arXiv subjects

Jeffrey C. Everts

Publications and source records attributed to Jeffrey C. Everts.

16 recordsLinked to original sources

arXiv2026

Exact results for dissipation and steady creeping flow in three-dimensional chiral active fluids

Chiral active fluids consist of self-spinning particles that rotate due to continuous energy injection at the microscopic scale (e.g. by activity or an external field). The hydrodynamics of such fluids is described by antisymmetric contributions in the viscosity tensor - called odd viscosity - which are allowed by symmetry due to the presence of a non-trivial spin angular momentum density. By generalising the Helmholtz minimum dissipation theorem to systems with odd viscosity, we show that incompressible three-dimensional odd fluids in the presence of sources that induce flow (e.g. surfaces that impose boundary conditions) admit a unique solution for their steady flow fields at low Reynolds number. Furthermore, we prove that such flows dissipate more energy than ordinary Stokes flow, provided that the flow field is affected by odd viscosity. As an example, we consider a model fluid described by one shear viscosity and one odd viscosity in the creeping flow regime. We explicitly compute the stress tensor for a fluid subjected to a point-force density. Finally, we compute exact results for the pressure and flow fields around a translating and rotating spherical particle from their singularity representations. From these solutions and our extended Helmholtz theorem, we explain why a translating sphere dissipates more energy when odd viscosity is present, whereas a rotating sphere does not.

physics.flu-dyn↗

arXiv2026

Linear odd electrophoresis of a sphere in a charged chiral active fluid

The electrophoresis of charged colloidal particles in fluids exhibiting odd viscosity represents a fundamental challenge in understanding transport phenomena within charge-stabilised chiral active suspensions. Here, we consider a charged chiral active fluid, where electrokinetics is coupled to odd Stokes flow, to explore how classical results from electrophoresis in Newtonian fluids are modified in the presence of odd viscosity. In particular, we derive a general expression for the electrophoretic mobility for particles of any shape, under weak external electric fields, using the Lorentz reciprocal theorem for odd fluids. By applying this result to a charged sphere at low zeta potentials, we obtain an exact, closed-form analytical expression for the electrophoretic mobility, valid for arbitrary values of the Debye screening length and the odd-viscosity coefficient. Similar to Newtonian fluids, we find that the electrophoretic mobility is proportional to the translational mobility of an uncharged sphere, modulated by the Henry function. However, unlike in Newtonian fluids, odd viscosity leads to directional asymmetries in the electrophoretic mobility tensor that persist even for thin electric double layers. This case contrasts significantly with a charged anisotropic particle suspended in an isotropic Newtonian fluid, where anisotropic effects would vanish under the same electrostatic-screening conditions.

cond-mat.soft↗

arXiv2026

Beyond uniform screening: electrostatic heterogeneity dictates solution structure of complex macromolecules

The complexity of biomolecular interactions necessitates advanced methodologies to accurately capture their behavior in solution. In this work, we focus on monoclonal antibodies and adopt a multi-scale coarse-graining strategy for their modeling, with particular emphasis on the role of electrostatic interactions. Using scattering experiments, theoretical analysis, and large-scale computer simulations, we explicitly compare two selected case studies-markedly different in their charge distributions. Through mutually corroborating lines of evidence, we demonstrate that conventional approaches relying on electrostatic screening and implicit charge representations fail to capture the structural and thermodynamic properties of antibody solutions when strong charge heterogeneity is present, even at a moderate (amino acid) level of coarse-graining. These findings highlight the importance of a correct treatment of electrostatic interactions and ion screening for heterogeneously- and oppositely-charged colloidal and protein systems. Such considerations are essential to move beyond descriptive models towards a truly predictive framework, with direct implications for the formulation of therapeutics and the treatment of other complex soft-matter systems.

