The liquid-glass-jamming transition in disordered ionic nanoemulsions

Glassy and compressed nanoemulsions stabilized with sodium dodecyl sulfate in the presence of poly(ethylene glycol)-diacrylate

The rheology of concentrated nanoemulsions is critical for their formulation in various applications, such as pharmaceuticals, foods, cosmetics, and templating advanced materials. The rheological properties of nanoemulsions depend on interdroplet interactions, Laplace pressure, dispersed phase volume fraction, and continuous phase properties. The interdroplet forces can be tuned by background electrolytes (i.e., charge screening), surfactant type, the excess surfactant micelle concentration, and depletant molecules such as polymer chains. In the current research, we study the effect of varying the content of poly(ethylene glycol)-diacrylate (PEGDA) on the interfacial tension of the water-oil phase and rheological properties of concentrated nanoemulsions with 50% and 60% volume fractions. Sodium dodecyl sulfate (SDS) is used as the ionic surfactant. The final concentrated nanoemulsions are repulsive according to overall interaction potentials and are in the glass and compressed states based on the effective volume fraction estimation. They contain nearly same SDS concentration on the droplet surface and also in the bulk, but a different amount of PEGDA. The scaled rheological properties of the glassy nanoemulsions show a higher dependency on the PEGDA content and the possible effect of polymer-surfactant complexations compared to those of the compressed ones. This dependency is more pronounced in small strain amplitudes but not in large strains in the non-linear regime. These results provide insights into formulating concentrated nanoemulsions with controlled rheology for expanded application areas.

Jamming and depletion in extremely bidisperse mixtures of microscale emulsions and nanoemulsions

While much attention has been given to jamming of granular and colloidal particles having monomodal size distributions, jamming of systems having more complex size distributions remains an interesting direction. We create concentrated, disordered binary mixtures of size-fractionated nanoscale and microscale oil-in-water emulsions, which are stabilized by the same common ionic surfactant, and measure the optical transport properties, microscale droplet dynamics, and mechanical shear rheological properties of these mixtures over a wide range of relative and total droplet volume fractions. Simple effective medium theories do not explain all of our observations. Instead, we show that our measurements are consistent with more complex collective behavior in extremely bidisperse systems, involving an effective continuous phase that governs nanodroplet jamming, as well as depletion attractions between microscale droplets induced by nanoscale droplets.

Signatures of nanoemulsion jamming and unjamming in stimulated-echo NMR

The unjamming of elastic concentrated nanoemulsions into viscous dilute nanoemulsions, through dilution with the continuous phase, offers interesting opportunities for a pulsed-field gradient (PFG) NMR, particularly if the nanoemulsion is designed to take advantage of the nuclear specificity offered by NMR. Here, we make and study size-fractionated oil-in-water nanoemulsions using a perfluorinated copolymer silicone oil that is highly insoluble in the aqueous continuous phase. By studying these nanoemulsions using ^{19}F stimulated-echo PFG-NMR, we avoid any contribution from the aqueous continuous phase, which contains a nonfluorinated ionic surfactant. We find a dramatic change in the ^{19}F PFG-NMR decays at high field-gradient strengths as the droplet volume fraction, ϕ, is lowered through dilution. At high ϕ, observed decays as a function of field-gradient strength exhibit decay-to-plateau behavior indicating the jamming of nanodroplets, which contain ^{19}F probe molecules, in an elastic material reminiscent of a nanoporous solid. In contrast, at lower ϕ, only a simple decay is observed, indicating that the nanodroplets have unjammed and can diffuse over much larger distances. Through a comparison with bulk mechanical rheometry, we show that this dramatic change coincides with the loss of low-frequency shear elasticity of the nanoemulsion.

Self-motion and heterogeneous droplet dynamics in moderately attractive dense emulsions

We show that diffusing wave spectroscopy (DWS) is sensitive to the presence of a moderate short-range attraction between droplets in uniform fractionated colloidal emulsions near and below the jamming point associated with monodisperse hard spheres. This moderate interdroplet attraction, induced by micellar depletion, has an energy of about ∼2.4k_BT, only somewhat larger than thermal energy. Although changes in the mean free path of optical transport caused by this moderate depletion attraction are small, DWS clearly reveals an additional secondary decay-to-plateau in the intensity autocorrelation function at long times that is not present when droplet interactions are nearly hard. We hypothesize that this secondary decay-to-plateau does not reflect the average self-motion of individual droplets experiencing Brownian excitations, but instead results from heterogeneous dynamics involving a sub-population of droplets that still experience bound motion yet with significantly larger displacements than the average. By effectively removing the contribution of this secondary decay-to-plateau, which is linked to greater local heterogeneity in droplet structure caused by the moderate attraction, we obtain self-motion mean square displacements (MSDs) of droplets that reflect only the initial primary decay-to-plateau. Moreover, we show that droplet self-motion primary plateau MSDs can be interpreted using the generalized Stokes-Einstein relation of passive microrheology, yielding quantitative agreement with plateau elastic shear moduli measured mechanically.

Glass and Jamming Rheology in Soft Particles Made of PNIPAM and Polyacrylic Acid

The phase behaviour of soft colloids has attracted great attention due to the large variety of new phenomenologies emerging from their ability to pack at very high volume fractions. Here we report rheological measurements on interpenetrated polymer network microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylic acid (PAAc) at fixed PAAc content as a function of weight concentration. We found three different rheological regimes characteristic of three different states: a Newtonian shear-thinning fluid, an attractive glass characterized by a yield stress, and a jamming state. We discuss the possible molecular mechanisms driving the formation of these states.

