Colloidal Forces Research Articles

Contributions of Colloidal Forces to the Heterogeneous Separation of Stable Oil-In-Water Emulsions.

We use theory to study the distribution of spherical emulsion oil droplets in water near a lipophilic surface as a guideline for designing membranes for oil/water phase separation. Heterogeneous phase separations are shown in our laboratory using hydrophilic and hydrophobic membrane designs, where the affinity of the membrane surface to one of the phases in the mixture locally increases its concentration. Considering a colloidal emulsion (nano- to microemulsions) of spherical and noncoalescing droplets, we assess the contribution of colloidal forces, i.e., van der Waals, electrical double layer, and hydrophobic interactions and the finite size of the droplets to the accumulation of spherical emulsion droplets near a surface. We use our theory to study an experiment-inspired case study and find that an isolated lipophilic membrane surface in contact with an oil-in-water emulsion supports the oil-enriched emulsion phase in a thin layer near the membrane surface, suggesting that a membrane pore size comparable to this thickness should support oil-enriched emulsion in the membrane pores and hence past the membrane.

Determination of interaction forces of (sub)-micron sized particles via the capillary rise method and colloidal probe atomic force microscopy: A combined approach

HypothesisThe adhesion forces of particles in the submicron range play a decisive role in many particle processes such as agglomeration. These forces are influenced by many factors such as particle size, surface roughness, and contact area. The quantification of these influences should be possible by linking adhesion forces, measured with colloidal probe atomic force microscopy (cp-AFM), and surface energy, measured with the capillary rise method, using different adhesion force models. ExperimentsSilica-silica (SiO2-SiO2), polystyrene-polystyrene (PS-PS) and mixed adhesive force contacts of micrometer-sized particles were measured by cp-AFM. The surface energy was determined by the capillary rise method using the Owens-Wendt-Rabel-Kaelble (OWRK) method. Various adhesive force models were used to link the experimentally measured forces. In the adhesive force models, the particle size, the distance between the particles, and the roughness were varied. FindingsAdhesion forces measured with the AFM can be linked to the OWRK method via adhesion force models such as the particle–particle Van-der-Waals (VDW) model, Rumpf’s roughness model, and a proprietary model for multiple particle–particle interactions with the surface energy. The main influencing factors are the substrate and particle roughness as well as their plastic or elastic behavior, which influences the contact area of the adhesive contact.

Adhesion Mechanics and Detachment Dynamics of Vanishing Surface Layers on Hydrogels.

Cross-linked hydrogel surfaces exhibit reduced stiffness when polymerized against polymeric hydrophobic surfaces. As such, these layers play a critical role in contact mechanics, particularly exhibiting strong relative adhesion with colloidal probes when the contact area is small. This prevents the use of continuum models of adhesive soft contact. To connect mechanisms of stretch to the force response, depth-controlled nanoindentation experiments were conducted on polyacrylamide (pAAM) hydrogel samples using colloidal probe atomic force microscopy (AFM). The pAAM sample had a high water content of >90% and was molded against polyoxymethylene (POM) to create a more dilute surface layer with thickness ∼0.5 μm. Indentations to multiple depths between 50 nm and 1.25 μm were repeated 10 times each. First, the force drops during the unloading, and separation segments of each indentation were characterized. This described the detachment progression for increasing areas of contact, revealing that the pull-off force for a single chain was in the single-pN range. Second, the stretched polymer network was modeled as an array of parallel, linear springs. Assuming a constant areal chain density of α = 100 chains/μm2, the maximum force of adhesion was plotted versus the volume of chains stretched upward, and the average chain stiffness was calculated from a linear fit to be 22.8 × 10-6 N/m. A Weibull distribution analysis of detachment events revealed a dependence of chain stiffness on maximum indentation depth (dmax), with higher stiffness at shallower depths approaching kchain ≈ 20 × 10-6 N/m. These findings on adhesion mechanics between a vanishing hydrogel surface and probe can guide the development of multifunctional hydrogels for various biomedical applications.

Anisotropic Stick-Slip Frictional Surfaces via Titania Nanorod Patterning.

