Colloidal Interactions Research Articles

Rheology and structural build-up of sodium silicate- and sodium hydroxide-activated GGBFS mixtures

Alkali-activated slag cements (AAS) are considered as an alternative to portland cement (PC) in many studies. A significant number of studies have focused on their mechanical and durability properties, but very few studies have concentrated on their rheological behavior. In this study, the effects of sodium silicate and sodium hydroxide on the rheological properties and structural build-up of alkali-activated GGBFS mixtures have been investigated. The sodium silicate-activated GGBFS mixtures exhibited a lower yield stress and a higher plastic viscosity than sodium hydroxide-activated GGBFS mixtures. The small-amplitude oscillatory shear (SAOS) tests indicated a negligible colloidal interaction between GGBFS particles in sodium silicate-activated system; therefore, the early increase in structuration was associated with the formation of reaction products due to the interaction between the dissolved calcium ions and the silicates originated from the activator solution. On the other hand, the stiffness and the early increase in structural build-up of sodium hydroxide-activated GGBFS system were attributed to the formation of well-percolated network.

Comprehensive view of microscopic interactions between DNA-coated colloids

The self-assembly of DNA-coated colloids into highly-ordered structures offers great promise for advanced optical materials. However, control of disorder, defects, melting, and crystal growth is hindered by the lack of a microscopic understanding of DNA-mediated colloidal interactions. Here we use total internal reflection microscopy to measure in situ the interaction potential between DNA-coated colloids with nanometer resolution and the macroscopic melting behavior. The range and strength of the interaction are measured and linked to key material design parameters, including DNA sequence, polymer length, grafting density, and complementary fraction. We present a first-principles model that screens and combines existing theories into one coherent framework and quantitatively reproduces our experimental data without fitting parameters over a wide range of DNA ligand designs. Our theory identifies a subtle competition between DNA binding and steric repulsion and accurately predicts adhesion and melting at a molecular level. Combining experimental and theoretical results, our work provides a quantitative and predictive approach for guiding material design with DNA-nanotechnology and can be further extended to a diversity of colloidal and biological systems.

Experimental study and modeling of the energy density and time-dependent rheological behavior of carbon nanotube nanofluids with sonication

Nanofluids are colloidal suspensions of nanoparticles in liquids. Nanoparticles have superior properties, which means that they can improve the performance characteristics of nanofluids. To press their advantages, the characteristics of nanofluids should be investigated to understand their underlying mechanisms. In this sense, fluidity is an important concept for examining the colloidal structure and interactions among nanoparticles based on their morphologies and intrinsic properties. Viscosity determines the fluidity of nanofluids, thereby indicating their degree of resistance to deformation and molecular dynamicity. Herein, we experimentally studied the rheological behavior of sonicated nanofluids with multi-walled carbon nanotubes (MWCNTs), which had long, tube-like, and flexible structures. The change in viscosity at an increasing energy density (Eρ) due to sonication at the specific total energy (ET) indicates that the nanotubes experienced a size phase change from “debundling” to “nano-cutting”. These phases agree well with the results of the hydrodynamic radii from dynamic light scattering (DLS) measurements. In the debundling phase, the unordered colloidal structure due to randomized entanglement increased the resistance to shear flow, thereby leading to a higher viscosity. With higher Eρ, the nanotubes broke owing to nano-cutting. In this phase, the short and exfoliated nanotubes functioned as rigid rods in the nanofluids and tended to align easily under shear flow. Finally, we discovered a well-fitted correlation between the viscosity of the nanofluids and sonication conditions (i.e., Eρ and ET). We found that the characteristic change in the colloidal structure of the MWCNT nanofluids was reflected in their rheological behavior during their preparation via sonication. Furthermore, the relationship between sonication conditions and viscosity helped us to predict the colloidal structure of the MWCNT nanofluids in the early stages of preparation. We expect that this work will contribute to promote the availability of MWCNTs in various application fields in terms of rheology and intermolecular structure.

