Self-assembly of ellipsoidal particles at fluid-fluid interfaces with an empirical pair potential

In-situ characterization of microgel monolayers: Controlling isostructural phase transitions for homogeneous crystal drying patterns

The self-assembly of microgels at fluid interfaces and transfer to solid substrates has proven valuable in fields like photonics, plasmonics, and nanofabrication. However, this process is constrained by the isostructural phase transition (IPT) that occurs under sufficiently high compression, disrupting the monolayer order. Understanding the mechanisms driving IPT is crucial to extend their applicability to a wider range of interparticle distances. We tackle this problem by studying the monolayer conformation via in-situ microscopy at the interface. We monitored the microgel monolayer throughout the different stages of the deposition onto a solid substrate. We found that neither the compression at the interface nor the capillary forces arising from the receding meniscus during the deposition triggered the IPT. In fact, the still wet deposited monolayers do not exhibit IPT regardless of the compression of the monolayer. Instead, the IPT occurs during the drying of the wet deposited monolayers, particularly when the capillary force overcomes the adhesion force. Additionally, we found a new mechanism to modulate the interparticle distance by light-induced Marangoni forces. Instead, IPT arises from capillary forces generated during the drying of the water film after the monolayer is transferred. We propose a theoretical model to estimate the adhesion force between the microgels and the substrate based on the compression curve of the monolayer. Furthermore, we suggest a novel method combining a Langmuir-Schaefer deposition with supercritical drying to fully prevent the IPT, resulting also in a new tool to study an otherwise inaccessible regime with highly compressed monolayers. Our findings advance the understanding of soft colloidal self-assembly at fluid interfaces and expand their applications, enabling the creation of larger substrates with highly ordered self-assembled microgel monolayers with tunable interparticle distance.

Self-assembly of defined core-shell ellipsoidal particles at liquid interfaces

HYPOTHESIS

Ellipsoidal particles confined at liquid interfaces exhibit complex self-assembly due to quadrupolar capillary interactions, favouring either tip-to-tip or side-to-side configurations. However, predicting and controlling which structure forms remains challenging. We hypothesize that introducing a polymer-based soft shell around the particles will modulate these capillary interactions, providing a means to tune the preferred self-assembly configuration based on particle geometry and shell properties.

EXPERIMENTS

We fabricate core-shell ellipsoidal particles with defined aspect ratios and shell thickness through thermo-mechanical stretching. Using interfacial self-assembly experiments, we systematically explore how aspect ratio and shell thickness affect the self-assembly configurations. Monte Carlo simulations and theoretical calculations complement the experiments by mapping the phase diagram of thermodynamically preferred structures as a function of core-shell properties.

FINDINGS

Pure ellipsoidal particles without a shell consistently form side-to-side "chain-like" assemblies, regardless of aspect ratio. In contrast, core-shell ellipsoidal particles exhibit a transition from tip-to-tip "flower-like" arrangements to side-to-side structures as aspect ratio increases. The critical aspect ratio for this transition shifts with increasing shell thickness. Our results highlight how we can engineer the self-assembly of anisotropic particles at liquid interfaces by tuning their physicochemical properties such as aspect ratio and shell thickness, allowing the deterministic realization of distinct structural configurations.

From Frustration to Order: Role of Fluid-Fluid Interfaces in Precision Assembly of Nanoparticles

Fluid-fluid interfaces are an attractive platform for self-assembling nanoparticles into low-dimensional materials. In this Perspective, we review recent developments in the use of interfaces to direct the assembly of spherical and anisotropic nanoparticles into diverse and sophisticated architectures. We illustrate how nanoparticle clusters, strings, networks, superlattices, chiral lattices, and quasicrystals can be self-assembled by harnessing the frustration between interfacial and interparticle forces. We highlight the role of polymeric ligands attached to the surface of nanoparticles in modulating assembly behavior by directly altering particle-fluid and particle-particle interactions or by deforming at interfaces and junctions between particles. We conclude by providing a roadmap of key questions and opportunities in this exciting field of interfacial assembly.

