlated oscillations is more than 10 times as large as for nanoscale chemical oscillators; (c) the relation between correlation time and variance of the period of the oscillations predicted for chemical oscillators in the weak noise limit is only fulfilled in a very restricted parameter range for the electrochemical nano-oscillator.

Volume dependence of baryon number cumulants and their ratios

We explore the influence of finite-volume effects on cumulants of baryon/quark number fluctuations in a nonperturbative chiral model. In order to account for soft modes, we use the functional renormalization group in a finite volume, using a smooth regulator function in momentum space. We compare the results for a smooth regulator with those for a sharp (or Litim) regulator, and show that in a finite volume, the latter produces spurious artifacts. In a finite volume there are only apparent critical points, about which we compute the ratio of the fourth- to the second-order cumulant of quark number fluctuations. When the volume is sufficiently small the system has two apparent critical points; as the system size decreases, the location of the apparent critical point can move to higher temperature and lower chemical potential.