Session M26: Soft Deformable and Compressible Particles: From Athermal to Thermal II

Crystallization and glass formation of soft compressible colloids

This talk will discuss when, how and why compressibility affects suspension behavior. We will use microgels as model soft and compressible particles at high particle number densities, and address both crystallization and glass formation. We will discuss the role of ions and their associated osmotic pressure, as well as the role of single-particle stiffness and deformability. Static light and small angle neutron scattering will be used to access structural aspects, while dynamical light scattering and rheology will enable quantifying suspension dynamics. Our results indicate that compressibility can fundamentally change the role of particle density and polydispersity in suspension behavior.   

Direct measurements of the pNipam Microgel Counter-ion Cloud via Small-Angle Neutron Scattering

Microgels are formed by cross-linked polymer networks and they are a good model system for soft colloids. However, unlike hard colloids, their behavior is not well understood at high concentrations. pNipam microgels carry charged groups and counter-ions at the periphery due to the initiator used during synthesis, which we have found to play a crucial role for the observed spontaneous deswelling behavior at high concentrations; microgels deswell below random close packing when the percolation of counter-ion clouds induces a suspension osmotic pressure that is greater than the particle bulk modulus[1]. Here, we present the first direct measurements of the counter-ion cloud via small-angle neutron scattering (SANS). Note that the form factor can be decomposed as P(q) = F_p²(q) + 2 F_p(q)F_c(q) + F_c²(q), where F_p(q) and F_c(q) are the scattering amplitudes of the pNIPAM polymer and the counter-ion cloud, respectively. We prepare one suspension with Na⁺ ions and the other with NH₄⁺ ions via dialysis to use the difference in the scattering-length densities of these ions to obtain an augmented scattering signal with structural information about the ion cloud and charged groups on a microgel. Using this method, we show that the counter-ion cloud is indeed located at the particle periphery. This result also corroborates our theory for microgel deswelling at high concentrations. Importantly, our theory relates the particle softness and surface charges with the phase behavior[2]. The findings are crucial for developing a better understanding of soft-deformable colloids at high concentrations and for formulating a model for the phase behavior of microgels that includes observed spontaneous deswelling.

[1]U.Gasser, A. Scotti, and A. Fernandez-Nieves, Physical Review E99, 042602 (2019)

[2]] A. Scotti, U. Gasser, E.S. Herman, J. Han, A. Menzel, L.A. Lyon, and A. Fernandez-Nieves, Phys. Rev. E 96, 032609 (2017)   

Lyotropic Dodecagonal Quasicrystals in Ternary Amphiphile/Oil/Water Microemulsions

Aqueous lyotropic liquid crystals (LLCs) arise from the water-driven self-assembly of ionic surfactants at moderate concentrations (typically, ≥ 20 wt% in H₂O). While privileged classes of amphiphiles are now known to self-assemble in water into topologically close-packed Frank-Kasper A15 and σ phases, one recent report demonstrated that the addition of a hydrocarbon oil to these aqueous LLCs also triggers the formation of previously unknown, aperiodically ordered, lyotropic dodecagonal quasicrystals (LDDQCs) of oil-swollen micelles. However, a predictive framework for reliably assessing the propensity for specific amphiphile structures to form soft LDDQCs is currently unknown, since only one such system has been disclosed to date. This talk will present studies of a new ternary surfactant/oil/water microemulsion system that forms exceptionally well-ordered LDDQCs, with attention to their thermodynamic (meta)stability and insights into new amphiphile molecular design principles that drive the formation of these complex oil-swollen micelle packings.   