New Structural Insights into Densely Assembled Reduced Graphene Oxide Membranes

AbstractDensely assembled graphene‐based membranes have attracted substantial interest for their widespread applications, such as compact capacitive energy storage, ion/molecular separation, gas barrier films, and flexible electronics. However, the multiscale structure of densely packed graphene membranes remains ambiguously understood. This article combines X‐ray and light scattering techniques as well as dynamic electrosorption analysis to uncover the stacking structure of the densely stacked reduced graphene oxide (rGO) membranes. The membranes are produced by reducing graphene oxide (GO) membranes with hydrazine, during which the colloidal interactions between GO sheets are modulated by the electrolyte solution. In contrast to the common notion that direct reduction of densely assembled GO sheets in parallel tends to result in significant “graphitization”, this article unexpectedly discovers that the resultant densely packed rGO membrane can still retain the interconnected network nanochannels and show good capacitive performances. This inspires the development of a hierarchical structural model to describe the densely packed rGO membranes. This article further shows that the nanochannel network can be fine‐tuned at the sub‐nanometer level by tailoring the salt concentration and the reduction temperature to render exceptional volumetric capacitance and good rate performance for rGO membranes even with increased packing density.

Comb-type polymer-hybridized MXene nanosheets dispersible in arbitrary polar, nonpolar, and ionic solvents.

Solution-based processing of two-dimensional (2D) nanomaterials is highly desirable, especially for the low-temperature large-area fabrication of flexible multifunctional devices. MXenes, an emerging family of 2D materials composed of transition metal carbides, carbonitrides, or nitrides, provide excellent electrical and electrochemical properties through aqueous processing. Here, we further expand the horizon of MXene processing by introducing a polymeric superdispersant for MXene nanosheets. Segmented anchor-spacer structures of a comb-type polymer, polycarboxylate ether (PCE), provide polymer grafting–like steric spacings over the van der Waals range of MXene surfaces, thereby reducing the colloidal interactions by the order of 103, regardless of solvent. An unprecedented broad dispersibility window for Ti3C2Tx MXene, covering polar, nonpolar, and even ionic solvents, was achieved. Furthermore, close PCE entanglements in MXene@PCE composite films resulted in highly robust properties upon prolonged mechanical and humidity stresses.

Quantitative characterization of non-DLVO factors in the aggregation of black soil colloids

The variable role and fate of soil colloids under different environmental conditions are derived from their dispersion and aggregation properties. In this work, dynamic and static light scattering were used to characterize the original size, aggregation kinetics of natural black soil colloids (BSCs) and structural features of aggregates in electrolytes with different cations (K+, Mg2+, Ca2+), respectively. For these three cations, the aggregation kinetics followed the trend of Ca2+ > Mg2+ > K+ and the critical coagulation concentration (CCC) followed the sequence: K+ (134.30 mmol L−1) > Mg2+ (13.27 mmol L−1) > Ca2+ (4.19 mmol L−1). The results indicated that the aggregation behavior in different valence cation systems followed the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) model qualitatively. However, the quantitative differences of CCC suggest the existence of ion-specific effects. The effective ionic charge coefficient 1.31, 2.20, and 2.78 of K+, Mg2+ and Ca2+ were proposed to consider of all the non-DLVO factors, which were obtained by forming a relationship based on mathematic between the electrostatic repulsion and the van der Waals attractive interaction at the CCC. The non-classical polarization of cations in a strong soil electric field is a primary mechanism of cation effects on soil colloid interactions, causing the difference in colloid interaction energy and further affecting soil colloid aggregation. This result is crucial for enriching the theory of charged colloidal interactions.

Interaction of Active Janus Colloids with Tracers.

Understanding the motion of artificial active swimmers in complex surroundings, such as a dense bath of passive particulate matter, is essential for their successful utilization as cargo (drug) carriers and sensors or for medical imaging, under microscopic domains. In this study, we experimentally investigated the motion of active SiO2-Pt Janus particles (JPs) in a two-dimensional bath of smaller silica tracers dispersed with varying areal densities. Our observations indicate that when an active JP undergoes a collision with an isolated tracer, their interaction can have a significant impact on the swimmer's motion. However, the overall impact of tracers on the active JPs' motion (translation and rotation) depends on the frequency of collisions and also on the nature of the collision, which is marked by the time-duration for which the particles maintain contact during the collisions. Further, in the high-density tracer bath, our experiments reveal that the motion of the active JP results in a novel organizational behavior of the tracers on the trailing Pt (depletion of tracers) and the leading SiO2 (accumulation of tracers) side. In laboratory frame the emergence and the subsequent vanishing of the depletion zone are discussed in detail.

Hydrotalcite Colloidal Stability and Interactions with Uranium(VI) at Neutral to Alkaline pH.

