arXiv subjects

Debra J. Searles

Publications and source records attributed to Debra J. Searles.

At least 19 recordsLinked to original sources

arXiv2026

Study of the Molecular Level Mechanism of Nanoscale Alternating Current Electrohydrodynamic Flow

This study investigates the molecular-level mechanism of Alternating Current Electrohydrodynamic (AC-EHD) flow in nanopores under high-frequency conditions, using molecular dynamics simulations. A gold-NaCl system with symmetric and asymmetric electrode configurations is used to analyze the flow patterns under high-frequency AC potentials. Our findings reveal localized heat generation near the electrode leading to a steep temperature gradient. An order parameter analysis indicates that the heat generation is due to the periodic change in the alignment of water molecules under AC potentials. At these high frequencies the influence of Na$^+$ and Cl$^-$ ions are negligible. The heat generation and temperature gradient are found to increase with the applied AC frequency. Three different electrode configurations were studied by varying the size and distance between the electrodes. A net directional flow develops in the asymmetric electrode structures. A possible mechanism for this is proposed by analyzing the flow patterns using velocity and temperature profiles, order parameters, streamline plots and mean square displacements. Different effects on the fluid were identified including those associated with temperature gradients, temperature-dependent fluid properties, and non-uniform electric fields. The asymmetric electrode structure created an imbalance in these effects and generated a net directional flow. These findings suggest the existence of a form of nanoscale AC-EHD flow that operates in a frequency regime above that of conventional electroosmotic and electrothermal mechanisms and that, unlike these mechanisms, occurs independently of ionic concentration. Thereby this work provides insights for optimizing AC-EHD flow in nanoscale systems where precise fluid manipulation is critical.

cond-mat.soft↗

arXiv2025

On the importance of numerical integration details for homogeneous flow simulation

The Sllod equations of motion enable modeling of homogeneous flow at the atomic scale, and are commonly used to predict fluid properties such as viscosity. However, few publicly available codes support such simulations, and those that do often do not implement a reversible numerical integration scheme or have other subtle problems. Here, we demonstrate a reversible and energy-conserving integration scheme for the Sllod equations of motion with error on the order of $δt^3$, in line with typical operator splitting integrators used in standard molecular dynamics simulations. We discuss various implementation details, and implement the scheme in LAMMPS where we find that our changes enable more accurate simulation of transient responses, mixed flows, and steady states, especially at high rates of flow. Importantly, we show that a lack of energy conservation can manifest as a systematic error in the direct ensemble average of the pressure tensor, leading to an error in the calculated viscosity which becomes significant at high flow rates.

cond-mat.soft↗

arXiv2025

Anomalous Diffusion and Emergent Universality in Coupled Memory-Driven Systems

Understanding how simple local interactions give rise to emergent exploration patterns is a fundamental question in statistical physics. We introduce a minimal model of two coupled agents that avoid retracing their own paths while being attracted to the trails left by one another. This system is inspired by, but not limited to, pheromone-guided insect navigation. The coupling of self-avoidance and attraction generates rich emergent behavior, including distinct anomalous diffusion regimes, non-Gaussian position distributions, and compressed exponential encounter statistics. Most notably, we identify new universality classes for coupled random walks, characterized by unique scaling laws and distributional properties that, to our knowledge, have not been previously reported. These findings advance the theoretical understanding of coupled stochastic processes with memory and interaction feedback, providing a framework for exploring transport phenomena in a broad range of multi-agent systems beyond biological contexts.

q-bio.PE↗

arXiv2025

Mechanical Degradation of Unentangled Polymer Melts under Uniaxial Extensional Flow

Complex flow fields govern the deformation of polymers in various manufacturing processes. However, high flow rates may trigger reaction events (i.e., bond breaking or undesirable reaction of mechanophores) in raw polymeric materials, leading to the mechanical or functional debasement of manufactured structures. Additionally, it is difficult to fully characterize such molecular-level flow in the laboratory due to time- and length-scale limits. In this study, we perform non-equilibrium molecular dynamics (NEMD) simulations to explore the rheological and mechanical degradation of unentangled polymer melts under uniaxial extensional flow (UEF), allowing for chain breaking. Our simulations demonstrate shear thickening-thinning-thickening stages with the increase of UEF extension rates, resulting from flow-induced changes of chain conformation. With further increasing UEF extension rates, a bond-breaking potential leads to another flow thinning stage. Interestingly, fracture kinetics is originally first-order owing to the need for highly stretched polymer chains before bond fracture. It is no longer first-order when bond fracture is instigated before chains are stretched. Our computational work provides insight into the optimal design of the manufacturing process for polymeric materials.

