Heterogeneous and anisotropic dynamics of a 2D gel

In situ aggregation and early states of gelation of gold nanoparticle dispersions

The aggregation and onset of gelation of PEGylated gold nanoparticles dispersed in a glycerol-water mixture is studied by small-angle X-ray scattering and X-ray photon correlation spectroscopy. Tracking structural dynamics with sub-ms time resolution over a total experimental time of 8 hours corresponding to a time windows larger than 108 Brownian times and varying the temperature between 298 K and 266 K we can identify three regimes. First, while cooling to 275 K the particles show Brownian motion that slows down due to the increasing viscosity. Second, upon further cooling the static structure changes significantly, indicated by a broad structure factor peak. We attribute this to the formation of aggregates while the dynamics are still dominated by single-particle diffusion. Finally, the relaxation functions become more and more stretched accompanied by an increased slow down of the dynamics. At the same time the structure changes continuously indicating the onset of gelation. Our observations further suggest that the colloidal aggregation and gelation is characterized first by structural changes with a subsequent slowing down of the systems dynamics. The analysis also reveals that the details of the gelation process and the gel structure strongly depend on the thickness of the PEG-coating of the gold nanoparticles.

Unveiling the structural relaxation of microgel suspensions at hydrophilic and hydrophobic interfaces

HYPOTHESIS

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles show considerable hydrophilicity below the lower critical solution temperature (LCST) while they become hydrophobic above LCST. We hypothesize that interfacial wettability could tune particle-surface interaction and subsequent structural relaxation of microgel suspensions at interfaces during the volume phase transition.

EXPERIMENTS

The evanescent-wave scattering images of microgels at hydrophilic and hydrophobic interfaces are analyzed by a density-fluctuation autocorrelation function (δACF) over a wide range of particle volume fraction ϕ. The structural relaxation is characterized by the decay behavior of δACF. The scattering images in bulk are also processed as a comparison.

FINDINGS

A two-step relaxation decay is observed at both hydrophilic and hydrophobic interfaces. Relative to fast decay, the rate of structural relaxation in slow decay is reduced by a factor of ∼ 500 and ∼ 50 at hydrophilic and hydrophobic interfaces, respectively. The relaxation times obey divergent power-law dependences on intermediate regime of observing length scales at the two interfaces. Besides, the distribution of fluctuation for relaxation time at different local regions reveals that the structural relaxation is much more homogenous at hydrophilic interfaces than that at hydrophobic interfaces, especially at high ϕ.

In Situ Produced Nanoparticles at the Oil-Water Interface for Conformance Control and Enhanced Oil Recovery

Nanoparticle-assisted enhanced oil recovery (Nano-EOR) has attracted intensive interest in the laboratory as a promising oil recovery technology. However, the nanoparticles' stability and long-distance delivery of nanoparticles (NPs) in large-scale reservoirs are two main challenges. In this work, we developed a novel concept of in situ synthesizing NPs at the oil-water interface inside the reservoir for EOR instead of injecting presynthesized NPs from outside. The pore-scale flooding experiments show that EOR efficiencies for tertiary flooding were 6.3% without reaction (Case 3), 14.6% for slow reaction (Case 1), and 25.4% for relatively quick reaction (Case 4). Examination of the EOR mechanism shows that in situ produced SiO₂ NPs in microchannels could alter the substrate wettability toward neutral wetting. Moreover, the produced NPs tended to assemble on the immiscible oil-water interface, forming a barrier toward interface deformation. As the reaction continued, excessive surface-modified NPs could also diffuse into aqueous brine and accumulate as a soft gel in the flowing path swept by brine. Collectively, these processes induced a "shut-off" effect and diverted displacing fluids to unswept areas, which consequently increased the sweep efficiency and improved the oil recovery efficiency. Auxiliary bulk-scale experiments also showed that the reaction-induced nanoparticle synthesis and assembly at an immiscible interface reduced the interfacial tension and generated an elastic oil-water interface.

Structural and Dynamical Studies of a Lipid-Nanoclay Composite Layer at the Air-Water Interface

