Attractive Particles Research Articles

The effect of surface roughness on the phase behavior of colloidal particles.

Shape anisotropy of colloidal particles can give rise to complex intermolecular interactions that determine particle packing and phase behavior. The vapor-liquid coexistence curves of attractive rough particles display a shift when compared to attractive smooth spherical particles. We use Integral Equation Theory (IET) to determine the vapor-liquid spinodal phase diagram of smooth and rough colloidal particles interacting through square-well attraction. Additionally, we use Gibbs Ensemble Monte Carlo (GEMC) simulations to locate their vapor-liquid coexistence curves. We model a rough colloidal particle as a spherical core with small beads embedded on its surface. The critical point of smooth spherical particle systems predicted by theory and simulations is in quantitative agreement. An increase in surface roughness due to an increase in either the number of beads or the diameter of the beads has a modest effect on the local structure of the system in the supercritical region. In contrast, increasing surface roughness consistently shifts the vapor-liquid coexistence curves to higher temperatures. The critical temperature is found to be a quadratic function of the number of beads. At a fixed bead size and number of beads, the critical temperature does not vary with the arrangement of beads on the core. Both IET and GEMC simulations predict that unlike critical temperatures, critical packing fractions vary non-monotonically with surface roughness. We find that the feasibility and accuracy of the integral equation theory depend sensitively on the chosen closure combination.

Time-rate-transformation framework for targeted assembly of short-range attractive colloidal suspensions

The aggregation of attractive colloids has been extensively studied from both theoretical and experimental perspectives as the fraction of solid particles is changed, and the range, type, and strength of attractive or repulsive forces between particles varies. The resulting gels, consisting of disordered assemblies of attractive colloidal particles, have also been investigated with regards to percolation, phase separation, and the mechanical characteristics of the resulting fractal networks. Despite tremendous progress in our understanding of the gelation process, and the exploration of different routes for arresting the dynamics of attractive colloids, the complex interplay between convective transport processes and many-body effects in such systems has limited our ability to drive the system toward a specific configuration. Here, we study a model attractive colloidal system over a wide range of particle characteristics and flow conditions undergoing aggregation far from equilibrium. The complex multiscale dynamics of the system can be understood using a time-rate-transformation diagram adapted from understanding of materials processing in block copolymers, supercooled liquids, and much stiffer glassy metals to direct targeted assembly of attractive colloidal particles.

Connecting particle clustering and rheology in attractive particle networks.

The structural properties of suspensions and other multiphase systems are vital to overall processability, functionality and acceptance among consumers. Therefore, it is crucial to understand the intrinsic connection between the microstructure of a material and the resulting rheological properties. Here, we demonstrate how the transitions in the microstructural conformations can be quantified and correlated to rheological measurements. We find semi-local parameters from graph theory, the mathematical study of networks, to be useful in linking structure and rheology. Our results, using capillary suspensions as a model system, show that the use of the clustering coefficient, in combination with the coordination number, is able to capture not only the agglomeration of particles, but also measures the formation of groups. These phenomena are tightly connected to the rheological properties. The present sparse networks cannot be described by established techniques such as betweenness centrality.

Using Patchy Particles to Prevent Local Rearrangements in Models of Non-equilibrium Colloidal Gels.

Simple models based on isotropic interparticle attractions often fail to capture experimentally observed structures of colloidal gels formed through spinodal decomposition and subsequent arrest: the resulting gels are typically denser and less branched than their experimental counterparts. Here, we simulate gels formed from soft particles with directional attractions (“patchy particles”), designed to inhibit lateral particle rearrangement after aggregation. We directly compare simulated structures with experimental colloidal gels made using soft attractive microgel particles, by employing a “skeletonization” method that reconstructs the three-dimensional backbone from experiment or simulation. We show that including directional attractions with sufficient valency leads to strongly branched structures compared to isotropic models. Furthermore, combining isotropic and directional attractions provides additional control over aggregation kinetics and gel structure. Our results show that the inhibition of lateral particle rearrangements strongly affects the gel topology and is an important effect to consider in computational models of colloidal gels.

