arXiv subjects

Victor Teboul

Publications and source records attributed to Victor Teboul.

16 recordsLinked to original sources

arXiv2025

A simple time coarse graining method for molecular dynamics simulations of liquids

Considering molecular dynamic simulations as a stochastic method, we investigate the possibility of time coarse graining the simulations. Similarly to Boltzmann inversion method in spatial coarse graining, which begins with a free energy called potential of mean force, we test the effect of a generalized potential of mean force that uses the distinct part of the Van Hove correlation function with a characteristic time different from zero. We show that the method is approximately equivalent to replace the hard core of the original potential by a smooth harmonic function. We then compare the results of simulations using the modified potential and the original one. Results show that this simple modification of the potential, namely replacing the short range wall with a smooth quadratic law, leads to a shift in the time step resulting in the same dynamics than the original potential function but with a much larger time step.

cond-mat.soft↗

arXiv2025

A facilitation-induced fluidization transition in supercooled water triggered by a few active molecules

Using an activation mechanism reproducing facilitation, a dynamic phase transition triggered by a few active molecules was recently found in a supercooled model liquid. Prompted by this finding we investigate the presence of a similar transition in supercooled water. We find the presence of the phase transition in water despite the numerous anomalies of water, suggesting universality of the transition. The transition appears at constant temperature, being only induced by the motion of a small percentage of active molecules and the system cooperativity. We observe that cooperative motions strongly increase at the transition and do not disappear when the medium viscosity drops. An increase of temperature is needed to make the cooperative motions disappear, suggesting a connection to the kinetic energy to potential energy ratio instead of the medium viscosity.

cond-mat.soft↗

arXiv2024

Transmission of mobility via cooperative mechanisms in soft active matter

When supercooled, liquids viscosity increases dramatically as the glass transition temperature is approached. While the physical origin of this behavior is still not understood, it is now well established that the addition of a few activated particles is able to reverse that increase in viscosity. Here we further raise the question of a limit in that fluidization process and of the differences between the fluidized liquid and its viscous counterpart. Results show that a few percent active molecules are enough to trigger a phase transition leading to diffusion coefficients typical of liquids while the medium retains cooperative properties of the viscous phase. The similarity between cooperative properties of the active and non active molecules suggests that the mobility of active molecules is transmitted to inactive ones via the medium cooperative mechanisms, a result in agreement with facilitation theories. This result is then confirmed by the compared behavior of the distinct van hove correlation functions of most mobile active and non active molecules. Interestingly enough, in our simulations the cooperative mechanisms are not induced or related to a decrease of the excitation concentration.

cond-mat.soft↗

arXiv2023

Dynamic phase transition induced by active molecules simulating a facilitation mechanism in a supercooled liquid

The purpose of this work is to use active particles to study the effect of facilitation on supercooled liquids. To this end we investigate the behavior of a model supercooled liquid doped with intermittently active and intermittently slowed particles. To simulate a facilitation mechanism, the active particles are submitted intermittently to a force following the mobility of the most mobile molecule around, while the slowed particles are submitted to a friction force. We observe upon activation, a fluidization of the whole medium simultaneously to a large increase of the dynamic heterogeneity. This effect is reminiscent of the fluidization observed with molecular motors doping of the medium. When the mobility characteristic time used in the facilitation mechanism matches the physical time characterizing the spontaneous mobility aggregation of the material, we observe a phase transition associated with a structural aggregation of active molecules. This transition is accompanied by a sharp increase of the fluidization and dynamic heterogeneity.

cond-mat.soft↗

arXiv2021

Simulation of a flat folding nano-swimmer confined in a nanopore

We use molecular dynamics simulations to investigate the displacement of a simple butterfly-like molecular motor inside nanopores of various radii filled with a viscous medium. The medium is modeled with a versatile potential that may be adjusted to represent a large number of materials. It was found previously that the motor folding not only increases its displacement but also creates elementary diffusion processes inside the medium, related to the opening angle of the motor folding. The presence of these processes changes the medium dynamics and in turn affects the motor displacement. Therefore we test the motor displacement with different activations of the medium inside the pore by varying the motor opening angles. We find that the optima of the motor displacement oscillate with pore sizes and that the optimal radii depend on the activation of the medium. These results imply that it is possible to choose the activation or opening angle that optimizes the motor displacement for a given pore size. Results also show that the activation decreases strongly the confinement hindering of the motor motion, in particular for small pores. Finally, analyzing the distribution probability of the motor position and the density of elementary motions we find that the motor is mainly located in the center of the pore. We find spikes in the density of elementary motions when the motor goes away from the center, suggesting important contributions of the motor bouncing motions on the pore walls.

