Session W16: Flow and Structure in Multiphase Systems

Aging of colloidal gels in microgravity

It is difficult to investigate the gelation process for long periods of time due to sedimentation under gravity. Sedimentation causes the delicate gel structures to collapse under their own weight, and we wish to understand what we'd see if this collapse does not occur. In this project, we look at microscope images of colloidal gels taken over 60 hours at the International Space Station by NASA. The gels use the depletion force, and the samples studied range from very sticky particles to barely sticky particles. We use particle tracking to study the structure and dynamics of the gels as they coarsen. We observe that with stronger attractive forces, particles form thick gel strands over time, whereas with weaker attractive forces, particles do not aggregate even at the longest timescales observed.   

Comparison of 2D and 3D images of colloidal gels

The Advanced Colloidal Experiment (ACE-M-1) was an experiment that was conducted by NASA to study colloidal gel structures in a microgravity context. Using the Light Microscopy Module, 2D images of the gels were generated. To quantify such gels’ properties, we conduct similar ground-based experiments, where we use colloidal polymethyl methacrylate (PMMA) particles and polystyrene as a depletant. We employ fluorescence confocal microscopy to take both 2D and 3D images of our colloidal gels.   We then consider how the 2D and 3D images are related.  In particular, we examine how the number of neighbors to each particle seen in 2D compares with the true number of 3D neighbors.  We additionally look at other structural quantities such as the pair correlation function.

    

The influence of silica nanoparticles on the microstructure and rheology of the suspensions containing wax network

Suspensions with jammed networks are found in different products such as chocolate, peanut butter, margarine in foods, creams, ointments in pharmaceuticals and consumer goods. Organic molecules present in these suspensions act as structuring agents. Network formation during the congealing step mostly governs the textural and rheological properties of the end product. Different additives are used to refine these properties by altering crystal morphology. Nanoparticles with different surface characteristics are reported to impact rheology by changing crystallite shape, size and size distribution.

We observe the effect of silica nanoparticles (SNPs) on wax in oil suspension. SNPs are synthesized using the Stöber method and hydrophobized via attachment of long alkyl chains. The influence of SNPs on wax crystal number density, size, morphology and suspension rheology is studied. Variation of alkyl chain grafting density, concentration and size of SNPs is also investigated.

    

Quasi-2d Emulsion Flow Through Hopper

We study the quasi-2d hopper flow of oil-in-water emulsions as they exit an orifice. Prior work on hopper dynamics has focused on the flow rate, which is defined as the number of oil droplets exiting per unit time. This has shown a general power law dependence between flow rate, Q, the ratio of the opening width, w, to the average diameter of droplet size, d, and the fitting constant k as such: Q~(w/d-k)^α. Prior work has seen various values for the exponent α, corresponding to different experimental conditions. Recent work (cite) has suggested that the range of values for the exponent α can explained by the ratio of the viscous drag force of particles moving in their medium to the kinetic friction of two particles sliding past each other. In two dimensions, for the low kinetic friction limit, this exponent should be ½. We experimentally verify this claim by studying the flow rate of silicon oil-in-water emulsions as they pass through an orifice over a range of w/d values. We find that the flow rate collapses to the general curve with α=0.49 and k=1.47.   

Describing surface tension and wetting behavior of emulsion droplets using the deformable particle model

The behavior of fluids in contact with the boundaries of microfluidic devices is primarily determined by the surface tensions at the triple line, where the solid, liquid, and gas phases connect. In this work, we implement surface tension into the deformable particle model (DPM) and compare the equilibrium shape of the particle with experiments. We also carry out discrete element method (DEM) simulations of shape-changing particles flowing through narrow channels under the influence of gravity and compare the results for the DPM simulations to those from experiments on quasi-2D flows of oil droplets in water through constrictions. We measure the droplet shape and velocity as a function of time during the flows and compare the droplet trajectories as a function of the surface tension.   

Dilatational response of an asphaltene-model molecule stabilizing the oil-water interface near the onset point of precipitation

Defined by their solubility class, asphaltenes represent the most polar, aromatic, and the heaviest fraction of crude oil. They are known to strongly adsorb at oil-water interfaces forming viscoelastic films that confer solid-like mechanical properties that stabilize water-in-oil emulsions. It is suggested that asphaltenes form the most stable water-in-oil emulsions close to the onset point of precipitation, in which soluble and insoluble asphaltene nanoaggregates are present in solution. It is speculated that at this point asphaltenes have the greatest heteroatom content and are highly surface active. The formation of these emulsions leads to undesired flow assurance problems for the oil and gas industry that require demulsification to prevent operational challenges and costs. Given the heterogeneity in chemical composition, structure, and molecular weight of natural asphaltenes, it remains challenging to identify how their aggregation, precipitation, and diffusion behavior at oil-water interfaces promote stability. To address this challenge, we use small-angle X-ray scattering (SAXS) to investigate the structure and aggregation behavior of asphaltenes and asphaltene-model molecule violanthrone-79 (VO-79). In addition, we compare the dilatational rheology response of soluble asphaltenes at the oil-water interface using oscillating pendant drop measurements. The direct connection between structure and the dilatational response of oil-water interfaces stabilized by soluble asphaltenes is important for understanding their interfacial behavior and their role in driving the emulsification process. This work contributes to providing insights for designing energy-efficient demulsification strategies.