In the United Kingdom, decommissioning of legacy spent fuel storage facilities involves the retrieval of radioactive sludges that have formed as a result of corrosion of Magnox nuclear fuel. Retrieval of sludges may re-suspend a colloidal fraction of the sludge, thereby potentially enhancing the mobility of radionuclides including uranium. The colloidal properties of the layered double hydroxide (LDH) phase hydrotalcite, a key product of Magnox fuel corrosion, and its interactions with U(VI) are of interest. This is because colloidal hydrotalcite is a potential transport vector for U(VI) under the neutral-to-alkaline conditions characteristic of the legacy storage facilities and other nuclear decommissioning scenarios. Here, a multi-technique approach was used to investigate the colloidal stability of hydrotalcite and the U(VI) sorption mechanism(s) across pH 7–11.5 and with variable U(VI) surface loadings (0.01–1 wt %). Overall, hydrotalcite was found to form stable colloidal suspensions between pH 7 and 11.5, with some evidence for Mg2+ leaching from hydrotalcite colloids at pH ≤ 9. For systems with U present, >98% of U(VI) was removed from the solution in the presence of hydrotalcite, regardless of pH and U loading, although the sorption mode was affected by both pH and U concentrations. Under alkaline conditions, U(VI) surface precipitates formed on the colloidal hydrotalcite nanoparticle surface. Under more circumneutral conditions, Mg2+ leaching from hydrotalcite and more facile exchange of interlayer carbonate with the surrounding solution led to the formation of uranyl carbonate species (e.g., Mg(UO2(CO3)3)2–(aq)). Both X-ray absorption spectroscopy (XAS) and luminescence analysis confirmed that these negatively charged species sorbed as both outer- and inner-sphere tertiary complexes on the hydrotalcite surface. These results demonstrate that hydrotalcite can form pseudo-colloids with U(VI) under a wide range of pH conditions and have clear implications for understanding the uranium behavior in environments where hydrotalcite and other LDHs may be present.

Mechanical attributes, colloidal interactions, and microstructure of meat batter influenced by flaxseed flour and tomato powder

The present investigation deals with the textural properties, colloidal interactions, and morphology of emulsified meat systems in the presence of flaxseed flour (FF) and tomato powder (TP). The results displayed that the emulsifying capacity and texture of raw and cooked meat batters were significantly affected following the addition of TP and FF. The cooked emulsified sausages containing 3% of FF and 3% (w/w) of TP showed the highest values for hardness and cohesiveness, as compared to the control and 6% (w/w) of FF samples. The outcomes of mechanical shearing forces and SEM showed the formation of a gel-type matrix around the unabsorbed protein in TP-FF batters. These patterns were then confirmed by the higher values of GN° (van Gurp Palmen) associated with an increase in the elasticity and the molecular entanglement. In contrast, large fat globules, low entanglement, and protein cross-linking were observed in meat batters with 6% FF.

Snap-in interactions between water droplets and hematite/quartz surfaces with various roughness after conditioning with soluble starch and DDA using a dynamic microbalance

• Hematite and quartz surfaces with different roughness were conditioned with starch and DDA. • Snap-in force of water droplets on the surfaces of the conditioned minerals was measured. • The largest variation in snap-in force was presented on the roughest surface. • Increasing roughness could enlarge the snap-in force difference for the two minerals. • There is a liner correlation between snap-in force and advancing contact angle. Surface roughness plays a vital role in flotation, because it has a pronounced impact on colloidal interactions, i.e., solid-liquid adhesion. In the previous studies, the natural mineral surfaces with various roughness were mainly taken as research object and the adhesion force of liquid on mineral surfaces with various roughness after conditioning with surfactants remains unknown. To fulfil this gap, in this study, the role of surface roughness in the snap-in force (An attractive force when the water droplet sharply and instantaneously spreads on the mineral surface in less than 0.1 s) of water droplets on hematite and quartz surfaces after conditioning with soluble starch and dodecylamine (DDA) was investigated using a high-sensitivity microelectronic mechanical balance. Results revealed that the effect of surface roughness on the snap-in force of water droplets on hematite surfaces was distinctly affected by the concentrations of both DDA and soluble starch, while the effect of surface roughness on the snap-in force of water droplets on quartz surfaces was only significantly affected by DDA concentration. After conditioning with 2 × 10 −4 mol/L soluble starch and 4 × 10 −4 mol/L DDA under pH value of 8.75, increasing surface roughness increased the snap-in force of water droplets on hematite surfaces and decreased the snap-in force of water droplets on quartz surfaces, suggesting that increasing surface roughness could enlarge the difference in the snap-in force of water droplets on the surfaces of the two minerals. In addition, the snap-in force was found to be a linear correlation with the advancing contact angle, providing evidence that surface roughness could affect the wettability and thereby the snap-in force.