cond-mat.soft↗

arXiv2021

Activation of a supercooled liquid confined in a nanopore

It is well stablished that confinement of supercooled liquids in nano-pores induces various effects as a strong modification of the dynamics and a layering of the local structure. In this work we raise the issue as how these confinement effects are modified when the liquid is out of equilibrium. To answer that question, we use molecular dynamics simulations to investigate the effect of confinement on a supercooled liquid activated by the periodic folding of a molecular motor. We find that the motor opening angle controls the activation of the medium and use that result to study the effect of different activations on the confined supercooled liquid. We observe an increase of the activation effect on the dynamics when the medium is confined. We find that the confinement slowing down dependence with the pore radius depends on the activation of the liquid. We argue that these findings result from a modification of dynamic correlation lengths with activation. In this picture the activation permits to control the liquid correlation length and dynamics inside the pore. Studying the local structure we observe a modification of the layering organization induced by the activation. We also find that the mobility inside the pore depends on the layers, being larger where the local density is small.

cond-mat.mes-hall↗

arXiv2017

Enhanced diffusion in finite-size simulations of a fragile diatomic glass former

Using molecular dynamics simulations we investigate the finite-size dependence of the dynamical properties of a diatomic supercooled liquid. The simplicity of the molecule permits us to access the microsecond time scale. We find that the relaxation time decreases simultaneously with the strength of cooperative motions when the size of the system decreases. While the decrease of the cooperative motions is in agreement with previous studies, the decrease of the relaxation time opposes what has been reported to date in monatomic glass formers and in silica. This result suggests the presence of different competing physical mechanisms in the relaxation process. For very small box sizes the relaxation times behavior reverses itself and increases strongly when the box size decreases, thus leading to a nonmonotonic behavior. This result is in qualitative agreement with defect and facilitation theories.

cond-mat.soft↗

arXiv2017

A simple diatomic potential that prevents crystallization in supercooled liquids simulations

We study a simple and versatile diatomic potential function coined to prevent crystallization in supercooled liquids. We show that the corresponding liquid does not crystallize even with very long simulation runs at the lowest temperature that we can access with ergodic simulations. The medium displays the usual features of supercooled materials and a non-Arrhenius dependence of the diffusion coefficient and alpha relaxation time with temperature. We also observe the breakdown of the Stokes-Einstein relation at low temperatures.

cond-mat.soft↗

arXiv2017

Stimuli Thresholds for Isomerization-induced Molecular Motions in Azobenzene Containing Materials

We use large scale molecular dynamics simulations of the isomerizations of azobenzene molecules diluted inside a simple molecular material, to investigate the effect of a modification of the cis isomer shape on the induced diffusion mechanism. To this end we simulate incomplete isomerizations, modifying the amplitude of the trans to cis isomerization. We find thresholds in the evolution of the host molecules mobility with the isomerization amplitude, a result predicted by the cage- breaking mechanism hypothesis (Teboul, V.; Saiddine, M.; Nunzi, J.M.; Accary, J.B. J. Chem. Phys. 2011, 134, 114517) and by the gradient pressure mechanism theory (Barrett, C.J.; Rochon, P.L.; Natansohn, A.L. J. Chem. Phys. 1998, 109, 1505-1516. ). Above the threshold the diffusion then increases linearly with the variation of the chromophore size induced by the isomerization.

cond-mat.soft↗

arXiv2017

How does the motion of the surrounding molecules depend on the shape of a folding molecular motor ?

Azobenzene based molecules have the property of isomerizing when illuminated. In relation with that photoisomerization property, azobenzene containing materials are the subject of unexplained massive mass transport. In this work we use an idealised rectangular chromophore model to study the dependence of the isomerization induced transport on the chromophore dimensions. Our results show the presence of a motor arm length threshold for induced transport, which corresponds to the host molecule size. Above the threshold, the diffusive motions increase proportionally to the chromophore length. Intriguingly, we find only a very small chromophore width dependence of the induced diffusive motions. Our very simplified motor reproduces relatively well the behavior observed using the real DR1 motor molecule, suggesting that the complex closing procedure and the detailed shape of the motor are not necessary to induce the molecular motions.

cond-mat.soft↗

arXiv2017

Folding time dependence of the motions of a molecular motor diluted inside an amorphous medium

We investigate the dependence of the displacements of a molecular motor embedded inside a glassy material on its folding characteristic time. We observe two different time regimes. For slow foldings (regime I) the diffusion evolves very slowly with the folding time, while for rapid foldings (regime II) the diffusion increases strongly with the folding time suggesting two different physical mechanisms. We find that in regime I the motor displacement during the folding process is counteracted by a reverse displacement during the unfolding, while in regime II this counteraction is much weaker. We notice that regime I behavior is reminiscent of the scallop theorem that holds for larger motors in a continuous medium. We find that the difference in the efficiency of the motor motion explains most of the observed difference between the two regimes. For fast foldings the motor trajectories differ significantly from the opposite trajectories induced by the following unfolding process, resulting in a more efficient global motion than for slow foldings. This result agrees with the fluctuation theorems expectation for time reversal mechanisms. In agreement with the fluctuation theorems we find that the motors are unexpectedly more efficient when they are generating more entropy, a result that can be used to increase dramatically the motor motion.

cond-mat.soft↗

arXiv2017

How do packing defects modify the cooperative motions in supercooled liquids ?