Nanoscale or microscale surface texturing is an effective technique to tailor the tribological properties between two surfaces that are rubbed against each other. In order to achieve the desired frictional properties by a patterned surface, one needs an in-depth understanding of the underlying mechanisms. Here, we demonstrate anisotropic stick-slip friction achieved via a nanotextured surface of tilted titania nanorods (TiNRs). The surface was developed by using the glancing angle deposition (GLAD) technique, and exhibited load-dependent variations in stick-slip friction as well as frictional anisotropy in different sliding directions. For studying the frictional properties of the newly developed surface, lateral force microscopy (LFM) was performed in three different reciprocal orientations (0° rotated, 45° rotated, 90° rotated) using a custom-made colloidal alumina atomic force microscopy (AFM) probe. The frictional behavior was found to vary significantly with the orientation. At 0° rotated position) a prominent "stick-slip" was observed when scanning opposite to the tilt direction, whereas the phenomenon reduced significantly when the nanotextured surface was scanned along the tilt direction or rotated to different angles (45 and 90°) with respect to the sliding direction of the AFM cantilever supporting the probe. The experimental findings were interpreted based on the classical solution for large deflections of tilted elastic rods. Overall, the textured surface, LFM-based frictional measurement, and the quantitative analysis presented here provide a fundamental understanding of how friction can be significantly varied on a surface patterned with tilted TiNRs at a length scale of about 1 μm, which can be comprehensively applied to nanorod patterns of other materials on different substrates.

Artificial chemotaxis under electrodiffusiophoresis

HypothesisThrough a large parameter space, electric fields can tune colloidal interactions and forces leading to diverse static and dynamical structures. So far, however, field-driven interactions have been limited to dipole-dipole and hydrodynamic contributions. Nonetheless, in this work, we propose that under the right conditions, electric fields can also induce interactions based on local chemical fields and diffusiophoretic flows. ExperimentsHerein, we present a strategy to generate and measure 3D chemical gradients under electric fields. In this approach, faradaic reactions at electrodes induce global pH gradients that drive long-range transport through electrodiffusiophoresis. Simultaneously, the electric field induces local pH gradients by driving the particle's double layer far from equilibrium. FindingsAs a result, while global pH gradients lead to 2D focusing away from electrodes, local pH gradients induce aggregation in the third dimension. Evidence points to a mechanism of interaction based on diffusiophoresis. Interparticle interactions display a strong dependence on surface chemistry, zeta potential and diameter of particles. Furthermore, pH gradients can be readily tuned by adjusting the voltage and frequency of the electric field. For large Péclet numbers, we observed a collective chemotactic-like collapse of particles. Remarkably, such collapse occurs without reactions at a particle's surface. By mixing particles with different sizes, we also demonstrate, through experiments and Brownian dynamics simulations, the emergence of non-reciprocal interactions, where small particles are more drawn towards large ones.

Self-Timed and Spatially Targeted Delivery of Chemical Cargo by Motile Liquid Crystal.

A key challenge underlying the design of miniature machines is encoding materials with time- and space-specific functional behaviors that require little human intervention. Dissipative processes that drive materials beyond equilibrium and evolve continuously with time and location represent one promising strategy to achieve such complex functions. This work reports how internal nonequilibrium states of liquid crystal (LC) emulsion droplets undergoing chemotaxis can be used to time the delivery of a chemical agent to a targeted location. During ballistic motion, hydrodynamic shear forces dominate LC elastic interactions, dispersing microdroplet inclusions (microcargo) within double emulsion droplets. Scale-dependent colloidal forces then hinder the escape of dispersed microcargo from the propelling droplet. Upon arrival at the targeted location, a circulatory flow of diminished strength allows the microcargo to cluster within the LC elastic environment such that hydrodynamic forces grow to exceed colloidal forces and thus trigger the escape of the microcargo. This work illustrates the utility of the approach by using microcargo that initiate polymerization upon release through the outer interface of the carrier droplet. These findings provide a platform that utilizes nonequilibrium strategies to design autonomous spatial and temporal functions into active materials.

