Session M40: DFD IV

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 chickpea) 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 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, influenced by interstitial fluid properties, finite drop size, and capillarity.   

Boiling Bifurcations and Hysteresis in Solid-state Nanopores

Boiling is a complex nonequilibrium dissipative process, which undergoes thermodynamic bifurcations and hysteresis. Although pool boiling has been widely studied, much less is known about thermodynamics at nanoscale, where imaging techniques suffer due to the diffraction limit. To fill this knowledge gap, we utilize the nanopore Joule heating system that enables the generation of nanobubbles and simultaneous diagnosis of their nanosecond resolution dynamics using resistive pulse sensing. When a bias voltage is applied across a silicon nitride nanopore immersed in aqueous salt solutions, Joule heat is generated owing to the flow of ionic current. With increasing voltage, the superheat within nanopore increases, leading to the first bifurcation point with the onset of periodic nucleate boiling. Upon further increasing the voltage, transition boiling commences with torus-shaped vapor films intermittently blanketing the pore surface. Finally, a stable vapor film is formed beyond a threshold voltage, marking the second bifurcation point. These boiling structures are summarized in the form of an 'M'-shaped boiling curve, experimentally obtained from the Joule heat variation with applied voltage. We also studied the hysteresis of bifurcation points for increasing and decreasing bias voltages at different voltage rise rates and pore sizes. These results offer insights into the nonlinear characteristics of nanoscale boiling, paving the way for further research using stochastic thermodynamics.   

Influence of interfacial properties on yield and morphology of biomass microbeads produced by a scalable emulsion procedure

Plastic microbeads are widely used as exfoliants and rheological modifiers to improve viscosity, bulking, and film formation in personal care consumer products (PCCPs). While almost all of the beads are removed by sedimentation processes during wastewater treatment, billions of these microplastics enter the environment daily in the US alone. To produce biomass microbeads in a scalable process with comparable sizes and stiffnesses – around 1 GPa and several hundred microns – solutions of Kraft lignin and cellulose in ionic liquid are emulsified in oil, and precipitated by dripping in an anti-solvent, before being filtered, washed, and extracted to remove residual ionic liquid. While this method accesses the desired size range, the chaotic mechanism of precipitation results in bead surfaces that are inhomogeneous, which is not conducive to their target as gentle exfoliants. Furthermore, these inhomogeneities can cause clumping during the filtration, leading to a reduction in effective yield of individual beads. To modify bead morphology and yield, we studied the influence of interfacial properties, as mediated by incorporation of surfactants and choice of oil, allowing the rational design of a wide range of microbeads for various consumer applications. We aim to use this new technique to produce a scalable, sustainably sourced and degradable alternative to a major source of primary microplastics.   

Rheology and Pinching Dynamics of Soapy Formulations

Commercial liquid hand soaps and body washes are rheologically complex fluids with additives that influence smell, color, viscosity, phase behavior, and overall consumer's sensory experience when dispensing or pumping onto the hand, body, or sponge. Most commercial hand soap dispersions are multicomponent fluids that contain wormlike micelles, and therefore, often exhibit shear banding and elastic instabilities. In this contribution, we characterize the shear rheology response of commercial hand soaps using torsional rheometry and characterize their pinching dynamics and extensional rheology using dripping-onto-substrate (DoS) rheometry protocols. To better understand the connection between rheological studies and consumer sensory experience, we complement these studies with stretched liquid bridge experiments carried using Cambridge Trimaster as well as a home-built device, and develop an understanding of stringiness and gloppiness.   

Diverse Fingering Morphologies Driven by Reaction-Viscosity Coupling in a Hele-Shaw Cell

Viscous fingering occurs when low viscosity fluid pushes high viscosity fluid, and it has been studied extensively in relation to mixing processes and petroleum extraction. In addition, previous research has shown that instabilities also develop by gel formation near the interface of pressure-driven viscosity-matched fluids. In this talk, we will present a novel system where the effects of gel formation, viscosity differences, and non-Newtonian behavior on interfacial instability are observed in concert, a scenario with potential applicability to subterranean carbon sequestration, manufacturing, and biological processes. In this system, the characteristics of the instability and resulting fingering patterns are determined by the competition of the diffusion and reaction timescales associated with gel formation and the advection timescale that drives viscous fingering. By adjusting flow rates, reactant concentrations, and viscosities in a Hele-Shaw cell, these timescales are manipulated to reveal a range of interesting finger morphologies.   

