Colloidal Dynamics Research Articles

Colloidal dynamics over a tilted periodic potential: Forward and reverse transition probabilities and entropy production in a nonequilibrium steady state.

We report a systematic study of the forward and reverse transition probability density functions (TPDFs) and entropy production in a nonequilibrium steady state (NESS). The NESS is realized in a two-layer colloidal system, in which the bottom-layer colloidal crystal provides a two-dimensional periodic potential U_{0}(x,y) for the top-layer diffusing particles. By tilting the sample at an angle with respect to gravity, a tangential component of the gravitational force F is applied to the diffusing particles, which breaks the detailed balance (DB) condition and generates a steady particle flux along the [1,0] crystalline orientation. While both the measured forward and reverse TPDFs reveal interesting space-time dependence, their ratio is found to be independent of time and obeys a DB-like relation. The experimental results are in good agreement with the theoretical predictions. This study thus provides a better understanding on how entropy is generated and heat is dissipated to the reservoir during a NESS transition process. It also demonstrates the applications of the two-layer colloidal system in the study of NESS transition dynamics.

Stable, Fluorescent Polymethylmethacrylate Particles for the Long-Term Observation of Slow Colloidal Dynamics.

Suspensions of solid micron-scale colloidal particles in liquid solvents are a foundational model system used to explore a wide range of phase transitions, including crystallization, gelation, spinodal decomposition, and the glass transition. One of the most commonly used systems for these investigations is the fluorescent spherical particles of polymethylmethacrylate (PMMA) suspended in a mixture of nonpolar solvents that match the density and the refractive index of the particles to minimize sedimentation and scattering. However, the particles can swell in these solvents, changing their size and density, and may leak the fluorescent dye over days to weeks; this constrains the exploration of slow and kinetically limited processes, such as near-boundary phase separation or the glass transition. In this paper, we produce PMMA colloidal particles that employ polymerizable and photostable cyanine-based fluorescent monomers spanning the range of visible wavelengths and a polymeric stabilizer prepared from polydimethylsiloxane, PDMS-graft-PMMA. Using microcalorimetry, we characterize the thermodynamics of an accelerated equilibration process for these dispersions in the buoyancy- and refractive-index-matching solvents. We use confocal differential dynamic microscopy to demonstrate that they behave as hard spheres. The suspended particles are stable for months to years, maintaining fixed particle size and density, and do not leak dye. Thus, these particles enable longer term experiments than may have been possible earlier; we demonstrate this by observing spinodal decomposition in a mixture of these particles with a depletant polymer in the microgravity environment of the International Space Station. Using fluorescence microscopy, we observe coarsening over several months and measure the growth of the characteristic length scale to be a fraction of a picometer per second; this rate is among the slowest observed in a phase-separating system. Our protocols should facilitate the synthesis of a variety of particles.

Structural and dynamic inhomogeneities induced by curvature gradients in elliptic colloidal halos of paramagnetic particles.

Paramagnetic colloidal particles distributed along an ellipse are used as a model system to study the effects of curvature gradients on the structure and dynamics of colloids in curved manifolds. Unlike what happens for circular and spherical systems, in the present case, the equilibrium one-particle distribution function displays inhomogeneities due to the changing curvature along the ellipse. The ensuing effects on the two-body correlations are also analyzed, leading to the observation of anisotropic and long-ranged effects. Another noticeable consequence is the slowing down of the self-diffusion of these particles, which for large eccentricities may induce metastable states; this is evaluated by means of the time-dependent self-distribution.

Velocity Fluctuations in Sedimenting Brownian Particles.

We report a gradual transition of dynamics in sedimenting suspensions of charge stabilized Brownian particles prior to the onset of the macroscopic sedimentation front. Using multispeckle ultrasmall-angle x-ray photon correlation spectroscopy (USA-XPCS), we show that well-defined advective motions dominate the colloid dynamics during the early stages of sedimentation. With elapsing time, these advective currents decay and diffusive motions become the dominating contribution in the dynamics. Probing the temporal development of these fluctuations at smaller Peclet numbers (<1) provides a new perspective for the mechanism determining the transient nature of velocity fluctuations in sedimentation and demonstrates new experimental capabilities enabled by multispeckle USA-XPCS.

