arXiv subjects

Rodrigo C. V. Coelho

Publications and source records attributed to Rodrigo C. V. Coelho.

At least 19 recordsLinked to original sources

arXiv2026

A diffuse-interface theory of active nematic interfaces: transport mechanisms and modal structure

We develop a long-wavelength theory for the linear stability of a flat interface between an active nematic and an isotropic fluid. Starting from a diffuse-interface Cahn--Hilliard--Landau--de Gennes description coupled to Brinkman-screened Stokes hydrodynamics, we project the linearized dynamics onto a small set of interfacial degrees of freedom: the conserved translation, or height, mode; a scalar profile distortion or amplitude mode; and a transverse orientational mode associated with director rotations. Eliminating the gapped scalar profile mode gives a reduced interfacial operator coupling the conserved height mode to the transverse orientational mode. The main result is that activity generates, in the screened diffuse-interface regime, a direct local contribution proportional to $q^2$ in the height sector. This term competes with the passive local diffusive capillary relaxation, which enters at order $q^4$, and defines a local active interfacial channel controlled by the internal structure of the diffuse interface. This mechanism is distinct from the non-analytic $|q|$ and $|q|q^2$ terms characteristic of weakly screened Hele--Shaw/Saffman--Taylor-type transport, which are controlled by long-ranged momentum transport in the surrounding fluid. This framework identifies a diffuse-interface route to active interfacial instability that can operate while the homogeneous active nematic remains linearly stable because of hydrodynamic screening. It also provides a basis for distinguishing local diffuse-interface instabilities, bulk-flow-driven hydrodynamic instabilities, and mixed regimes in active nematic--isotropic interfaces.

cond-mat.soft↗

arXiv2026

Porosity and Material Disorder Drive Distinct Channelization Transition

Flow through porous media can reshape the medium through erosion and deposition, producing preferential flow channels across a wide range of natural and industrial systems. Yet the mechanisms by which spatial disorder triggers channelization remain unclear. Here we derive a continuum description for the coupled evolution of flow and porosity by coarse-graining pore-scale dynamics and validating the resulting model with pore-scale simulations. Using this framework, we show that different sources of disorder lead to qualitatively distinct behaviors. Disorder in erosion resistance produces a discontinuous transition to localized flow, with permanent channels appearing only above a finite disorder strength. In contrast, even extremely weak fluctuations in the initial porosity destabilize homogeneous flow and trigger persistent channelization. These results reveal an unexpected sensitivity of evolving porous media to structural heterogeneity, suggesting that channelization can arise generically even in nearly uniform materials.

physics.flu-dyn↗

arXiv2026

Spatiotemporal crystallization of an active fluid

The emergence of long-range spatiotemporal order from intrinsic chaos is a central challenge in far-from-equilibrium physics. In active fluids, such as cytoskeletal networks driving cellular motion, self-generated flows typically produce "active turbulence", lacking translational symmetry. Here we show that a chaotic active nematic can self-organize into a spatiotemporal crystal, forming a regular lattice of density, orientation, and vorticity that breaks both spatial and temporal translational symmetry. Using a microtubule/kinesin active nematic interfaced with a lamellar liquid crystal and confined in microfluidic channels, we observe robust spatiotemporal lattices without external forcing. The ordering emerges from spontaneous synchronization of intrinsic flow instabilities, mediated by confinement and feedback between the active layer and the passive anisotropic interface. Continuum nematohydrodynamics simulations support our interpretation, highlighting how intrinsic length and time scales shape the active crystals. These results reconcile chaos and crystallinity in active matter and provide a strategy for engineering order in self-driven, far-from-equilibrium soft materials.

cond-mat.soft↗

arXiv2025

Soft Colloidal Robots: Magnetically Guided Liquid Crystal Torons for Targeted Micro-Cargo Delivery

Quasiparticles in liquid crystals, such as torons and skyrmions, represent a new class of topologically protected solitonic excitations, offering a promising route toward soft microrobotics. Here we demonstrate that torons can be propelled by modulated electric fields and magnetically steered with full directional control, thus achieving programmable trajectories without net liquid flow. Within microfluidic architectures, we guide ensembles of torons through confined channels and realize targeted pick-up, transport, and release of colloidal cargo. By combining experiments and numerical simulations, we uncover how magnetic alignment reshapes toron structure, speed, and stability, while confinement within microchannels gives rise to novel transport regimes. Unlike conventional colloidal inclusions, torons are intrinsically uniform, soft, and reconfigurable, establishing them as both an ideal model system for studying emergent phenomena in active topological matter and a versatile platform for next-generation soft robots, adaptive delivery systems, and smart active matter.

