Session P31: Gels, Emulsions and Complex Fluids

Multi-scale relaxation dynamics of arrested patchy particle gels

Patchy particle interactions enable the design of so-called ‘equilibrium gels’, a system where arrest is achieved without an underlying phase separation, resulting in structurally equilibrated gels which do not undergo coarsening-induced aging. Here, we study the multi-scale relaxation dynamics of a model patchy particle gel consisting of nanoparticles linked with end-functionalized polymers by using stopped-flow spectro-photometry, x-ray photon correlation spectroscopy, and linear viscoelastic measurements. We show that, despite the fast dissociation dynamics of an individual polymer linker from the nanoparticle, the relaxation of the system becomes slow and arrested at larger length-scales due to multi-functional associations between the nanoparticles. We show that the nanoscale primary clusters undergo a combination of super-diffusive and diffusive relaxation modes, which is contrasted by the sub-diffusive and stretched-exponential relaxation of the network at the macroscale. We show that the super-diffusive dynamics of the primary clusters are highly intermittent, which supports the interpretation that avalanche dynamics is a general feature associated with the microscopic dynamics of arrested soft materials, irrespective of the material’s route to arrest.   

Nanosheets and Metallo-Hydrogel Formed by 2-nm Metal-Organic Cages based on Electrostatic Interaction

Macroionic solutions, including metal-organic cages, would form the single layered hollow sperical blackberry-type structure based on counterion-mediated attraction. Here, we observed a hydrogel formation process from 2-nm emissive, low molecular-weight metal-organic cages at low concentrations (>15 mg/mL) based on counterion-mediated attraction and π-π/hydrophobic interactions. With addition of small electrolytes, the cages in aqueous solution will follow the counterion-mediated attraction and self-assemble into 2D nanosheets, like normal macroionic solutions. However, the unique molecular structure and the charge distribution of the cages make the bending of nanosheets into spheres difficult, leading to stable, standing alone MOC nanosheets in solution and their very large excluded volumes lead to gelation at very low (~1.5 wt%) cage concentrations, with the helps from hydrophobic and partially π-π interactions similar to the gelation of graphene oxides.   

Emergence of Multiscale Dynamics in Colloidal Gels

To gain insight into the kinetics of colloidal gel evolution at low particle volume fractions φ, we utilize differential dynamic microscopy to investigate particle aggregation, geometric percolation, and the subsequent transition to nonergodic dynamics. We report the emergence of unexpectedly rich multiscale dynamics upon the onset of nonergodicity, which separates the wave vectors q into three different regimes. In the high-q domain, the gel exhibits φ-independent internal vibrations of fractal clusters. The intermediate-q domain is dominated by density fluctuations at the length scale of the clusters, as evidenced by q-independence of the relaxation time τ. In the low-q domain, the scaling of τ as q^-3 suggests that the network appears homogeneous. The transitions between these three regimes introduce two dynamical characteristic length scales, distinct from the cluster size.
  

Size-Dependent Tracer Diffusion in Colloidal Gels

Colloid diffusion in porous media is ubiquitous in industrial and biophysical systems, such as the extracellular matrix hydrogels of cell tissues and the nucleoid region inside prokaryotic cells, where specific pore morphologies enable selective filtering. Obtaining a relation between pore size and colloid diffusion in porous media will lead to the development of models that will elucidate how cells regulate biomolecule transport. In this study, we seek a fundamental understanding of how the diffusion of tracer colloids of various sizes are hindered by a porous medium. We present our results from simulations of tracer colloids undergoing Brownian diffusion in the voids of a colloidal gel. Detailed characterization of tracer diffusion reveals that all tracers experience sub-diffusive motion over short to intermediate length scales. Small tracers eventually recover the diffusive regime over longer length scales while large tracers remain in a local cavity. We additionally present algorithms to identify the characteristic length scale that marks the onset of the recovery to normal diffusion and the cutoff tracer size that determines whether a tracer is too large to break through the void network.   

Connecting the viscoelastic response of nanosheet gels to the elastic properties of the particles

Micron-sized nanosheets such as graphene, graphene oxide or clay platelets can be used to make conducting inks or as fillers in composites to enhance their mechanical properties. At high concentrations beyond rigidity percolation, nanosheet suspensions become yield stress fluids with a finite storage modulus. In this regime the elastic response of nano-sheet suspensions appears to be universal. The storage modulus plateau of few-layer graphene in NMP solvent, aqueous graphene oxide gels and clays exhibit a power law exponent close to 3 as a function of relative volume fraction.