Diffusive and Arrestedlike Dynamics in Currency Exchange Markets.

This work studies the symmetry between colloidal dynamics and the dynamics of the Euro-U.S. dollar currency exchange market (EURUSD). We consider the EURUSD price in the time range between 2001 and 2015, where we find significant qualitative symmetry between fluctuation distributions from this market and the ones belonging to colloidal particles in supercooled or arrested states. In particular, we find that models used for arrested physical systems are suitable for describing the EURUSD fluctuation distributions. Whereas the corresponding mean-squared price displacement (MSPD) to the EURUSD is diffusive for all years, when focusing in selected time frames within a day, we find a two-step MSPD when the New York Stock Exchange market closes, comparable to the dynamics in supercooled systems. This is corroborated by looking at the price correlation functions and non-Gaussian parameters and can be described by the theoretical model. We discuss the origin and implications of this analogy.

Asymmetrical flow field-flow fractionation of white wine chromophoric colloidal matter.

Two analytical separation methods-size-exclusion chromatography and asymmetrical flow field-flow fractionation-were implemented to evaluate the integrity of the colloidal composition of Chardonnay white wine and the impact of pressing and fermentations on the final macromolecular composition. Wine chromophoric colloidal matter, representing UV-visible-absorbing wine macromolecules, was evaluated by optical and structural measurements combined with the description of elution profiles obtained by both separative techniques. The objective of this study was to apply these two types of fractionation on a typical Chardonnay white wine produced in Burgundy and to evaluate how each of them impacted the determination of the macromolecular chromophoric content of wine. UV-visible and fluorescence measurements of collected fractions were successfully applied. An additional proteomic study revealed that grape and microorganism proteins largely impacted the composition of chromophoric colloidal matter of Chardonnay wines. Asymmetrical flow field-flow fractionation appeared to be more reliable and less invasive with respect to the native chemical environment of chromophoric wine macromolecules, and hence is recommended as a tool to fractionate chromophoric colloidal matter in white wines. Graphical Abstract An innovative macromolecular separation method based on Asymmetrical Flow Field-Flow Fractionation was developed to better control colloidal dynamics across Chardonnay white winemaking.

Seasonal Variation of Colloid Particles in the Shallow Well Water of a Small Watershed of Purple Soil

Colloids are the major carriers of various contaminants during the downward transport into groundwater.To explore the long-term dynamics of colloid in the groundwater and its influencing factors,a one-year monitoring of colloid concentration and water physicochemical parameters was carried out in the shallow wells in a purple soil dominated watershed in the hilly region of central Sichuan.The results indicated that colloid concentrations within a year had a strong seasonal variation with the coefficient of variation being larger than 0.5.The maximum of colloid mass concentration could reach as high as 14.68 mg·L-1(the corresponding number concentration being 1.34×109 L-1),which occurred in the non-rainy season and was caused by the physical perturbations from water taken.Physical perturbations of rainfall led to the distinct peaks of colloid concentrations in the well water at the early stages of rainy season while it did not occur at the end of the rainy season even upon heavy storm.Water chemistries (EC,Ca2+,Mg2+,DOC) showed the dominant role in determining colloid concentrations and status in the well.The facilitated transport of contaminants (i.e.pesticides and heavy metals) due to the increased colloids in the shallow well water at the early stages of rainy season would potentially pose a great threat to the drinking water safety in the study area.Therefore,it is strongly suggested to increase the monitoring frequencies in terms of colloid concentrations and well water physicochemical parameters following the large rainfall events during this period.

Collective colloid diffusion under soft two-dimensional confinement.