We modulate the adsorption affinities of nanoclay particles for the air-water interface by changing the cationic surface charge composition of the lipid monolayer and thereby tune the attractive electrostatic interaction between the positively charged lipid layer and the zwitterionic nanoclay particles in the water subphase. Our findings emphasize the significance of electrostatic interaction between lipids and the nanoclay, as well as its impact on the structural and viscoelastic features of the composite layer. We use surface pressure (Π)-mean molecular area (A) isotherms, atomic force microscope (AFM), Brewster angle microscopy (BAM), and energy dispersive X-ray spectrsocopy (EDXS) measurements to analyze the structure phases of lipid and lipid-nanoclay composite interfacial layer. The Π-A isotherm curve shows that the lipid-nanoclay composite layer has a larger lift-off area than the neat lipid layer, indicating that nanoparticles adsorb at the lipid layer via electrostatic interaction between lipid and nanoclay molecules. The surface density of the adsorbed nanoclay particles increases with an increase in the composition of the cationic lipid molecules. The stress relaxation response of the composite layer, measured using step compression measurements, exhibits exponential decay and ubiquitous dependence on the cationic dimyristoy-trimethylammonium propane (DMTAP) composition in the lipid layer with crossover to faster relaxation dynamics at DMTAP > 0.75. The power-law study of the frequency-dependent dynamic viscoelastic responses of the interfacial layer, measured using the barrier oscillation method, reveals a transition from glass-like response from neat lipid layer to gel-like dynamic response for the lipid-nanoclay composite layer. A solid-like behavior is evident for all the interface layers with dilation elastic modulus (E') > dilational viscous modulus (E″); however, the dynamic response of the neat layer is largely frequency-independent, whereas lipid-nanoclay composite layers with DMTAP > 0.75 reveal a frequency-dependent dynamic responses. The frequency-dependent power-law exponent of E', E″ increases on increasing the fractional composition of cationic DMTAP from 0.1 to 1.0, which forms a saturated interface of laponite particles and behaves as a viscoelastic gel in 2D.

Relaxation and Aging of Nanosphere Assemblies at a Water-Oil Interface

The relaxation and aging of an assembly of spherical nanoparticles (NPs) at a water-oil interface are characterized in situ by grazing incidence X-ray photon correlation spectroscopy. The dynamics of the interfacial assembly is measured while the interface saturates with NPs. Weak attractions between NPs lead to gel-like structures in the assembly, where the in-plane ordering is inhibited by the broad size distribution of the NPs. Structural rearrangements on the length scale of the NP-NP center-to-center distances proceed by intermittent fluctuations instead of continuous cooperative motions. The coexistence of rapid and slow NP populations is confirmed, as commonly observed in soft glass-forming materials. Dynamics are increasingly slowed as the NPs initially segregate to the locally clustered interface. The structural relaxation of the NPs in these localized clusters is 5 orders of magnitude slower than that of free particles in the bulk. When the interface is nearly saturated, the time for relaxation increases suddenly due to the onset of local jamming, and the dynamics slow exponentially afterward until the system reaches collective jamming by cooperative rearrangements. This investigation provides insights into structural relaxations near the glass transition and the evolution of the structure and dynamics of the assemblies as they transition from an isotropic liquid to a dense disordered film.

A broad perspective to particle-laden fluid interfaces systems: from chemically homogeneous particles to active colloids

Particles adsorbed to fluid interfaces are ubiquitous in industry, nature or life. The wide range of properties arising from the assembly of particles at fluid interface has stimulated an intense research activity on shed light to the most fundamental physico-chemical aspects of these systems. These include the mechanisms driving the equilibration of the interfacial layers, trapping energy, specific inter-particle interactions and the response of the particle-laden interface to mechanical perturbations and flows. The understanding of the physico-chemistry of particle-laden interfaces becomes essential for taking advantage of the particle capacity to stabilize interfaces for the preparation of different dispersed systems (emulsions, foams or colloidosomes) and the fabrication of new reconfigurable interface-dominated devices. This review presents a detailed overview of the physico-chemical aspects that determine the behavior of particles trapped at fluid interfaces. This has been combined with some examples of real and potential applications of these systems in technological and industrial fields. It is expected that this information can provide a general perspective of the topic that can be exploited for researchers and technologist non-specialized in the study of particle-laden interfaces, or for experienced researcher seeking new questions to solve.

Elastic and Dynamic Heterogeneity in Aging Alginate Gels

Anomalous aging in soft glassy materials has generated a great deal of interest because of some intriguing features of the underlying relaxation process, including the emergence of "ultra-long-range" dynamical correlations. An intriguing possibility is that such a huge correlation length is reflected in detectable ensemble fluctuations of the macroscopic material properties. We tackle this issue by performing replicated mechanical and dynamic light scattering (DLS) experiments on alginate gels, which recently emerged as a good model-system of anomalous aging. Here we show that some of the monitored quantities display wide variability, including large fluctuations in the stress relaxation and the occasional presence of two-step decay in the DLS decorrelation functions. By quantifying elastic fluctuation through the standard deviation of the elastic modulus and dynamic heterogeneities through the dynamic susceptibility, we find that both quantities do increase with the gel age over a comparable range. Our results suggest that large elastic fluctuations are closely related to ultra-long-range dynamical correlation, and therefore may be a general feature of anomalous aging in gels.