We present a new analytical model that explains this behaviour and connects the bulk response to the elastic properties of single nanosheets and their size. This allows is to infer the magnitude of the bending stiffness of single nano-sheets from our rheological data.

This model opens up the possibility to infer elastic properties of different nanosheets from rheological data of the suspension rather than performing AFM experiments on single nano-sheets. Furthermore, this model may explain the mechanical enhancement of nanosheet composites where a similar exponent can be observed.   

Tunable viscoelasticity of double gel networks: colloidal gels embedded in a hydrogel matrix

We create double gel networks consisting of a colloidal gel embedded within a hydrogel matrix. Depending on the relative strength between the two networks, we can tune the rheology to be either dominated by the colloidal gel, the hydrogel, or to exhibit features characteristic of both systems. This allows us to adjust both the sample modulus and the yielding behavior. Looking beyond the macroscopic mechanical characteristics, we discuss the changes in the microscopic dynamics of the colloidal gel occurring as the hydrogel network forms around it. We use differential dynamic microscopy and confocal fluorescence microscopy to access the variations in the structure and network fluctuations of the colloidal gel.   

Tuning vaporization threshold of perfluorocarbon by interfacial melting in endoskeletal droplets

Perfluorocarbon (FC) droplets have been extensively used as phase-change contrast agents for biomedical ultrasound imaging and therapy. Several studies have aimed at understanding the vapor embryo nucleation and vaporization behavior of these droplets. However, these studies have not looked at tuning the thermodynamic limit of stability (spinodal) by using multiphase mixtures. We investigated the vaporization behavior of endoskeletal perfluoropentane (C₅F₁₂) droplets by incorporating solid FC and solid hydrocarbon (HC) skeletons. Multiple geometries were generated, including endoskeletal (solid-in-liquid) as well as exoskeletal (liquid-in-solid) droplets. Vaporization of these droplets was measured over a range of temperatures both optically using a microscope and acoustically with clinical ultrasound scanner. We show that C₅F₁₂ stability can be tuned by controlling the intermolecular interactions, as captured quantitatively by the exchange parameter. Using a simple statistical thermodynamics lattice model, we demonstrate that the presence of the FC strengthens the intermolecular attraction and the presence of HC breaks the intermolecular attraction between the liquid molecules, making it possible to finely tune the spinodal and consequently the vaporization temperature.   

Flow of Quasi-2D Emulsion Droplets Through Small Openings

We examine how various parameters affect the flow rate of quasi-2D soft particles moving through tight openings. To observe this, we create thin hopper-shaped chambers which have a small exit for the particles to flow through. We fill the chambers with monodisperse oil-in-water emulsions with a soap surfactant. We use microscopy to observe the droplets moving through the chamber. We find faster flow rates when there are more droplets in the chamber (providing a larger pressure head) and with larger exit sizes. We fit the flow rate to a modified Beverloo equation with an extra term for the height of the particles pushing the droplets through the opening.   

A granular material spreading as a liquid: growth dynamics of 3D aggregates of oil droplets

A continuous medium like a puddle has a characteristic critical height set by the capillary length, a balance between gravity and surface tension. In this study, monodisperse frictionless and lightly attractive oil droplets (radius approximately 10 microns) are produced one-by-one in an aqueous solution. Droplets are buoyant and accumulate underneath a glass slide which acts as the top of a liquid cell, forming 3D aggregates. The droplets initially accumulate vertically. As a critical height is reached, the aggregate collapses and spreads horizontally on the glass slide. Despite the aggregates being granular in nature, we find that the equilibrium shape of such aggregates follows the physics of continuous media. Indeed, the height of the aggregate reaches a critical value which is set by a balance between adhesion between the droplets and buoyancy. Both parameters can be tuned in the experiment to vary the geometry of the droplet aggregates. We developed a model that captures the shape of the aggregate with a simple parameter analogous to the capillary length.   

Spreading of a 2D granular analogue of a liquid puddle: Predicting structure through a “granular capillary length”

The structure of an accumulation of granular material, such as a pile of sand, is understood through the balance between gravity, which acts to limit the height, and inter-grain friction, which holds the particles together. In contrast, for the case of a continuous medium, the height of a puddle is dictated by the capillary length which balances gravity and surface tension. Here we develop an experimental model that allows us to probe the structure of a 2D pile of monodisperse microscopic oil droplets. The droplets are buoyant, adhesive, and friction is negligible. Oil droplets are deposited within a chamber and accumulate at a barrier. The droplets accumulate to a certain critical height at which point the pile collapses and spreads across the barrier. This process is reminiscent of the spreading of a liquid even though the pile is granular and 2D in nature. We define a parameter, analogous to the capillary length, which determines the height of the pile and is dictated by the balance between buoyant and adhesive forces. These parameters can be controlled experimentally in order to modify the height of the pile. We developed a simple model that can predict the shape of the pile based on the balance of adhesion and buoyancy.   

