arXiv subjects

Carles Calero

Publications and source records attributed to Carles Calero.

At least 19 recordsLinked to original sources

arXiv2025

Hydrodynamic Mechanism of Colloidal Propulsion through Momentum Exchange

Propulsion of colloidal particles due to momentum transfer from localized surface reactions is investigated by solving the exact unsteady Stokes equation. We model the effect of surface reactions as either a {\it force dipole} acting on the fluid or a {\it pair force} acting on both the colloid and the fluid. Our analysis reveals that after a single reaction event the colloid's velocity initially decays as $\sim t^{-1/2}$, followed by a long-time tail decay $\sim t^{-5/2}$. This behavior is distinct from the $\sim t^{-3/2}$ decay seen for simple impulsively forced particles, a result of the force-free nature of the reaction mechanism. The velocity and transient dynamics are strongly controlled by the distance of the reaction from the colloid surface. For a colloid subject to periodic reactions, the theory predicts a steady-state velocity that is comparable to experimental results and previous simulations, suggesting that direct momentum transfer is a relevant mechanism for self-propulsion in systems like Janus particles. Finally, our study shows that fluid compressibility is not required for momentum transfer to produce colloidal propulsion.

cond-mat.soft↗

arXiv2025

Colloidal Model for Investigating Optimal Efficiency in Weakly Coupled Ratchet Motors

We investigate the transport of superparamagnetic colloidal particles along self-assembled tracks using a periodically applied magnetic field as a model for ratchet-like mechanisms. Through video microscopy and simulations, we examine how factors such as particle size, track roughness, and the shape, intensity, and frequency of the applied potential influence transport efficiency. The findings reveal that processive motion can be achieved without residual attraction, with optimal transport efficiency governed by the combined effects of particle size ratios, actuation frequency, track roughness, and asymmetry in the applied potential. Additionally, we explore alternative strategies, including weak residual attraction and alternating magnetic fields, to further enhance efficiency. These findings provide valuable insights for the development of synthetic micro/nanomotors with potential applications in drug delivery and environmental remediation.

cond-mat.soft↗

arXiv2024

Enhancement of swimmer diffusion through regular kicks: analytic mapping of a scale independent parameter space

Depending on their mechanism of self-propulsion, active particles can exhibit a time-dependent, often periodic, propulsion velocity. The precise propulsion velocity profile determines their mean square displacement and their effective diffusion coefficient at long times. Here we demonstrate that any periodic propulsion profile results in a larger diffusion coefficient than the corresponding case with constant propulsion velocity. We investigate in detail the case of periodic exponentially decaying velocity pulses, expected in propulsion mechanisms based on sudden absorption of finite amounts of energy. We show both analytically and with numerical simulations that in these cases the effective diffusion coefficient can be arbitrarily enhanced with respect to the case with constant velocity equal to the average speed. Our results may help interpret in a new light observations on the diffusion enhancement of active particles.

cond-mat.soft↗

arXiv2023

Water-methanol mixture confined in a graphene slit-pore

Efficient and sustainable techniques for separating water-methanol mixtures are in high demand in the industry. Recent studies have revealed that membranes and 2D materials could achieve such separation. In our research, we explore the impact of a nanoconfining graphene slit-pore on the dynamics and structure of water-methanol mixtures. By Molecular Dynamics simulations of a coarse-grained model for water mixtures containing up to 25% methanol, we show that, for appropriate pore sizes, water tends to occupy the center of the pore. In contrast, methanol's apolar moiety accumulates near the hydrophobic walls. Additionally, modifying the pore's width leads to a non-monotonic change in the diffusivity of each component. However, water always diffuses faster than methanol, implying that it should be possible to identify an optimal configuration for water-methanol separation based on physical mechanisms. Our calculations indicate that one of the more effective pore sizes, 12.5Å, is also mechanically stable, minimizing the energy cost of a possible filtering membrane.