Colloidal and electrostatic interactions in free-standing membranes

Phospholipid bilayer elastic properties govern numerous biological processes, ranging from the deformation necessary during endocytosis and exocytosis, to the lateral diffusion of membrane proteins. As a result, there have been a multitude of studies using artificial techniques to reconstitute phospholipids into phospholipid bilayers and quantify membrane material properties. In this presentation, we will discuss recent advances in our lab applying a novel technique to create free-standing membranes to understand membrane elasticity and how membrane material properties dictate biological function.

Probing concentration and time dependent conformational changes in poly acrylic acid stabilized magnetic nanoemulsion using magnetic chaining-based inter-droplet force measurement

Emulsions are stabilized with surfactants, polymers, or polyelectrolytes to extend their shelf-life and colloidal stability. Electro-steric interaction in polyelectrolyte stabilized emulsions makes them ideal systems for probing the conformational dynamics. Here, magnetic chaining technique is exploited for inter-droplet force measurement in poly acrylic acid (PAA) stabilized magnetic nanoemulsions, and to probe the conformational changes of the stabilizing moieties as a function of time and PAA concentration. Force measurement studies indicate an increased inter-droplet spacing for higher concentration of PAA due to an increase in osmotic pressure with concentration. Further, for a fixed concentration, the inter-droplet spacing decreases with time due to the slow diffusion of the polymer chains resulting in conformational changes. This is the first experimental report on conformational dynamics of PAA stabilized oil-in-water magnetic nanoemulsions. The obtained results are beneficial for understanding the nature of colloidal interaction forces in polyelectrolyte stabilized nanoemulsions.

Chemical design of self-propelled Janus droplets

Chemical design of self-propelled Janus droplets

Inverse design of soft materials via a deep learning-based evolutionary strategy.

Colloidal self-assembly—the spontaneous organization of colloids into ordered structures—has been considered key to produce next-generation materials. However, the present-day staggering variety of colloidal building blocks and the limitless number of thermodynamic conditions make a systematic exploration intractable. The true challenge in this field is to turn this logic around and to develop a robust, versatile algorithm to inverse design colloids that self-assemble into a target structure. Here, we introduce a generic inverse design method to efficiently reverse-engineer crystals, quasicrystals, and liquid crystals by targeting their diffraction patterns. Our algorithm relies on the synergetic use of an evolutionary strategy for parameter optimization, and a convolutional neural network as an order parameter, and provides a way forward for the inverse design of experimentally feasible colloidal interactions, specifically optimized to stabilize the desired structure.

Effect of packing conditions and materials properties on loose packing fraction of rigid particles: a theoretical and experimental review

This paper focuses on the effect of packing conditions and materials properties on loose packing fraction of particles from theoretical and experimental perspectives. First, the difference in loose packing of particles was measured in different conditions. In the second part, we obtained the main factor that dominated the loose packing fraction in different particle scales. We concluded that kinetics energy induced by the gravitational potential energy almost has no influence on the loose packing fraction for particles smaller than a few millimeters. On the contrary, the wall effect became more and more evident with the increase of particle size. Particle morphology and PSD significantly affect the loose packing of particles above the sub-millimeter size. Moreover, for micron-sized particles, the Van der Waals force was shown to dominate all other colloidal interactions, and the ratio between Van der Waals force and effective gravity determined the loose packing fraction of fine powders.

The role of hydrodynamic interactions on the aggregation kinetics of sedimenting colloidal particles.

The aggregation kinetics of sedimenting colloidal particles under fully destabilized conditions has been investigated over a wide range of particle volume fractions (Φ) and Péclet numbers (Pe) using the recent PSE algorithm implementing the Rotne-Prager-Yamakawa (RPY) approximation for long-range Hydrodynamic Interactions (HI). Fast Lubrication Dynamics (FLD) and simple Brownian Dynamics (BD) methods have also been employed to assess the importance of long range hydrodynamic interactions on the resulting dynamics. It has been observed that long-range hydrodynamic interactions are essential to capture the fast aggregation rates induced by the increase in sedimentation rate of clusters with increasing mass, which manifests with an explosive-like cluster growth after a given induction time. On the contrary, simulations employing only short-range hydrodynamic interactions (such as FLD) and BD (which neglects completely hydrodynamic interactions) are incapable of predicting this very rapid kinetics, because sedimentation simply leads to all particles and clusters moving vertically with identical velocity. It has been observed that at high volume fractions and low Pe values, a gel point can be formed and a phase diagram predicting when gelation is reached has been obtained. It was also observed that, as Pe increases, the anisotropy of the resulting clusters decreases, suggesting that denser clusters with spherical-like morphology are formed due to cluster breakage and restructuring. We can conclude that long-range hydrodynamic effects are of crucial importance in understanding the aggregation dynamics of settling clusters, revealing important features of the complex interplay between sedimentation, and colloidal interactions.