Bio-inspired optimization algorithms for real underwater image restoration

Underwater image restoration algorithms usually take into account the physical model of the acquisition medium to obtain a restored image with good quality. This way, they compensate for the specific degradations introduced by the physical medium. These models often contain several parameters, which represent the type and strength of the medium degradations (e.g., absorption, scattering, and many others). Naturally, the quality of the restored image will depend on the correct estimation of these model parameters. In this work, we propose a restoration algorithm that estimates the model parameters using bio-inspired optimization metaheuristics, whose cost function (objective function) is a No-Reference Image Quality metric (NR-IQA). In this case, Opposition-based Particle Swarm Optimization (OPSO), Repulsive–Attractive Particle Swarm Optimization (RAPSO), Artificial Bee-Colony Algorithm (ABC), Opposition-based Artificial Bee Colony (OABC), and Differential Evolution (DE) metaheuristics have been tested. Since most quality metrics are not designed for underwater scenarios, a study was carried out to choose the best metric for this type of scenario, which is used as the cost function during the optimization process. To do that, an underwater image dataset was built, containing a set of images with underwater degradations and their corresponding subjective quality scores. The subjective quality scores were obtained by performing a subjective quality assessment experiment with voluntary participants that rated the quality of the test images. The proposed study has found out that NIQE metric presents the highest SRCC value, being therefore chosen as the cost function for the optimization algorithms that estimate the physical parameters of Barros’s inverse model. The performed experiments have demonstrated the suitability of our approach for underwater image restoring, showing that among the tested metaheuristics methods the PSO and ABC are the better, which provided restored images with a good visual quality.

Hydrodynamics control shear-induced pattern formation in attractive suspensions

Dilute suspensions of repulsive particles exhibit a Newtonian response to flow that can be accurately predicted by the particle volume fraction and the viscosity of the suspending fluid. However, such a description fails when the particles are weakly attractive. In a simple shear flow, suspensions of attractive particles exhibit complex, anisotropic microstructures and flow instabilities that are poorly understood and plague industrial processes. One such phenomenon, the formation of log-rolling flocs, which is ubiquitously observed in suspensions of attractive particles that are sheared while confined between parallel plates, is an exemplar of this phenomenology. Combining experiments and discrete element simulations, we demonstrate that this shear-induced structuring is driven by hydrodynamic coupling between the flocs and the confining boundaries. Clusters of particles trigger the formation of viscous eddies that are spaced periodically and whose centers act as stable regions where particles aggregate to form flocs spanning the vorticity direction. Simulation results for the wavelength of the periodic pattern of stripes formed by the logs and for the log diameter are in quantitative agreement with experimental observations on both colloidal and noncolloidal suspensions. Numerical and experimental results are successfully combined by means of rescaling in terms of a Mason number that describes the strength of the shear flow relative to the rupture force between contacting particles in the flocs. The introduction of this dimensionless group leads to a universal stability diagram for the log-rolling structures and allows for application of shear-induced structuring as a tool for assembling and patterning suspensions of attractive particles.

Numerical simulation of particulate cake formation in cross-flow microfiltration: Effects of attractive forces

We present a two-dimensional simulation model to explore cake formation in cross-flow filtration. The model uses the lattice Boltzmann method (LBM) for fluid computation and the discrete element method (DEM) for particle computation; they were fully coupled with the smoothed profile method. We verified our model by simulating filtration under different transmembrane pressures. We then investigated the effects of attractive forces and particle concentration on the cake formation mechanism. Generally, as the attractive interaction and particle concentration increased, the particles formed a cake layer with a looser body and rough surface, due to the decrease in the mobility of the particles in contact with the cake surface. It is concluded that the effects of particle concentration are affected by the different conditions of attractive interactions between the particles.