cond-mat.soft↗

arXiv2023

Size-pore-dependent methanol sequestration from water-methanol mixtures by an embedded graphene slit

The separation of liquid mixture components is relevant to many applications -- ranging from water purification to biofuel production -- and is a growing concern related to the UN Sustainable Development Goals (SDGs), such as ``Clean water and Sanitation'' and ``Affordable and clean energy''. One promising technique is using graphene slit-pores as filters, or sponges, because the confinement potentially affects the properties of the mixture components in different ways, favoring their separation. However, no systematic study has shown how the size of a pore changes the thermodynamics of the surrounding mixture. Here, we focus on water-methanol mixtures and explore, using Molecular Dynamics simulations, the effects of a graphene pore, with size ranging from 6.5 to 13 Å, for three compositions: pure water, 90\%-10\%, and 75\%-25\% water-methanol. We show that tuning the pore size can change the mixture pressure, density and composition in bulk due to the size-dependent methanol sequestration within the pore. Our results can help in optimizing the graphene pore size for filtering applications.

cond-mat.mes-hall↗

arXiv2021

Nanoconfined fluids: Uniqueness of water compared to other liquids

Nanoconfinement can drastically change the behavior of liquids, puzzling us with counterintuitive properties. Moreover, it is relevant in applications, including decontamination and crystallization control. It still lacks a systematic analysis for fluids with different bulk properties. Here we fill this gap. We compare, by molecular dynamics simulations, three different liquids in a graphene slit pore: (A) A simple fluid, such as argon, described by a Lennard-Jones potential; (B) An anomalous fluid, such as a liquid metal, modeled with an isotropic core-softened potential; (C) Water, the prototypical anomalous liquid, with directional hydrogen bonds. We study how the slit-pore width affects the structure, thermodynamics, and dynamics of the fluids. We check that all the fluids, as expected, show similar oscillating properties by changing the pore size. However, the nature of the free-energy minima for the three fluids is quite different: i) only for the simple liquid all the minima are energy-driven, while their structural order increases with decreasing slit-pore width; ii) only for the isotropic core-softened potential all the minima are entropy-driven, while the energy in the minima increases with decreasing slit-pore width; iii) only the water has a changing nature of the minima: the monolayer minimum is entropy-driven, at variance with the simple liquid, while the bilayer minimum is energy-driven, at variance with the other anomalous liquid. Also, water diffusion has a large increase for sub-nm slit-pores, becoming faster than bulk. Instead, the other two fluids have diffusion oscillations much smaller than water slowing down for decreasing slit-pore width. Our results clarify that nanoconfined water is unique compared to other (simple or anomalous) fluids under similar confinement, and are possibly relevant in nanopores applications, e.g., in water purification from contaminants.

cond-mat.soft↗

arXiv2021

Water under extreme confinement in graphene: Oscillatory dynamics, structure, and hydration pressure explained as a function of the confinement width

Graphene nanochannels are relevant for their possible applications, as in water purification, and for the challenge of understanding how they change the properties of confined liquids. Here, we use all-atom molecular dynamics simulations to investigate water confined in an open graphene slit-pore as a function of its width $w$, down to sub-nm scale. We find that the water translational and rotational dynamics exhibits an oscillatory dependence on $w$, due to water layering. The oscillations in dynamics correlate with those in hydration pressure, which can be negative (hydrophobic attraction), or as high as $\sim 1$ GPa, as seen in the experiments. At pore widths commensurable with full layers (around $7.0$ Å and $9.5$ Å for one and two layers, respectively), the free energy of the system has minima, and the hydration pressure vanishes. These are the separations at which the dynamics of confined water slows down. Nevertheless, the hydration pressure vanishes also where the free energy has maxima, i.e., for those pore-widths which are incommensurable with the formation of well-separated layers, as $w\simeq 8.0$ Å. Around these values of $w$, the dynamics is faster than in bulk, with water squeezed out from the pore. This behavior has not been observed for simple liquids under confinement, either for water in closed nano-pores. The decomposition of the free energy clarifies the origins of the dynamics speedups and slowdowns. In particular, we find that the nature of the slowdown depends on the number of water layers: for two layers, it is due to the internal energy contribution, as in simple liquids, while for one layer, it has an entropic origin possibly due to the existence of a hydrogen-bond network in water. Our results shed light on the mechanisms ruling the dynamics and thermodynamics of confined water and are a guide for future experiments.

