Session T30: Drops: Complex Fluids

Producing Shape-Engineered Alginate Particles Using Viscoplastic Fluids

Non-spherical hydrogel particles are of fundamental interest and can find use in a variety of applications ranging from pharmaceuticals to biomedical to food. Here, we report a new method that leverages the yield stress property of viscoplastic fluids to synthesize shape-engineered alginate particles. By dripping an aqueous viscoplastic solution composed of sodium alginate and a yield-stress material into an ionic gelation bath, droplets are controllably deformed and crosslinked, producing a wide assortment of shapes. We find that by tuning the yield stress of the solution and the nozzle tip orientation, a range of shapes from symmetric and near-spherical, to asymmetric and anisotropic (e.g., egg-, rice grain-, arc-, ring-, snail shell-, tear-, and tadpole-like) can be produced. We explain our observations using scaling analysis of the forces exerted on the droplet at different stages of particle production. We show that the main factors that determine the degree of droplet deformation during bath entry and the final appearance of the alginate particles are the initial shape of the droplets, the timescales of the viscoplastic fluid relaxation versus the crosslinking reaction, and the physico-chemical properties of the yield-stress material.   

Retraction of a highly viscous shear-thinning liquid sheet

The retraction of liquid sheets arises in many applications, such as atomization and coating. For example, atomization processes involve a cascade from fluid volumes to sheets to filaments to droplets and require an understanding of the retraction dynamics. While the retraction dynamics of Newtonian sheets have been studied extensively, that of non-Newtonian sheets, characterized by complex rheological behavior, remains largely unexplored. Here, we analyze the retraction of a highly viscous shear-thinning liquid sheet. Using the Carreau constitutive model and 1D slender-sheet equations, we derive asymptotic solutions for the evolution of the tip velocity and maximum film thickness for three different regimes of the Carreau number, representing the relative importance of shear thinning to viscous-capillary effects. Our analysis also provides insight into the retraction of shear-thickening sheets. We show that while shear-thinning rheology expedites the retraction dynamics of highly viscous sheets, the shear thickening slows it down. We supplement our analytical results with numerical simulations and find excellent agreement.

    

Contraction velocity of viscoelastic filaments

Liquid filaments/ligaments---elongated drops---are omnipresent in diverse applications, including ink-jet printing, crop spraying, and atomization coating. In these and other applications, the working fluids often contain polymer additives which render the fluids non-Newtonian. Motivated by the fact that in experiments and applications the filament is often highly stretched axially at the initial instant when it is just beginning to contract, the effect of a nonzero initial stress condition is examined on the retraction of axisymmetric viscoelastic (VE) filaments described by the Oldroyd-B model. Among others, a feature of great practical and fundamental importance in all filament retraction processes is the filament's contraction velocity. Here, we compare the contraction velocity of VE filaments to that of Newtonian ones and show that initially pre-stressed VE filaments experience unexpectedly large accelerations during recoil. Simulation results are probed to furnish a physically-based explanation as to how the initial pre-stressing can result in such large accelerations and hence increased contraction velocities.   

Axisymmetric numerical simulations of Rayleigh-Ohnesorge Jetting Rheometer (ROJER) with FENE-P rheology

Spray formation is an important aspect of many industrial and biological processes like inkjet printing, the dispersal of fertilizers and pesticides, and human subject sneezing. Sprays are the result of an atomization process where the liquids are destabilized and undergo breakup into several fragments, i.e. ligaments and liquid threads, and eventually droplets, where complex topological transitions of the interface which feature “pinch-off singularities” emerge. Here, we perform axisymmetric simulations of an impulsively-started viscoelastic jet exiting a nozzle and entering a stagnant gas phase using the open-source code Basilisk. This code allows for efficient computations through an adaptively-refined volume-of-fluid technique to capture the interface. We use the FENE-P constitutive equation to describe the viscoelasticity of the fluid and employ the log-conformation transformation, which provides a stable solution for the conformation tensor. The entire jetting and breakup process is simulated, including the flow through the nozzle, which results in an inhomogeneous initial radial stress distribution that affects the subsequent breakup dynamics. The evolution of the velocity field and the elastic stresses in the nozzle are validated against analytical solutions and the early-stage dynamics of the jet is compared favourably to linear instability predictions. We elucidate how the thinning rate of the viscoelastic filament is determined by both the jet speed and the polymeric extensibility, highlighting their effect on the total stress balance of the non-Newtonian ligament. We also probe how the secondary droplet formation can be controlled by the finite extensibility of the polymeric chains, as well as the wavenumber of the forced oscillation of the injected liquid at the nozzle inlet.

    

Modeling necking dynamics of viscoelastic microgels

Microgels can deform and interpenetrate and display colloid/polymer duality. Our previous studies show that the effective interaction of microgels in the collapsed state is governed by the interplay of polymer–solvent interfacial tension and bulk elasticity. A connecting neck is observed to mediate microgel interaction, but its temporal evolution has not been addressed. In this study, we systematically studied the necking dynamics using mesoscale hydrodynamic simulations and theoretical modeling. The results reveal a crossover in the coalescence dynamics reflecting the viscoelastic signature of microgels. In contrast to the common knowledge that viscoelastic materials respond elastically on short time scales, the early expansion of the microgel neck exhibits a linear behavior, similar to the viscous coalescence of liquid droplets. However, the late regime with arrested dynamics resembles the sintering of solid particles. Through an analytical model relating microgel dynamics to neck growth, we show that the long-term behavior is governed by stress relaxation of the polymers in the neck region and predict an exponential decay in the rate of growth, which agrees favorably with the simulation. Different from coalescence, the thread thinning in microgel breakup primarily highlights its polymeric characteristics.   