Exploring the mechanisms of calcium carbonate deposition on various substrates with implications for effective anti-scaling material selection

The unexpected scaling phenomena have resulted in significant damages to the oil and gas industries, leading to issues such as heat exchanger failures and pipeline clogging. It is of practical and fundamental importance to understand the scaling mechanisms and develop efficient anti-scaling strategies. However, the underlying surface interaction mechanisms of scalants (e.g., calcite) with various substrates are still not fully understood. In this work, the colloidal probe atomic force microscopy (AFM) technique has been applied to directly quantify the surface forces between calcite particles and different metallic substrates, including carbon steel (CR1018), low alloy steel (4140), stainless steel (SS304) and tungsten carbide, under different water chemistries (i.e., salinity and pH). Measured force profiles revealed that the attractive van der Waals (VDW) interaction contributed to the attachment of the calcium carbonate particles on substrate surfaces, while the repulsive electric double layer (EDL) interactions could inhibit the attachment behaviors. High salinity and acidic pH conditions of aqueous solutions could weaken the EDL repulsion and promote the attachment behavior. The adhesion of calcite particles with CR1018 and 4140 substrates was much stronger than that with SS304 and tungsten carbide substrates. The bulk scaling tests in aqueous solutions from an industrial oil production process showed that much more severe scaling behaviors of calcite was detected on CR1018 and 4140 than those on SS304 and tungsten carbide, which agreed with surface force measurement results. Besides, high salinity and acidic pH can significantly enhance the scaling phenomena. This work provides fundamental insights into the scaling mechanisms of calcite at the nanoscale with practical implications for the selection of suitable anti-scaling materials in petroleum industries.

Rate dependence in adhesive particle–particle contacts affect ceramic suspension bulk flow behavior

Particle–fluid mixtures are known for displaying a wide variety of interesting flow mechanisms, such as thickening and thinning under shear. It is now becoming increasingly evident that even molecular scale effects are relevant for this macroscopic flow behavior of suspensions. Here we show that even 50μm ceramic beads in a non-density matched solvent are affected by the pH of the solvent. We trace the origin of the pH dependence to the contact mechanics using colloidal probe atomic force microscopy (CP-AFM). The gel-like structure of the pH 3 samples suggests that adhesion effects play a major role, but which adhesion timescale competes with shear rate to affect the rheology is not obvious. We test here two time scales in the contact mechanics: total contact time tc and contact retraction speed vr. We observe that contact time is the most important variable to correlate with rheology. The ceramic particle suspension so serves as a model system to help understand which adhesive mechanism affects flow behavior in the suspension.

Dynamic Light Scattering and Its Application to Control Nanoparticle Aggregation in Colloidal Systems: A Review.

Colloidal systems and their control play an essential role in daily human activities, but several drawbacks lead to an avoidance of their extensive application in some more productive areas. Some roadblocks are a lack of knowledge regarding how to influence and address colloidal forces, as well as a lack of practical devices to understand these systems. This review focuses on applying dynamic light scattering (DLS) as a powerful tool for monitoring and characterizing nanoparticle aggregation dynamics. We started by outlining the core ideas behind DLS and how it may be used to examine colloidal particle size distribution and aggregation dynamics; then, in the last section, we included the options to control aggregation in the chemically processed toner. In addition, we pinpointed knowledge gaps and difficulties that obstruct the use of DLS in real-world situations. Although widely used, DLS has limits when dealing with complicated systems, including combinations of nanoparticles, high concentrations, and non-spherical particles. We discussed these issues and offered possible solutions and the incorporation of supplementary characterization approaches. Finally, we emphasized how critical it is to close the gap between fundamental studies of nanoparticle aggregation and their translation into real-world applications, recognizing challenges in colloidal science.

Separation of Nanoparticle Seed Pseudoisomers via Amplification of Their Crystallographic Differences.

Seed-mediated syntheses rely on small nanoparticle (NP) precursors that act as templates for growth but are often inhomogeneous with respect to their internal twinning structures (e.g., single crystalline, multiply twinned), leading to nonuniform product morphologies. To address this, we developed a method for separating seed NPs of the same approximate size (∼ 10 nm) but with different interior twinning (i.e., NP "pseudoisomers") by exaggerating their crystallographic differences through heteroexpitaxial metal overgrowth. Specifically, single crystalline and pentatwinned Au seeds that are natively inseparable via traditional methods exhibit drastically different Ag shell morphologies that allow for their selective precipitation through colloidal depletion forces. Oxidation of the Ag shell from separated particles results in seeds that are both size uniform and crystallographically pure (>99%), allowing for the controlled synthesis of a library of Oh- and D5h-symmetric gold NPs bearing {111}, {110}, {730}, {310}, {720}, and {100} facets, several of which have no precedent in the literature. These results lay the foundation for precision nanosynthesis by establishing a new paradigm for the purification of NP precursors.