We use molecular dynamic simulations to investigate the relation between the presence of packing defects in a glass-former and the spontaneous cooperative motions called dynamic heterogeneity. For that purpose we use a simple diatomic glass-former and add a small number of larger or smaller diatomic probes. The diluted probes modify locally the packing, inducing structural defects in the liquid, while we find that the number of defects is small enough not to disturb the average structure. We find that a small packing modification around a few molecules can deeply influence the dynamics of the whole liquid, when supercooled. When we use small probe molecules, the dynamics accelerates and the dynamic heterogeneity decreases. In contrast, for large probes the dynamics slows down and the dynamic heterogeneity increases. The induced heterogeneities and transport coefficient modification increase when the temperature decreases and disappear around the onset temperature of the cage dynamics.

cond-mat.dis-nn↗

arXiv2014

Induced cooperative motions in a medium driven at the nanoscale: Searching for an optimum excitation period

Recent results have shown the appearance of induced cooperative motions called dynamic heterogeneity during the isomerization of diluted azobenzene molecules in a host glass-former. In this paper we raise the issue of the coupling between these "artificial" heterogeneities and the isomerization period. How do these induced heterogeneities differ in the saturation regime and in the linear response regime ? Is there a maximum of the heterogeneous motion versus isomerization rate and why ? Are the heterogeneity evolution with the isomerization rate connected with the diffusion or relaxation time evolution ? We use out of equilibrium molecular dynamics simulations to answer these questions. We find that the heterogeneity increases in the linear response regime for large isomerization periods and small perturbations. In contrast the heterogeneity decreases in the saturation regime, i.e. when the isomerization half-period ($τ_{p}/2$) is smaller than the relaxation time of the material ($τ_α$). This result makes possible a test of the effect of cooperative motions on the dynamics using the chromophores as Maxwell demons that destroy or stimulate the cooperative motions. Because the heterogeneities increase in the linear regime and then decrease in the saturation regime, we find a maximum for $τ_{p}/2 \approx τ_α$. The induced excitations concentration follows a power law evolution versus the isomerization rate and then saturates. As a consequence the $α$ relaxation time is related to the excitation concentration with a power law, a result in qualitative agreement with recent findings in constrained models. This result supports a common origin for the heterogeneities with constrained models and a similar relation to the excitation concentration.

cond-mat.soft↗

arXiv2014

A toy model mimicking cage effect, structural fluctuations and kinetic constraints in supercooled liquids

The cage effect is widely accepted as the basic microscopic mechanism underlying the physics of supercooled liquids in contrast with usual liquids which are governed by molecular interactions only. In this work we implement a new toy model coined to reproduce the cage effect with variants including structural fluctuations and kinetic constraints. We use this new model to investigate which glass-transition features are directly due to the cage effect and which are due to more complex mechanisms.

cond-mat.dis-nn↗

arXiv2007

Self-organization of supercooled liquids confined inside nano-porous materials

Large scale molecular dynamics simulations are used to investigate the structural and dynamical modifications of supercooled water when confined inside an hydrophilic nanopore. We then investigate the evolution of the auto-organization of the most and the least mobile molecules (dynamical heterogeneity and string-like cooperative motions) when supercooled water is confined. Our calculations use the recent TIP5P intermolecular potential for water. We observe a strong slowing down of the dynamical properties when the liquid is confined, although the liquid structure is found to remain unchanged when corrected from the pore geometry. We then study cooperative motions inside supercooled confined water in comparison with bulk water. We observe strong modifications of the cooperative motions when the liquid is confined. We observe that dynamical heterogeneities and the associated correlation lengths are strongly increased as well as string-like motions in the confined liquid. This result, which is in opposition with the expected limitation of the correlation length by the confinement procedure, may explain (or be explained by) the slowing down of the dynamics. However the comparison of the dynamical heterogeneities at constant diffusion coefficient shows that the slowing down of the dynamics is not sufficient to explain the increase of the correlation lengths.

cond-mat.dis-nn↗

arXiv2005

Cooperative motions in a finite size model of liquid silica: an anomalous behavior

Finite size effects on dynamical heterogeneity are studied in liquid silica with Molecular Dynamics simulations using the BKS potential model. When the system size decreases relaxation times are found to increase in accordance with previous results in finite-size simulations and confined liquids. It has been suggested that this increase may be related to a modification of the cooperative motions in confined liquids. In agreement with this hypothesis we observe a decrease of the dynamical heterogeneities associated to the most and the least mobile atoms when the size L decreases. However we find that the decrease of the dynamical aggregation associated to the least mobile atoms is much more important than the decrease associated to the most mobile atoms. This result is surprising as the liquid is slowed down. The decrease of the heterogeneous behavior is also in contradiction with the increase of the heterogeneities observed in liquids confined in nanopores. However an increase of the non-Gaussian parameter appears both in nanopores and in the finite size simulations. As the non-Gaussian parameter is usually associated with dynamical heterogeneities, the increase of the non-Gaussian parameter together with a decrease of dynamical heterogeneity is also surprising.

cond-mat.dis-nn↗