Impacts of Colloidal and Interfacial Interactions on Cohesion Strength of Rubberized Rap Asphalt Binder

Impacts of Colloidal and Interfacial Interactions on Cohesion Strength of Rubberized Rap Asphalt Binder

Line optical tweezers as controllable micromachines: techniques and emerging trends.

In the past three decades, the technology of optical tweezers has made significant contributions in various scientific areas, including optics, photonics, and nanosciences. Breakthroughs include manipulating particles in both static and dynamic ways, particle sorting, and constructing controllable micromachines. Advances in shaping and controlling the laser beam profile enable control over the position and location of the trap, which has many possible applications. A line optical tweezer (LOT) can be created by rapidly moving a spot optical tweezer using a tool such as a galvanometer mirror or an acousto-optic modulator. By manipulating the intensity profile along the beam line to be asymmetric or non-uniform, the technique can be adapted to various specific applications. Among the many exciting applications of line optical tweezers, in this work, we discuss in detail applications of LOT, including probing colloidal interactions, transporting and sorting of colloidal microspheres, self-propelled motions, trapping anisotropic particles, exploring colloidal interactions at fluid-fluid interfaces, and building optical thermal ratchets. We further discuss prospective applications in each of these areas of soft matter, including polymeric and biological soft materials.

The role of small separation interactions in ferrofluid structure

Interparticle interactions in colloids are traditionally modeled by means of the DLVO theory, which includes van der Waals and electrical double layer (EDL) interactions However, the validity range limitations become critical in biocompatible magnetic colloids, requiring a more detailed description of the interactions, especially at small intersurface separations. As magnetic colloids, ferrofluids require an extended DLVO (XDLVO) model that includes magnetic interactions. Moreover, the nanoparticles of biocompatible ferrofluids are usually ionic-surfacted, such that their charged surfactants interact both electrically and sterically. In some of such particles, the charge is usually not located at the surface, but at the outer extremities of the surfactant molecules, and this feature restricts the EDL model validity to larger separation distances. We addressed this problem by means of a model proposed by Schnitzer and Morozov, which employs a generalized Derjaguin approximation that makes the EDL repulsion expression valid for all separations. The van der Waals expression of the DLVO theory is also problematic because it shows an unphysical divergence as the intersurface separation tends to zero, a problem that was circumvented by replacing the expression at small separations with another expression based on cohesion energy and the Born-Mayer repulsion. The modifications proposed here are of interest for research on colloids in general and our Monte Carlo simulations show that they acquire even greater importance when it comes to ferrofluids. The influence of magnetic interparticle interactions on the colloid structure is better gauged using these modifications, which prevent magnetic interactions from being obfuscated by artificially large van der Waals and EDL interactions. This conclusion makes the small separation treatment particularly important for the study of magnetic colloids.

Improved Magneto-Microfluidic Separation of Nanoparticles through Formation of the β-Cyclodextrin-Curcumin Inclusion Complex.

Molecular adsorption to the nanoparticle surface may switch the colloidal interactions from repulsive to attractive and promote nanoparticle agglomeration. If the nanoparticles are magnetic, then their agglomerates exhibit a much stronger response to external magnetic fields than individual nanoparticles. Coupling between adsorption, agglomeration, and magnetism allows a synergy between the high specific area of nanoparticles (∼100 m2/g) and their easy guidance or separation by magnetic fields. This yet poorly explored concept is believed to overcome severe restrictions for several biomedical applications of magnetic nanoparticles related to their poor magnetic remote control. In this paper, we test this concept using curcumin (CUR) binding (adsorption) to β-cyclodextrin (βCD)-coated iron oxide nanoparticles (IONP). CUR adsorption is governed by host-guest hydrophobic interactions with βCD through the formation of 1:1 and, possibly, 2:1 βCD:CUR inclusion complexes on the IONP surface. A 2:1 stoichiometry is supposed to promote IONP primary agglomeration, facilitating the formation of the secondary needle-like agglomerates under external magnetic fields and their magneto-microfluidic separation. The efficiency of these field-induced processes increases with CUR concentration and βCD surface density, while their relatively short timescale (<5 min) is compatible with magnetic drug delivery application.