PC -symmetry-protected edge states in interacting driven-dissipative bosonic systems

A main objective of topological photonics is the design of disorder-resilient optical devices. Many prospective applications would benefit from nonlinear effects, which not only are naturally present in real systems but also are needed for switching in computational processes, while the underlying particle interactions are a key ingredient for the manifestation of genuine quantum effects. A particularly attractive switching mechanism of dynamical systems are infinite-period bifurcations into limit cycles, as these set on with a finite amplitude. Here we describe how to realize this switching mechanism by combining attractive and repulsive particle interactions in a driven-dissipative Su-Schrieffer-Heeger model, such as realized in excitonic lasers and condensates so that the system displays a nonhermitian combination of parity and charge-conjugation (PC) symmetry. We show that this symmetry survives in the nonlinear case and induces infinite-period and limit-cycle bifurcations (distinct from a Hopf bifurcation) where the system switches from a symmetry-breaking stationary state into a symmetry-protected power-oscillating state of finite amplitude. These protected dynamical solutions display a number of characteristic features, among which are their finite amplitude at onset, their arbitrary long oscillation period close to threshold, and the symmetry of their frequency spectrum which provides a tuneable frequency comb. Phases with different transition scenarios are separated by exceptional points in the stability spectrum, involving nonhermitian degeneracies of symmetry-protected excitations.

Transmutable Colloidal Crystals and Active Phase Separation via Dynamic, Directed Self-Assembly with Toggled External Fields

A diverse set of functional materials can be fabricated by assembling dispersions of colloids and nanoparticles. Two principal engineering challenges prevent efficient production of these materials: first, scalable synthesis of particles with carefully tailored interactions required to generate complex structures, and second, the propensity of such materials to arrest in undesirable metastable states. Active assembly processes, such as dynamic, directed self-assembly in which the interactions among particles are externally controlled and vary over time, offer a promising method to address these challenges. For dispersions of polarizable dielectric or paramagnetic nanoparticles, an effective mode of active assembly can be achieved by toggling an external electric or magnetic field, which induces attractive particle interactions, on and off cyclically over time. Here, we develop computational and theoretical models for such active assembly processes and find that cyclically toggling the external field leads to growth of colloidal crystals at significantly faster rates and with many fewer defects than for assembly in a steady field. The active process stabilizes phases that are only metastable in steady fields, including a dense fluid phase and body-centered orthorhombic crystals. The growth mechanism and terminal structure of the dispersion are easily controlled by the toggling protocol, and the toggle parameters can be used to continuously transmute between crystal structures with different lattice parameters. Finally, we show how results from linear irreversible thermodynamics can be used to predict the dissipative terminal states of the active assembly process in terms of parameters of the toggling protocol.

On the Position-Dependent Effective Mass in Bose Condensates of Photons and Polaritons in an Optical Microcavity Trap

The effect of the particle position-dependent effective mass on the behavior of quasi-two-dimensional photon and exciton-polariton condensates in a parabolic trap in a semiconductor optical microcavity is investigated. It is demonstrated that the correct inclusion of the coordinate dependence of the effective mass in the kinetic-energy operator modifies the effective confining potential for the particles. Corrections to the energy and wavefunction of the photon Bose condensate are derived. For exciton-polaritons, it is shown that the choice of specific mirror geometry can lead to the change from repulsive to attractive particle–particle interaction.

Sticky Matters: Jamming and Rigid Cluster Statistics with Attractive Particle Interactions.

While the large majority of theoretical and numerical studies of the jamming transition consider athermal packings of purely repulsive spheres, real complex fluids and soft solids generically display attraction between particles. By studying the statistics of rigid clusters in simulations of soft particles with an attractive shell, we present evidence for two distinct jamming scenarios. Strongly attractive systems undergo a continuous transition in which rigid clusters grow and ultimately diverge in size at a critical packing fraction. Purely repulsive and weakly attractive systems jam via a first-order transition, with no growing cluster size. We further show that the weakly attractive scenario is a finite size effect, so that for any nonzero attraction strength, a sufficiently large system will fall in the strongly attractive universality class. We therefore expect attractive jamming to be generic in the laboratory and in nature.