Nonlinear measurements of kinetics and generalized dynamical modes. I. Extracting the one-dimensional Green's function from a time series

Often, a single correlation function is used to measure the kinetics of a complex system. In contrast, a large set of k-vector modes and their correlation functions are commonly defined for motion in free space. This set can be transformed to the van Hove correlation function, which is the Green's function for molecular diffusion. Here, these ideas are generalized to other observables. A set of correlation functions of nonlinear functions of an observable is used to extract the corresponding Green's function. Although this paper focuses on nonlinear correlation functions of an equilibrium time series, the results are directly connected to other types of nonlinear kinetics, including perturbation-response experiments with strong fields. Generalized modes are defined as the orthogonal polynomials associated with the equilibrium distribution. A matrix of mode-correlation functions can be transformed to the complete, single-time-interval (1D) Green's function. Diagonalizing this matrix finds the eigendecays. To understand the advantages and limitation of this approach, Green's functions are calculated for a number of models of complex dynamics within a Gaussian probability distribution. Examples of non-diffusive motion, rate heterogeneity, and range heterogeneity are examined. General arguments are made that a full set of nonlinear 1D measurements is necessary to extract all the information available in a time series. However, when a process is neither dynamically Gaussian nor Markovian, they are not sufficient. In those cases, additional multidimensional measurements are needed.

From Femtoseconds to Hours—Measuring Dynamics over 18 Orders of Magnitude with Coherent X-rays

X-ray photon correlation spectroscopy (XPCS) enables the study of sample dynamics between micrometer and atomic length scales. As a coherent scattering technique, it benefits from the increased brilliance of the next-generation synchrotron radiation and Free-Electron Laser (FEL) sources. In this article, we will introduce the XPCS concepts and review the latest developments of XPCS with special attention on the extension of accessible time scales to sub-μs and the application of XPCS at FELs. Furthermore, we will discuss future opportunities of XPCS and the related technique X-ray speckle visibility spectroscopy (XSVS) at new X-ray sources. Due to its particular signal-to-noise ratio, the time scales accessible by XPCS scale with the square of the coherent flux, allowing to dramatically extend its applications. This will soon enable studies over more than 18 orders of magnitude in time by XPCS and XSVS.

Particle-laden fluid/fluid interfaces: physico-chemical foundations

Particle-laden fluid/fluid interfaces are ubiquitous in academia and industry, which has fostered extensive research efforts trying to disentangle the physico-chemical bases underlying the trapping of particles to fluid/fluid interfaces as well as the properties of the obtained layers. The understanding of such aspects is essential for exploiting the ability of particles on the stabilization of fluid/fluid interface for the fabrication of novel interface-dominated devices, ranging from traditional Pickering emulsions to more advanced reconfigurable devices. This review tries to provide a general perspective of the physico-chemical aspects associated with the stabilization of interfaces by colloidal particles, mainly chemical isotropic spherical colloids. Furthermore, some aspects related to the exploitation of particle-laden fluid/fluid interfaces on the stabilization of emulsions and foams will be also highlighted. It is expected that this review can be used for researchers and technologist as an initial approach to the study of particle-laden fluid layers.

Shukur MF, Manaka T. Effect of nanoparticles concentration on electromagnetic-assisted oil recovery using ZnO nanofluids

Utilization of metal-oxide nanoparticles (NPs) in enhanced oil recovery (EOR) has generated substantial recent research interest in this area. Among these NPs, zinc oxide nanoparticles (ZnO-NPs) have demonstrated promising results in improving oil recovery due to their prominent thermal properties. These nanoparticles can also be polarized by electromagnetic (EM) field, which offers a unique Nano-EOR approach called EM-assisted Nano-EOR. However, the impact of NPs concentrations on oil recovery mechanism under EM field has not been well established. For this purpose, ZnO nanofluids (ZnO-NFs) of two different particle sizes (55.7 and 117.1 nm) were formed by dispersing NPs between 0.01 wt.% to 0.1 wt.% in a basefluid of sodium dodecylbenzenesulfonate (SDBS) and NaCl to study their effect on oil recovery mechanism under the electromagnetic field. This mechanism involved parameters, including mobility ratio, interfacial tension (IFT) and wettability. The displacement tests were conducted in water-wet sandpacks at 95˚C, by employing crude oil from Tapis. Three tertiary recovery scenarios have been performed, including (i) SDBS surfactant flooding as a reference, (ii) ZnO-NFs flooding, and (iii) EM-assisted ZnO-NFs flooding. Compare with incremental oil recovery from surfactant flooding (2.1% original oil in place/OOIP), nanofluid flooding reaches up to 10.2% of OOIP at optimal 0.1 wt.% ZnO (55.7 nm). Meanwhile, EM-assisted nanofluid flooding at 0.1 wt.% ZnO provides a maximum oil recovery of 10.39% and 13.08% of OOIP under EM frequency of 18.8 and 167 MHz, respectively. By assessing the IFT/contact angle and mobility ratio, the optimal NPs concentration to achieve a favorable ER effect and interfacial disturbance is determined, correlated to smaller hydrodynamic-sized nanoparticles that cause strong electrostatic repulsion between particles.

