Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session L45: Suspensions: Rheology and Confined Flows 
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Chair: Irmgard Bischofberger, MIT Room: 209A 
Monday, November 20, 2023 8:00AM  8:13AM 
L45.00001: Yieldstress transition in suspensions of deformable droplets Giuseppe Negro, Alexander Morozov, Davide Marenduzzo, Giuseppe Gonnella, Livio Nicola Carenza Yieldstress materials, which require a sufficiently large forcing to flow, are currently illunderstood theoretically. To gain insight into their yielding transition, we study numerically the rheology of a suspension of deformable droplets in 2D. We show that the suspension displays yieldstress behavior, with droplets remaining motionless below a critical bodyforce. In this phase, droplets jam to form an amorphous structure, whereas they order in the flowing phase. Yielding is linked to a percolation transition in the contacts of dropletdroplet overlaps and requires strict conservation of the droplet area to exist. Close to the transition, we find strong oscillations in the droplet motion that resemble those found experimentally in confined colloidal glasses. We show that even when droplets are static, the underlying solvent moves by permeation so that the viscosity of the composite system is never truly infinite, and its value ceases to be a bulk material property of the system. 
Monday, November 20, 2023 8:13AM  8:26AM 
L45.00002: Memory of shear flow in jammed suspensions H. A A Vinutha, Manon Marchand, Marco Caggioni, Vishwas Vasisht, Emanuela Del Gado, Veronique Trappe Shearing jammed suspensions of soft particles results in heterogeneous and correlated dynamics leading to stress heterogeneities. To assess the role of such complex dynamics in encoding memory of shear, we perform shear cessation simulations and experiments at various shear rates. Consistent with previous work we find that the preshearrates set the time scales of stressrelaxation at early times, which is correlated with a ballisticlike motion, reminiscent of the nonaffine shorttime particle dynamics observed during shear. An investigation of the correlations of the shorttime nonaffine displacement fluctuations observed just before and after shear cessation reveals remarkable similarities in the dynamical correlations observed in both regimes. This indicates that the spatially heterogeneous dynamics during shear imprints stress inhomogeneities that in turn drive spatially heterogeneous dynamics upon shear cessation. Beyond the shorttime behavior, we find that the longtime relaxation towards a residual stress is connected to the evolution of local stiffness. Our findings suggest that steadystate shear is a process through which a memory of the imposed shear rate and of the distribution of local yield stresses is built into the material, a memory that can be then effectively read out in a shear cessation test. 
Monday, November 20, 2023 8:26AM  8:39AM Withdrawn 
L45.00003: Abstract Withdrawn

Monday, November 20, 2023 8:39AM  8:52AM 
L45.00004: Rheological fingerprints of noninertial debris flows Shravan Pradeep, Paulo E Arratia, Douglas Jerolmack Climate change has exacerbated the frequency of landslide events causing huge loss to human life and infrastructure. One major challenge in predicting such events is the absence of constitutive models to explain the flow and failure properties of natural heterogeneous dense suspension mixtures. From a suspension rheology perspective, the onset of a landslide is dictated by the rearrangement dynamics of the constituent materials and the magnitude of external forcing. Here, we use a minimum ingredient complex fluid mixture  a suspension of silica sand and kaolin clay suspended in deionized water  to probe the effect of material properties on the yielding phenomena in dense suspensions. Silica sand particles primarily interact through frictional contacts, while the kaolin clay particles are attractive and form systemspanning percolated structures. We systematically vary the clay to total solids (clay ratio), and the overall suspension volume fraction to generate steady shear rheological flow curves. By rescaling the curves using yield stress and microscopic rearrangement times, we obtain universal yielding curves that only depend on the clay ratio parameter. By estimating the strain energy required to fluidize the mixture, we show that the sandrich suspensions are “annealed” and exhibit more “brittlelike” yielding behavior compared to the clayrich suspensions. Here, we developed a preliminary rheological framework to explore the effects of frictional interactions in model mud mixtures using a minimum ingredient complex fluid. We believe that our work reconciles previously contradictory observations in debris flow rheology and can help modify the existing models that assess the hazard potentials of extreme environmental flows in the future. 