cond-mat.soft↗

arXiv2025

Stokes drag on a sphere in a three-dimensional anisotropic porous medium

We study the hydrodynamic drag force exerted on a sphere in a static anisotropic porous medium. This problem is analysed using the Brinkman-Debye-Bueche equations with an axisymmetric shielding (or permeability) tensor. Using the exact Green's functions for this model fluid within a single-layer boundary element formulation, we numerically compute the friction tensor for a translating sphere subjected to stick boundary conditions. Furthermore, we derive approximate analytical expressions for small anisotropy using the Lorentz reciprocal theorem. By benchmarking this result against the numerical solutions, we find that a linear approximation is valid in a broad parameter regime. Our results are important for studying self-diffusion in general anisotropic porous media, but can also be applied to small tracers in nematic fluids composed of disk- or rod-like crowders.

cond-mat.soft↗

arXiv2025

Brownian motion at various length scales with hydrodynamic and direct interactions

Brownian motion is essential for describing diffusion in systems ranging from simple to complex liquids. Unlike simple liquids, which consist of only a solvent, complex liquids, such as colloidal suspensions or the cytoplasm of a cell, are mixtures of various constituents with different shapes and sizes. Describing Brownian motion in such multiscale systems is extremely challenging because direct and many-body hydrodynamic interactions (and their interplay) play a pivotal role. Diffusion of small particles is mainly governed by a low viscous character of the solution, whereas large particles experience a highly viscous flow of the complex liquid on the macro scale. A quantity that encodes hydrodynamics on both length scales is the wave-vector-dependent viscosity. Assuming this quantity to be known -- in contrast to most studies in which the solvent shear viscosity is given -- provides a new perspective on studying the diffusivity of a tracer, especially in situations where the tracer size can vary by several orders of magnitude. Here, we start systematic studies of exact formal microscopic expressions for the short- and long-time self-diffusion coefficients of a single probe particle in a complex liquid in terms of short-ranged hydrodynamic response kernels. We study Brownian motion as a function of the probe size, contrasting most theories that focus on self-diffusion as a function of the crowder volume fraction. We discuss the limits of small and large probe sizes for various levels of approximations in our theory, and discuss the current successes and shortcomings of our approach.

cond-mat.soft↗

arXiv2024

Dissipative effects in odd viscous Stokes flow around a single sphere

Odd viscosity (OV) is a transport coefficient in, for example, fluids of self-spinning (active) particles or electrons in an external magnetic field. The key feature of OV is that it does not contribute to dissipation in two spatial dimensions. In contrast, we explicitly show that in the three-dimensional case, OV can contribute indirectly to dissipation by modifying the fluid flow. We quantify the dissipation rate of a single spherical particle moving through a fluid with OV via an exact analytical solution of the generalised stationary creeping flow equations. Our results provide a novel way to quantify the effects of OV by measuring the solid-body motion of a single spherical particle. Moreover, we explicitly demonstrate how complex fluids can be designed in terms of their rheological properties by mixing passive particles with self-spinning active particles.

cond-mat.soft↗

arXiv2023

Realisation of the Brazil-nut effect in charged colloids without external driving

Sedimentation is a ubiquitous phenomenon across many fields of science, such as geology, astrophysics, and soft matter. Sometimes, sedimentation leads to unusual phenomena, such as the Brazil-nut effect, where heavier (granular) particles reside on top of lighter particles after shaking. We show experimentally that a Brazil-nut effect can be realised in a binary colloidal system of long-range repulsive charged particles driven purely by Brownian motion and electrostatics without the need for activity. Using theory, we argue that not only the mass-per-charge for the heavier particles needs to be smaller than the mass-per-charge for the lighter particles, but that at high overall density, the system can be trapped in a long-lived metastable state, which prevents the occurrence of the equilibrium Brazil-nut effect. Therefore, we envision that our work provides valuable insights into the physics of strongly interacting systems, such as partially glassy and crystalline structures. Finally, our theory, which quantitatively agrees with the experimental data, predicts that the shapes of sedimentation density profiles of multicomponent charged colloids are greatly altered when the particles are charge regulating with more than two ion species involved. Hence, we hypothesise that sedimentation experiments can aid in revealing the type of ion-adsorption processes that determine the particle charge and possibly the value of the corresponding equilibrium constants.