    

The emergence of soft-glassy dynamics in foams

Many seemingly simple materials such as foams and emulsions exhibit complex physical and rheological properties whose physical origins have largely defied understanding. Soft-sphere models have proven very useful in understanding various mechanical and rheological properties of soft-glassy systems like foams and emulsions. Previous experimental studies, however, have failed to capture various complex phenomena such as intermittent dynamics, power-law rheology, super-diffusive bubble motion and others in their entirety. We simulate a viscously damped soft-sphere bubble model, subjected to Ostwald ripening, over a wide range of viscosity. The results capture the various contrasting phenomena observed in previous experimental studies. Systems with higher effective viscosity/damping produce less intermittent motion with configuration structures farther away from jamming. Conversely, systems without little to no viscosity/damping display highly intermittent dynamics, heavy-tailed bubble displacement distribution functions and power-law rheology. Lastly, this model produces interesting damping correlated aging dynamics for perturbed systems, with time scales for recovery to steady state vanishing in the quasi-static limit (zero viscosity) indicating little to no self organization in these systems.   

Aggregation and Gelation in a Tunable Aqueous Colloid-Polymer Bridging System

We investigate the phase behavior of a colloid-polymer mixture with attractive bridging interactions in which the strength of the polymer adsorption can be tuned through polymer molecular weight, and normalized polymer concentration c/c^*. Bridging interactions were induced between trifluoromethyl methacrylate-co-tert-butyl methacrylate (TtMA) particles by introducing poly(acrylic acid) (PAA) to the system. The formation of hydrogen bonds between PAA and stabilizers on the surface of the particles decreases as pH is increased. We find that the particles form flocs and networks at low c/c^* (pH ≈ 3.9)  and the structure and extent of flocculation depends on the polymer size. Floc size is larger at higher molecular weight due to the larger polymer radius of gyration. By quantifying dynamics and structure, we show that the bridging suspensions transition from gel-like to fluid-like upon increasing c/c^*. These results provide insight into flocculation processes in applications such as wastewater treatment and creation of dense markers for indirect detection of diseases such as HIV and influenza.   

Topological Origins of Yielding in Colloidal Gels

Yielding of colloidal gels under applied deformation is accompanied by various microstructural changes in the particulate network including rearrangement, bond rupture, anisotropy, and reformation of secondary structures. While much work has been done to understand the physical underpinnings of yielding in colloidal gels, its topological origins remain poorly understood. The question of which bonds/particles are primarily prone to these break-up events and ultimately responsible for the yielding of a gel remains unanswered. Here, we seek to understand the bond characteristics that correlate with break-up events and provide a topological origin to the yielding mechanism in colloidal gels. Here, employing a series of large-scale dynamic simulations and network science tools, we characterize the bonds using their orientation and network centrality. We find that bonds with higher centralities in the network are ruptured the most at all applied deformation rates. This suggests that a network analysis of the particulate structure can be used to predict the failure points in colloidal gels a priori.   

Modeling the approach to statistical self-similarity for systems that coarsen by the diffusion of material between neighboring bubbles, droplets, or grains

Aqueous foams are commonly believed to coarsen by gas diffusion between neighboring bub- bles into a statistically self-similar scaling state, such that the shape of size distributions is time- independent. This allows prediction of average growth rates as well as size-topology relations. Integro-differential PDE models for phase separating systems, as first shown by Lifshitz and Sly- ozov, can be written down for the evolution of the size distribution but are generally intractable. Here we show that essential features of the approach to the scaling state can be captured by an exactly-solvable pair of coupled differential equations for the evolution of the average bubble size and of the critical bubble size, which instantaneously neither grows nor shrinks. To test our sim- plified model, we compare with data for two-dimensional dry foams created with different initial polydispersities. This allows us to readily identify the critical radius from the average area of six- sided bubbles, whose growth rate is zero by the von Neumann law. Preliminary results show good agreement. Our approach is applicable to 3d foams, as well to dilute phase-separating systems like very wet froths. We hope it will aid in analysis of data recently-collected aboard the International Space Station for the coarsening of 3d foams with different liquid content.   