This work presents a numerical and theoretical investigation of the collective dynamics of colloids in an unbounded solution but trapped in a harmonic potential. Under strict two-dimensional confinement (infinitely stiff trap) the collective colloidal diffusion is enhanced and diverges at zero wave number (like k^{-1}), due to the hydrodynamic propagation of the confining force across the layer. The analytic solution for the collective diffusion of colloids under a Gaussian trap of width δ still shows enhanced diffusion for large wavelengths kδ<1, while a gradual transition to normal diffusion for kδ>1. At intermediate and short wavelengths, we illustrate to what extent the hydrodynamic enhancement of diffusion is masked by the conservative forces between colloids. At very large wavelengths, the collective diffusion becomes faster than the solvent momentum transport and a transition from Stokesian dynamics to inertial dynamics takes place. Using our inertial coupling method code (resolving fluid inertia), we study this transition by performing simulations at small Schmidt number. Simulations confirm theoretical predictions for the k→0 limit [Phys. Rev. E 90, 062314 (2014)PLEEE81539-375510.1103/PhysRevE.90.062314] showing negative density-density time correlations. However, at finite k simulations show deviations from the theory.

Phase separation dynamics of polydisperse colloids: a mean-field lattice-gas theory.

New insights into phase separation in colloidal suspensions are provided via a dynamical theory based on the polydisperse lattice-gas model. The model gives a simplified description of polydisperse colloids, incorporating a hard-core repulsion combined with polydispersity in the strength of the attraction between neighbouring particles. Our mean-field equations describe the local concentration evolution for each of an arbitrary number of species, and for an arbitrary overall composition of the system. We focus on the predictions for the dynamics of colloidal gas-liquid phase separation after a quench into the coexistence region. The critical point and the relevant spinodal curves are determined analytically, with the latter depending only on three moments of the overall composition. The results for the early-time spinodal dynamics show qualitative changes as one crosses a 'quenched' spinodal that excludes fractionation and so allows only density fluctuations at fixed composition. This effect occurs for dense systems, in agreement with a conjecture by Warren that, at high density, fractionation should be generically slow because it requires inter-diffusion of particles. We verify this conclusion by showing that the observed qualitative changes disappear when direct particle-particle swaps are allowed in the dynamics. Finally, the rich behaviour beyond the spinodal regime is examined, where we find that the evaporation of gas bubbles with strongly fractionated interfaces causes long-lived composition heterogeneities in the liquid phase; we introduce a two-dimensional density histogram method that allows such effects to be easily visualized for an arbitrary number of particle species.

Phoretic motion of colloids in a phase separating medium.

The enhanced motion of dispersed particles driven by a concentration gradient is the basis for diffusiophoresis. Here we present the dynamics of colloids in a phase separating medium probed by X-Ray Photon Correlation Spectroscopy (XPCS) in the ultra-small angle scattering range. Charge stabilized silica colloids suspended in a binary mixture of 3-methylpyridine and water/heavy water are preferentially wetted by 3-methylpyridine and consequently display a phoretic motion towards that phase upon demixing. This activity lasts for hundreds of seconds before the phase separation is complete and the enhanced motion is arrested as the colloids return to normal diffusive dynamics.

Rotating colloids in rotating magnetic fields: Dipolar relaxation and hydrodynamic coupling.

Video microscopy (VM) experiments and Brownian dynamics (BD) simulations were used to measure and model superparamagnetic colloidal particles in rotating magnetic fields for interaction energies on the order of the thermal energy, kT. Results from experiments and simulations were compared for isolated particle rotation, particle rotation within doublets, doublet rotation, and separation within doublets vs field rotation frequency. Agreement between VM and BD results was obtained at all frequencies and amplitudes only by including exact two-body hydrodynamic interactions and relevant relaxation times of magnetic dipoles. Frequency-dependent particle forces and torques cause doublets to rotate at low frequencies via dipolar interactions and at high frequencies via hydrodynamic translation-rotation coupling. By matching measurements and simulations for a range of conditions, our findings unambiguously demonstrate the quantitative forms of dipolar and hydrodynamic interactions necessary to capture nonequilibrium, steady-state dynamics of Brownian colloids in magnetic fields.