Ion-Mediated Cross-linking of Biopolymers Confined at Liquid/Liquid Interfaces Probed by In Situ High-Energy Grazing Incidence X-ray Photon Correlation Spectroscopy

As manifested in biological cell membranes, the confinement of chemical reactions at the 2D interfaces significantly improves the reaction efficacy. The interface between two liquid phases is used in various key processes in industries, such as in food emulsification and floatation. However, monitoring the changes in the mechanics and dynamics of molecules confined at the liquid/liquid interfaces still remains a scientific challenge because it is nontrivial to access the interface buried under a liquid phase. Herein, we report the in situ monitoring of the cross-linking of polyalginate mediated by Ca²⁺ ions at the oil/water interface by grazing incidence X-ray photon correlation spectroscopy (GIXPCS). We first optimized the reaction conditions with the aid of interfacial shear rheology and then performed GIXPCS using a high-energy synchrotron X-ray beam (22 keV) that guarantees sufficiently high transmittance through the oil phase. The intensity autocorrelation functions implied that the formation of a percolated network of polyalginate is accompanied by increasing relaxation time. Moreover, the relaxation rate scales linearly with the momentum transfer parallel to the interface, suggesting that the process is driven by hyperdiffusive propagation but not by Brownian diffusion. Our data indicated that high-energy GIXPCS has potential for in situ monitoring of changes in the dynamics of polymers confined between two liquid phases.

Anisotropic and heterogeneous dynamics in an aging colloidal gel

We investigate the out-of-equilibrium dynamics of a colloidal gel obtained by quenching a suspension of soft polymer-coated gold nanoparticles close to and below its gelation point using X-ray Photon Correlation Spectroscopy (XPCS). A faster relaxation process emergent from the localized motions of the nanoparticles reveals a dynamically-arrested network at the nanoscale as a key signature of the gelation process. We find that the slower network dynamics is hyperdiffusive with a compressed exponential form, consistent with stress-driven relaxation processes. Specifically, we use direction-dependent correlation functions to characterize the anisotropy in dynamics. We show that the anisotropy is greater for the gel close to its gelation point than at lower temperatures, and the anisotropy decreases as the gel ages. We quantify the anisotropic dynamical heterogeneities emergent in such a stress-driven dynamical system using higher order intensity correlations, and demonstrate that the aging phenomenon contributes significantly to the properties evaluated by the fluctuations in the intensity correlations. Our results provide important insights into the structural origin of the emergent anisotropic and cooperative heterogeneous dynamics, and we discuss analogies with previous work on other soft disordered systems.

Dynamical Heterogeneity near Glass Transition Temperature under Shear Conditions

We experimentally studied the shear effect on dynamical heterogeneity near glass transition temperature. X-ray photon correlation spectroscopy was utilized to study the dynamics of polyvinyl acetate with tracer particles near its glass transition temperature, to determine the local shear rate from the anisotropic behavior of the time autocorrelation function and to calculate the dynamical heterogeneity using higher-order correlation function. The obtained results show a decrease in the dynamical heterogeneity and faster dynamics with increasing shear rate. This is the first experimental result that proved the predictions of previous molecular dynamics simulations.

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

This article aims to provide an overview of broad range of applications of synchrotron scattering methods in the investigation of nanoscale materials. These scattering techniques allow the elucidation of the structure and dynamics of nanomaterials from sub-nm to micron size scales and down to sub-millisecond time ranges both in bulk and at interfaces. A major advantage of scattering methods is that they provide the ensemble averaged information under in situ and operando conditions. As a result, they are complementary to various imaging techniques which reveal more local information. Scattering methods are particularly suitable for probing buried structures that are difficult to image. Although, many qualitative features can be directly extracted from scattering data, derivation of detailed structural and dynamical information requires quantitative modeling. The fourth-generation synchrotron sources open new possibilities for investigating these complex systems by exploiting the enhanced brightness and coherence properties of X-rays.