A mechanistic study on the effectiveness of star-like and comb-like polycarboxylate superplasticizers in cement pastes

In this paper, star-like polycarboxylate superplasticizer (SPCE) and comb-like polycarboxylate superplasticizer (CPCE) were prepared via atom transfer radical polymerization (ATRP) to study the mechanism of the effectiveness enhancement resulting from this transformation of topological structure. It was found that SPCE molecules acted higher adsorption amount and more Ca2+ complexes, and formed a lower and more compact adsorbed layer and larger surface coverage with a flat conformation on cement surface, depending on more anchored points per unit volume. Through this mechanism, cement particles adsorbed by SPCE showed stronger dispersible capability and colloidal interaction forces (the steric force was dominant), resulting in lower yield stress and plastic viscosity of the cement paste containing SPCE which behaved as an ideal Bingham fluid. The aim of this study is to clarify the mechanistic correlation between topological structure evolution and effectiveness promotion for PCE.

Adsorptive chito-beads for control of membrane fouling

Chitosan has excellent antimicrobial, adsorption, heavy metal removal, and adhesion properties, making it a good substitute for microplastic-based cleaners. Here, chitosan microbeads (chito-beads) of various sizes ranging from 32 μm to 283 μm were prepared via emulsion using a liquid on oil method and the feasibility of using them as an essential constituent in a chemical cleaning solution for a reverse-osmosis (RO) membrane-fouling-control process was assessed. Prior to the assessment the cleaning efficiency of a solution containing chito-beads, the interaction energy between chitosan and a representative organic foulant (humic acid (HA)) in a RO membrane fouling was analyzed using colloidal atomic force microscopy, and the strongest attraction between chitosan and HA was observed in an aqueous solution. When comparing the membrane cleaning efficiency of cleaning solutions with and without chito-beads, smaller chito-beads (32 μm and 70 μm) were found to have higher cleaning efficiency. Applications of chito-beads to the membrane cleaning process can enhance the cleaning efficiency through the physicochemical interaction with organic foulants. This study can widen the use of chito-beads as an additive to membrane chemical cleaning solutions to control membrane fouling in other membrane processes as well.

Optical Tweezers-Based Measurements of Colloidal Forces between Asphaltene Thin Films: Effect of Ultrasonication.

Oil production and processing often involve the treatment of water-in-oil emulsions stabilized by asphaltenes. The asphaltenes adsorb irreversibly at the water-oil interface and, by self-association at the interface, form a viscoelastic film that stabilizes the emulsions mechanically and sterically. Hydrophobic forces associated with these films may also contribute to the emulsion stability. A key step in treating these emulsions is to weaken the asphaltene film at the interface, and one way to do so is with ultrasonic treatment. The effect of ultrasonic waves on the interactions between asphaltene films was investigated at a silica-water interface using optical tweezers. Silica microparticles were aged in asphaltene solutions to form asphaltene coatings on their surfaces. The particles were dispersed in water, and interparticle force measurements were performed with optical tweezers to capture the steric force and hydrophobic force contributions. The asphaltene coating thickness and hydrophobic coefficient (a factor resembling the strength of the hydrophobic interaction) were obtained from fitting these forces. The effect of ultrasonication on the thickness of the asphaltene films on the surfaces of the particles was investigated. No change in the hydrophobic coefficient was observed upon changing the interfacial asphaltene concentration. The asphaltene film thickness increased with the concentration of the asphaltene solution and aging time. After treatment of the dispersion with ultrasonic waves for different durations (between 5 and 40 min), a significant reduction in the coating thickness was observed. This reduction was confirmed by thermogravimetric analysis (TGA) measurements. It is hypothesized that cavitation at the interface removed part of the surface layer of asphaltenes from the coated particles. Based on these findings, we proved that a low-power ultrasound field can effectively break asphaltene-stabilized water-in-oil emulsions.