Emergent search strategies from the physics of fluid-driven branching

Biological search strategies in complex environments alternate between exploration and exploitation, and are suggestive of the behavior of a sentient organism. We demonstrate that a simple physical system, in which self-organized branched patterns form upon injection of a low-viscosity fluid into a Hele-Shaw cell containing a yield-stress fluid, might allow for a useful interpretation in terms of this complex behavioral strategy. As we inject water from a point source into a sealed cell with one point sink, the expansion of the branched pattern towards the sink appears to change from a direct (exploitative) to an indirect (exploration) strategy with an increase in flow-rate. We show that this transition is connected to a switch from a viscosity-dominated to an elasticity-dominated response from the yield-stress fluid to the invading fluid and that the corresponding flow-rate can be predicted. Moreover, we find that at this transition, the invasion strategy is most cost-effective in terms of the amount of material that needs to be displaced to reach the sink. Overall, our results suggest how a combination of local rules and global constraints in a mundane physical system can lead to concrete examples of embodied adaptation, exploration and learning.   

Control of viscoelastic instabilities by symmetry in microfluidic channels

We leverage the symmetry of the pillars and their organization in microfluidic arrays to control the existence and characteristics of viscoelastic fluctuations. We study these fluctuations in the form of waves that we observed recently in DNA at concentrations exceeding the overlap concentration flowing through square arrays with circular pillars [1] and that appear in two mirrored variants (LEFT and RIGHT). Breaking the lateral symmetry using triangular pillars, the waves form only in one of the mirrored orientations (LEFT or RIGHT).

Enhanced mixing is demonstrated at low Reynolds numbers using these waves by introducing DNA stained with different colors for each of two inlets in the microfluidic device. We observe the waves using epi-fluorescence microscopy and quantify the mixing by the overlap of the two DNA samples as they flow along the channel. We also quantify the mixing of the aqueous solvent by monitoring the change in fluorescence of an ion sensitive dye that is added to one of the DNA samples while a monovalent ion is added to the other sample.

Efficient suppression of the viscoelastic fluctuations is realized by alternating every second row in the pillar array, thereby creating a situation where a LEFT wave is initiated immediately followed by a RIGHT wave that neutralizes it.   

Textured Frozen Polymer Droplets

Ice templating provides a means of generating textures with a well-defined topography. While typically executed with colloids, our research explores using polymers to manipulate the morphology and texture of frozen droplets. Our experiments indicate that frozen polymer droplets' macroscopic shape and texture vary with the molecular weight and polymer concentration under unidirectional freezing conditions. Distinctly, a white cloudy halo forms along the freezing direction upon freezing, delineating two droplet zones exhibiting different freezing behaviors and suggesting phase separation occurring. Concomitantly, detailed in-situ microscopic observations revealed that the roughening of the freezing front also depends on polymer concentration and molecular weight, suggesting a connection between the microscopic roughness of the freezing front and the macroscopic roughness of the frozen droplet. By combining polymer physics and heat transfer, our findings offer novel strategies for ice templating and microstructure fabrication.   

Simulation of concentrated polymers in flow using a fluctuating field theory

In the flow of concentrated polymer systems, interactions among the polymers can couple with elasticity to change the rheology and flow instabilities. Simulations using continuum field theories are useful because of the high polymer concentration. We have used a fluctuating field theory called a stochastic kinetic theory, which is a relatively new approach for simulating nonequilibrium situations using field equations. Although it is coarse-grained relative to equilibrium polymer field theory, this approach naturally applies to nonequilibrium situations. It also provides more detail than present in two-fluid models. This new method is particularly important for systems in which fluctuations lead to structure formation or instabilities. In this talk, we will introduce the application of stochastic kinetic theories to concentrated polymers. We will quantify the accuracy of the numerical method and show how it can be used to examine structure formation and instabilities in flow.   

Viscoelastic fingering during the sedimentation of a sphere

The motion of particles in complex fluids is ubiquitous in biological and physical processes. The most popular toy model for understanding the physics of such systems is the settling of a solid sphere in a viscoelastic fluid. There is a general agreement that an elastic wake develops downstream of the sphere, causing the breakage of fore-and-aft symmetry, while the flow remains axisymmetric, independent of fluid viscoelasticity and flow conditions. Using a continuum mechanics model, we reveal that axisymmetry holds only for weak viscoelastic flows. Beyond a critical value of the settling velocity, steady, nonaxisymmetric disturbances develop peripherally of the sphere rear pole, giving rise to a 4-lobed fingering instability. The transition to nonaxisymmetric flow is characterized by a regular bifurcation and solely depends on the interplay between shear and extensional properties of the viscoelastic fluid under different flow regimes. At higher settling velocities, each lobe tip is split into two new lobes, resembling fractal fingering in interfacial flows. For the first time, we capture a fingering instability under steady-state conditions and provide the missing information for understanding and predicting such instabilities in the response of soft media.   