Depletion Potentials in Colloid-Polymer Mixtures: Influence of Solvent Quality

The classic Asakura-Oosawa (AO) model first explained how effective attractive forces between colloidal particles induced by depletion of nonadsorbing polymers can drive demixing into colloid-rich and polymer-rich phases, with relevance for water purification, stability of foods and pharmaceuticals, and macromolecular crowding in biological cells. In previous work [1, 2], we extended the AO model to incorporate aspherical polymer conformations and showed that fluctuating shapes of random-walk coils depend sensitively on solvent quality and can significantly modify depletion potentials. Here we use Monte Carlo simulation to analyze how solvent quality affects depletion potentials in mixtures of hard-sphere colloids and nonadsorbing polymers, modeled as ellipsoids whose principal radii fluctuate according to statistics of self-avoiding (good solvent) or non-self-avoiding (theta solvent) random walks. We find that depletion of polymers of equal molecular weight induces much stronger attraction between colloids in good solvents than in theta solvents.

[1] W. J. Davis and A. R. Denton, J. Chem. Phys. 149, 124901 (2018).

[2] W. K. Lim and A. R. Denton, Soft Matter 12, 2247 (2016); J. Chem. Phys. 144, 024904 (2016).   

Computational Modeling of Microgel-Nanoparticle Mixtures

Microgels are soft, compressible, colloidal particles composed of crosslinked polymer networks that become swollen in a good solvent. The ability to change their size and internal structure in response to changes in external stimuli (e.g., temperature, pH, concentration) allows microgels to absorb and release nanoparticles or macromolecules, suiting them well to applications as “smart” particles, such as biosensors and drug delivery vehicles. Applying Monte Carlo and molecular dynamics simulation methods to coarse-grained models of mixtures of spherical microgels [1] and hard-sphere nanoparticles, we analyze (1) how nanoparticles affect the swelling/deswelling of microgels and (2) how properties of single microgels (e.g., crosslink density) and of bulk suspensions (e.g., concentration, solvent quality) affect the ability of nanoparticles to penetrate microgels. We present simulation results for the density distribution of nanoparticles inside and outside of microgels and for pair correlation functions in these highly asymmetric soft matter mixtures.

[1] M. Urich and A. R. Denton, Soft Matter 12, 9086 (2016).   

Formation of Complex Spherical Packing Phases in Hard Spheres with SALR Interactions

Complex spherical packing phases, namely the Frank-Kasper (FK) phases, have been discovered in various soft matter systems such as block copolymers and surfactant solutions. A generic and simple model for the formation of spherical packing phases in these systems comprises hard spheres with short-range attraction and long-range repulsion (SALR). In the SALR systems, the attractive head promotes the colloids to form clusters, while the repulsive tail prevents the clusters from growing infinitely. The resultant finite-sized clusters could pack onto a crystal lattice forming a cluster crystal. It is anticipated that the ability of the clusters to change their volume and shape could enable the formation of stable complex spherical packing phases. In the current work, the formation of the FK σ and A15 phases in a system of hard spheres with SALR interactions is studied using density functional theory. A set of phase diagrams with different SALR potentials are constructed showing that the stabilities of σ and A15 phases are highly sensitive to the potential. The key factor stabilizing the FK phases is also discussed. Our results provide a first step in understanding the universality of the existence of the complex spherical packing phases in a broader range of soft matter systems.   

Understanding and accounting for different yielding rates in soft materials

Many soft materials show the transition from solid-like behavior to liquid-like behavior, but how this yielding transition occurs can vary significantly. Understanding the physics behind yielding is of great interest for the behavior of biological, environmental, and industrial materials. Some materials yield smoothly and gradually while others yield abruptly. We refer to this abrupt yielding as being “brittle”. The key rheological signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a sharp increase in loss modulus during oscillatory tests. We account for brittility within our recently proposed continuum model for yield stress materials (Phys. Rev. Lett. 126, (2021)). The original formulation describes the plastic viscosity as being dependent on the total strain rate; plastic flow is aided by the rate at which elastic deformation is acquired. We account for brittility by scaling the contribution of the recoverable component, which impacts the rate at which yielding occurs, by an amount related to the softness of the constituents relative to the bulk. The model predictions are successfully compared to results of different rheological protocols from several model yield stress fluids. Our study shows that the brittility, and therefore the softness of the constituents, plays a critical role in the transient nonlinear rheology of soft materials.   