cond-mat.soft↗

arXiv2021

Re-defining the concept of hydration water in water under soft confinement

Water shapes and defines the properties of biological systems. Therefore, understanding the nature of the mutual interaction between water and biological systems is of primary importance for a proper assessment of biological activity and the development of new drugs and vaccines. A handy way to characterize the interactions between biological systems and water is to analyze their impact on water density and dynamics in the proximity of the interfaces. It is well established that water bulk density and dynamical properties are recovered at distances in the order of $\sim1$~nm from the surface of biological systems. Such evidence led to the definition of \emph{hydration} water as the thin layer of water covering the surface of biological systems and affecting-defining their properties and functionality. Here, we review some of our latest contributions showing that phospholipid membranes affect the structural properties and the hydrogen bond network of water at greater distances than the commonly evoked $\sim1$~nm from the membrane surface. Our results imply that the concept of hydration water should be revised or extended, and pave the way to a deeper understanding of the mutual interactions between water and biological systems.

cond-mat.soft↗

arXiv2020

Propulsion and energetics of a minimal magnetic microswimmer

In this manuscript we describe the realization of a minimal hybrid microswimmer, composed of a ferromagnetic nanorod and a paramagnetic microsphere. The unbounded pair is propelled in water upon application of a swinging magnetic field that induces a periodic relative movement of the two composing elements, where the nanorod rotates and slides on the surface of the paramagnetic sphere. When taken together, the processes of rotation and sliding describe a finite area in the parameter space, which increases with the frequency of the applied field. We develop a theoretical approach and combine it with numerical simulations, which allow us to understand the dynamics of the propeller and explain the experimental observations. Furthermore, we demonstrate a reversal of the microswimmer velocity by varying the length of the nanorod, as predicted by the model. Finally, we determine theoretically and in experiments the Lighthill's energetic efficiency of this minimal magnetic microswimmer.

cond-mat.soft↗

arXiv2020

Collective Transport of Magnetic Microparticles at a Fluid Interface through Dynamic Self-Assembled Lattices

The transport of motile entities across modulated energy landscapes plays an important role in a range of phenomena in biology, colloidal science and solid-state physics. Here, an easily implementable strategy that allows for the collective and monitored transport of microparticles at fluid-fluid interfaces is introduced. Adsorbed magnetic microparticles are carried on time-dependent magnetic potentials, generated by a dynamic self-assembled lattice of different-sized magnetic particles. In such binary systems, the sudden reorientation of the applied field triggers the rapid exchange between attractive and repulsive configurations, enabling for the ballistic transfer of the carriers through the lattice. As the number of motile entities increases, the induced current increases, before reaching a maximum, while the loaded interface gradually displays bidirectional transport. The described methodology can be tuned through the applied field and exploited for the monitored guidance of adsorbed molecules on liquid surfaces, the segregation of colloidal mixtures, the induced motion of defects in photonic crystals or the design of new self-assembled microrobots.

cond-mat.soft↗

arXiv2020

Self-thermophoresis at the nanoscale using light induced solvation dynamics

Downsizing microswimmers to the nanoscale, and using light as an externally controlled fuel, are two important goals within the field of active matter. Here we demonstrate using all-atom molecular dynamics simulations that solvation relaxation, the solvent dynamics induced after visible light electronic excitation of a fluorophore, can be used to propel nanoparticles immersed in polar solvents. We show that fullerenes functionalized with fluorophore molecules in liquid water exhibit substantial enhanced mobility under external excitation, with a propulsion speed proportional to the power dissipated into the system. We show that the propulsion mechanism is quantitatively consistent with a molecular scale instance of self-thermophoresis. Strategies to direct the motion of functionalized fullerenes in a given direction using confined environments are also discussed.

