Session G31: Colloids

Shape and Interaction Decoupling for Colloidal Pre-Assembly

Creating materials with a structural hierarchy that is independently controllable at a range of scales requires breaking naturally occurring hierarchies. Breaking natural hierarchies is possible if building block attributes can be decoupled from the structure of pre-assembled, mesoscale building blocks that form the next level in the structural hierarchy. Here, we show that pre-assembled colloidal structures achieving geometric and interaction decoupling can be prepared in emulsions of silica superballs, which are cubic-like particles with rounded edges. We show that, for clusters of up to nine particles, colloidal superballs pack consistently like spheres, despite the presence of shape anisotropy and facets in the cubic-like particles. We compare our results with clusters prepared with magnetic superballs and find good qualitative agreement, suggesting that the cluster geometries are solely determined by the shape of the constituent particles. Our findings demonstrate that highly shape-anisotropic building blocks, under suitable conditions, can be pre-assembled into structures that are not found in bulk, thereby achieving a decoupling that can be further exploited for hierarchical materials development.   

Topological states of hard rods in extreme annular confinement

Hard particles are a standard model for colloidal systems and can be effectively studied within classical density functional theory (DFT). Fundamental mixed measure theory (FMMT) allows to predict the phase behavior of a hard-body fluid solely from the shape of individual particles.

Recent experimental advances allow for the synthesis of colloids with a nearly hard interaction that can be analyzed on the single-particle level. Slices of a system of such silica rods confined in a three-dimensional chamber under gravity can be considered a quasi-two-dimensional fluid that exhibits typical liquid-crystal behavior in confinement.

Applying FMMT to hard discorectangles in two dimensions, we map out a full phase diagram. Then we focus on a smectic fluid in extreme complex confinement, where the optimal bulk layer spacing competes with the extrinsic geometric and topological constraints. As a result, we characterize a variety of topologically different states in an annular geometry, also observed in particle-resolved experiments with silica rods. By further comparing the free energy of the different states, naturally provided by our DFT, we map out a topological phase diagram, indicating the stable topology depending on the details of the annular confinement.   

Do cavities matter? Suppression of crystallization in hollow microgel solutions

Microgels are crosslinked polymeric networks in the colloidal domain that exhibit phase behavior comparable to soft spheres. Solutions of microgels have been widely used as model systems to study self-assembly of condensed matter and complex fluids. Here we study the phase behavior of hollow microgels, characterized by a solvent-filled cavity in their center. Surprisingly, the phase behavior of these microgels, which lack a dense polymeric core, does not show crystal formation. The absence of crystals is independent of both the softness of the network and the size of the cavity. By means of small-angle neutron and X-ray scattering, both the form factors and the interparticle separations of hollow microgels are measured in overcrowded environments, revealing a complex interplay between interpenetration and deswelling. We confirm these findings by Monte Carlo simulations of microgel solutions modeled by the Flory-Rehner single-particle free energy and the Hertz pair potential.   

Buckling of 2D complex plasma crystals

Complex plasmas (or dusty plasmas) consist of microsized dust particles and a weakly ionized gas and as such they are often regarded as the "plasma state of soft matter". Apart from their particularly strong coupling, such systems exhibit also other surprising properties, such as a non-Hamiltonian behavior, due to the emergent plasma wakes. In this talk I will present some very recent theoretical results about the ways in which the existence of plasma wakes affects the structure of complex plasma crystals under a harmonic confinement. The main focus thereof will be on the buckling transition from a 2D monolayer to bilayer or triple-layer complex plasma crystals, taking place as the confinement frequency is decreased. Such structural transitions provide further insight to the underexplored properties of systems with effective non-reciprocal interactions occuring often in non-equilibrium soft matter.   