Axion isocurvature perturbations in low-scale models of hybrid inflation

The quantum chromodynamics (QCD) axion solves the strong $CP$ problem and represents an attractive particle candidate for cold dark matter (CDM). However, quantum fluctuations of the axion field during inflation easily result in large CDM isocurvature perturbations that are in conflict with observations of the cosmic microwave background. In this paper, we demonstrate how this problem can be solved in low-scale models of hybrid inflation that may emerge from supersymmetric grand unified theories. We consider both F-term hybrid inflation and D-term hybrid inflation in supergravity, explicitly taking into account the effect of hidden-sector supersymmetry breaking. We discuss the production of cosmic strings and show how the soft terms in the scalar potential readily allow us to achieve the correct scalar spectral index. In both cases, we are able to identify large regions in parameter space that are consistent with all constraints. In particular, we find that evading the CDM isocurvature constraint always requires a small Yukawa or gauge coupling of $\mathcal{O}({10}^{\ensuremath{-}3})$ or smaller. This translates into upper bounds on the gravitino mass of $\mathcal{O}({10}^{5})\text{ }\text{ }\mathrm{GeV}$ in F-term hybrid inflation and $\mathcal{O}({10}^{9})\text{ }\text{ }\mathrm{GeV}$ in D-term hybrid inflation. Our results point to interesting scenarios in well-motivated parameter regions that will be tested in future axion and cosmic microwave background experiments.

Nucleation of weakly attractive aggregates in microgravity

We investigate the incipient stages of the formation of aggregates of weakly attractive particles in microgravity. Using colloidal particles interacting with tunable critical Casimir forces, we drive the system increasingly out of equilibrium starting from near-equilibrium gas-solid coexistence, and follow the early stages of aggregation with near-field scattering. Contrary to measurements on ground that show a rapid exponential growth of aggregates, our microgravity measurements indicate that under pure diffusion-limited conditions, the formation of aggregates is still governed by a nucleation process over the investigated range of moderate attractive strength of a few kBT. By studying in detail the decrease of the critical radius with the increase of the attractive strength, we determine a surface tension of 115 nN/m, or , with σ the particle diameter, of the order of but larger than that for equilibrium colloidal liquids and crystals. These results highlight the importance of nucleation in the formation of non-equilibrium solid aggregates bound by weakly attractive forces.

Effects of Added Physical Cross‐Linkers on Mechanical Properties of Polymer Networks

AbstractThe effects of added physical cross‐linkers on the mechanical properties of polymer networks are studied by a computer simulation and a theoretical calculation. The physical cross‐linkers are modeled with small attractive particles and are adsorbed on the networks. The particle adsorption on the networks serves as sacrificial bonds and enhances the mechanical strength and the extensibility of the networks. The subchains of the networks change the conformation from collapsed states to expanded states with increasing particle content. This conformational change leads the non‐monotonic behavior of the mechanical properties of the networks. If the deformation rate is less than the inverse of the lifetime of the adsorption, the stress shows a relaxation with the conformational change of the subchains and the restructuring of inter‐subchain physical bonds via particle adsorption. In contrast, if the deformation rate is fast, the networks are deformed before the structural change, and show higher mechanical strengths.

Hydrodynamic Effects on Particle Diffusion in Polymeric Hydrogels with Steric and Electrostatic Particle–Gel Interactions

We consider the diffusion of a particle in a polymeric hydrogel in the presence of steric, electrostatic, and hydrodynamic interactions. We in particular investigate hydrodynamic effects by comparing Brownian dynamics simulations with and without hydrodynamic interactions. Inclusion of hydrodynamics improves agreement of the diffusion constant between simulations and published experimental data and yields quantitative agreement for gels with electrostatic interactions. Repulsive particle–gel interactions decrease the effect of hydrodynamic interactions on the particle diffusivity compared to purely steric gels, while attractive particle–gel interactions strongly increase hydrodynamic effects. This asymmetry is due to spatial particle–gel correlations.

Kinetic Monte Carlo modeling of Y2O3 nano-cluster formation in radiation resistant matrices

Abstract As known, Y2O3 nano-clusters considerably increase radiation resistance of reactor construction materials. To model the nano-cluster formation kinetics, we propose the simplest possible mathematical model and perform kinetic Monte Carlo (KMC) simulations. We extended the KMC simulated results to the experimentally relevant times using autoregressive integrated moving average forecasting. Within the model, we have studied prototypical attractive interaction energies and particle concentrations, and compared the simulations with experiments. We have observed the standard Lifshitz-Slyozov-Wagner (LSW) theory, predicting the average cluster radius growth with time, R ‾ ∼ t 1 / p , with p = 3 in the long-time limit, for weak (0.1 eV) mutual particle attraction. However, the respective cluster growth rates in these KMC simulations are overestimated compared to the experiments. The best agreement with experiment is obtained for a medium (0.3 eV) and strong (0.5 eV) attractions, when nano-cluster formation occurs during intermediate asymptotic time scale, where power order p ranges from 5 to 7.6 depending on interaction, without reaching actually the LSW long-time limit. Such a stronger interaction leads also to a more compact {110}–faceted nano-clusters.