Effect of flow structure and colloidal forces on aggregation rate of small solid particles suspended in aqueous solutions

Effect of flow structure and colloidal forces on aggregation rate of small solid particles suspended in aqueous solutions

Modulating particle-particle interaction in phyllosilicate serpentine aqueous suspensions using sodium citrate

The depletion of high-grade nickel sulfide ores has led to an increased interest in low-grade ultramafic ores, which are widely available but challenging to process due to their high content of phyllosilicate serpentine. The anisotropic surface charge and shape of serpentine particles lead to attractive interactions, which increase the viscosity of ore slurry and associated operational and equipment costs. To overcome this challenge, we investigate the application of an environmentally benign reagent, sodium citrate, to modify the particle-particle interactions in serpentine aqueous suspensions. Our results show that at alkaline pH (10), sodium citrate selectively adsorbs on the brucite plane of serpentine, making the entire particle overall negatively charged, which weakens the interparticle attraction and reduces the suspension's shear viscosity, yield stress, and storage modulus. We confirmed these results using colloidal probe atomic force microscopy (AFM) technique, which showed a significant weakening of attraction between silica and brucite surfaces upon the addition of citrate. This work enhances the understanding of surface interaction mechanisms of phyllosilicate serpentine particles in aqueous media with the addition of sodium citrate and provides useful implications for the application of green reagents in mineral operations involving phyllosilicate gangues. Our findings suggest that sodium citrate has great potential as an effective and sustainable reagent in the processing of abundantly available low grade ultramafic nickel ores.

Nanoparticle Assembly: From Self-Organization to Controlled Micropatterning for Enhanced Functionalities.

Nanoparticles form long-range micropatterns via self-assembly or directed self-assembly with superior mechanical, electrical, optical, magnetic, chemical, and other functional properties for broad applications, such as structural supports, thermal exchangers, optoelectronics, microelectronics, and robotics. The precisely defined particle assembly at the nanoscale with simultaneously scalable patterning at the microscale is indispensable for enabling functionality and improving the performance of devices. This article provides a comprehensive review of nanoparticle assembly formed primarily via the balance of forces at the nanoscale (e.g., van der Waals, colloidal, capillary, convection, and chemical forces) and nanoparticle-template interactions (e.g., physical confinement, chemical functionalization, additive layer-upon-layer). The review commences with a general overview of nanoparticle self-assembly, with the state-of-the-art literature review and motivation. It subsequently reviews the recent progress in nanoparticle assembly without the presence of surface templates. Manufacturing techniques for surface template fabrication and their influence on nanoparticle assembly efficiency and effectiveness are then explored. The primary focus is the spatial organization and orientational preference of nanoparticles on non-templated and pre-templated surfaces in a controlled manner. Moreover, the article discusses broad applications of micropatterned surfaces, encompassing various fields. Finally, the review concludes with a summary of manufacturing methods, their limitations, and future trends in nanoparticle assembly.

Particle-Bubble Interactions: an Investigation of the Three-Phase Contact Line by Atomic Force Microscopy.

The dynamics of the three-phase contact line during particle-bubble interactions determine the stability of particle-bubble aggregates in flotation. The interaction of particles and sessile gas bubbles can be studied by colloidal probe atomic force microscopy (CP-AFM). This paper demonstrates a method to obtain the contact angle, the position of the three-phase contact line on the particle, and the bubble profile by utilizing the full information contained in AFM force-distance curves, i.e., force and CP-position information as well as the work done to move the three-phase contact line on the CP-particle. The proposed method does not require any assumption of a constant contact angle or a constant opening angle. This is achieved by the combined solution of the particle force balance and an expression for the work required to move the three-phase contact line over the colloid probe. The applicability to AFM force-distance measurements was demonstrated for the interaction of a hydrophobic SiO2 or a hydrophobic Al2O3 colloidal probe particle with sessile gas bubbles having radii between 45 and 80 μm.

Particle Network Self-Assembly of Similar Size Sub-Micron Calcium Alginate and Polystyrene Particles Atop Glass.