cond-mat.soft↗

arXiv2024

Local temperature measurement in molecular dynamics simulations with rigid constraints

Constraining molecules in simulations (such as with constant bond lengths and/or angles) reduces their degrees of freedom (DoF), which in turn affects temperature calculations in those simulations. When local temperatures are measured, e.g. from a set of atoms in a subvolume or from velocities in one Cartesian direction, the result can appear to unphysically violate equipartition of the kinetic energy if the local DoF are not correctly calculated. Here we determine how to correctly calculate local temperatures from arbitrary Cartesian component kinetic energies, accounting for general geometric constraints, by self-consistently evaluating the DoF of atoms subjected to those constraints. The method is validated on a variety of test systems, including systems subject to a temperature gradient and those confined between walls. It is also shown to provide a sensitive test for the breakdown of kinetic energy equipartition caused by the approximate nature of numerical integration or insufficient equilibration times. As a practical demonstration, we show that kinetic energy equipartition between C and H atoms connected by rigid bonds can be violated even at the commonly-used time step of 2 fs, and that this equipartition violation appears to usefully indicate configurational overheating.

cond-mat.stat-mech↗

arXiv2024

Fluctuation theorems and distribution functions for polar molecules in an electric field

In this perspective we consider how modern statistical mechanics and response theory can be applied to understand the response of polar molecules to an applied electric field and the fluctuations in these systems. Results that are consistent with thermodynamics and physical expectations are derived, as well as a new fluctuation relation that is tested in molecular simulations. It is demonstrated that a deterministic approach leads to distribution functions that continually evolve, even when system properties have relaxed, which has implications for treatment of nonequilibrium steady states.

cond-mat.stat-mech↗

arXiv2023

Interfacial Thermal Transport and Electrical Performance of Supercapacitors with Graphene/Carbon Nanotube Composite Electrodes

Advanced supercapacitors have great potential to transform how we store and utilize energy, leading to more efficient and sustainable energy systems. This study reveals the structural features influencing the interfacial thermal transport and electrical performances of supercapacitors, using the constant potential and constant charge molecular dynamics simulation techniques. Thermal and electrical properties were calculated for graphene/carbon nanotube composite electrodes and ionic liquid electrolytes with different nanotube diameter, number, layers, and alignments of the nanotubes. The effect of application of a constant potential on the Kapitza resistance is determined for the first time. The vertically aligned CNT structures exhibited higher electrical performance, while the horizontal arrangement showed better thermal performance. Optimum electrode configurations were identified by considering thermal and electrical performance, along with other design factors, such as structural stability, ease of manufacturing, and scalability. After considering all these factors, the horizontally stacked multi-layer CNT arrangement emerged as the optimal electrode structure. The insights gained from this study aid in comprehending the effects of variations in electrode structure, thereby enabling efficient supercapacitor electrode design.

physics.app-ph↗

arXiv2023

Machine learning a time-local fluctuation theorem for nonequilibrium steady states

Fluctuation theorems (FTs) quantify the thermodynamic reversibility of a system, and for deterministic systems they are defined in terms of the dissipation function. However, in a nonequilibrium steady state of deterministic dynamics, the phase space distribution is unknown, making the dissipation function difficult to evaluate without extra information. As such, steady state FTs for deterministic systems to date have required either that the trajectory segment of interest is relatively long, or that information is available about the entire trajectory surrounding that segment. In this work, it is shown that a simple machine learning model trained to predict whether a given steady state trajectory segment is being played forward or backward in time calculates a function that satisfies an FT and relies solely on information within the segment of interest. The FT is satisfied even for very short trajectory segments where the approximate relation derived from theory breaks down, for systems far from equilibrium, and for various nonequilibrium dynamics. It is further demonstrated that any function that is a well-calibrated predictor of time's arrow must satisfy a fluctuation theorem, and that a local FT can be derived which depends only on local dissipation and its correlations with the surrounding non-local dissipation.

cond-mat.stat-mech↗

arXiv2023

Constant Potential and Constrained Charge Ensembles for Simulations of Conductive Electrodes

