The rheologically-complex fluid beauty of nail lacquer formulations

Pinching dynamics, extensional rheology, and stringiness of saliva substitutes

Saliva substitutes are human-made formulations extensively used in medicine, food, and pharmaceutical research to emulate human saliva's biochemical, tribological, and rheological properties. Even though extensional flows involving saliva are commonly encountered in situations such as swallowing, coughing, sneezing, licking, drooling, gleeking, and blowing spit bubbles, rheological evaluations of saliva and its substitutes in most studies rely on measured values of shear viscosity. Natural saliva possesses stringiness or spinnbarkeit, governed by extensional rheology response, which cannot be evaluated or anticipated from the knowledge of shear rheology response. In this contribution, we comprehensively examine the rheology of twelve commercially available saliva substitutes using torsional rheometry for rate-dependent shear viscosity and dripping-onto-substrate (DoS) protocols for extensional rheology characterization. Even though most formulations are marketed as having suitable rheology, only three displayed measurable viscoelasticity and strain-hardening. Still, these too, failed to emulate the viscosity reduction with the shear rate observed for saliva or match perceived stringiness. Finally, we explore the challenges in creating saliva-like formulations for dysphagia patients and opportunities for using DoS rheometry for diagnostics and designing biomimetic fluids.

Rheology and dispensing of real and vegan mayo: the chickpea or egg problem

The rheology, stability, texture, and taste of mayonnaise, a dense oil-in-water (O/W) emulsion, are determined by interfacially active egg lipids and proteins. Often mayonnaise is presented as a challenging example of an egg-based food material that is hard to emulate using plant-based or vegan ingredients. In this contribution, we characterize the flow behavior of animal-based and plant-based mayo emulsions, seeking to decipher the signatures that make the real mayonnaise into such an appetizing complex fluid. We find that commercially available vegan mayos can emulate the apparent yield stress and shear thinning of yolk-based mayonnaise by the combined influence of plant-based proteins (like those extracted from chickpeas) and polysaccharide thickeners. However, we show that the dispensing and dipping behavior of egg-based and vegan mayos display striking differences in neck shape, sharpness, and length. The ratio of apparent extensional to shear yield stress value is found to be larger than the theoretically predicted square root of three for all mayo emulsions. The analysis of neck radius evolution of these extension thinning yield stress fluids reveals that even when the power law exponent governing the intermediate pinching dynamics is similar to the exponent obtained from the shear flow curve, the terminal pinching dynamics show strong local effects, possibly influenced by interstitial fluid properties, finite drop size and deformations, and capillarity.

Polymer-Surfactant Complexes Impact the Stratification and Nanotopography of Micellar Foam Films

Freestanding films of soft matter drain via stratification due to confinement-induced structuring and layering of supramolecular structures such as micelles. Neutral polymers, added as rheology modifiers to cosmetics, foods, pharmaceuticals, and petrochemical formulations, often interact with monomers and micelles of surfactants, forming polymer-surfactant complexes. Despite many studies that explore interfacial and bulk rheological properties, the corresponding influence of polymer-surfactant complexes on foam drainage and lifetime is not well understood and motivates this study. Here, we report the discovery and evidence of drainage via stratification in foam films formed with polymer-surfactant (PEO-SDS) complexes. We show that the stratification trifecta of coexisting thick-thin regions, stepwise thinning, and nanoscopic topological features such as nanoridges and mesas can be observed using IDIOM (interferometry, digital imaging, and optical microscopy) protocols we developed for nanoscopic thickness mapping. We determine that for polymer concentrations below overlap concentration and surfactant concentrations beyond the excess micelle point, polymer-surfactant complexation impact the nanoscopic topography but not the step size, implying the amplitude of disjoining pressure changes, but periodicity remains unchanged.

Transition to the viscoelastic regime in the thinning of polymer solutions

In this study, we investigate the transition between the Newtonian and the viscoelastic regimes during the pinch-off of droplets of dilute polymer solutions and discuss its link to the coil-stretch transition. The detachment of a drop from a nozzle is associated with the formation of a liquid neck that causes the divergence of the local stress in a vanishingly small region. If the liquid is a polymer solution, this increasing stress progressively unwinds the polymer chains, up to a point where the resulting increase in the viscosity slows down drastically the thinning. This threshold to a viscoelastic behavior corresponds to a macroscopic strain rate. In the present study, we characterize the variations of with respect to the polymer concentration and molar weight, to the solvent viscosity, and to the nozzle size, i.e., the weight of the drop. We provide empirical scaling laws for these variations. We also analyze the thinning dynamics at the transition and show that it follows a self-similar dynamics controlled by the time scale _c^-1. This characteristic time is different and always shorter than the relaxation time of the polymer.

Small-volume extensional rheology of concentrated protein and protein-excipient solutions

Limited studies measure extensional rheology in protein solutions due to volume constraints and measurement challenges. We developed a small-volume, dripping-onto-substrate (DoS) extensional rheology device to measure the capillary thinning of protein and protein-excipient solutions via DoS for the first time. Ovalbumin (OVA) was used as a model system, examined via DoS both with and without excipient poloxamer 188 (P188). Water and dilute OVA break apart rapidly and demonstrate inertiocapillary (IC) thinning behavior, where longer breakup times in OVA can be attributed to lower surface tension. Further increasing OVA content leads to longer breakup times and deviations from IC thinning at the start of thinning, however, no evidence of elastic behavior is observed. P188 more effectively lowers the droplet surface tension than OVA, transitioning from IC behavior in dilute solution to weakly elastic behavior at higher concentrations. Combined protein/excipient formulations act synergistically at low concentrations, where breakup times are identical to those of the individual components despite the higher total concentration. However concentrated protein/excipient formulations exhibit elasticity, where extensional rheology parameters depend on P188 content and total concentration. These findings imply that excipients intended to stabilize proteins in shear flow can cause undesirable behavior in extensional flows like injection.

Dynamics and extensional rheology of polymer-surfactant association complexes

Understanding and characterizing the influence of polymers and surfactants on rheology, application, and processing is critical for designing complex fluid formulations for enhanced oil recovery, pharmaceuticals, cosmetics, foods, inks, agricultural sprays, and coatings. It is well-established that the addition of anionic surfactant like sodium dodecyl sulfate (SDS) to an aqueous solution of an oppositely-charged or uncharged polymer like poly(ethylene oxide) (PEO) can result in the formation of the polymer-surfactant association complexes (P0S-ACs) and a non-monotonic concentration-dependent variation in zero shear viscosity. However, the extensional rheology response of polymer-surfactant mixtures remains relatively poorly understood, partially due to characterization challenges that arise for low viscosity, low elasticity fluids, even though the response to strong extensional flows impacts drop formation and many processing operations. In this article, we use the recently developed dripping-onto-substrate (DoS) rheometry protocols to characterize the pinching dynamics and extensional rheology response of aqueous P0S- solutions formulated with PEO (P0) and SDS (S-), respectively. We find the PEO-SDS mixtures display a significantly weaker concentration-dependent variation in the extensional relaxation time, filament lifespan, and extensional viscosity values than anticipated by the measured shear viscosity.