cond-mat.soft↗

arXiv2023

Nematronics: Reciprocal coupling between ionic currents and nematic dynamics

Adopting a spintronics-inspired approach, we study the reciprocal coupling between ionic charge currents and nematic texture dynamics in a uniaxial nematic electrolyte. Assuming quenched fluid dynamics, we develop equations of motion analogously to spin torque and spin pumping. Based on the principle of least dissipation of energy, we derive the adiabatic "nematic torque" exerted by ionic currents on the nematic director field as well as the reciprocal motive force on ions due to the orientational dynamics of the director. We discuss several simple examples that illustrate the potential functionality of this coupling. Furthermore, using our phenomenological framework, we propose a practical means to extract the coupling strength through impedance measurements on a nematic cell. Exploring further applications based on this physics could foster the development of nematronics -- nematic iontronics.

cond-mat.soft↗

arXiv2021

Ionically charged topological defects in nematic fluids

Charge profiles in liquid electrolytes are of crucial importance for applications, such as supercapacitors, fuel cells, batteries, or the self-assembly of particles in colloidal or biological settings. However, creating localised (screened) charge profiles in the bulk of such electrolytes, generally requires the presence of surfaces -- for example, provided by colloidal particles or outer surfaces of the material -- which poses a fundamental constraint on the material design. Here, we show that topological defects in nematic electrolytes can perform as regions for local charge separation, forming charged defect cores and in some geometries even electric multilayers, as opposed to the electric double layers found in isotropic electrolytes. Using a Landau-de Gennes-Poisson-Boltzmann theoretical framework, we show that ions highly effectively couple with the topological defect cores via ion solvability, and with the local director-field distortions of the defects via flexoelectricity. The defect charging is shown for different defect types -- lines, points, and walls -- using geometries of ionically screened flat isotropic-nematic interfaces, radial hedgehog point defects and half-integer wedge disclinations in the bulk and as stabilised by (charged) colloidal particles. More generally, our findings are relevant for possible applications where topological defects act as diffuse ionic capacitors or as ionic charge carriers.

cond-mat.soft↗

arXiv2021

Charge-, salt- and flexoelectricity-driven anchoring effects in nematics

We investigate the effects of electric double layers and flexoelectricity on the surface anchoring in general nematic fluids. Within a simplified model, we demonstrate for a nematic electrolyte how the surface anchoring strength can be affected by the surface charge, bulk ion concentration and/or flexoelectricity, effectively changing not only the magnitude of the anchoring but also the anchoring type, such as from planar to tilted. In particular, we envisage possible tuning of the anchoring strength by the salt concentration in the regime where sufficiently strong electrostatic anchoring, as controlled by the (screened) surface charge, can compete with the non-electrostatic anchoring. This effect is driven by the competing energetic-torque couplings between nematic director and the emergent electrostatic potential, due to surface charge, ions and flexoelectricity. Our findings propose a way of influencing surface anchoring by using electrostatic effects, which could be used in various aspects, including in the self-assembly of colloidal particles in nematic fluids, optical and display patterns, and sensing.

cond-mat.soft↗

arXiv2021

Anisotropic electrostatic screening of charged colloids in nematic solvents

The physical behaviour of anisotropic charged colloids is determined by their material dielectric anisotropy, affecting colloidal self-assembly, biological function and even out-of-equilibrium behaviour. However, little is known about anisotropic electrostatic screening, which underlies all electrostatic effective interactions in such soft or biological materials. In this work, we demonstrate anisotropic electrostatic screening for charged colloidal particles in a nematic electrolyte. We show that material anisotropy behaves markedly different from particle anisotropy: The electrostatic potential and pair interactions decay with an anisotropic Debye screening length, contrasting the constant screening length for isotropic electrolytes. Charged dumpling-shaped near-spherical colloidal particles in a nematic medium are used as an experimental model system to explore the effects of anisotropic screening, demonstrating competing anisotropic elastic and electrostatic effective pair interactions for colloidal surface charges tunable from neutral to high, yielding particle-separated metastable states. Generally, our work contributes to the understanding of electrostatic screening in nematic anisotropic media.