Constant potential method molecular dynamics simulation (CPM MD) enables the accurate modelling of atomistic electrode charges when studying the electrode-electrolyte interface at the nanoscale. Here we extend the theoretical framework of CPM MD to the case in which the total charge of each conductive electrode is controlled, instead of their potential differences. We show that the resulting thermodynamic ensemble is distinct from that sampled in with a fixed potential difference, but rigorously related as conjugate ensembles. This theoretical correspondence, which we demonstrate using simulations of an ionic liquid supercapacitor, underpins the success of recent studies with fixed total charges on the electrodes. We show that equilibration is usefully sped up in this ensemble and outline some potential applications of these simulations in future.

cond-mat.mtrl-sci↗

arXiv2022

A new framework for computing a general local self-diffusion coefficient using statistical mechanics

Widely applicable, modified Green-Kubo expressions for the local diffusion coefficient ($D_l$) are obtained using linear response theory. In contrast to past definitions in use, these expressions are statistical mechanical results. Molecular simulations of systems with anisotropic diffusion and an inhomogeneous density profile confirm the validity of the results. Diffusion coefficients determined from different expressions in terms of currents and velocity correlations agree in the limit of large systems. Furthermore, they apply to arbitrarily small local regions, making them readily applicable to nanoscale and inhomogeneous systems where knowledge of $D_l$ is important.

cond-mat.stat-mech↗

arXiv2022

Fully Periodic, Computationally Efficient Constant Potential Molecular Dynamics Simulations of Ionic Liquid Supercapacitors

Molecular dynamics (MD) simulations of complex electrochemical systems, such as ionic liquid supercapacitors, are increasingly including the constant potential method (CPM) to model conductive electrodes at specified potential difference, but the inclusion of CPM can be computationally expensive. We demonstrate the computational savings available in CPM MD simulations of ionic liquid supercapacitors when the usual non-periodic slab geometry is replaced with fully periodic boundary conditions. We show how a doubled cell approach, previously used in non-CPM MD simulations of charged interfaces, can be used to enable fully periodic CPM MD simulations. Using either a doubled cell approach, or a finite field approach previously reported by others, fully periodic CPM MD simulations produce comparable results to the traditional slab geometry simulations with a nearly double speed-up in computational time. Indeed, these savings can offset the additional cost of the CPM algorithm, resulting in periodic CPM MD simulations that are faster than the non-periodic, fixed-charge equivalent simulations for the ionic liquid supercapacitors studied here.

cond-mat.mtrl-sci↗

arXiv2019

Diffusion of Lithium Ions in Lithium-Argyrodite Solid-State Electrolytes from Equilibrium and Nonequilibrium Molecular Dynamics Simulations

The use of solid-state electrolytes to provide safer, next-generation rechargeable batteries is becoming more feasible as new materials with greater stability and higher ionic diffusion coefficients are designed. However, accurate determination of diffusion coefficients in solids is problematic and reliable calculations are highly sought-after. In this paper we compare diffusion coefficients calculated using nonequilibrium and equilibrium ab initio molecular dynamics simulations for highly diffusive solid-state electrolytes for the first time, to demonstrate the accuracy that can be obtained. Moreover, we show that ab initio nonequilibrium molecular dynamics can be used to determine diffusion coefficients when the diffusion is too slow for it to be feasible to obtain them using ab initio equilibrium simulations. Thereby, using ab initio nonequilibrium molecular dynamics simulations we are able to obtain accurate estimates of the diffusion coefficients of Li ions in Li$_6$PS$_5$Cl and Li$_5$PS$_4$Cl$_2$, two promising electrolytes for all-solid-state batteries. Furthermore, these calculations show that the diffusion coefficient of lithium ions in Li$_5$PS$_4$Cl$_2$ is higher than many other potential all-solid-state electrolytes, making it promising for future technologies. The reasons for variation in conductivities determined using computational and experimental methods are also discussed. It is demonstrated that small degrees of disorder and vacancies can result in orders of magnitude differences in diffusivities of Li ions in Li$_6$PS$_5$Cl, and these factors are likely to contribute to inconsistencies observed in experimentally reported values. Notably, the introduction of Li-vacancies and disorder can enhance the ionic conductivity of Li$_6$PS$_5$Cl.

cond-mat.mtrl-sci↗

arXiv2019

A Live Maxwell's Demon

The first direct experimental replication of the Maxwell Demon thought experiment is outlined. The experiment determines the velocity/kinetic energy distribution of the particles in a sample by a novel interpretation of the results from a standard time-of-flight (TOF) small angle neutron scattering (SANS) procedure. Perspex at 293 K was subjected to neutrons at 82.2 K. The key result is a TOF velocity distribution curve that is a direct spatial and time-dependent microscopic probe of the velocity distribution of the Perspex nuclei at 293 K. Having this curve, one can duplicate the Demon's approach by selecting neutrons at known kinetic energies. One example is given: namely, two reservoirs -- hot and cold reservoirs -- were generated from the 293 K source without disturbing its original 293 K energy distribution.