Characterizing the effects of Temperature and Ethanol Concentration on the louche of Absinthe.

Oil flavored alcohols are traditionally served by mixing them with cold water to form a louche: a microemulsion that turns the drink opaque because of the interactions of the alcohol with water, which it dissolves, and the oil, which is disperses. When there is sufficient water the oil phase precipitates, forming the louche. Thus, characterization of the louche depends on the concentrations of water, alcohol and oil. Absinthe has not been as well characterized as other oil flavored alcohols such as Limoncello or Ouzo with no studies on either a ternary phase diagram or on the louche phase. This work examines the emergence of the louche phase in absinthe by measuring the optical transmittance of the solution as a function of both: (1) the concentration of ethanol and (2) the temperature, which can be separately varied. The laser transmission was measured through a temperature-controlled sample of absinthe. The change in transmission was recorded as water was added to the sample, revealing an inverse relationship between temperature and ethanol concentration at which louche forms. There are also two different steps in the louche suggesting two different micro-emulsions form depending on the concentrations.
  

Surface active microgels: dual functioning soft stabilisers

Emulsions are stabilised by either surfactants or solid particles which sit at the interface between the phases. Particles irreversibly adsorb to an interface whereas surfactants detach under small perturbations. Due to their enhanced stability, particle-stabilised emulsions are used as a base for many products such as foods and pharmaceuticals. Furthermore, particles impart elasticity to interfaces, a quality which enhances the sensory performance of foods without the need for excessive amounts of fat [1]. Personal care products are another potential application as they require enhanced sensory properties and tuneable stability.

However, particle-stabilised interfaces can become unstable when dilated or compressed, processes which are common in processing. This instability, known as buckling, occurs when colloids jam together causing films to collapse.

We present work done on a commercial microgel system, composed of an amphiphilic polymer called Sepimax Zen (SZ). We show that SZ microgels stabilise an oil-water interface forming elastic films which are immune to buckling and can also be used to thicken suspensions.

[1] Husband, F. et al. Gums and Stabilisers for the Food Industry 17: The Changing Face of Food Manufacture: The Role of Hydrocolloids 17, 230 (2014).   

Clustering Colloids with Polymer Bridges

Colloidal model systems are typically highly uniform, yet structurally simple. One design principle inspired from organic chemistry is to take rudimentary atomic building blocks and combine them into more complex and functional molecules. An analogous colloidal reaction is the heteroaggregation of core and shell particles into clusters that have new shapes, which can foster assembly in a directional fashion. Some colloidal pairings are naturally complementary, such as oppositely charged particles, but it is difficult to combine distinct like-charged suspensions in a similar manner. Highly tailored surface chemistry can bind particles together, however, severely impact throughput along the way. Here, a general approach to heteroaggregation is described where particles are speciated as either polymer-coated or bare and made to bind specifically to their counterpart through polymer bridging. Flexibly selecting from a variety of materials for the cores and shells creates a veritable zoo of different cluster products with large-scale production potential. Harnessing this colloidal bond has led to advancements in particle shape, surface pattern, and regioselective binding, which are all necessary control parameters for furthering our capabilities in synthetic self-assembling systems.   

Universal elasticity in nearly floppy soft matter

In many soft matter systems, mechanics is governed by the balance between microscopic degrees of freedom and internal constraints on motion. Constraints on relative motion parallel to the bond between two particles, or between two nodes in a network, can be modeled with springs. However, other, non-central force constraints are often present as well — examples include constraints on sliding in frictional granular media and composites, constraints on bending in biopolymers and covalent glasses, and constraints on shape distortions in confluent tissues, foams, and emulsions. We consider the case where non-central force constraints are present, but carry a weaker energetic cost than their spring-like counterparts. We will show that the elasticity of these systems has universal features, independent of the origin of the non-central force constraint. These features can be explained in terms of the floppy modes that appear in the limit where non-central forces are "turned off." We derive scaling relations for the shear modulus and validate them numerically in three model systems — pre-tensioned spring networks, semi-flexible fibre networks, and composite packings of hard and soft particles.