Anisotropic and heterogeneous dynamics in stretched elastomer nanocomposites

We use X-ray photon correlation spectroscopy (XPCS) to investigate the dynamics of a stretched elastomer by means of probe particles. The particles dispersed in the elastomer were carbon black or silica aggregates classically used for elastomer reinforcement but their volume fraction is very low (φ < 10-2). We show that their dynamics is slower in the direction of the tensile strain than in the perpendicular one. For hydroxylated silica which is poorly wetted by the elastomer, there is no anisotropy. Two-time correlation functions confirm anisotropic dynamics and suggest dynamical heterogeneity already expected from the q-1 behavior of the relaxation times. The height χ* of the peak of the dynamical susceptibility, determined by the normalized variance of the instantaneous correlation function, is larger in the direction parallel to the strain than in the perpendicular one. It also appears that its q dependence changes with the morphology of the probe particle. Therefore, the heterogeneous dynamic probed by the particles is not related only to that of the strained elastomer matrix. In fact, it results from modification of the dynamics of the polymer chains near the surface of the particles and within the aggregate porosity (bound polymer). It is concluded that XPCS is a powerful method for investigating the dynamics, at a given strain, of the bound polymer-particle units which are responsible, at large volume fractions, for the reinforcement.

Sub-microsecond-resolved multi-speckle X-ray photon correlation spectroscopy with a pixel array detector

Small-angle X-ray photon correlation spectroscopy (XPCS) measurements spanning delay times from 826 ns to 52.8 s were performed using a photon-counting pixel array detector with a dynamic range of 0-3 (2 bits). Fine resolution and a wide dynamic range of time scales was achieved by combining two modes of operation of the detector: (i) continuous mode, where data acquisition and data readout are performed in parallel with a frame acquisition time of 19.36 µs, and (ii) burst mode, where 12 frames are acquired with frame integration times of either 2.56 µs frame^-1 or 826 ns frame^-1 followed by 3.49 ms or 1.16 ms, respectively, for readout. The applicability of the detector for performing multi-speckle XPCS was demonstrated by measuring the Brownian dynamics of 10 nm-radius gold and 57 nm-radius silica colloids in water at room temperature. In addition, the capability of the detector to faithfully record one- and two-photon counts was examined by comparing the statistical distribution of photon counts with expected probabilities from the negative binomial distribution. It was found that in burst mode the ratio of 2 s to 1 s is markedly smaller than predicted and that this is attributable to pixel-response dead-time.

Physico-chemical foundations of particle-laden fluid interfaces

Particle-laden interfaces are ubiquitous nowadays. The understanding of their properties and structure is essential for solving different problems of technological and industrial relevance; e.g. stabilization of foams, emulsions and thin films. These rely on the response of the interface to mechanical perturbations. The complex mechanical response appearing in particle-laden interfaces requires deepening on the understanding of physico-chemical mechanisms underlying the assembly of particles at interface which plays a central role in the distribution of particles at the interface, and in the complex interfacial dynamics appearing in these systems. Therefore, the study of particle-laden interfaces deserves attention to provide a comprehensive explanation on the complex relaxation mechanisms involved in the stabilization of fluid interfaces.

Stress relaxation in quasi-two-dimensional self-assembled nanoparticle monolayers

We experimentally probed the stress relaxation of a monolayer of iron oxide nanoparticles at the water-air interface. Upon drop-casting onto a water surface, the nanoparticles self-assembled into islands of two-dimensional hexagonally close packed crystalline domains surrounded by large voids. When compressed laterally, the voids gradually disappeared as the surface pressure increased. After the compression was stopped, the surface pressure (as measured by a Wilhelmy plate) evolved as a function of the film aging time with three distinct timescales. These aging dynamics were intrinsic to the stressed state built up during the non-equilibrium compression of the film. Utilizing x-ray photon correlation spectroscopy, we measured the characteristic relaxation time (τ) of in-plane nanoparticle motion as a function of the aging time through both second-order and two-time autocorrelation analysis. Compressed and stretched exponential fitting of the intermediate scattering function yielded exponents (β) indicating different relaxation mechanisms of the films under different compression stresses. For a monolayer compressed to a lower surface pressure (between 20 mN/m and 30 mN/m), the relaxation time (τ) decreased continuously as a function of the aging time, as did the fitted exponent, which transitioned from being compressed (>1) to stretched (<1), indicating that the monolayer underwent a stress release through crystalline domain reorganization. However, for a monolayer compressed to a higher surface pressure (around 40 mN/m), the relaxation time increased continuously and the compressed exponent varied very little from a value of 1.6, suggesting that the system may have been highly stressed and jammed. Despite the interesting stress relaxation signatures seen in these samples, the structural ordering of the monolayer remained the same over the sample lifetime, as revealed by grazing incidence x-ray diffraction.