cond-mat.soft↗

arXiv2020

Screened Coulomb interactions of general macroions with nonzero particle volume

A semianalytical approach is developed to calculate the effective pair potential of rigid arbitrarily shaped macroions with a nonvanishing particle volume, valid within linear screening theory and the mean-field approximation. The essential ingredient for this framework is a mapping of the particle to a singular charge distribution with adjustable effective charge and shape parameters determined by the particle surface electrostatic potential. For charged spheres this method reproduces the well-known Derjaguin-Landau-Verwey-Overbeek (DLVO) potential. Further exemplary benchmarks of the method for more complicated cases, like tori, triaxial ellipsoids, and additive torus-sphere mixtures, leads to accurate closed-form integral expressions for all particle separations and orientations. The findings are relevant for determining the phase behaviour of macroions with experiments and simulations for various particle shapes.

cond-mat.soft↗

arXiv2020

Topological-defect-induced surface charge heterogeneities in nematic electrolytes

We show that topological defects in an ion-doped nematic liquid crystal can be used to manipulate the surface charge distribution on chemically homogeneous, charge-regulating external surfaces, using a minimal theoretical model. In particular, the location and type of the defect encodes the precise distribution of surface charges and the effect is enhanced when the liquid crystal is flexoelectric. We demonstrate the principle for patterned surfaces and charged colloidal spheres. More generally, our results indicate an interesting approach to control surface charges on external surfaces without changing the surface chemistry.

cond-mat.soft↗

arXiv2018

Complex electric double layers in charged topological colloids

Charged surfaces in contact with liquids containing ions are accompanied in equilibrium by an electric double layer consisting of a layer of electric charge on the surface that is screened by a diffuse ion cloud in the bulk fluid. This screening cloud determines not only the interactions between charged colloidal particles or polyelectrolytes and their self-assembly into ordered structures, but it is also pivotal in understanding energy storage devices, such as electrochemical cells and supercapacitors. However, little is known to what spatial complexity the electric double layers can be designed. Here, we show that electric double layers of non-trivial topology and geometry -including tori, multi-tori and knots- can be realised in charged topological colloidal particles, using numerical modelling within a mean-field Poisson-Boltzmann theory. We show that the complexity of double layers -including geometry and topology- can be tuned by changing the Debye screening length of the medium, or by changing the shape and topology of the (colloidal) particle. More generally, this work is an attempt to introduce concepts of topology in the field of charged colloids, which could lead to novel exciting material design paradigms.

cond-mat.soft↗

arXiv2017

Colloid-oil-water-interface interactions in the presence of multiple salts: charge regulation and dynamics

We theoretically and experimentally investigate colloid-oil-water-interface interactions of charged, sterically stabilized, poly(methyl-methacrylate) colloidal particles dispersed in a low-polar oil (dielectric constant $ε=5-10$) that is in contact with an adjacent water phase. In this model system, the colloidal particles cannot penetrate the oil-water interface due to repulsive van der Waals forces with the interface whereas the multiple salts that are dissolved in the oil are free to partition into the water phase. The sign and magnitude of the Donnan potential and/or the particle charge is affected by these salt concentrations such that the effective interaction potential can be highly tuned. Both the equilibrium effective colloid-interface interactions and the ion dynamics are explored within a Poisson-Nernst-Planck theory, and compared to experimental observations.

cond-mat.soft↗

arXiv2016

Alternating strings and clusters in suspensions of charged colloids

We report the formation of alternating strings and clusters in a binary suspension of repulsive charged colloids with double layers larger than the particle size. Within a binary cell model we include many-body and charge-regulation effects under the assumption of a constant surface potential, and consider their repercussions on the two-particle interaction potential. We find that the formation of induced dipoles close to a charge-reversed state may explain the formation of these structures. Finally, we will touch upon the formation of dumbbells and small clusters in a one-component system, where the effective electrostatic interaction is always repulsive.

cond-mat.soft↗