Particle-mediated self-assembly, such as nanocomposites, microstructure formation in materials, and core-shell coating of biological particles, offers precise control over the properties of biological materials for applications in drug delivery, tissue engineering, and biosensing. The assembly of similar-sized calcium alginate (CAG) and polystyrene sub-micron particles is studied in an aqueous sodium nitrate solution as a model for particle-mediated self-assembly of biological and synthetic mixed particle species. The objective is to reinforce biological matrices by incorporating synthetic particles to form hybrid particulate networks with tailored properties. By varying the ionic strength of the suspension, the authorsalter the energy barriers for particle attachment to each other and to a glass substrate that result from colloidal surface forces. The particles do not show monotonic adsorption trend to glass with ionic strength. Hence, apart from DLVO theory-van der Waals and electrostatic interactions-the authors further consider solvation and bridging interactions in the analysis of the particulate adsorption-coagulation system. CAG particles, which support lower energy barriers to attachment relative to their counterpart polystyrene particles, accumulate as dense aggregates on the glass substrate. Polystyrene particles adsorb simultaneously as detached particles. At high electrolyte concentrations, where electrostatic repulsion is largely screened, the mixture of particles covers most of the glass substrate; the CAG particles form a continuous network throughout the glass substrate with pockets of polystyrene particles. The particulate structure is correlated with the adjustable energy barriers for particle attachment in thesuspension.

Emerging Trends in Nanomaterial-Based Biomedical Aspects

Comprehending the interfacial interaction of nanomaterials (NMs) and biological systems is a significant research interest. NMs comprise various nanoparticles (NPs) like carbon nanotubes, graphene oxides, carbon dots, graphite nanopowders, etc. These NPs show a variety of interactions with biological interfaces via organic layers, therapeutic molecules, proteins, DNA, and cellular matrices. A number of biophysical and colloidal forces act at the morphological surface to regulate the biological responses of bio-nanoconjugates, imparting distinct physical properties to the NMs. The design of future-generation nano-tools is primarily based on the basic properties of NMs, such as shape, size, compositional, functionality, etc., with studies being carried out extensively. Understanding their properties promotes research in the medical and biological sciences and improves their applicability in the health management sector. In this review article, in-depth and critical analysis of the theoretical and experimental aspects involving nanoscale material, which have inspired various biological systems, is the area of focus. The main analysis involves different self-assembled synthetic materials, bio-functionalized NMs, and their probing techniques. The present review article focuses on recent emerging trends in the synthesis and applications of nanomaterials with respect to various biomedical applications. This article provides value to the literature as it summarizes the state-of-the-art nanomaterials reported, especially within the health sector. It has been observed that nanomaterial applications in drug design, diagnosis, testing, and in the research arena, as well as many fatal disease conditions like cancer and sepsis, have explored alongwith drug therapies and other options for the delivery of nanomaterials. Even the day-to-day life of the synthesis and purification of these materials is changing to provide us with a simplified process. This review article can be useful in the research sector as a single platform wherein all types of nanomaterials for biomedical aspects can be understood in detail.

Electrostatic interactions control the adsorption of extracellular vesicles onto supported lipid bilayers

Communication between cells located in different parts of an organism is often mediated by membrane-enveloped nanoparticles, such as extracellular vesicles (EVs). EV binding and cell uptake mechanisms depend on the heterogeneous composition of the EV membrane. From a colloidal perspective, the EV membrane interacts with other biological interfaces via both specific and non-specific interactions, where the latter include long-ranged electrostatic and van der Waals forces, and short-ranged repulsive “steric-hydration” forces. While electrostatic forces are generally exploited in most EV immobilization protocols, the roles played by various colloidal forces in controlling EV adsorption on surfaces have not yet been thoroughly addressed. In the present work, we study the adsorption of EVs onto supported lipid bilayers (SLBs) carrying different surface charge densities using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal laser scanning microscopy (CLSM). We demonstrate that EV adsorption onto lipid membranes can be controlled by varying the strength of electrostatic forces and we theoretically describe the observed phenomena within the framework of nonlinear Poisson-Boltzmann theory. Our modelling results confirm the experimental observations and highlight the crucial role played by attractive electrostatics in EV adsorption onto lipid membranes. They furthermore show that simplified theories developed for model lipid systems can be successfully applied to the study of their biological analogues and provide new fundamental insights into EV-membrane interactions with potential use in developing novel EV separation and immobilization strategies.