Prolate spheroidal polystyrene nanoparticles: matrix assisted synthesis, interface properties, and scattering analysis

Shape-anisotropic colloids are increasingly attracting attention for the fabrication of nano- and mesostructured materials. Polymer-based prolate spheroids are typically accessible through a two-step fabrication procedure comprising the synthesis of monodisperse particles of initially spherical shape and their stretching into elongated, ellipsoidal-like objects. The particle stretching is conducted within a matrix polymer, most commonly polyvinylalcohol, which allows heating beyond the glass transition temperature of the polymer particles, e.g. polystyrene. Here, we investigate various aspects of the synthesis and their consequences for the resulting colloids. Loading the stretching matrix with a high amount of polymer particles results in small particle clusters, which are separated during the mechanical stretching step. At the same time, the matrix polymer physisorbs at the particle surface which can be removed via a rigorous work-up procedure. Overall, this process allows for a precise adjustment of the aspect ratio of the prolate spheroids with a small size distribution and retained electrostatic stabilization. We analyse these particles with a range of microscopic and scattering techniques, including depolarized dynamic light scattering that gives access to the rotational diffusion coefficients.

Effect of Particle Size on the Orientation and Order of Assemblies of Functionalized Microscale Cubes Formed at the Water/Air Interface

The impact of the particle size and wettability on the orientation and order of assemblies obtained by self-organization of functionalized microscale polystyrene cubes at the water/air interface is reported. An increase in the hydrophobicity of 10- and 5-μm-sized self-assembled monolayer-functionalized polystyrene cubes, as assessed by independent water contact angle measurements, led to a change of the preferred orientation of the assembled cubes at the water/air interface from face-up to edge-up and further to vertex-up, irrespective of microcube size. This tendency is consistent with our previous studies with 30-μm-sized cubes. However, the transitions among these orientations and the capillary force-induced structures, which change from flat plate to tilted linear and further to close-packed hexagonal arrangements, were observed to shift to larger contact angles for smaller cube size. Likewise, the order of the formed aggregates decreased significantly with decreasing cube size, which is tentatively attributed to the small ratio of inertial force to capillary force for smaller cubes in disordered aggregates, which results in more difficulties to reorient in the stirring process. Experiments with small fractions of larger cubes added to the water/air interface increased the order of smaller homo-aggregates to values similar to neat 30 μm cube assemblies. Hence, collisions of larger cubes or aggregates are shown to play a decisive role in breaking metastable structures to approach a global energy minimum assembly.

Hybrid Nanoparticles at Fluid-Fluid Interfaces: Insight from Theory and Simulation

Hybrid nanoparticles that combine special properties of their different parts have numerous applications in electronics, optics, catalysis, medicine, and many others. Of the currently produced particles, Janus particles and ligand-tethered (hairy) particles are of particular interest both from a practical and purely cognitive point of view. Understanding their behavior at fluid interfaces is important to many fields because particle-laden interfaces are ubiquitous in nature and industry. We provide a review of the literature, focusing on theoretical studies of hybrid particles at fluid-fluid interfaces. Our goal is to give a link between simple phenomenological models and advanced molecular simulations. We analyze the adsorption of individual Janus particles and hairy particles at the interfaces. Then, their interfacial assembly is also discussed. The simple equations for the attachment energy of various Janus particles are presented. We discuss how such parameters as the particle size, the particle shape, the relative sizes of different patches, and the amphiphilicity affect particle adsorption. This is essential for taking advantage of the particle capacity to stabilize interfaces. Representative examples of molecular simulations were presented. We show that the simple models surprisingly well reproduce experimental and simulation data. In the case of hairy particles, we concentrate on the effects of reconfiguration of the polymer brushes at the interface. This review is expected to provide a general perspective on the subject and may be helpful to many researchers and technologists working with particle-laden layers.