cond-mat.soft↗

arXiv2025

Spontaneous rotation and propulsion of suspended capsules in active nematics

We investigate the dynamics of elastic capsules suspended in two-dimensional active nematic fluids using lattice Boltzmann simulations. The capsules, modeled as flexible membranes enclosing active internal regions, exhibit a rich variety of behaviors shaped by their geometry and the interplay between internal and external activity. Circular capsules with active interiors undergo persistent rotation driven by internally confined +1/2 topological defects. Axisymmetric capsules, such as boomerangs, develop directed motion along their axis of symmetry due to unbalanced active forces generated by defect distributions near their boundaries. We further show that capsule flexibility suppresses motility and rotation, as active stresses are dissipated into shape deformations. These findings reveal how shape, deformability, and defect dynamics cooperate to produce emergent motility in soft active matter, with potential applications in the design of microswimmers and drug delivery vehicles.

cond-mat.soft↗

arXiv2025

Shape transitions of sedimenting confined droplets and capsules: from oblate to bullet-like geometries

The transport and deformation of confined droplets and flexible capsules are central to diverse phenomena and applications, from biological flows in microcapillaries to industrial processes in porous media. Inspired by experiments, we perform numerical simulations to investigate their shape dynamics under varying levels of confinement and particle flexibility. A transition from an oblate to a bullet-like shape is observed at a confinement threshold, independent of flexibility, which agrees with our analytical calculations. A fluid-structure interaction analysis reveals two regimes: a pressure-dominated and a viscous-dominated regime. For highly flexible particles, the pressure-dominated regime prevails and the deformation is enhanced. These findings offer new insights into the transport of flexible particles in confined environments, with implications for biomedical applications, filtration technologies, and multiphase fluid mechanics.

cond-mat.soft↗

arXiv2025

Active nematic pumps

Microfluidics involves the manipulation of flows at the microscale, typically requiring external power sources to generate pressure gradients. Alternatively, harnessing flows from active fluids, which are usually chaotic, has been proposed as a paradigm for the development of micro-machines. Here, by combining experimental realizations and simulations, we demonstrate that the addition of triangular-shaped inclusions into an active nematic gel can locally break the fore-aft symmetry of active turbulence and stabilize flow fields with self-pumping capabilities. The proposed strategy has enabled us to generate wall-free and self-powered microfluidic systems capable of both cargo transport and mixing along with the downstream flow. We analyze the performance of these active pumps, both isolated and within cooperative ensembles in terms of their output velocity and hydrostatic pressure buildup. Finally, we demonstrate strategies to incorporate them into specifically designed microfluidic platforms to advantageously tailor the geometry of active flows. Our results reveal new possibilities for leveraging the self-organized mechanodynamics of active fluids.

cond-mat.soft↗

arXiv2025

Phase Separation in Mixtures of Nematic and Isotropic Fluids

Mixtures of nematic liquid crystals and isotropic fluids display a diverse range of phase behaviors, arising from the coupling between orientational order and concentration fluctuations. In this review, we introduce a simplified mathematical framework that integrates the Landau-de Gennes free energy for nematic ordering with the Cahn-Hilliard free energy for phase separation. We derive the corresponding governing equations and analyze the stability of uniform phases, along with the resulting interfacial phenomena. The review concludes with a brief discussion highlighting key differences in phase separation between mixtures of isotropic fluids with passive and active nematics.

cond-mat.soft↗

arXiv2025

Efficient single-precision simulations of nematohydrodynamics

Simulations of nematohydrodynamics on graphics processing units (GPUs) are typically performed using double precision, which ensures accuracy but significantly increases computational cost. However, consumer-grade GPUs are optimized for single-precision calculations, making double-precision simulations inefficient on widely available hardware. In this work, we demonstrate that single-precision simulations can achieve the same accuracy as double-precision methods while delivering a 27-fold increase in computational speed. To achieve this, we introduce two key improvements: (i) the shifted distribution function in the lattice Boltzmann method, which mitigates precision loss at low velocities, and (ii) the use of larger time steps in the finite-difference solver, which reduces numerical errors and improves overall accuracy. We find that, unlike in double precision, accuracy in single-precision simulations follows a non-monotonic trend with respect to the finite-difference time step, revealing an optimal regime for precise computations. To illustrate the effectiveness of our approach, we simulate the dynamics of single and multiple skyrmionic tubes in Poiseuille flow. Our results confirm that optimized single-precision simulations enable fast and accurate modeling of complex nematohydrodynamic systems, making large-scale simulations feasible on standard gaming GPUs.