Drainage of Vegan Foams

Milk foams are fragile objects, often stabilized in frothy cappuccinos by proteins such as caseins and whey or by derivatives such as sodium casienate. The life and death of these desirable foams are scripted to a large extent by the forces that drive the drainage and rupture of the thin foam films that separate individual gas pockets. In this study, the bulk foam drainage kinetics of two animal-based milks (cow and goat) was compared to that of the most commonly sold plant-based milks: almond, oat, soy, pea, coconut, and rice. Foam creation methods involving mechanical agitation (via electric frother) and sparging (via foam rise method/bubbling test) at different temperatures were employed and compared to obtain quantitative measures of foamability and foam stability of the various milks. An understanding of the temperature-dependent differences in bulk drainage behavior for animal and plant-based milks sheds light onto the macromolecular interactions and networks of proteins/lipids at liquid-air interfaces, and can ultimately lead to the improvement of vegan milks.   

Rheology and Texture of Animal and Plant-based Mayo Emulsions

Mayonnaise is an O/W emulsion used as a dressing, a dip, and a base for sauces. The conventional recipe of mayonnaise uses egg that influences the texture, taste, stability, and rheology of these animal protein-based concoctions. Often mayo is presented as a challenging example of food materials made with animal-based proteins that are hard to emulate using plant-based proteins. To understand the role of protein substitution, we characterize the shear and extensional rheology response of animal-based and plant-based mayo emulsions, seeking to decipher the signatures that make real mayonnaise into such an appetizing complex fluid.   

Collective bubble growth in water and water+oil systems: How to make bubbles grow indefinitely

When cold, air-saturated water is put into a glass, air bubbles will form, as observed by millions of people in the freshly poured glasses of water on their bedside tables.This is a well-known effect caused by the higher solubility of air at lower temperatures. When the glass is left alone, the air bubbles will subsequently slowly dissolve due to the slightly higher pressure caused by their surface tension, which creates a mildly elevated gas concentration inside the bubble with respect to the environment that drives the transport. In this work, we change the atmospheric boundary condition by adding a thin layer of immiscible oil on top of the water and show that under certain conditions we can make the air bubbles grow indefinitely. We investigate these conditions in a set of humidity-controlled experiments. Finally, we present a mass transfer model that fully accounts for the observed phenomena.   

Drainage via stratification in freestanding micellar films

Ultrathin foam films containing supramolecular structures like micelles in bulk, and adsorbed surfactant at the liquid-air interface, undergo drainage via stratification. At a fixed surfactant concentration, the stepwise decrease in average film thickness of a stratifying micellar film yields a characteristic step-size that also describes the quantized thickness difference between coexisting thick-thin flat regions. It is well-established that step-size is inversely proportional to the cubic root of SDS concentration, and cannot be estimated by adding micelle size to Debye length, as the latter is inversely proportional to the square root of SDS concentration. Recently we contrasted the step-size obtained from the analysis of nanoscopic thickness variations and transitions in stratifying SDS micellar foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols (that we developed) with the intermicellar distance obtained using small-angle X-ray scattering. We found that step-size equals to intermicellar distance obtained using scattering from bulk solutions, and stratification driven by the confinement-induced layering of micelles within the liquid-air interfaces of a foam film provides a sensitive probe of non-DLVO oscillatory forces and micellar interactions. In this contribution, we examine the concentration dependency of step-size and layer number for ionic surfactants, including SDS, SDBS, and NaN.   

Multi-scale characterization of hydrogels of biological interest

Hydrogels are three-dimensional elastic materials consisting of a porous matrix swollen with a large amout of water. They are characterized by a strong porosity and thus constitute water reservoirs which can absorb or release water when they are subjected to external stimuli. They must be able to withstand the stresses due to drying (water removal) and swelling (water imbibition). For these reasons, gels are used in many biomedical applications (controlled drug release systems).

We study here the way to form a homogeneous bead of colloidal hydrogel. We control the gelation kinetics of silica nanoparticles as well as the drying. This process leads to the formation of a gel with well-defined mechanical properties and permeability. A phase diagram evidences two regions in the parameter plane, i.e. the physicochemical properties versus the evaporation rate: a region where the drop undergoes an isotropic shrinkage and forms an homogeneous gel and a region where mechanical instabilities (cracking, buckling) appear. We proceed to a multiscale analysis from the microscopic scale by studying the structure of silica gels (X-ray scattering) to the macroscopic scale through mechanical properties measurements (indentation testing).

Finally, we have shown that the addition of a model protein (Bovine Serum Albumine) in the hydrogel bead, results in the reinforcement of the structure, involving a controlled deformability. Such a process has potential applications in drug delivery