Monday, November 20, 2023 8:52AM  9:05AM 
L45.00005: A Theoretical Framework for Determining the Yield Stress of Nanoparticle Organic Hybrid Materials Sotoodeh Rassouli, Mehryar Jannesari Ghomsheh, Anubhab Roy, Donald L Koch, Sarah Hormozi Designing materials with desirable yield stresses is of interest due to their applications in additive manufacturing, soft robotics, and mimicking biological yieldstress fluids. One model class of materials with yield stress is Nanoparticle Organic Hybrid Materials (NOHMs), which are made up of inorganic nanoparticles with tethered oligomers on their surfaces. The apparent yield stress of NOHMs comes from the entropic penalty experienced by the oligomers that must fill the space between a deformed array of cores. We derive the evolution of the free energy of NOHMs in a deforming periodic array in the absence of a solvent as well as in the presence of an added solvent. For a solventfree case, the tethered oligomers are at equilibrium for a given configuration of nanoparticles. This enables us to find the oligomer concentration field by minimizing the free energy of the system. Low values of yield stress are achievable by adding a solvent. In this case, the free energy is a result of oligomer configurational entropy, enthalpic affinity with solvent molecules, and solvent molecules configurational entropy. We model selfsuspended poly (ethylene glycol)tethered NOHMs and their solutions in water. We obtain the yield stress as the minimum stress necessary to continually deform the periodic array. We also obtain a simple estimate of the free energy barrier to thermally induced relaxation of the NOHMs microstructure as a function of core volume fraction, solvent concentration, and grafting density of oligomers. 
Monday, November 20, 2023 9:05AM  9:18AM 
L45.00006: Transient, nonlinear rheology and apparent yield stress of nanoparticleorganic hybrid materials Mehryar Jannesari Ghomsheh, Sotoodeh Rassouli, Anubhab Roy, Donald L Koch, Sarah Hormozi Nanoparticleorganic hybrid materials (NOHMs) consist of hard inorganic nanocores densely grafted with polymer chains. Even in the absence of solvents, NOHMs can exhibit fluid behavior since the grafted polymers fill the interstitial space between the cores like an incompressible fluid. Experiments indicate that NOHMs show rheological characteristics similar to soft glassy materials [P. Agarwal, H. Qi, and L.A. Archer, Nano Lett. 10, 111 (2010)]. We use the soft glassy rheology (SGR) model to predict the NOHMs rheology with and without solvent. Within this framework, we model NOHMs as mesoscopic elements trapped in cages formed by their neighbors. Using a density functional theory, we obtain the associated energy barrier as an input parameter to the SGR model as a function of the nanocores, polymers, and solvent properties. Mechanical loading reduces the energy barrier thus facilitating hops out of the trap by thermal fluctuations. Due to thermal and mechanical relaxations, NOHMs show Newtonian behavior over very long time scales. However, over a broader range of time scales of usual rheological characterization (or more importantly processing flow times), they act like yield stress materials. We examine how the apparent yield stress depends on the method of characterization such as startup shear or strain sweep oscillatory flow and on the time scale (shear rate, frequency) of the rheological probe. 
Monday, November 20, 2023 9:18AM  9:31AM 
L45.00007: Rigidity development in shearthickening suspensions Jeffrey F Morris, Aritra Santra, Michel Orsi, Bulbul Chakraborty Suspensions in shear flow may undergo a sharp shearthickening transition, which is closely related to the jamming transition. Recent work has converged on the view that this transition results from stressinduced frictional contacts. These contacts reduce the degrees of freedom for the motion of the suspended particles, eventually resulting in a jammed state in which only global rigid body motions of bulk translation and rotation are allowed. We study the development of the rigid structures in sheared suspensions in two dimensions (2D). using an established simulation tool that captures lubrication hydrodynamics, repulsive conservative interparticle forces. The 2D suspensions exhibit the same phenomenology in terms of shear thickening rheology as do threedimensional suspensions. The crucial advantage of two dimensions. for this study is that in. 2D, a pebble game algorithm can be applied to rigorously identify rigid clusters; an added benefit is that larger length scales are accessible at a given computational cost, which allows more effective exploration of scaling relationships involving the system size. The rigid clusters found as a function of stress and solid fraction will be presented, and the finding of a critical concentration at which rigidity occurs will be demonstrated. The finitesize scaling of the results at large stress will be presented, along with evidence allowing the transition to be associated with. a known universality class. 
Monday, November 20, 2023 9:31AM  9:44AM 
L45.00008: Influence of the constriction angle on the clogging of dilute and dense suspensions Sacha Escudier, Nathan Vani, DeokHoon Jeong, Alban Sauret Suspensions flowing through confined systems are present in various applications, from additive manufacturing to filtration processes. However, the passage of a suspension through a constriction, where a reduction of the crosssection of a channel occurs, is particularly prone to clogging. In this study, we consider the clogging by bridging, i.e., through the formation of a stable arch of particles at a constriction that hinders the transport of particles downstream of the clog. To characterize the role of the angle of the constriction on the clogging dynamics, we study the flow of suspensions of nonBrownian particles through quasibidimensional 3Dprinted millifluidic devices that allow us to track the particles as they flow and form arches at a constriction. We report the conditions for clogging by bridging when varying the constriction width to particle diameter ratio for different angles of the constriction for both dilute and dense suspensions. Understanding the role of the channel geometry in the formation of clogs is an important step for optimizing engineering design and developing more reliable dispensing systems. 