physics.flu-dyn↗

arXiv2024

Solute dispersion in pre-turbulent confined active nematics

We investigate the dispersion of solutes in active nematic fluids confined in narrow channels based on simulations of nematohydrodynamics. The study focuses on two pre-turbulent regimes: oscillatory flow, with net mass flux, and dancing flow, without net flux. Our findings reveal that the longitudinal dispersion of solutes, in both regimes, is determined by the second moments of the longitudinal and transverse components of the velocity field and proportional to the activity, suggesting a common mechanism for dispersion in these distinct flows. Finally, we found that the dispersion coefficient may increase up to one order of magnitude with respect to molecular diffusion in the dancing flow regime and that it is also enhanced in oscillatory flows, with potential for applications in natural biological environments and engineered devices.

cond-mat.soft↗

arXiv2024

Wetting and pressure gradient performance in a lattice Boltzmann color gradient model

An accurate implementation of wetting and pressure drop is crucial to correctly reproducing fluid displacement processes in porous media. Although several strategies have been proposed in the literature, a systematic comparison of them is needed to determine the most suitable for practical applications. Here, we carried out numerical simulations to investigate the performance of two widely used wettability schemes in the lattice Boltzmann color gradient model, namely the geometrical wetting scheme by Leclaire et al. (Scheme-I) and the modified direction of the color gradient scheme by Akai et al. (Scheme-II). We showed that Scheme-II was more accurate in simulating static contact angles of a fluid droplet on a solid surface. However, Scheme-I was more accurate in simulating a dynamic case of a binary fluid flow in a horizontal capillary tube described by the Washburn equation. Moreover, we investigated the performance of two popular pressure gradient implementation types. Type-I used the so-called Zou-He pressure boundary conditions at the inlet and the outlet of the domain, while Type-II used an external body force as a pressure gradient. We showed that the Type-I implementation was slightly more accurate in simulating a neutrally wetting fluid in a horizontal capillary tube described by the Washburn equation. We also investigated the differences between the two types of pressure gradient implementation in simulating two fluid displacement processes in a Bentheimer sandstone rock sample: the primary drainage and the imbibition displacement processes.

physics.flu-dyn↗

arXiv2024

Liquid crystal torons in Poiseuille-like flows

Three-dimensional (3D) simulations of the structure of liquid crystal (LC) torons, topologically protected distortions of the LC director field, under material flows are rare but essential in microfluidic applications. Here, we show that torons adopt a steady-state configuration at low flow velocity before disintegrating at higher velocities, in line with experimental results. Furthermore, we show that under partial slip conditions at the boundaries, the flow induces a reversible elongation of the torons, also consistent with the experimental observations. These results are in contrast with previous simulation results for 2D skyrmions under similar flow conditions, highlighting the need for a 3D description of this LC soliton in relation to its coupling to the material flow. These findings pave the way for future studies of other topological solitons, like hopfions and heliknotons, in flowing soft matter systems.

cond-mat.soft↗

arXiv2024

"Hall" transport of liquid crystal solitons in Couette flow

Topology establishes a unifying framework for a diverse range of scientific areas including particle physics, cosmology, and condensed matter physics. One of the most fascinating manifestations of topology in the context of condensed matter is the topological Hall effect, and its relative: the Skyrmion Hall effect. Skyrmions are stable vortex-like spin configurations in certain chiral magnets, and when subject to external electric currents can drift in the transverse direction to the current. These quasi-particles are characterised by a conserved topological charge which in the Skyrmion Hall effect plays the role of electric charges in the ordinary Hall effect. Recently, it has been shown that liquid crystals endowed with chiral properties serve as an ideal testbed for the fundamental investigation of topological solitons, including their two- and three-dimensional realisations. Here, we show experimentally and numerically that three-dimensional solitons aka "torons" exhibit a Hall-like effect when driven by shear flows: the torons are deflected in the direction perpendicular to the shear plane. The experimental results are rationalised in terms of the dynamic Ericksen-Leslie equations, which predict the emergence of the transverse component of the net mass flow, the magnitude of which scales as the 3rd power of the shear rate. The perturbation analysis highlights an interplay of the viscous and chiral elastic torques as the mechanism for the emergence of net transverse currents. Numerical simulations demonstrate, however, that torons are not merely dragged by the flow but move with their own transverse speed, much larger than the average flow velocity in the transverse direction. Our findings may enable responsive microfluidic applications relying on soft topological solitons.

cond-mat.soft↗

arXiv2024

Active nematic-isotropic interfaces on flat surfaces: effects of anchoring, ordering field and activity