Vibrational properties of quasi-two-dimensional colloidal glasses with varying interparticle attraction.

We measure the vibrational modes and particle dynamics of quasi-two-dimensional colloidal glasses as a function of interparticle interaction strength. The interparticle attractions are controlled via a temperature-tunable depletion interaction. Specifically, the interparticle attraction energy is increased gradually from a very small value (nearly hard-sphere) to moderate strength (∼4k_{B}T), and the variation of colloidal particle dynamics and vibrations are concurrently probed. The particle dynamics slow monotonically with increasing attraction strength, and the particle motions saturate for strengths greater than ∼2k_{B}T, i.e., as the system evolves from a nearly repulsive glass to an attractive glass. The shape of the phonon density of states is revealed to change with increasing attraction strength, and the number of low-frequency modes exhibits a crossover for glasses with weak compared to strong interparticle attraction at a threshold of ∼2k_{B}T. This variation in the properties of the low-frequency vibrational modes suggests a new means for distinguishing between repulsive and attractive glass states.

Dynamics of highly polydisperse colloidal suspensions as a model system for bacterial cytoplasm.

There are various kinds of macromolecules in bacterial cell cytoplasm. The size polydispersity of the macromolecules is so significant that the crystallization and the phase separation could be suppressed, thus stabilizing the liquid state of bacterial cytoplasm. On the other hand, recent experiments suggested that the macromolecules in bacterial cytoplasm should exhibit glassy dynamics, which should be also affected significantly by the size polydispersity of the macromolecules. In this work, we investigate the anomalous and slow dynamics of highly polydisperse colloidal suspensions, of which size distribution is chosen to mimic Escherichia coli cytoplasm. We find from our Langevin dynamics simulations that the diffusion coefficient (D_{tot}) and the displacement distribution functions (P(r,t)) averaged over all colloids of different sizes do not show anomalous and glassy dynamic behaviors until the system volume fraction ϕ is increased up to 0.82. This indicates that the intrinsic polydispersity of bacterial cytoplasm should suppress the glass transition and help maintain the liquid state of the cytoplasm. On the other hand, colloids of each kind show totally different dynamic behaviors depending on their size. The dynamics of colloids of different size becomes non-Gaussian at a different range of ϕ, which suggests that a multistep glass transition should occur. The largest colloids undergo the glass transition at ϕ=0.65, while the glass transition does not occur for smaller colloids in our simulations even at the highest value of ϕ. We also investigate the distribution (P(θ,t)) of the relative angles of displacement for macromolecules and find that macromolecules undergo directionally correlated motions in a sufficiently dense system.

Coherent X-ray scattering reveals nature of dynamical transitions in nanoparticle–polymer suspensions

We report the microrheological behavior and dynamics of the suspensions of polystyrene-grafted gold nanoparticles (PGNPs) mixed with linear polystyrene (PS) as a function of the PS concentration and size asymmetry parameter, xi (=M-g/M-m, with M-g and M-m being the grafted and free chain molecular weights, respectively) = 2.76 and 0.14. Diffusive wave spectroscopy (DWS) is used for measuring the frequency dependent micro-rheology parameter, the loss modulus G `'(omega)(similar to omega(alpha)). For suspensions with xi = 0.14 a transition of a, from similar to 1 to similar to 1.6, is observed at a critical fraction of added polymers. On the other hand, a is always similar to 1 for the suspensions with xi = 2.76. Corresponding x-ray photon correlation spectroscopy (XPCS) data on the same samples reveals a sharp transition in the nature of dynamics of PGNPs, from exponential to logarithmic, in suspensions with x = 0.14, while for suspensions with xi = 2.76 this dynamical transition is not observed. Interestingly, the dynamical transition in xi = 0.14 based samples occurs at the same fraction of added polymers, where the transition of a from similar to 1 to similar to 1.6 was observed, thus providing a clear correlation between microscopic dynamics and micro-rheology. The conformational transitions of the grafted polymers is found to be the reason behind the observed differences in the dynamic behavior of the two systems. The important role of advanced x-ray scattering techniques in unravelling subtle aspects of nanoscale complex polymer and soft colloidal dynamics is thus revealed. (C) 2016 Elsevier Ltd. All rights reserved.