Effects of DMF cosolvent addition on the extensional rheology of PNIPAM solutions

Poly(N-isopropylacrylamide) (PNIPAM), a thermoresponsive polymer known for its lower critical solution temperature (LCST) of ~32 °C in aqueous media, has been studied extensively for a variety of applications including drug delivery, sensing, and smart coatings. PNIPAM chains undergo a dramatic conformational change from hydrated coils in solution below the LCST to collapsed globules that often aggregate above the LCST. Polymer chain conformation affects the amenability of solutions to extensional flow-dominated processes such as spraying and printing, which are of interest for manufacturing coatings and devices at-scale. Adding a cosolvent can change the PNIPAM solution phase windows and thus be leveraged to tune processing windows. Herein, we use dripping-onto-substrate extensional rheometry (DoS) to measure the extensional flow behavior of PNIPAM in dimethylformamide (DMF)/water mixtures. We demonstrate the effects of DMF fraction on solution elasticity. Most notably, while the LCST exhibits a maximum with increasing DMF fraction as determined by turbidimetry measurements, the extensional relaxation time increases monotonically within the same range. Our findings demonstrate that preferential PNIPAM-DMF interactions evolve with solvent composition, in line with other studies.   

The Randomness of Rain

The local spatiotemporal structure of rainfall is of critical importance to radar-based measurements of weather, erosion prediction, splash-based plant and fungal reproduction, and other droplet scale processes. Past studies using optical disdrometers (2DVDs) have observed structure in rain on the spatial scale of storms and rain cells, but little is known about the droplet scale due to tapering instrument resolution. In this work, we directly image raindrop arrival times and locations to investigate patterns and interactions on the centimeter scale. We analyze the droplet fall pattern through a pair correlation function and calculate a fractal dimension to determine spatial and temporal clustering. We hypothesis that raindrops will cluster as a function of drop size distribution and therefore storm type, convective or stratiform. We aim to apply our  results on rain patterning at the smallest scale to understanding splash-based reproduction in early land plants.   

Emerging patterns of phoretic active matter: from crystalline solids to active turbulence

Phoretic microswimmers, such as Janus colloids and isotropic active droplets, are promising constituents for self-organized active materials. However, the underlying mechanisms governing their self-organization remain unclear, despite numerous experimental observations in various regimes. Here, we perform large-scale simulations to investigate a paradigmatic suspension of isotropic phoretic disks representing active droplets, explicitly resolving their many-body, long-range hydrochemical interactions. We observe that they exhibit diverse collective phenomena, including the formation of crystalline solids resembling Wigner crystals, melting, gas-like chain formation, active transition and turbulence. Our work reproduces and reconciles seemingly conflicting experimental observations on chemically active systems, emphasizing the importance of incorporating full physicochemical hydrodynamics. Remarkably, altering activity alone induces solid-fluid phase transition and, subsequently, the laminar-turbulent transition of the fluid. These two progressively emerging transitions, hitherto unreported, bring us closer to understanding the parallels between active matter and traditional matter. Our findings will help enhance the understanding of long-range, many-body interactions among phoretic agents, offer new insights into non-equilibrium collective behaviors, and provide potential guidelines for designing reconfigurable materials.   

Simulation and Modeling of the Settling Behavior of Polydisperse Gas-Solid Flows With Application to Pyroclastic Density Currents

Sedimenting flows occur in a wide range of industrial and natural systems, such as circulating fluidized bed reactors and pyroclastic density currents (PDCs). In systems with sufficiently high mass loading, momentum coupling between the phases gives rise to mesoscale behavior, such as clustering. These structures can generate and sustain turbulence in the carrier phase and directly impact large-scale quantities of interest, such as settling time. As an added complexity, many flows of interest consist of a polydisperse particulate phase. In this talk, we characterize the sedimentation behavior of a range of polydisperse gas-solid flows, sampled from a parameter space typically associated with PDCs. Highly resolved data is collected using an Euler-Lagrange framework and polydisperse settling behavior is contrasted with Stokes settling and analogous ensembles of monodisperse particles and a new settling model is proposed that takes into account the effect of polydispersity.