arXiv subjects

Juan de Pablo

Publications and source records attributed to Juan de Pablo.

9 recordsLinked to original sources

arXiv2025

Out-of-equilibrium modeling of lyotropic liquid crystals: from binary simulations to multi-component theory

We present a thermodynamically consistent theoretical framework for lyotropic liquid crystals (LCs) based on the GENERIC (General Equation for the Non-Equilibrium Reversible-Irreversible Coupling) formalism. This formalism ensures conservation of energy and production of entropy, while coupling concentration, momentum balance, and liquid crystalline order. Starting from a binary nematic-isotropic mixture, we derive a theory for these key variables, which is then extended to multi-component systems. The binary equations are solved numerically using a Julia-based solver that relies on an upwind finite-difference scheme, enabling stable and efficient simulations capable of handling multiple time scales while satisfying fundamental mathematical constraints. The results of simulations are consistent with experimental observations of topological core defects in chromonic LCs, as well as flow-driven droplet shape transitions under Couette and Poiseuille flows. This work provides a platform for simulations of multi-component lyotropic LCs that can be extended to systems with multiple interfaces, active materials, and materials subject to external fields.

cond-mat.soft↗

arXiv2025

Omnidirectionally manipulated skyrmions in an orientationally chiral system

Skyrmions, originally from condensed matter physics, have been widely explored in various physical systems, including soft matter. A crucial challenge in manipulating topological solitary waves like skyrmions is controlling their flow on demand. Here, we control the arbitrary moving direction of skyrmions in a chiral liquid crystal system by adjusting the bias of the applied alternate current electric field. Specifically, the velocity, including both moving direction and speed can be continuously changed. The motion control of skyrmions originates from the symmetry breaking of the topological structure induced by flexoelectric-polarization effect. The omnidirectional control of topological solitons opens new avenues in light-steering and racetrack memories.

cond-mat.soft↗

arXiv2022

Entanglements via Slip-Springs with Soft, Coarse-Grained Models for Systems Having Explicit Liquid-Vapor Interfaces

Recent advances in nano-rheology require that new methods and models be developed to describe the equilibrium and non-equilibrium properties of entangled polymeric materials and their interfaces at a molecular level of detail. In this work we present a Slip-Spring (SLSP) model capable of describing the dynamics of entangled polymers at interfaces, including explicit liquid-vapor and liquid-solid interfaces. The highly coarse-grained approach adopted with this model enables simulation of entire nano-rheological characterization systems within a particle-level base description. Many-body dissipative particle dynamics (MDPD) non-bonded interactions allow for explicit liquid-vapor interfaces, and compensating potential within the SLSP model ensures unbiased descriptions of the shape of the liquid-vapor interface. The usefulness of the model has been illustrated by studying the deposition of polymer droplets onto a substrate, where it s shown that the wetting dynamics is strongly dependent on the degree of entanglement of the polymer. More generally, the model proposed here provides a foundation for the development of digital twins of experimentally relevant systems, including a new generation of nano-rheometers based on nano- or micro-droplet deformation.

cond-mat.soft↗

arXiv2020

Statistical mechanics of a chiral active fluid

Statistical mechanics provides the foundation for describing complex materials using only a few thermodynamic variables. No such framework currently exists far from equilibrium. In this Letter, we demonstrate how thermodynamics emerges far from equilibrium, using fluids composed of active spinners as a case study. Activity gives rise to a single effective temperature that parameterizes both the equation of state and the emergent Boltzmann statistics. The same effective temperature, renormalized by velocity correlations, controls the linear response through canonical Green-Kubo relations for both the familiar shear viscosity and the odd (or Hall) viscosity observed in chiral fluids. The full frequency dependence of these viscosities can be derived analytically by modeling the active-spinner fluid as a random walker undergoing cyclotron motion in shear-stress space. More generally, we provide a first-principles derivation of the Green-Kubo relations valid for a broader class of fluids far from equilibrium. Besides advancing non-equilibrium thermodynamics, our work demonstrates in silico a non-invasive microrheology of active fluids.

cond-mat.soft↗

arXiv2020

Structural transformations in tetravalent nematic shells induced by a magnetic field

The role of applied fields on the structure of liquid crystals confined to shell geometries has been studied in past theoretical work, providing strategies to produce liquid crystal shells with controlled defect structure or valence. However, the predictions of such studies have not been experimentally explored yet. In this work, we study the structural transformations undergone by tetravalent nematic liquid crystal shells under a strong uniform magnetic field, using both experiments and simulations. We consider two different cases in terms of shell geometry and initial defect symmetry: i) homogeneous shells with four s = +1/2 defects in a tetrahedral arrangement, and ii) inhomogeneous shells with four s = +1/2 defects localized in their thinner parts. Consistently with previous theoretical results, we observe that the initial defect structure evolves into a bipolar one, in a process where the defects migrate towards the poles. Interestingly, we find that the defect trajectories and dynamics are controlled by curvature walls that connect the defects by pairs. Based on the angle between Bs, the local projection of the magnetic field on the shell surface, and n+ 1/2 , a vector describing the defect orientations, we are able to predict the nature and shape of those inversion walls, and therefore, the trajectory and dynamics of the defects. This rule, based on symmetry arguments, is consistent with both experiments and simulations and applies for shells that are either homogeneous or inhomogeneous in thickness. By modifying the angle between Bs and n+1/2, we are able to induce, in controlled way, complex routes towards the final bipolar state. In the case of inhomogeneous shells, the specific symmetry of the shell allowed us to observe a hybrid splay-bend Helfrich wall for the first time.