Monday, November 20, 2023 9:44AM  9:57AM 
L45.00009: Let it flow: preventing clogging of constricted channels during suspension flow DeokHoon Jeong, Remi Thomas, Nathan Vani, Alban Sauret Particles flowing in confined channels may clog at constrictions depending on the size of the particles compared to the channel constriction. Clogging is noticeably uneconomical, time and energyconsuming, as it can cause engineering problems such as blocking 3D printer nozzles or biomedical devices. However, predicting the probability of clogging in threedimensional channels by monodisperse and polydisperse suspensions remains challenging. Indeed, the clogging probability depends upon many parameters, such as the physical properties and concentration of the particles in the suspension, the flow, and the geometry of the system. Here, we consider clogging by "bridging" – the case in which multiple particles form a stable arch at a constriction. Building upon previous studies on quasitwodimensional flows, we characterize the role of the geometry on clogging in threedimensional flows. We also report how the polydispersity of the particles in the suspension influences the clogging probability compared to the monodisperse case. 
Monday, November 20, 2023 9:57AM  10:10AM 
L45.00010: Success and breakdown of Tanner's law for drops of dense granular suspensions Alice Pelosse, Elisabeth L Guazzelli, Matthieu Roché The spreading of viscous droplets of densitymatched suspensions on a solid surface is experimentally investigated at the global drop scale. 
Monday, November 20, 2023 10:10AM  10:23AM 
L45.00011: Suppression of fragmentation of drop chains in confined geometries Sagnik Singha, Abhilash Reddy Malipeddi, Mauricio ZuritaGotor, Kausik Sarkar, Jerzy Blawzdziewicz Droplet chains in confined suspension flows exhibit complex collective dynamics that can range from densitywave propagation to diffusive relaxation, to chain fragmentation. For an externally imposed Poiseuille flow, a unidirectional density wave propagation occurs because the particle interactions inherit the vectorlike nature of the incident flow. In contrast, in a Couette flow the leadingorder particle interactions are symmetric, which leads to either a stable diffusive behavior or an unstable one when the diffusion constant is negative. The instability that causes the fragmentation of lowdensity chains is driven by the symmetric quadrupolar interparticle hydrodynamic interactions. Using a simple simulation model with dipolar, quadrupolar, and repulsive nearfield hydrodynamic interactions, we show that in some cases lowdensity chains initially undergo fragmentation but are subsequently stabilized by a spontaneously formed train of short solitary waves. A propagating soliton, consisting of a group of closely spaced particles, absorbs the disordered particles in front of it and leaves equally spaced drops in its wake, thus stabilizing the dropchain structure. Such a stabilization mechanism may be useful in microfluidic manipulation of drop chains. 
Monday, November 20, 2023 10:23AM  10:36AM 
L45.00012: Inertial migration of nonneutrally buoyant particles in unidirectional shearing flows Raksha Mahalinkam, Ganesh Subramanian The science behind the separation of particles of different sizes and densities via the action of shearinduced lift forces, has various practical applications. The present line of research stems from the need to extend recent investigations on the inertial migration of neutrally buoyant particles (buoyancy parameter B_{y} = 0) in plane Poiseuille flow and Couette flow to nonneutrally buoyant particles (B_{y} ≠ 0) by including the effects of gravity. The study is conducted at small particle Reynolds numbers and confinement ratios, but for arbitrary B_{y} and channel Reynolds numbers (Re_{c}). The objective is to organize the equilibrium positions and relevant scaling regimes in the B_{y}Re_{c} plane. One may have different orientations of gravity, and we focus on the flowaligned case. For plane Poiseuille flow, as B_{y} increases, we observe the migration of the offcenter equilibria from the original SegreSilberberg locations at B_{y} = 0 towards the centerline, albeit in a nonmonotonic fashion. For plane Couette flow, the recently discovered supercritical pitchfork bifurcation for B_{y} = 0 transforms to a broken pitchfork resembling a disfigured 's' for small but finite B_{y}, that then opens up with increasing B_{y}. Our calculations should provide a rational basis for particle separation protocols. 
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