Tuning Interactions between Charged Structured Colloids and Hydrophobic Salts for the Production of Pickering Emulsifiers

Pickering emulsifiers have been demonstrated as advantageous alternatives to traditional small molecule surfactants, owing to their high interfacial adsorption energy and mechanical functionality. However, translating these advantages on an industrial scale requires continuous fabrication processes. Flash nanoprecipitation (FNP) has been demonstrated as a scalable process for the production of nanocolloids with a rich library of morphologies including homogeneous, core-shell, and Janus structures, the last of which presents an opportunity for generating amphiphilic Pickering emulsifiers. In this work, the FNP of homopolymers with ionomer analogs is presented as a route for the tunable expression of charge groups on distinct hemispheres of Janus particles. Utilizing this enhanced surface charge, the interaction between colloids and hydrophobic salts is studied, showing non-monotonic trends in size stability. Lastly, the surface activity of various colloid-salt combinations is demonstrated via the formation of highly stable Pickering emulsions.   

Bayesian inference of particle size distributions from dynamic light scattering

Autocorrelation functions from dynamic light scattering experiments have previously been analyzed either by performing least-squares fits to determine the mean and variance of the particle size distribution, or by using constrained regularization techniques to infer the size distribution. We present open-source tools for performing Bayesian inference of particle size distributions while rigorously incorporating smoothness and non-negativity constraints on the inferred distributions. We successfully apply these tools to simulated autocorrelation functions at multiple scattering angles. We intend to release these tools for use by the soft condensed matter and biophysics communities in the near future.   

Confinement of colloidal smectic phases

We report on the confinement of colloidal liquid crystals in three dimensional chambers. We exploit silica rods’ relatively large density difference with respect to the dispersing solvent to study isotropic, nematic and smectic phases confined into a single chamber. Combining laser scanning confocal microscopy and soft-lithography techniques enables us to characterize the configurations down to the single particle level. We focus on the smectic phase observed in direct vicinity of the bottom wall. We first consider chambers with square footprint finding occurrences of the smectic bridge state, characterized by two parallel disclination lines, in good agreement with simulation of 2-dimensional hard rods. We then examine the case of topologically different annular chambers. By progressively varying the confinement from a disk to a thin annulus we induce a transition from the smectic bidge state, formely found in square, to the smectic ring state with distinctive edge dislocations. The results are compared to very recent density functional theory calculations.   

Molecular dynamics simulations resolve the encapsulation of testosterone propionate by both solid and liquid lipid nanoparticles

Solid lipid nanoparticles (SLNs) have a crystalline lipid core which is stabilised in solution by interfacial surfactants. They are considered favourable candidates for future drug delivery vehicles as they are capable of storing and release bioactive molecules. Following our successful study on the self-assembly of the SLN and our characterisation of the resulting structure, we have expanded our work to investigate the formation of triolein-based liquid lipid nanoparticles (LLNs) stabilised by the Brij O10 surfactant. LLNs are, like SLNs, of interest for their potential applications in drug delivery and so a thorough understanding of their structure is vital in the development and optimisation of drug delivery formulations. In this work we have characterised the structure and formation of LLNs by means of small angle neutron scattering and atomistic molecular dynamics simulations. We have also characterised and compared the processes involved in the encapsulation and localisation of a steroidal drug, testosterone propionate, for both the SLN and the LLN. By comparing the two nanostructures and the localisation of the drug within them we have shed light on the relationship between a nanoparticle’s internal structure and its role in drug delivery.   

Dynamics and contact microstructure of rough colloids

Colloidal particles with hard-sphere interactions have been used to study crystallization and glassy dynamics. Here we synthesize smooth and rough poly(methyl methacrylate) colloids and disperse them in an index-matched solvent, squalene, at volume fractions (Φ) ranging from dilute (Φ=0.1) to random close packing (RCP). We use confocal microscopy to obtain 3D image-stacks of the colloidal suspensions and process them to obtain particle centroids with subpixel precision. We estimate the contact numbers for smooth and rough colloids at RCP to be 6 and 4 respectively, as predicted by simulations of frictional granules. In both the systems, scaling of the form <z>~Φ^a was observed, where parameter 'a' varies with surface roughness. We hypothesize that the power law variations are related to the phase behaviors and associated dynamics (mean-squared displacement) of these particles at various Φ. We believe our approach to connect the contact microstructure to dynamics in suspensions will enable better understanding of the effect of surface anisotropy on colloidal crystallization, rheological phenomena, and osmotic pressure of suspensions.   