cond-mat.soft↗

arXiv2018

Pair and Many-body Interactions Between Ligated Au Nanoparticles

We report the results of molecular dynamics simulations of the properties of a pseudo-atom model of dodecane thiol ligated 5-nm diameter gold nanoparticles (AuNP) in vacuum as a function of ligand coverage and particle separation in three state of aggregation: the isolated AuNP, an isolated pair of AuNPs and a square assembly of AuNPs. Our calculations show that for all values of the coverage the ligand density along a radius emanating from the core of an isolated AuNP oscillates along the chain up to the fourth pseudo-atom, then smoothly decays to zero. We examine the ligand envelope as a function of the coverage and demonstrate that the deformation of that envelope generated by interaction between the NPs is coverage-dependent, so that the shape, depth and position of the minimum of the potential of mean force displays a systematic dependence on the coverage. We propose an accurate analytical description of the calculated potential of mean force with parameters that scale linearly with the ligand coverage. We define and calculate an effective pair potential of mean force for a square configuration of particles; our definition contains, implicitly, both the three- and four-particle contributions to deviation from additivity. We find that the effective pair potential of mean force in this configuration has a different minimum and a different well depth than the isolated pair potential of mean force. Previous work has found that the three-particle contribution to deviation from additivity is monotone repulsive, whereas we find that the combined three- and four-particle contributions have an attractive well, implying that the three- and four-particle contributions are of comparable magnitude but opposite sign, thereby suggesting that even higher order correction terms likely play a significant role in the behavior of assemblies of many nanoparticles.

physics.chem-ph↗

arXiv2016

Controlling placement of nonspherical (boomerang) colloids in nematic cells with photopatterned director

Placing colloidal particles in predesigned sites represents a major challenge of the current state-of-the-art colloidal science. Nematic liquid crystals with spatially varying director patterns represent a promising approach to achieve a well-controlled placement of colloidal particles thanks to the elastic forces between the particles and the surrounding landscape of molecular orientation. Here we demonstrate how the spatially varying director field can be used to control placement of non-spherical particles of boomerang shape. The boomerang colloids create director distortions of a dipolar symmetry. When a boomerang particle is placed in a periodic splay-bend director pattern, it migrates towards the region of a maximum bend. The behavior is contrasted to that one of spherical particles with normal surface anchoring, which also produce dipolar director distortions, but prefer to compartmentalize into the regions with a maximum splay. The splay-bend periodic landscape thus allows one to spatially separate these two types of particles. By exploring overdamped dynamics of the colloids, we determine elastic driving forces responsible for the preferential placement. Control of colloidal locations through patterned molecular orientation can be explored for future applications in microfluidic, lab on a chip, sensing and sorting devices.

cond-mat.soft↗

arXiv2016

Age and Structure of a Model Vapor-Deposited Glass

Glass films prepared by a process of physical vapor deposition have been shown to have thermodynamic and kinetic stability comparable to those of ordinary glasses aged for thousands of years. A central question in the study of vapor-deposited glasses, particularly in light of new knowledge regarding anisotropy in these materials, is whether the ultra-stable glassy films formed by vapor deposition are ever equivalent to those obtained by liquid cooling. We present a computational study of vapor deposition for a two-dimensional glass forming liquid using a methodology which closely mimics experiment. We find that for the model considered here, structures that arise in vapor-deposited materials are statistically identical to those observed in ordinary glasses, provided the two are compared at the same inherent structure energy. We also find that newly deposited hot molecules produce cascades of hot particles that propagate far into the film, possibly influencing the relaxation of the material.

cond-mat.soft↗

arXiv2016

An O(N) and parallel approach to integral problems by a kernel-independent fast multipole method: Application to polarization and magnetization of interacting particles

Large classes of materials systems in physics and engineering are governed by magnetic and electrostatic interactions. Continuum or mesoscale descriptions of such systems can be cast in terms of integral equations, whose direct computational evaluation requires O(N^2) operations, where N is the number of unknowns. Such a scaling, which arises from the many-body nature of the relevant Green's function, has precluded wide-spread adoption of integral methods for solution of large-scale scientific and engineering problems. In this work, a parallel computational approach is presented that relies on using scalable open source libraries and utilizes a kernel-independent Fast Multipole Method to evaluate the integrals in O(N) operations, with O(N) memory cost, thereby substantially improving the scalability and efficiency of computational integral methods. We demonstrate the accuracy, efficiency, and scalability of our approach in the contest of two examples. In the first, we solve a boundary value problem for a ferroelectric/ferromagnetic volume in free space. In the second, we solve an electrostatic problem involving polarizable dielectric bodies in an unbounded dielectric medium. The results from these test cases show that our parallel approach, which is built on a kernel-independent FMM, can enable highly efficient and accurate simulations and allow for considerable flexibility in a broad range of applications.

physics.comp-ph↗