Transition in Dynamics as Nanoparticles Jam at the Liquid/Liquid Interface

Nanoparticles (NPs) segregated to the liquid/liquid interface form disordered or liquid-like assemblies that show diffusive motions in the plane of the interface. As the areal density of NPs at the interface increases, the available interfacial area decreases, and the interfacial dynamics of the NP assemblies change when the NPs jam. Dynamics associated with jamming was investigated by X-ray photon correlation spectroscopy. Water-in-toluene emulsions, formed by a self-emulsification at the liquid/liquid interface and stabilized by ligand-capped CdSe-ZnS NPs, provided a simple, yet powerful platform, to investigate NP dynamics. In contrast to a single planar interface, these emulsions increased the number of NPs in the incident beam and decreased the absorption of X-rays in comparison to the same path length in pure water. A transition from diffusive to confined dynamics was manifested by intermittent dynamics, indicating a transition from a liquid-like to a jammed state.

Diffusive dynamics during the high-to-low density transition in amorphous ice

Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high- (HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nanometer length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid-liquid transition in the ultraviscous regime.

Ultra-long-range dynamic correlations in a microscopic model for aging gels

We use large-scale computer simulations to explore the nonequilibrium aging dynamics in a microscopic model for colloidal gels. We find that gelation resulting from a kinetically arrested phase separation is accompanied by "anomalous" particle dynamics revealed by superdiffusive particle motion and compressed exponential relaxation of time correlation functions. Spatiotemporal analysis of the dynamics reveals intermittent heterogeneities producing spatial correlations over extremely large length scales. Our study is a microscopically resolved model reproducing all features of the spontaneous aging dynamics observed experimentally in soft materials.

On the use of two-time correlation functions for X-ray photon correlation spectroscopy data analysis

Multi-time correlation functions are especially well suited to study non-equilibrium processes. In particular, two-time correlation functions are widely used in X-ray photon correlation experiments on systems out of equilibrium. One-time correlations are often extracted from two-time correlation functions at different sample ages. However, this way of analysing two-time correlation functions is not unique. Here, two methods to analyse two-time correlation functions are scrutinized, and three illustrative examples are used to discuss the implications for the evaluation of the correlation times and functional shape of the correlations.

Hydrophobic Silica Nanoparticles Induce Gel Phases in Phospholipid Monolayers

Silica nanoparticles (SiNP) can be incorporated in phospholipid layers to form hybrid organic-inorganic bidimensional mesostructures. Controlling the dynamics in these mesostructures paves the way to high-performance drug-delivery systems. Depending on the different hydrophobicity/hydrophilicity of SiNP, recent X-ray reflectivity experiments have demonstrated opposite structural effects. While these are reasonably well understood, less is known about the effects on the dynamics, which in turn determine molecular diffusivity and the possibility of drug release. In this work we characterize the dynamics of a mixed Langmuir layer made of phospholipid and hydrophobic SiNP. We combine X-ray photon correlation spectroscopy and epifluorescence discrete Fourier microscopy to cover more than 2 decades of Q-range (0.3-80 μm(-1)). We obtain evidence for the onset of an arrested state characterized by intermittent stress-relaxation rearrangement events, corresponding to a gel dominated by attractive interactions. We compare this with our previous results from phospholipid/hydrophilic SiNP films, which show an arrested glassy phase of repulsive disks.

2D dynamical arrest transition in a mixed nanoparticle-phospholipid layer studied in real and momentum spaces

We investigate the interfacial dynamics of a 2D self-organized mixed layer made of silica nanoparticles interacting with phospholipid (DPPC) monolayers at the air/water interface. This system has biological relevance, allowing investigation of toxicological effects of nanoparticles on model membranes and lung surfactants. It might also provide bio-inspired technological solutions, exploiting the self-organization of DPPC to produce a non-trivial 2D structuration of nanoparticles. The characterization of interfacial dynamics yields information on the effects of NPs on the mechanical properties, important to improve performances of systems such as colloidosomes, foams, creams. For this, we combine micro-tracking in real-space with measurement in momentum-space via x-ray photon-correlation spectroscopy and Digital Fourier Microscopy. Using these complementary techniques, we extend the spatial range of investigation beyond the limits of each one. We find a dynamical transition from Brownian diffusion to an arrested state driven by compression, characterized by intermittent rearrangements, compatible with a repulsive glass phase. The rearrangement and relaxation of the monolayer structure results dramatically hindered by the presence of NPs, which is relevant to explain some the mechanical features observed for the dynamic surface pressure response of these systems and which can be relevant for the respiratory physiology and for future drug-delivery composite systems.