Liquid crystal-mediated growth and assembly of polymer colloids

Templating the director field of liquid crystals (LCs) has been shown to organize colloids into a variety of ordered assemblies, including chain-like structures and hexagonal lattices. Most systems produce LC-mediated assemblies by pre-mixing colloids in host mesogens or by depositing colloids at LC interfaces. In our work, we demonstrate simultaneous growth and assembly of polymer colloids at LC-air interfaces. The polymer colloids are produced by polymerization of acrylate monomer mixed in non-reactive liquid crystal mesogens. Colloids spontaneously form ordered assemblies at the LC-air interface. Systematic experiments varying the reaction time were conducted to monitor the nucleation, growth, and assembly. Simultaneous colloid growth and assembly in LCs can open new opportunities to utilize LC-templated polymerization for the preparation of large-scale assemblies or continuous network structures.   

Holographic Perfusion Porosimetry of Individual Colloidal Particles

The in-line hologram of a micrometer-scale colloidal sphere can be analyzed with the Lorenz-Mie theory of light scattering to obtain precise measurements of the sphere's diameter and refractive index. The same technique also can be used to characterize porous and irregularly shaped colloidal particles provided that the extracted parameters are interpreted with effective-medium theory to represent the properties of an equivalent effective sphere. We demonstrate through experiments on mesoporous silica spheres, protein aggregates and nanoparticle agglomerates that the effective-sphere model consistently accounts for the influence of the medium on the particle's measured effective refractive index.This dependence yields information on the particles' structure and composition that cannot be obtained in other ways, including their porosity, the polydispersity of their porosity, and the size distribution and connectivity of their pores.   

Controlling the flexible polymer - hard colloid duality of temperature-responsive microgels at interfaces by the adsorption pathway

We study the effect of the adsorption pathway on the morphology and temperature-induced collapse of PNIPAM microgels at the solid/liquid interface by means of atomic force microscopy (AFM). Two samples with extreme differences in softness are probed: i) regularly 5 mol% BIS, and ii) ultra-low cross-linked microgels (ULC). ULC microgels are strongly deformed when being deposited either by spin-coating or by Langmuir-Blodgett technique from an oil-water interface. After rehydration, the ULC microgels cannot collapse as entire objects instead small globules are formed. Such a strong deformation can be avoided by in-situ adsorption to the substrate. Then, the ULC microgels exhibit half-ellipsoidal shapes with a smooth surface similar to more cross-linked microgels. Due to the extreme softness of ULC microgels, they can be selectively trapped either in a more particle-like or in a more polymer-like behavior. Coatings with strongly different topographies and properties can be prepared by one and the same ultra-low cross-linked microgel.
  

Localization to delocalization transitions in size asymmetric mixtures of colloidal particles with grafted chains

We study binary systems of polymer-grafted colloidal particles, in which the diameter of one species is substantially larger than the other. It has recently been shown [1] that, in these systems, the larger colloids order to form a lattice while the smaller ones may form a sublattice or delocalize within the interstitial space, depending on system parameters. We develop a simplified molecular dynamics model to study these colloidal systems and the nature of the delocalization of the smaller species under varied conditions. Using this model, we explore phase transitions between different lattice and sublattice types as a function of temperature.
[1] M Girard et al., Science 364, 1174–1178 (2019)   

Two Coupled Mechanisms Produce Fickian, yet non-Gaussian Diffusion in Heterogeneous Media

In several biological and soft matter systems, Fickian yet non-Gaussian diffusion has been observed, where the mean square displacement remains linear in nature, but the displacement distribution is non-Gaussian. The underlying reasons behind this strange behavior still remain speculative. Here, we perform a set of controlled experiments that quantitatively explore the effect of spatial heterogeneities on the appearance of non-Gaussianity in Fickian diffusion. Specifically, we study the diffusion of fluorescent colloidal particles in a matrix of lithographically fabricated micropillar arrays having a range of structural configurations: from completely ordered to completely random. Structural randomness and density are found to be the two most decisive factors in making diffusion non-Gaussian. We show that non-Gaussianity emerges as a direct consequence of two coupled physical mechanisms that produce a superstatistical behavior of the ensemble in a structurally heterogeneous environment. The two mechanisms identified here are relevant for many systems of crowded heterogeneous environments where non-Gaussian diffusion is frequently observed, for example in biological systems, polymers, gels and porous materials.