Anisotropic Microgels Show Their Soft Side

Anisotropic, submicrometer-sized particles are versatile systems providing interesting features in creating ordering in two-dimensional systems. Combining hard ellipsoids with a soft shell further enhances the opportunities to trigger and control order and alignment. In this work, we report rich 2D phase behavior and show how softness affects the ordering of anisotropic particles at fluid oil-water interfaces. Three different core-shell systems were synthesized such that they have the same elliptical hematite-silica core but differ with respect to thickness and stiffness of the soft microgel shell. Compression isotherms, the shape of individual core-shell microgels, and their 2D order at a decane-water interface are investigated by means of the Langmuir-Blodgett technique combined with ex-situ atomic force microscopy (AFM) imaging as well as dissipative particle dynamics (DPD) simulations. We show how the softness, size, and anisotropy of the microgel shell affect the side-to-side vs tip-to-tip ordering of anisotropic hybrid microgels as well as the alignment with respect to the direction of compression in the Langmuir trough. A large, soft microgel shell leads to an ordered structure with tip-to-tip alignment directed perpendicular to the direction of compression. In contrast, a thin and harder microgel shell leads to side-to-side ordering orientated parallel to the compression direction. In addition, the thin and harder microgel shell induces clustering of the microgels in the dilute state, indicating the presence of strong capillary interactions. Our findings highlight the relevance of softness for the complex ordering of anisotropic hybrid microgels at interfaces.

Computer simulations of self-assembly of anisotropic colloids

Computer simulations have played a significant role in understanding the physics of colloidal self-assembly, interpreting experimental observations, and predicting novel mesoscopic and crystalline structures. Recent advances in computer simulations of colloidal self-assembly driven by anisotropic or orientation-dependent inter-particle interactions are highlighted in this review. These interactions are broadly classified into two classes: entropic and enthalpic interactions. They mainly arise due to shape anisotropy, surface heterogeneity, compositional heterogeneity, external field, interfaces, and confinements. Key challenges and opportunities in the field are discussed.

Interfacial Assembly of Anisotropic Core-Shell and Hollow Microgels

Microgels, cross-linked polymers with submicrometer size, are ideal soft model systems. While spherical microgels have been studied extensively, anisotropic microgels have hardly been investigated. In this study, we compare the interfacial deformation and assembly of anisotropic core-shell and hollow microgels. The core-shell microgel consists of an elliptical core of hematite covered with a thin silica layer and a thin shell made of poly(N-isopropylacrylamide). The hollow microgels were obtained after a two-step etching procedure of the inorganic core. The behavior of these microgels at the oil-water interface was investigated in a Langmuir-Blodgett trough combined with ex situ atomic force microscopy. First, the influence of the architecture of anisotropic microgels on their spreading at the interface was investigated experimentally and by dissipative particle dynamic simulations. Hereby, the importance of the local shell thickness on the lateral and longitudinal interfacial deformation was highlighted as well as the differences between the core-shell and hollow architectures. The shape of the compression isotherms as well as the dimensions, ordering, and orientation of the microgels at the different compressions were analyzed. Due to their anisotropic shape and stiffness, both anisotropic microgels were found to exhibit significant capillary interactions with a preferential side-to-side assembly leading to stable microgel clusters at low interfacial coverage. Such capillary interactions were found to decrease in the case of the more deformable hollow anisotropic microgels. Consequently, anisotropic hollow microgels were found to distribute more evenly at high surface pressure compared to stiffer core-shell microgels. Our findings emphasize the complex interplay between the colloid design, anisotropy, and softness on the interfacial assembly and the opportunities it therefore offers to create more complex ordered interfaces.