cond-mat.mes-hall↗

arXiv2019

Direct Measurement of Lighthill's Energetic Efficiency of a Minimal Magnetic Microswimmer

The realization of artificial microscopic swimmers able to propel in viscous fluids is an emergent research field of fundamental interest and vast technological applications. For certain functionalities, the efficiency of the microswimmer in converting the input power provided through an external actuation into propulsive power output can be critical. Here we use a microswimmer composed by a self-assembled ferromagnetic rod and a paramagnetic sphere and directly determine its swimming efficiency when it is actuated by a swinging magnetic field. Using fast video recording and numerical simulations we fully characterize the dynamics of the propeller and identify the two independent degrees of freedom which allow its propulsion. We then obtain experimentally the Lighthill's energetic efficiency of the swimmer by measuring the power consumed during propulsion and the energy required to translate the propeller at the same speed. Finally, we discuss how the efficiency of our microswimmer could be increased upon suitable tuning of the different experimental parameters.

cond-mat.soft↗

arXiv2018

Water between membranes: Structure and Dynamics

An accurate description of the structure and dynamics of interfacial water is essential for phospholipid membranes, since it determines their function and their interaction with other molecules. Here we consider water confined in stacked membranes with hydration from poor to complete, as observed in a number of biological systems. Experiments show that the dynamics of water slows down dramatically when the hydration level is reduced. All-atom molecular dynamics simulations identify three (inner, hydration and outer) regions, within a distance of approximately 1 nm from the membrane, where water molecules exhibit different degrees of slowing down in the dynamics. The slow-down is a consequence of the robustness of the hydrogen bonds between water and lipids and the long lifetime of the hydrogen bonds between water molecules near the membrane. The interaction with the interface, therefore, induces a structural change in the water that can be emphasized by calculating its intermediate range order. Surprisingly, at distances as far as ~ 2.5 nm from the interface, although the bulk-like dynamics is recovered, the intermediate range order of water is still slightly higher than that in the bulk at the same thermodynamic conditions. Therefore, the water-membrane interface has a structural effect at ambient conditions that propagates further than the often-invoked 1 nm length scale. Membrane fluctuations smear out this effect macroscopically, but an analysis performed by considering local distances and instantaneous configurations is able to reveal it, possibly contributing to our understanding of the role of water at biomembrane interfaces.

cond-mat.soft↗

arXiv2016

Large Slowdown of Water Dynamics at Stacked Phospholipid Membranes for Increasing Hydration Level: All-Atoms Molecular Dynamics

Water hydrating phospholipid membranes determine their stability and function, as well as their interaction with other molecules. In this article we study, using all-atom molecular dynamics simulations, the rotational and translational dynamical properties of water molecules confined in stacked phospholipid membranes at different levels of hydration, from poorly hydrated to a completely hydrated membrane. We find that both the translational and the reorientation dynamics of water are dramatically slowed down as the hydration is reduced. Analyzing in details the structure and dynamics of the hydrogen bond at the interface, we show that both those among water molecules and those between water and lipids slow down by decreasing the hydration, however the latter are always slower than the former. By increasing hydration, water saturates all the possible hydrogen bonds with the lipids and, by further increase of hydration, the hydrogen bonds among waters becomes the majority. However, the dynamics of the water-lipids hydrogen bonds becomes up to one order of magnitude slower than that of the water-water hydrogen bonds, inducing a large slowing down of the dynamics of the entire system even at large hydration level.

cond-mat.soft↗

arXiv2012

On-the-fly coarse-graining methodology for the simulation of chain formation of superparamagnetic colloids in strong magnetic fields