Statistical fluid theory for systems of variable range interacting via triangular-well pair potential

The interest for describing the thermodynamic properties of fluids is growing in many fields from pure theoretical to those with technological applications. In this work, we have developed an analytical expression for the Helmholtz free energy of particles interacting via the triangular-well pair potential (TW), in the Barker-Henderson framework. This theoretical equation of state (EoS) uses the analytical radial distribution function of the hard-sphere (HS) fluid coupled with the Barker-Henderson perturbation theory. Thermodynamic properties as vapor-liquid coexistence, vapor pressures, internal energies and critical properties are obtained for attractive particles whose interaction potential range lies between 1.2 ≤ λ ≤ 2.6. The performance of the TW equation of state is assessed by comparing with Monte Carlo (MC) simulation data. Finally, this theoretical approach is used to model real fluids as methane, oxygen, fluoromethane and hydrogen sulfide, in a wide range of temperature and pressures.

One-step ultrasound producing O/W emulsions stabilized by chitosan particles

Deprotonated chitosan nanoparticles have shown a potential to act as a food-grade particle stabilizer of oil-in-water (O/W) Pickering emulsions. The effect of one-step emulsification conditions using ultrasonic device was studied changing time and ultrasonication (US) power. The physicochemical properties of chitosan particles and O/W Pickering emulsions produced at the same time from different process conditions were evaluated. The surface activity of chitosan particles was evidenced with the reduction of interfacial tension between oil-water phases. This behavior was associated to the great capacity of hydrophobic groups acting onto the interface during the ultrasonic emulsification. The combined intensification of time and US power also led to an increase of hydrophobicity and polydispersity, changes in zeta potential and reduction on size of chitosan particles. At higher US power, the decrease of droplet size favored the interaction between oil droplets through weak attractive forces and particles sharing (bridging flocculation) leading to an increase in viscosity of emulsions. Thus, the major finding of this work was to elucidate the emulsion stability mechanism by deprotonated chitosan particles, which was associated to an increase of their hydrophobicity and the formation of a droplet network structure. In addition, a one-step homogenization could be performed allowing to produce these emulsions using less energy and process time.

The Relentless Hunt for Dark Matter

The latest results from two dark matter searches have further ruled out many theoretically attractive dark matter particle candidates.

Effects of a “bound” substrate layer on the dynamics of supported polymer films

It is widely appreciated that an attractive polymer-substrate interaction can slow relaxation in thin supported polymer films and polymer nanocomposites. Recent measurements and simulations on nancomposites have indicated that this slowing of polymer dynamics occurs more strongly near a highly attractive particle surface where a "bound" layer having a much lower mobility can form, strongly influencing the thermodynamics and dynamics of the film. Here we use molecular simulations to show that a bound interfacial layer having a very similar nature arises in thin supported polymer films when the polymer-polymer attraction is stronger than the polymer-polymer interaction strength. This bound polymer layer effectively insulates the remainder of the film from the strong interfacial interactions, and the resulting thermodynamically determined Tg is relatively insensitive to the polymer-substrate interaction strength when it exceeds that of the polymer-polymer interactions. The presence of this layer gives rise to an additional relaxation process in the self-intermediate scattering function that is not observed in the bulk material and leads to a slowing down of the average relaxation time of the film as a whole. On the other hand, the average relaxation time of the film outside the bound layer does not grow in proportion to the strength of the substrate attraction due to the weak coupling of the substrate relaxation to the relaxation in the interior of the film. At large substrate attraction, the bound layer effectively "cloaks" the substrate, reducing the effect of the polymer-surface interaction on Tg.