Phase Inversion of Ellipsoid-Stabilized Emulsions

The efficacy of anisotropic particles in Pickering emulsion stabilization, attributed to shape-induced capillary interactions, is well-documented in the literature. In this contribution, we show that the surface of hematite ellipsoids can be modified in situ by the addition of oleic acid to effect transitional phase inversion of Pickering emulsions. Interestingly, incorporation of oleic acid results in the formation of nonspherical emulsion drops. The phase inversion of oil-in-water to water-in-oil and the transition in shape of emulsion drops from spherical to nonspherical is observed in two different particle systems, namely, nanoellipsoids and microellipsoids. The surface of spherical emulsion drops stabilized by particles or particles along with high concentration of oleic acid is found to consist of ellipsoids arranged in a close-packed configuration with their major axis parallel to the interface. In contrast, at intermediate oleic acid concentration, the surface of nonspherical emulsion drops is observed to be covered with loosely packed particle monolayer, with the ellipsoids at the oil/water interface taking up many different orientations. Using contact angle goniometry, the change in the wettability of hematite particles due to adsorption of oleic acid is established to be the mechanism responsible for the phase inversion of Pickering emulsions.

Shear-induced transitions in colloidal and polymeric liquids

This review reflects different aspects of wide current studies of the phenomena related to the shear-induced structure transformation in various complex liquids. Experimental data, being the basis of this discussion, were obtained for polymeric liquids (melts, blends, solutions) and different dispersions (colloidal solutions, suspensions, emulsions). The general initial input of shearing is the creation of inhomogeneities which can continue to remain as separate domains, become the nuclei of new phases, or become diffuse, leading to phase separation. The following effects are discussed: diffusion-induced phase separation, phase transitions occurring mainly due to the deformation-driven orientation of polymer chains and worm-like micelles that results in the formation of a liquid-crystal state, as well as self-assembly effects. It can be stressed that the appearance of regular structures takes place in systems that can coexist in different concentrations or phase states at the same stress or shear rate. This is linked with the existence of two-value points on flow curves (part of a flow curve with negative slope) or transient regimes of deformation that lead to instability of the flow. The described experimental facts are briefly discussed on the basis of the application of different constitutive molecular or phenomenological rheological models.

Detachment work of prolate spheroidal particles from fluid droplets: role of viscous dissipation

The force-displacement curve for removal of an elongated solid particle from the surface of liquid droplets or gas bubbles is calculated and compared to our previous reported results for spherical particles. The surface adsorption energy for prolate particles is known to be larger than that for spheres. We show that in fact the minimum possible work done upon removal of an elongated particle from surface can be less than that for a sphere. This result is obtained when the dissipation of interfacial energy, stored in the fluid film, attaching the particles to the surface during their displacement, is properly accounted for. This dissipation is unavoidable, even if the particles are removed infinitely slowly. Once the particle actually leaves the surface, the formed liquid bridge relaxes thus dissipating any stored interfacial energy as the surface returns to its original undistorted state. The difference between the work of removal of a particle from surface and its adsorption energy is seen to become increasingly larger with smaller particle to droplet size ratios. For example, for a size ratio of 1 : 100, the work of removal is 1.93 times greater than the adsorption energy. However, we also find that for any given size ratio, there is a value of particle aspect ratio for which the work of removal of particles (combined dissipated and adsorbed energy) attains its minimum value.

Molecularly Designed Interfacial Viscoelasticity by Dendronized Polymers: From Flexible Macromolecules to Colloidal Objects

The thermodynamic and rheological properties of densely packed dendronized polymers (DPs) at water-air interfaces were studied here for first- and fourth-generation DPs (PG1, PG4) with both small (P_n ≈ 50) and large (P_n ≈ 500) backbone degrees of polymerization. The excellent control over the structural characteristics of these polymers enabled us to investigate how the interfacial properties change as we go from thin, flexible macromolecules toward thicker molecular objects that display colloidal features. The effects of the dendron generation, affecting the persistence length, as well as the degree of polymerization and surface pressure on the formation of DP layers at the water-air interface were studied. Surface pressure measurements and interfacial rheology suggest the existence of significant attractive interactions between the molecules of the higher generation DPs. While all DPs featured reproducible Π-A diagrams, successive compression-expansion cycles and surface pressure relaxation experiments revealed differences in the stability of the formed films, which are consistent with the variations in shape persistence and interactions between the studied DPs. Atomic force microscopy after Langmuir-Blodgett transfer of the films displayed a nanostructuring that can be attributed to the increase in attractive forces with increasing polymer generation and anisotropy. The importance of such structures on the surface properties was probed by interfacial shear rheology, which validated the existence of strong albeit brittle structures for fourth-generation DPs. Ultimately, we demonstrate how in particular rod-like DPs can be used as robust foam stabilizers.