The aim of this work is the description of the chain formation phenomena observed in colloidal suspensions of superparamagnetic nanoparticles under high magnetic fields. We propose a new methodology based on an on-the-fly Coarse-Grain (CG) model. Within this approach, the coarse grain objects of the simulation are not fixed a priori at the beginning of the simulation but rather redefined on the fly. The motion of the CG objects (single particles or aggregates) is described by an anisotropic diffusion model and the magnetic dipole-dipole interaction is replaced by an effective short range interaction between CG objects. The new methodology correctly reproduces previous results from detailed Langevin Dynamics simulations of dispersions of superparamagnetic colloids under strong fields whilst requiring an amount of CPU time orders of magnitude smaller. This substantial improvement in the computational requirements allows the simulation of problems in which the relevant phenomena extends to time scales inaccessible with previous simulation techniques. A relevant example is the waiting time dependence of the relaxation time T_2 of water protons observed in Magnetic Resonance experiments containing dispersions of superparamagnetic colloids, which is correctly predicted by our simulations. Future applications may include other popular real-world applications of superparamagnetic colloids such as the magnetophoretic separation processes.

cond-mat.soft↗

arXiv2010

A First Passage Time Analysis of Atomic-Resolution Simulations of the Ionic Transport in a Bacterial Porin

We have studied the dynamics of chloride and potassium ions in the interior of the OmpF porin under the influence of an external electric field. From the results of extensive all-atom molecular dynamics simulations of the system we computed several first passage time (FPT) quantities to characterize the dynamics of the ions in the interior of the channel. Such FPT quantities obtained from MD simulations demonstrate that it is not possible to describe the dynamics of chloride and potassium ions inside the whole channel with a single constant diffusion coefficient. However, we showed that a valid, statistically rigorous, description in terms of a constant diffusion coefficient D and an effective deterministic force Feff can be obtained after appropriate subdivison of the channel in different regions suggested by the X-ray structure. These results have important implications for popular simplified descriptions of channels based on the 1D Poisson-Nernst-Planck (PNP) equations. Also, the effect of entropic barriers on the diffusion of the ions is identified and briefly discussed.

cond-mat.soft↗

arXiv2010

Ionic partition and transport in multi-ionic channels: A Molecular Dynamics Simulation study of the OmpF bacterial porin

We performed all-atom molecular dynamics simulations studying the partition of ions and the ionic current through the bacterial porin OmpF and two selected mutants. The study is motivated by new interesting experimental findings concerning their selectivity and conductance behaviour at neutral pH. The mutations considered here are designed to study the effect of removal of negative charges present in the constriction zone of the wild type OmpF channel (which contains on one side a cluster with three positive residues and on the other side two negatively charged residues). Our results show that these mutations induce an exclusion of cations from the constriction zone of the channel, substantially reducing the flow of cations. In fact, the partition of ions inside the mutant channels is strongly inhomogeneous, with regions containing excess of cations and regions containing excess of anions. Interestingly, the overall number of cations inside the channel is larger than the number of anions in the two mutants, as in the OmpF wild type channel. We found that the differences in ionic charge inside these channels are justified by the differences in electric charge between the wild type OmpF and the mutants, following an electroneutral balance.

cond-mat.mtrl-sci↗

arXiv2010

Molecular Dynamics simulations of concentrated aqueous electrolyte solutions

Transport properties of concentrated electrolytes have been analyzed using classical molecular dynamics simulations with the algorithms and parameters typical of simulations describing complex electrokinetic phenomena. The electrical conductivity and transport numbers of electrolytes containing monovalent (KCl), divalent (MgCl$_2$), a mixture of both (KCl + MgCl$_2$), and trivalent (LaCl$_3$) cations have been obtained from simulations of the electrolytes in electric fields of different magnitude. The results obtained for different simulation parameters have been discussed and compared with experimental measurements of our own and from the literature. The electroosmotic flow of water molecules induced by the ionic current in the different cases has been calculated and interpreted with the help of the hydration properties extracted from the simulations.

cond-mat.mtrl-sci↗