Record dynamics in the parking-lot model.

We present an analytical and numerical study of the parking lot model (PLM) of granular relaxation and make a connection to the aging dynamics of dense colloids. As we argue, the PLM is a Kinetically Constrained Model which features astronomically large equilibration times and displays a characteristic aging behavior on all observable time scales. The density of parked cars displays quasi-equilibrium Gaussian fluctuations interspersed by increasingly rare intermittent events, quakes, which can lead to an increase of the density to new record values. Defining active clusters as the shortest domains of parked cars which must be rearranged to allow further insertions, we find that their typical length grows logarithmically with time for low enough temperatures and show how the number of active clusters on average gradually decreases as the system approaches equilibrium. We further characterize the aging process in terms of the statistics of the record-sized fluctuations in the interstitial free volume which lead to quakes and show that quakes are uncorrelated and that they can be approximately described as a Poisson process in logarithmic time.

Relative Importance of Mineralogy and Organic Matter Characteristics on Macroaggregate and Colloid Dynamics in MG‐Silicate Dominated Soils

AbstractSoil aggregation and organic matter (OM) conservation are important in the prevention of land degradation. Aggregation processes and OM turnover influence each other and depend on the characteristics of both minerals and organic pools. We assessed the relative importance of the organic and mineral phases at the macroaggregate and colloidal scale in two soils (CHL and SRP, chlorite and serpentine‐rich, respectively) where Mg‐silicates dominated, by incubating them with a relatively degraded and oxidized organic fraction, that is the humic acids (HAs) extracted from the organic horizons of both CHL and SRP. The HA from SRP were more aromatic and richer in phenolic groups, whereas HA from CHL were N‐richer, more aliphatic and richer in carboxyl groups. The SRP soil formed larger amounts of macroaggregates, more stable than in CHL. At the colloidal scale, SRP was more flocculated and clay had a lower electrophoretic mobility than CHL. HA enhanced aggregate formation in both samples but improved aggregate stability only in CHL. In CHL, slight differences in electrophoretic mobility were visible, while in SRP, differences were more pronounced, with a point of zero charge at lower pH and larger hydrodynamic diameter. The abundance of Mg in SRP may have favoured the formation of weaker outer‐sphere interactions and the release of clay‐HA associations upon water dispersion, while in CHL Ca formed more stable bonds with HA. In SRP, ligand exchange reactions can be ruled out, conversely to the dominant bonding mechanism occurring in Al‐silicate dominated soils, with important consequences on the release of OM‐loaded clay particles. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Irradiation effects in CaF2 probed by Raman scattering

The formation conditions and dynamics of Ca colloids and point defects that appear in irradiated single crystals of CaF2 were investigated by Raman spectroscopy. The intensity changes in the Raman spectra because of the presence of different concentrations of point defects and Ca colloids that emerged in CaF2 after irradiation with 2.2 GeV Au ions were used to study their distribution and stability under illumination with three laser wavelengths (473, 532 and 633 nm) at different output powers (2 to 200 mW). A damage saturation at a fluence of 6 × 1011 ion cm−2 was observed. The dependence of the spectral changes on the ion fluence can be described by a core/halo damage cross‐section model. A radius of 13–18 nm was obtained for the outer (halo) cylinder, in agreement with previous swelling studies. Illuminations of irradiated samples with blue (473 nm) and green (532 nm) lasers were found to be extremely efficient in bleaching the samples, while illumination with a red (633 nm) laser did not lead to a sample recovery. This indicates that the bleaching process is governed by recombination of point defects that have to overcome an energy barrier. Typical time constants for the processes involved are presented. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Characterizing the Adsorption of Poly(vinyl alcohol) on Colloidal Silica with Aggregation-Induced Emission Fluorophore.