Changes to the Stiffness and Compressibility of Soft Phytoglycogen Nanoparticles Through Acid Hydrolysis

Phytoglycogen (PG) is a glucose-based polymer that is naturally produced by sweet corn in the form of compact nanoparticles with an underlying dendritic architecture. Their deformability and porous structure combined with their non-toxicity and digestibility make them ideal for applications in personal care, nutrition and biomedicine. PG nanoparticles can be modified using chemical procedures such as acid hydrolysis, which reduces both the size and density of the particles. We used atomic force microscopy (AFM) force spectroscopy to collect high resolution maps of the Young’s modulus E of acid hydrolyzed PG nanoparticles in water, and we compare these results to those obtained on native PG nanoparticles.¹ Acid hydrolysis produced distinctive changes to the particle morphology and significant decreases in E. These measurements highlight the tunability of the physical properties of PG nanoparticles using simple chemical modifications. 

¹ B. Baylis et al., Biomacromolecules 2021, 22, 2985.   

Effect of Acid Hydrolysis on the Softness and Fragility of Phytoglycogen Nanoparticle Glasses

Phytoglycogen (PG) is a natural polysaccharide produced in the form of compact, 44 nm diameter nanoparticles in the kernels of sweet corn. Its highly branched, dendritic structure and soft, compressible nature leads to interesting and useful properties that make the particles ideal as unique additives in personal care, nutrition, and biomedical formulations. These applications are particularly dependent on the softness of PG, which can be controlled through chemical modifications. We consider the effect of acid hydrolysis on the softness of PG by characterizing the fragility of acid hydrolyzed PG glasses: as acid hydrolyzed PG particles are dispersed in water at packing densities approaching their soft colloidal glass transition, the dependence of the zero-shear viscosity on effective volume fraction abruptly changes from behaviour well-described by the Vogel-Fulcher-Tammann equation to more Arrhenius-like behaviour. This result is consistent with stronger glass behaviour for acid hydrolyzed PG relative to that for native PG, suggesting that acid hydrolysis of PG makes the particles softer.   

Method to simultaneously probe the bulk modulus and structure of soft compressible objects using small-angle neutron scattering with contrast variation

The bulk modulus of an object quantifies its resistance to an isotropic compression. For soft deformable colloids the bulk modulus must be known to predict their response to crowding.  Here, we will present a new approach to obtain partially-deuterated,  high molecular weight,  polyethylene glycol (dPEG), which is used to exert osmotic stress on soft objects.  In this study, microgels were used as a model system for soft compressible spheres and their bulk modulus is determined by means of small-angle neutron scattering with contrast matching.  By partial deuteration the scattering length density of the dPEG was matched in pure heavy water.  Consequently, no contribution of the osmotic stress polymer is measured during the scattering experiments, and the form factor of the microgels was directly measured.  Furthermore, in addition to the total radius, the variation of the different parts of the microgels can be also measured as a function of the external osmotic stress.  Therefore, using this method the different elasticity along a single particle - highly important for soft colloids such as a proteins or viruses - can be determined directly.   

Phase Behavior of Mixtures of Hard and Soft Penetrable Particles

The effective interactions between a pair of amphiphilic dendrimers is found to be approximately expressed within a generic model called as Generalized Exponential Model (GEM-n) U(r) = ε exp[-(r/σ)]n with n≈3. These potentials represent bounded and purely repulsive interactions. The absence of divergence at origin in the interaction potentials between these macromolecules, allows them to penetrate each other, and their centers of mass can overlap. At high enough densities, these systems form cluster crystals.

We have studied mixtures of soft and hard particles at different densities and with different fraction of hard particles in the system. Both hard and soft particles are well dispersed at low densities for all compositions. When the mixture has a high fraction of hard or soft particles, it macrophase separates into a hard-rich and soft-rich region. The system undergoes microphase separation for intermediate fraction of hard particles, in which there are alternating soft-rich, and hard-rich domains. We find that this self-assembled state is disordered and bicontinuous. Within the hard-rich domains, the hard particles are ordered into FCC crystal structure. The soft particles form clusters in both micro and macro phase separated domains.