arXiv subjects

José López-Molina

Publications and source records attributed to José López-Molina.

4 recordsLinked to original sources

arXiv2026

Non-equilibrium (thermo)dynamics of colloids under mobile piston compression

We investigate the non-equilibrium compression of a confined hard-sphere colloidal fluid driven by a mobile boundary within dynamical density functional theory. The system consists of a fluid confined between two parallel walls, one acting as an overdamped piston subjected to a sudden increase in external pressure. The piston motion is controlled by a mobility parameter $K$, which sets the relative timescale between mechanical driving and diffusive relaxation. By varying $K$ over several orders of magnitude, we identify a crossover from quasi-static compression to a diffusion-limited strongly driven regime. For small $K$, the system evolves close to equilibrium and the total injected work approaches the equilibrium free-energy difference. For large $K$, the piston rapidly adjusts and the dynamics becomes governed by diffusive relaxation, leading to saturation in the piston trajectory, pressure--position relation, particle currents, and center-of-mass velocity. In this regime, the injected work and entropy production are bounded, reflecting constraints imposed by diffusive transport. The maximum injected power scales linearly with $K$, while the entropy-production peak exhibits a crossover from quadratic growth to saturation, with peak times displaying $1/K$ scaling. The entropy change of the thermal bath interpolates between a reversible limit and a strongly driven dissipative regime. Finally, the evolution of configurational entropy and external potential energy reveals a dynamical decoupling between confinement and structural relaxation, including transient non-monotonic behavior. These results provide a quantitative thermodynamic characterization of boundary-driven compression.

cond-mat.soft↗

arXiv2024

Measuring Absolute Velocities from Non-Equilibrium Oscillations via Single-Detector 3D Dynamic Light Scattering

Single-detector 3D dynamic light scattering (3D-DLS) emerges as a reliable technique to determine the drift velocity of out-of-equilibrium colloidal particles. In particular, our investigation reveals the appearance of oscillations of a well-defined frequency in the autocorrelation function of the scattered intensity when particles are immersed in a medium exposed to thermally induced convection. These oscillations arise as a consequence of the directed motion of particles due to the convective motion of the fluid. The experimental results obtained for different colloidal systems are corroborated by a theoretical model and thoroughly validated with fluid dynamics and Brownian dynamics simulations. The excellent agreement between experimental, theoretical and simulation data allows us to provide a solid and comprehensive explanation of the observed physical phenomena. This study via advanced dynamic light scattering (DLS) technique offers insights into the field of non-equilibrium particle dynamics, applicable not only to colloidal suspension affected by steady-state diffusion-convection but also to other non-equilibrium situations, such as systems driven by external fields (gravitational, electric or magnetic fields, among others).

cond-mat.soft↗

arXiv2024

Nonequilibrium relaxation of soft responsive colloids

Stimuli-responsive macromolecules display large conformational changes during their dynamics, sometimes switching states, and are an integral property for the development of soft functional materials. Here, we introduce a mean-field dynamical density functional theory (DDFT) for a model of responsive colloids (RCs) to study the nonequilibrium dynamics of a colloidal dispersion in time-dependent external fields, with a focus on the coupling of translational and conformational dynamics during their relaxation. Specifically, we consider soft Gaussian particles with a bimodal size distribution between two confining walls with time-dependent (switching-on and off) external gravitational and osmotic fields. We find a rich relaxation behavior of the systems in excellent agreement with particle-based Brownian dynamics (BD) computer simulations. In particular, we find time-asymmetric relaxations of integrated observables (wall pressures, mean size, liquid center-of-mass) for activation/deactivation of external potentials, respectively, which are tuneable by the ratio of translational and conformational diffusion time scales. Our work paves thus the way for studying the nonequilibrium relaxation dynamics of complex soft matter with multiple degrees of freedom and hierarchical relaxations.

cond-mat.soft↗

arXiv2023

Diffusion and interaction effects on molecular release from collapsed microgels

The transport of biomolecules, drugs, or reactants encapsulated inside stimuli-responsive polymer networks in aqueous media is fundamental for many material and environmental science applications, including drug delivery, biosensing, catalysis, nanofiltration, water purification, and desalination. The transport is particularly complex in dense polymer media, such as collapsed hydrogels, where the molecules strongly interact with the polymer network and diffuse via a hopping mechanism. In this study, we employ Dynamical Density Functional Theory (DDFT) to investigate the non-equilibrium release kinetics of non-ionic subnanometer-sized molecules initially uploaded inside collapsed microgel particles. The theory is consistent with previous molecular dynamics simulations of collapsed poly($N$-isopropylacrylamide) (PNIPAM) polymer matrices, accommodating molecules of varying shapes and sizes. We found that, despite the intricate physico-chemical properties involved in the released process, the kinetics is predominantly dictated by two material parameters: the diffusion coefficient of the molecules inside the microgel ($D^*$) and the interaction free energy of the molecules with the microgel ($ΔG$). Our results reveal two distinct limiting regimes: For large, slowly diffusing molecules weakly attracted to the polymer network, the release is primarily driven by diffusion, with a release time that scales as $τ_{1/2} \sim 1/D^*$. Conversely, for small molecules strongly attracted to the polymer network, the release time is dominated by the interaction, scaling as $τ_{1/2} \sim \exp(-ΔG/k_{\textrm{B}} T)$. Our DDFT calculations are directly compared with an analytical equation for the half-release time, demonstrating excellent quantitative agreement. This equation represents a valuable tool for predicting release kinetics of non-ionic molecules from collapsed microgels.

cond-mat.soft↗