Recent developments in emulsion characterization: Diffusing Wave Spectroscopy beyond average values

We report here an overview of current trends and a selection of recent results regarding the characterization of emulsions by Diffusing Wave Spectroscopy (DWS). We provide a synopsis of the state of the art of the DWS technique, and a critical discussion of experiments performed on samples in which Brownian and ballistic dynamics coexist. A novel analysis scheme is introduced for DWS experiments on creaming or sedimenting emulsions, allowing to extract not only average values for drop size and drop dynamics - as usual in DWS - but also properties related to the width of the distributions governing these quantities. This analysis scheme starts from a realistic Monte Carlo simulation of light diffusing in the volume of the sample and reaching the detector. This simulation is more accurate than the analytical expressions available for the idealized geometries normally used in DWS interpretation. By disentangling Brownian and ballistic motions we directly access the variance of velocity distribution, σ_v. In relatively unstable emulsions σ_v governs the frequency of drop-drop collisions and subsequent coalescence events. Furthermore, when gravity dominates dynamics, as in emulsions subject to sedimentation or creaming, σ_v is strongly related to the 2nd and 4th moments of drop size distribution. This novel analysis scheme is exemplified investigating freshly formed model emulsions. Results are validated by comparison with microscopy imaging. This analysis is then extended to emulsions with a much broader drop size distribution, resembling those that are planned to be investigated in microgravity by the Soft Matter Dynamics facility onboard the International Space Station (ISS). This review is concluded by sketching some promising directions, and suggesting useful complementarities between DWS and other techniques, for the characterization of transient regimes in emulsions, and of destabilization processes of great practical importance.

Packing and dynamics of a protein solution approaching the jammed state

The packing of proteins and their collective behavior in crowded media is crucial for the understanding of biological processes. Here we study the structural and dynamical evolution of solutions of the globular protein bovine serum albumin with increasing concentration via drying using small angle X-ray scattering and dynamic light scattering. We probe an evolving correlation peak on the scattering profile, corresponding to the inter-protein distance, ξ, which decreases following a power law of the protein volume fraction, φ. The rate of decrease in ξ becomes faster above a protein concentration of ∼200 mg ml-1 (φ = 0.15). The power law exponent changes from 0.33, which is typical of colloidal or protein solutions, to 0.41. During the entire drying process, we observe the development and the growth of two-step relaxation dynamics with increasing φ as revealed by dynamic light scattering. We find three different regimes of the dependence of ξ as a function of φ. In the dilute regime (φ < 0.22), protein molecules are far apart from each other compared to their size. In this case, the dynamics mainly corresponds to Brownian motion. At an intermediate concentration (0.22 < φ < 0.47), inter-protein distances become comparable to the size of protein molecules, leading to a preferential orientation of the ellipsoidal protein molecules along with a possible deformation. In this regime, the dynamics shows two distinct relaxation times. At a very high concentration (φ > 0.47), the system reaches a jammed state. Subsequently, the secondary relaxation time in this state becomes extremely slow. In this state, the protein molecules have approximately one hydration layer. This study contributes to the understanding of protein molecular packing in crowded environments and the phenomenon of density-driven jamming for soft matter systems.

Diffusing wave microrheology of highly scattering concentrated monodisperse emulsions

Motivated by improvements in diffusing wave spectroscopy (DWS) for nonergodic, highly optically scattering soft matter and by cursory treatment of collective scattering effects in prior DWS microrheology experiments, we investigate the low-frequency plateau elastic shear moduli [Formula: see text] of concentrated, monodisperse, disordered oil-in-water emulsions as droplets jam. In such experiments, the droplets play dual roles both as optical probes and as the jammed objects that impart shear elasticity. Here, we demonstrate that collective scattering significantly affects DWS mean-square displacements (MSDs) in dense colloidal emulsions. By measuring and analyzing the scattering mean free path as a function of droplet volume fraction φ, we obtain a φ-dependent average structure factor. We use this to correct DWS MSDs by up to a factor of 4 and then calculate [Formula: see text] predicted by the generalized Stokes-Einstein relation. We show that DWS-microrheological [Formula: see text] agrees well with mechanically measured [Formula: see text] over about three orders of magnitude when droplets are jammed but only weakly deformed. Moreover, both of these measurements are consistent with predictions of an entropic-electrostatic-interfacial (EEI) model, based on quasi-equilibrium free-energy minimization of disordered, screened-charge-stabilized, deformable droplets, which accurately describes prior mechanical measurements of [Formula: see text] made on similar disordered monodisperse emulsions over a wide range of droplet radii and φ. This very good quantitative agreement between DWS microrheology, mechanical rheometry, and the EEI model provides a comprehensive and self-consistent view of weakly jammed emulsions. Extensions of this approach may improve DWS microrheology on other systems of dense, jammed colloids that are highly scattering.

Crossover between Athermal Jamming and the Thermal Glass Transition of Suspensions

The non-Newtonian flow behavior of thermal and athermal disordered systems of dispersed uniform particles at high densities have strikingly similar features. By investigating the flow curves of yield-stress fluids and colloidal glasses having different volume fractions, particle sizes, and interactions, we show that both thermal and athermal systems exhibit power-law scaling with respect to the glass and jamming point, respectively, with the same exponents. All yield-stress flow curves can be scaled onto a single universal curve using the Laplace pressure as the stress scale for athermal systems and the osmotic pressure for the thermal systems. Strikingly, the details of interparticle interactions do not matter for the rescaling, showing that they are akin to usual phase transitions of the same universality class. The rescaling allows us to predict the flow properties of these systems from the volume fraction and known material properties.