The adsorption of polymer on colloidal particle has significant influence on colloid structure and dynamics. Here we introduce a new method to monitor the adsorption in situ, based on the different emission behavior of aggregation-induced emission (AIE) luminogen in different micro environments. Poly(vinyl alcohol) (PVA) and colloidal silica (CS) were used as a model system. It was found that AIE molecules exhibited extremely low fluorescence intensity in water and PVA solution, while their emission efficiency was enhanced when adsorbed on CS, and became significantly boosted when PVA was adsorbed on CS at the same time. The fluorescence intensity increases with the amount of added PVA and reaches a saturation point, which is earlier than that obtained by the well-established solvent relaxation NMR method, due to their different sensitivities for adsorption segments in specific conformation. This new method is advantageous in quick response, where the measurement can be finished within 2 min, while others usually take hours. Therefore, it is expected that this new method may be used to monitor the dynamical adsorption process of polymer on colloidal particles.

Time- and ensemble-averages in evolving systems: the case of Brownian particles in random potentials

Anomalous diffusion is a ubiquitous phenomenon in complex systems. It is often quantified using time- and ensemble-averages to improve statistics, although time averages represent a non-local measure in time and hence can be difficult to interpret. We present a detailed analysis of the influence of time- and ensemble-averages on dynamical quantities by investigating Brownian particles in a rough potential energy landscape (PEL). Initially, the particle ensemble is randomly distributed, but the occupancy of energy values evolves towards the equilibrium distribution. This relaxation manifests itself in the time evolution of time- and ensemble-averaged dynamical measures. We use Monte Carlo simulations to study particle dynamics in a potential with a Gaussian distribution of energy values, where the long-time limit of the diffusion coefficient is known from theory. In our experiments, individual colloidal particles are exposed to a laser speckle pattern inducing a non-Gaussian roughness and are followed by optical microscopy. The relaxation depends on the kind and degree of roughness of the PEL. It can be followed and quantified by the time- and ensemble-averaged mean squared displacement. Moreover, the heterogeneity of the dynamics is characterized using single-trajectory analysis. The results of this work are relevant for the correct interpretation of single-particle tracking experiments in general.

Hydrogel-colloid interfacial interactions: a study of tailored adhesion using optical tweezers.

Dynamics of colloidal particles adhering to soft, deformable substrates, such as tissues, biofilms, and hydrogels play a key role in many biological and biomimetic processes. These processes, including, but not limited to colloid-based delivery, stitching, and sorting, involve microspheres exploring the vicinity of soft, sticky materials in which the colloidal dynamics are affected by the fluid environment (e.g., viscous coupling), inter-molecular interactions between the colloids and substrates (e.g., Derjaguin-Landau-Verwey-Overbeek (DLVO) theory), and the viscoelastic properties of contact region. To better understand colloidal dynamics at soft interfaces, an optical tweezers back-focal-plane interferometry apparatus was developed to register the transverse Brownian motion of a silica microsphere in the vicinity of polyacrylamide (PA) hydrogel films. The time-dependent mean-squared displacements are well described by a single exponential relaxation, furnishing measures of the transverse interfacial diffusion coefficient and binding stiffness. Substrates with different elasticities were prepared by changing the PA crosslinking density, and the inter-molecular interactions were adjusted by coating the microspheres with fluid membranes. Stiffer PA hydrogels (with bulk Young's moduli ≈1-10 kPa) immobilize the microspheres more firmly (lower diffusion coefficient and position variance), and coating the particles with zwitterionic lipid bilayers (DOPC) completely eliminates adhesion, possibly by repulsive dispersion forces. Remarkably, embedding polyethylene glycol-grafted lipid bilayers (DSPE-PEG2k-Amine) in the zwitterionic fluid membranes produces stronger adhesion, possibly because of polymer-hydrogel attraction and entanglement. This study provides new insights to guide the design of nanoparticles and substrates with tunable adhesion, leading to smarter delivery, sorting, and screening of micro- and nano-systems.