Magnetic microwire probes for the magnetic rod interfacial stress rheometer

The magnetic needle interfacial shear rheometer is a valuable tool for the study of the mechanical properties of thin fluid films or monolayers. However, it is difficult to differentiate the interfacial and subphase contributions to the drag on the needle. In principle, the problem can be addressed by decreasing the needle diameter, which decreases the bulk contribution while the interfacial contribution remains essentially the same. Here we show the results obtained when using a new type of needle, that of magnetic microwires with diameter approximately 10 times thinner than for commercial needles. We show that the lower inertia of the microwires calls for a new calibration procedure. We propose such a new calibration procedure based on the flow field solution around the needle introduced in refs 1 and 2. By measuring thin silicone oil films with well-controlled interfacial viscosities as well as eicosanol (C20) and pentadecanoic acid (PDA, C15) Langmuir monolayers, we show that the new calibration method works well for standard needles as well as for the microwire probes. Moreover, we show that the analysis of the force terms contributing to the force on the needle helps to ascertain whether the measurements obtained are reliable for given surface shear viscosity values. We also show that the microwire probes have at least a 10-fold-lower resolution limit, allowing one to measure interfacial viscosities as low as 10(-7) N·m/s.

Relaxation in yield stress systems through elastically interacting activated events

We study consequences of long-range elasticity in thermally assisted dynamics of yield stress materials. Within a two-dimensional mesoscopic model we calculate the mean-square displacement and the dynamical structure factor for tracer particle trajectories. The ballistic regime at short time scales is associated with a compressed exponential decay in the dynamical structure factor, followed by a subdiffusive crossover prior to the onset of diffusion. We relate this crossover to spatiotemporal correlations and thus go beyond established mean field predictions.

X-ray photon correlation spectroscopy studies of surfaces and thin films

The technique of X-ray Photon Correlation Spectroscopy (XPCS) is reviewed as a method for studying the relatively slow dynamics of materials on time scales ranging from microseconds to thousands of seconds and length scales ranging from microns down to nanometers. We focus on the application of this technique to study dynamical fluctuations of surfaces, interfaces and thin films. We first discuss instrumental issues such as the effects of partial coherence (or alternatively finite instrumental resolution) and optimization of signal-to-noise ratios in the experiments. We then review what has been learned from recent XPCS studies of capillary wave fluctuations on liquid surfaces and polymer films, of nanoparticles used as probes to study the interior dynamics of polymer films, of liquid crystals and multilamellar surfactant films, and of metal surfaces, and magnetic domain wall fluctuations in antiferromagnets. We then discuss studies of non-equilibrium dynamics described by 2-time correlation functions. Finally, we briefly speculate on possible future XPCS experiments at new synchrotron sources currently under development including studies of dynamics on time scales down to femtoseconds.

Two-dimensional DPPC based emulsion-like structures stabilized by silica nanoparticles

We studied the mechanical and structural properties of mixed surface layers composed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and silica nanoparticles (NPs). These layers are obtained by spreading a DPPC Langmuir monolayer on a colloidal silica dispersion. The transfer/incorporation of NPs into the DPPC monolayer, driven by electrostatic interactions, alters the molecular orientation, the mechanisms of domain formation, and consequently the phase behavior of the surface layer during compression. The investigation of these systems by means of complementary techniques (Langmuir trough, fluorescence microscopy, ellipsometry, and scanning electron microscopy (SEM)) shows that the incorporated NPs preferentially distribute along the liquid expanded phase of DPPC. The layer assumes the stable and homogeneous bidimensional structure of a two-dimensional (2D) analogue of a Pickering emulsion. In fact, the presence of particles provides a circular shape to the DPPC domains and stabilizes them against growth and coalescence during the monolayer compression.

Silica nanoparticles as tracers of the gelation dynamics of a natural biopolymer physical gel

The gelation of methylcellulose in water has been studied by X-ray photon correlation spectroscopy, electrophoresis and rheological measurements by looking into the dynamics of silica nanoparticles as tracers in the polymer matrix. The temperature and scattering vector dependence of the structural relaxation time is investigated at the nanometric length scale during the formation of the strong gel state. We find a stress-dominated dynamics on approaching the gel state, characterized by a hyper-diffusive motion of the silica particles. These results support the idea of a unifying scenario for the dynamics of complex out of equilibrium soft materials.

Direction-dependent freezing of diamagnetic colloidal tracers suspended in paramagnetic ionic liquids

The dynamic behavior of an inverse ferrofluid consisting of diamagnetic, spherical silica particles suspended in the paramagnetic ionic liquid (EMIm)2[Co(NCS)4] is investigated by means of x-ray photon correlation spectroscopy in the presence of an external magnetic field. Dipole-dipole interactions between the diamagnetic holes in the paramagnetic continuum of the suspending medium induce a direction-dependence of the diffusive motion of the colloidal particles: due to a magnetic repulsion perpendicular to the direction of an external field the diffusive motion of the colloidal particles is selectively frozen in this direction.