cond-mat.stat-mech↗

arXiv2019

Doping Effects on the Performance of Paired Metal Catalysts for the Hydrogen Evolution Reaction

Metal heteroatoms dispersed in nitrogen-doped graphene display promising catalytic activity for fuel cell reactions such as the hydrogen evolution reaction (HER). Here we explore the effects of dopant concentration on the synergistic catalytic behaviour of a paired metal atom active site comprised of Co and Pt atoms. The metals are coordinated to six atoms in a vacancy of N-doped graphene. We find that HER activity is enhanced with increasing N concentration, where the free energy of hydrogen atom adsorption ranges from 0.23 to -0.42 eV as the doping changes from a single N atom doped in the pore, to fully doped coordination sites. The results indicated that the effect of N is to make the Co atom more active towards H adsorption and presents a means through which transition metals can be modified to make more effective and sustainable fuel cell catalysts.

cond-mat.mtrl-sci↗

arXiv2016

Equilibrium binding energies from fluctuation theorems and force spectroscopy simulations

Brownian dynamics simulations are used to study the detachment of a particle from a substrate. Although the model is simple and generic, we attempt to map its energy, length and time scales onto a specific experimental system, namely a bead that is weakly bound to a cell and then removed by an optical tweezer. The external driving force arises from the combined optical tweezer and substrate potentials, and thermal fluctuations are taken into account by a Brownian force. The Jarzynski equality and Crooks' fluctuation theorem are applied to obtain the equilibrium free energy difference between the final and initial states. To this end, we sample non--equilibrium work trajectories for various tweezer pulling rates. We argue that this methodology should also be feasible experimentally for the envisioned system. Furthermore, we outline how the measurement of a whole free energy profile would allow the experimentalist to retrieve the unknown substrate potential by means of a suitable deconvolution. The influence of the pulling rate on the accuracy of the results is investigated, and umbrella sampling is used to obtain the equilibrium probability of particle escape for a variety of trap potentials.

cond-mat.soft↗

arXiv2016

Ergodicity of non-Hamiltonian equilibrium systems

It is well known that ergodic theory can be used to formally prove a weak form of relaxation to equilibrium for finite, mixing, Hamiltonian systems. In this Letter we extend this proof to any dynamics that preserves a mixing equilibrium distribution. The proof uses an approach similar to that used in umbrella sampling, and demonstrates the need for a form of ergodic consistency of the initial and final distribution. This weak relaxation only applies to averages of physical properties. It says nothing about whether the distribution of states relaxes towards the equilibrium distribution or how long the relaxation of physical averages takes.

cond-mat.stat-mech↗

arXiv2016

On the relaxation to nonequilibrium steady states

The issue of relaxation has been addressed in terms of ergodic theory in the past. However, the application of that theory to models of physical interest is problematic, especially when dealing with relaxation to nonequilibrium steady states. Here, we consider the relaxation of classical, thermostatted particle systems to equilibrium as well as to nonequilibrium steady states, using dynamical notions including decay of correlations. We show that the condition known as ΩT-mixing is necessary and sufficient to prove relaxation of ensemble averages to steady state values. We then observe that the condition known as weak T-mixing applied to smooth observables is sufficient for relaxation to be independent of the initial ensemble. Lastly, weak T-mixing for integrable functions makes relaxation independent of the ensemble member, apart from a negligible set of members enabling the result to be applied to observations from a single physical experiment. The results also allow us to give a microscopic derivation of Prigogine's principle of minimum entropy production in the linear response regime. The key to deriving these results lies in shifting the discussion from characteristics of dynamical systems, such as those related to metric transitivity, to physical measurements and to the behaviour of observables. This naturally leads to the notion of physical ergodicity.

cond-mat.stat-mech↗

arXiv2013

Optimal protocols and the Jarzynski equality

The Jarzynski Equality is a well-known and widely used identity, relating the free energy difference between two states of a system to the work done over some arbitrary, nonequilibrium transformation between the two states. Despite being valid for both stochastic and deterministic systems, we show that the optimal transformation protocol for the deterministic case seems to differ from that predicated from an analysis of the stochastic dynamics. In addition, it is shown that for certain situations, more dissipative processes can sometimes lead to better numerical results for the free energy differences.

cond-mat.stat-mech↗