Interfacial aggregation of Janus rods in binary polymer blends and their effect on phase separation

Janus particles interfacially self-assemble into different structures when incorporated into multiphase systems. Dissipative particle dynamics simulations are employed herein to investigate the interplay between aggregation mechanisms and phase separation in polymer blends. Shorter rods with a standing configuration become increasingly "caged" or trapped in larger aggregates as weight fraction increases, which is reflected in the way that their diffusion is coupled to their aggregation rates. Janus rods of higher aspect ratios that are tilted at the interface aggregate side-by-side and are able to hinder phase separation kinetics. This is due to a combination of individual Janus rod conformations at the interface, their intrinsic aggregation mechanisms, aggregate fractal dimension, and aggregation rates, and can also be traced back to the scaling of the diffusion coefficient of aggregates with their size. Findings presented provide insight into the mechanisms governing two dimensionally growing colloidal aggregates at fluid interfaces, more specifically, those associated with Janus particles, and shed light on the potential of these systems in paving the way for designing new functional materials.

Interfacial Rheology of Charged Anisotropic Cellulose Nanocrystals at the Air-Water Interface

Cellulose nanocrystals (CNCs) have received attention as a biological alternative for the stabilization of fluid interfaces, yielding biocompatible and sustainable emulsions, foams, and aerogels. The interfacial behavior of nanoparticles with shape anisotropy and surface charge like CNCs is still poorly understood, although it ultimately dictates the mechanical properties and stability of the macroscopic colloidal material. Here, we report on the linear and nonlinear interfacial dilatational and shear rheology of CNCs at the air-water interface. We observed the formation of viscoelastic CNC layers at comparably low surface coverage, which was attributed to the shape anisotropy of CNCs. Further, the interfacial elasticity of CNC layers can be modulated by salt-induced charge screening, thereby shifting the interplay of repulsive and attractive CNC interactions. CNC layers had a viscous character without salt, followed by increasing viscoelasticity upon salt addition. CNC layers display strain hardening during compression and show a yield stress followed by flow under shear. The observed interfacial behavior is discussed in the context of CNC-stabilized foam and emulsion properties. We conclude that understanding the CNC interfacial behavior may help improve the performance of CNC-stabilized colloidal materials.

Dynamic heterogeneity in complex interfaces of soft interface-dominated materials

Complex interfaces stabilized by proteins, polymers or nanoparticles, have a much richer dynamics than those stabilized by simple surfactants. By subjecting fluid-fluid interfaces to step extension-compression deformations, we show that in general these complex interfaces have dynamic heterogeneity in their relaxation response that is well described by a Kohlrausch-Williams-Watts function, with stretch exponent β between 0.4-0.6 for extension, and 0.6-1.0 for compression. The difference in β between expansion and compression points to an asymmetry in the dynamics. Using atomic force microscopy and simulations we prove that the dynamic heterogeneity is intimately related to interfacial structural heterogeneity and show that the dominant mode for stretched exponential relaxation is momentum transfer between bulk and interface, a mechanism which has so far largely been ignored in experimental surface rheology. We describe how its rate constant can be determined using molecular dynamics simulations. These interfaces clearly behave like disordered viscoelastic solids and need to be described substantially different from the 2d homogeneous viscoelastic fluids typically formed by simple surfactants.