Revealing the fast atomic motion of network glasses

Still very little is known on the relaxation dynamics of glasses at the microscopic level due to the lack of experiments and theories. It is commonly believed that glasses are in a dynamical arrested state, with relaxation times too large to be observed on human time scales. Here we provide the experimental evidence that glasses display fast atomic rearrangements within a few minutes, even in the deep glassy state. Following the evolution of the structural relaxation in a sodium silicate glass, we find that this fast dynamics is accompanied by the absence of any detectable aging, suggesting a decoupling of the relaxation time and the viscosity in the glass. The relaxation time is strongly affected by the network structure with a marked increase at the mesoscopic scale associated with the ion-conducting pathways. Our results modify the conception of the glassy state and asks for a new microscopic theory.

Controlling the dynamics of a bidimensional gel above and below its percolation transition

The morphology and the microscopic internal dynamics of a bidimensional gel formed by spontaneous aggregation of gold nanoparticles confined at the water surface are investigated by a suite of techniques, including grazing-incidence x-ray photon correlation spectroscopy (GI-XPCS). The range of concentrations studied spans across the percolation transition for the formation of the gel. The dynamical features observed by GI-XPCS are interpreted in view of the results of microscopic imaging; an intrinsic link between the mechanical modulus and internal dynamics is demonstrated for all the concentrations. Our work presents an example of a transition from a stretched to a compressed correlation function actively controlled by quasistatically varying the relevant thermodynamic variable. Moreover, by applying a model proposed some time ago by Duri and Cipelletti [Europhys. Lett. 76, 972 (2006)] we are able to build a master curve for the shape parameter, whose scaling factor allows us to quantify a "long-time displacement length." This characteristic length is shown to converge, as the concentration is increased, to the "short-time localization length" determined by pseudo-Debye-Waller analysis of the initial contrast. Finally, the intrinsic dynamics of the system is then compared with that induced by means of a delicate mechanical perturbation applied to the interface.

Stabilizing liquid drops in nonequilibrium shapes by the interfacial jamming of nanoparticles

Nanoparticles assemble at the interface between two fluids into disordered, liquid-like arrays where the nanoparticles can diffuse laterally at the interface. Using nanoparticles dispersed in water and amine end-capped polymers in oil, nanoparticle surfactants are generated in situ at the interface overcoming the inherent weak forces governing the interfacial adsorption of nanoparticles. When the shape of the liquid domain is deformed by an external field, the surface area increases and more nanoparticles adsorb to the interface. Upon releasing the field, the interfacial area decreases, jamming the nanoparticle surfactants and arresting further shape change. The jammed nanoparticles remain disordered and liquid-like, enabling multiple, consecutive deformation and jamming events. Further stabilization is realized by replacing monofunctional ligands with difunctional versions that cross-link the assemblies. The ability to generate and stabilize liquids with a prescribed shape poses opportunities for reactive liquid systems, packaging, delivery, and storage.

Direction dependent diffusion of aligned magnetic rods by means of x-ray photon correlation spectroscopy

Rodlike hematite particles in suspension align perpendicular to an external magnetic field due to a negative anisotropy of their magnetic susceptibility Δχ. The diffusion tensor consists of two principal constants D(∥) and D(⊥) for the diffusion parallel and perpendicular to the long particle axis. X-ray photon correlation spectroscopy is capable of probing the diffusive motion in optically opaque suspensions of rodlike hematite particles parallel to the direction of the scattering vector Q. Choosing Q parallel or perpendicular to the direction of an external magnetic field H the direction dependent intermediate scattering function is measured by means of x-ray photon correlation spectroscopy. From the intermediate scattering function in both directions the principal diffusion constants D(∥) and D(⊥) are determined. The ratio D(∥)/D(⊥) increases with increasing aspect ratio of the particles and can be described via a rescaled theoretical approach for prolate ellipsoids of revolution.

Random walk in chemical space of Cantor dust as a paradigm of superdiffusion

We point out that the chemical space of a totally disconnected Cantor dust K(n) [Symbol: see text E(n) is a compact metric space C(n) with the spectral dimension d(s) = d(ℓ) = n > D, where D and d(ℓ) = n are the fractal and chemical dimensions of K(n), respectively. Hence, we can define a random walk in the chemical space as a Markovian Gaussian process. The mapping of a random walk in C(n) into K(n) [Symbol: see text] E(n) defines the quenched Lévy flight on the Cantor dust with a single step duration independent of the step length. The equations, describing the superdiffusion and diffusion-reaction front propagation ruled by the local quenched Lévy flight on K(n) [Symbol: see text] E(n), are derived. The use of these equations to model superdiffusive phenomena, observed in some physical systems in which propagators decay faster than algebraically, is discussed.