Elastocapillary Worthington jets

The impact of droplets of viscoelastic liquids on non-wetting substrates is relevant for several deposition processes such as pesticide treatment and inkjet printing. The retraction of an impacting droplet is associated with the formation of a Worthington jet, which progressively stretches as the liquid retracts. We show that the viscoelasticity of the liquid results in a hitherto unknown elastocapillary regime in the stretching Worthington jet. We identify the impact conditions for observing elastocapillary Worthington jets, and show that these jets exhibit a linear (in time) variation of the strain rate, which can be explained by a simple theoretical model. Upon further extension, the jet exhibits beads-on-a-string structures, characteristic of elastocapillary thinning of slender viscoelastic filaments. The elastocapillary Worthington jet is not only relevant for a droplet impact on a solid substrate, but can also be expected in other configurations where a Worthington jet is observed, such as drop impact on a liquid pool and bubble bursting at an interface. 

    

The effect of polymers on the retraction of impacting droplets.

Viscoelastic properties can drastically change the dynamics of droplets impacting a hydrophobic surface. While long polymers in aqueous drops have no effect on drop spreading, the retraction speed of dilute polymer solutions can be up to two orders of magnitude slower. Since the discovery of this phenomenon more than two decades ago, the mechanism governing this slower retraction speed has been up for debate. Latest insights point towards interactions between the polymer molecules and the substrate, but a prediction for the retraction velocity based on the fluid and substrate properties is still lacking.

 

We revisited the viscoelastic drop retraction problem and found two different modes of retraction for dilute polymer solutions. Additionally, evaporation of the droplet before impact was found to be of great influence on the retraction dynamics.

    

Air entrainment dynamics under bouncing Boger droplets

Recent studies have revealed that air entrainment plays a crucial role in preventing contact between an impacting droplet and a smooth surface. In this study, we explore the air entrainment dynamics for different types of Boger droplets with similar zero-shear viscosities but varying elastic moduli. The submicron air layer is visualized during impact on an oil film (thickness ~ 3 μm) at We ≈ 9 with a high-speed total internal reflection microscopy (TIRM) technique. We observe that droplets with increasing elastic modulus enhance the thickness gradient of the air film. However, the droplet's top surface collapse is surprisingly uninhibited by an increase in the elastic modulus.

    

Ultrasound single droplet generation and deposition of a nanocellulose bioink

Drop-on-demand devices and bioinks are critical components of state-of-the-art platforms for bioprinting applications. Commercial nanocellulose bioinks have recently attracted much attention due to their biocompatibility, excellent mixing and rheological properties. In this research, we construct two compact acoustic droplet ejectors using 3D printed Fresnel acoustic lenses. Liquid ejectors with ultrasound frequencies at 2.3 MHz and 4.0 MHz are studied. For droplet generation, we use aqueous suspensions of cellulose nanofibers (CNF), which present remarkably stable shear-thinning and elastic properties. We vary the solid content concentration of our liquid CNF suspension to identify a regime of single droplet ejection. We find that CNF suspensions at concentrations between 0.5% w/v – 0.6% w/v and pH of 8.5 are ideal for single droplet generation. Our acoustic ejectors produce 1.0 mm (4.0 MHz) and 1.5 mm (2.3 MHz) droplet diameters with high reproducibility of greater than 95% accuracy.

    

The Ouzo Effect: adding a splash of dynamics to the oil/water/ethanol phase diagram

Ouzo is a colorless aniseed-flavor liquor which turns cloudy when diluted with water. This process, known as spontaneous emulsification, occurs as the added water dilutes the ethanol, leading to precipitation of oil-rich microdroplets. Despite much work on the dynamics of this process, the full equilibrium phase diagram for water-ethanol-anise oil has not previously been determined experimentally. We present data determining the two-phase region and the spinodal curve separating unstable from metastable behavior. We also measure the composition of coexisting phases, indicated on the phase diagram by tie lines, identify the plait point (critical point) and determine densities and surface tensions of coexisting phases. We then use this equilibrium data to better understand spontaneous emulsification and other complex dissolution dynamics.

    

On the Morphology and Viscosity of Water-in-Crude Oil Emulsions Generated by Impingement of Breaking Waves on Oil Slicks

This study examines the time evolution and effect of photo-oxidation on the microscopic morphology and bulk viscosity of salt water-in-crude oil emulsions generated by wave impingement on oil slicks. Slicks of Alaskan North Slope (ANS) and Hibernia crude oils with an initial thickness of 2 mm are emulsified by 40 s periodic plunging breaking waves over a 12-day period. Rheology is used to determine the viscosity, and fluorescence microscopy, followed by a machine-learning-based analysis, to measure the droplet size and spatial distributions of emulsion samples collected daily. Results show that ANS emulsifies water faster than Hibernia oil, resulting in a greater concentration of water droplets and a more interconnected droplet structure. Consequently, the (non-Newtonian) viscosity of ANS-based emulsion is at least three times higher than that of Hibernia. Additionally, photo-oxidation results in faster emulsification, accelerated formation of droplet clusters, larger characteristic droplet size, and higher water volume fraction. Hence, photo-oxidation increases the viscosity of Hibernia oil-based emulsion. These trends are consistent with our previously determined Ohnesorge number-based empirical relationships between the emulsion morphology and its viscosity.