A surface in contact with the isotropic phase of a passive liquid crystal can induce nematic order over distances that range from microscopic to macroscopic when the nematic-isotropic interface undergoes an orientational-wetting transition. If the nematic is active, what happens to the interface? Does it propagate and, if it does, is its structure different from the passive one? In this paper, we address these questions. We investigate how the active nematic-isotropic interface is affected by the anchoring strength of the surface, the bulk ordering field and the activity. We find that while passive interfaces are one-dimensional the active ones exhibit two dynamical regimes: a passive-like regime and a propagating regime where the interfaces propagate until the entire domain is active nematic. Active interfaces break the translational symmetry within the interfacial plane above a threshold activity, where the active nematic fluctuations, which are ultimately responsible for the emergence of an active turbulent nematic phase, drive non-steady dynamical interfacial regimes.

cond-mat.soft↗

arXiv2024

Lattice Boltzmann simulation of deformable fluid-filled bodies: progress and perspectives

With the rapid development of studies involving droplet microfluidics, drug delivery, cell detection, and microparticle synthesis, among others, many scientists have invested significant efforts to model the flow of these fluid-filled bodies. Motivated by the intricate coupling between hydrodynamics and the interactions of fluid-filled bodies, several methods have been developed. The objective of this review is to present a compact foundation of the methods used in the literature in the context of lattice Boltzmann methods. For hydrodynamics, we focus on the lattice-Boltzmann method due to its specific ability to treat time and spatial-dependent boundary conditions and to incorporate new physical models in a computationally efficient way. We split the existing methods into two groups with regard to the interfacial boundary: fluid-structure and fluid-fluid methods. The fluid-structure methods are characterised by the coupling between fluid dynamics and mechanics of the flowing body, often used in applications involving membranes and similar flexible solid boundaries. We further divide fluid-structure-based methods into two subcategories, those which treat the fluid-structure boundary as a continuum medium and those that treat it as a discrete collection of individual springs and particles. Next, we discuss the fluid-fluid methods, particularly useful for the simulations of fluid-fluid interfaces. We focus on models for immiscible droplets and their interaction in a suspending fluid and describe benchmark tests to validate the models for fluid-filled bodies.

cond-mat.soft↗

arXiv2023

Continuum model for extraction and retention in porous media

Several natural and industrial processes involve the extraction or retention of a solute by a fluid invading a network of channels. Examples include aquifer contamination, chemical filtration, and coffee extraction. We propose a continuum equation to model these processes, parametrized by the Péclet number and the rate of mass transfer between the solid and the fluid. We study the time dependence of the extracted mass for different values of the parameter space. The continuum description is validated by combining extraction experiments with coffee and computational fluid dynamics. An analytical solution is derived for the limit of slow mass transfer, which is corroborated by numerical simulations.

physics.flu-dyn↗

arXiv2023

Active nematics on flat surfaces: from droplet motility and scission to active wetting

We consider the dynamics of active nematics droplets on flat surfaces, based on the continuum hydrodynamic theory. We investigate a wide range of dynamical regimes as a function of the activity and droplet size on surfaces characterized by strong anchoring and a range of equilibrium contact angles. The activity was found to control a variety of dynamical regimes, including the self-propulsion of droplets on surfaces, scission, active wetting and droplet evaporation. Furthermore, we found that on a given surface (characterized by the anchoring and the equilibrium contact angle) the dynamical regimes may be controlled by the active capillary number of suspended droplets. We also found that the active nematics concentration of the droplets varies with the activity, affecting the wetting behaviour weakly but ultimately driving droplet evaporation. Our analysis provides a global description of a wide range of dynamical regimes reported for active nematics droplets and suggests a unified description of droplets on surfaces. We discuss the key role of the finite size of the droplet and comment on the suppression of these regimes in the infinite size limit, where the active nematics is turbulent at any degree of activity.

cond-mat.soft↗

arXiv2023

Sculpting liquid crystal skyrmions with external flows

We investigate, using experiments and numerical simulations, the distortions and the alignment of skyrmions in the liquid crystal under external flows for a range of average flow velocities. The simulations are based on the Landau-de Gennes $Q$ tensor theory both for isolated as well as for systems with many skyrmions. We found striking flow-driven elongation of an isolated skyrmion and flow alignment of skyrmions in the many-skyrmion system, both of which are also observed in the experiments. In the simulations, particular attention was given to the dissipation rate and to the various dissipation channels for a single skyrmion under external flow. This analysis provides insight on the observed scaling regime of the elongation of isolated flowing skyrmions and revealed a surprising plastic response at very short times, which may be relevant in applications based on the alignment of soft structures such as liquid crystal skyrmions.

cond-mat.soft↗