Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q26: Microscale NonNewtonian and Complex Flows IFocus Recordings Available

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Sponsoring Units: DSOFT DPOLY GSNP Chair: Ivan Christov, Purdue University Room: McCormick Place W187B 
Wednesday, March 16, 2022 3:00PM  3:36PM 
Q26.00001: Passive and active particle dynamics in microhydrodynamic flows of complex fluids Invited Speaker: Gwynn J Elfring There is a rich history in the literature on the effects of particles in Newtonian flows, but when the suspending fluid is also complex, as it is in many applications from environmental flows such as avalanches or mudslides, to energy applications such as downhole scenarios in the oilfield, to biophysical flows such as cells in the human body, the rheology of these suspensions is far less understood. Particles can interact with nonlinear nonNewtonian stresses in these complex suspensions to significantly alter flows and an ongoing effort is to characterize and understand these effects. 
Wednesday, March 16, 2022 3:36PM  3:48PM 
Q26.00002: Heterogeneity and the role of entanglements during the extrusion of fiber suspensions Zehao Pan, Janine K. Nunes, Howard A Stone The flow of fiber suspensions occurs in industrial processes such as paper making and filtration. Recently, flexible (high aspect ratio) polymeric microfiber suspensions have been demonstrated to form gellike materials under shear stress or during extrusion. Using microscopic imaging, we systematically studied the extrusion process in three dimensions. We found that during extrusion and dependent of the extrusion flow rate the extrudate volume fraction varies relative to the initial suspension volume fraction. At a small flow rate, the channel is clogged and the extrudate is purely the solvent. At a higher flow rate, the microfibers are extruded but at a higher volume fraction than their preextrusion state. The extrudate volume fraction reaches a plateau when the preextrusion volume fraction exceeds a critical value. Using rheometric measurements, we observed that the critical volume fraction is related to the emergence of yield stress in the suspension. A model to rationalize the experimentally generated extrusion phase diagram will be presented. 
Wednesday, March 16, 2022 3:48PM  4:00PM 
Q26.00003: Tuning interfacial rheology and particle microstructure with Janus particles Yiming Qiao, Nathan C Keim, Xiang Cheng Particleladen fluid interfaces are ubiquitous in nature and have attracted considerable research interest, due to their exceptional ability to stabilize interfacerich materials such as emulsions, foams and cocontinuous blends. Here, we explore the effect of a small amount of additive on the interfacial rheology and the microscopic structure of such an interface. Using a custombuilt interfacial stress rheometer, we show that the addition of a small amount of platinumpolystyrene (PtPS) Janus particles (1:40 ratio of Janus versus PS) within a monolayer of PS colloids can lead to more than an orderofmagnitude increase in the surface modulus of the monolayer with enhanced elasticity, greatly improving the stability of the interface. This drastic change in interfacial rheology is associated with the formation of local particle clusters surrounding each Janus particle. We further develop a simple model based on interparticle interactions, which qualitatively explains the origin of local particle clusters and the increase of the interfacial elasticity. Our systematic experiments demonstrate a new way to tune the microstructure and stability of particleladen fluid interfaces. 
Wednesday, March 16, 2022 4:00PM  4:12PM 
Q26.00004: Twist And Snap: Heterogeneous Defect Nucleation via FrankRead Sources in Nematic Liquid Crystals Matthew J Deutsch, Robin L Selinger Topological defects can nucleate heterogeneously, as when a pinned dislocation segment in a crystal bows out under stress and snaps off repeatedly, emitting concentric dislocation loops, known as a FrankRead source. A similar mechanism can arise in a nematic liquid crystal containing a pinned disclination. We consider a disclination halfloop pinned to a planar anchoring layer [1]. When director twist is imposed on the top substrate, the halfloop expands and snaps off new disclination loops leaving the original halfloop intact [2,3]. We model this mechanism via a 3D LebwohlLasher rotor model, representing a uniaxial nematic with equal Frank constants. We study the FrankRead mechanism and explore the effects of temperature and twist rate. Using a materialsbydesign approach, we propose that a liquid crystal cell with a patterned array of FrankRead sources will demonstrate a rheological response that depends on disclination halfloop sizes, density, orientation, and pattern. Defect halfloops pinned on colloids may also serve as sources. We discuss the importance of this mechanism in rheology of both passive and active nematics. [1] Guo et al doi:10.1002/adom.202100181, [2] Long et al doi:10.1039/d0sm01899f, [3] Angelo http://tiny.cc/jangelo. 
Wednesday, March 16, 2022 4:12PM  4:24PM 
Q26.00005: Local deformation fields in active soft matter induced by internal stresses Mehdi Molaei, Steven A Redford, Wenhung Chou, Danielle Scheff, Margaret Gardel In many active soft materials, force generation, storage, and dissipation occur across length and time scales. This multiscale interaction between active force generating units and dissipative structural units is the defining character of active materials. Understanding how deformations induced by internal stresses propagate across these materials can provide insights into the underpinning mechanisms of this behavior. Here we present a method to measure such deformation fields. Specifically we use microscopy images to investigate systems driven by transient internal active stresses. We illustrate the application of this method in various actomyosin networks. 
Wednesday, March 16, 2022 4:24PM  4:36PM 
Q26.00006: Fluorescent streak velocimetry of nonNewtonian fluids Brendan C Blackwell, Han Lin, Connor Call, Michelle R Driscoll, Jeffrey J Richards Here, we demonstrate how fluorescent streak velocimetry can be used to characterize flow in nonNewtonian fluids. By seeding a fluid with 2 μm colloids and imaging its flow using long exposure times (20100 ms), we generate data which encodes information about flow velocity and acceleration in individual images. To analyze these images, we have created a userfriendly and opensource computational package. We demonstrate this technique by characterizing the channel flow profiles of several nonNewtonion fluids: micellar Cetylpyridinium Chloride solution, Carbopol 940, and Polyethylene Glycol. Our measurement resolution is sufficient to distinguish the distinct nonNewtonian flow profiles from each other and from the Newtonian profile. We will also present results of this method from more complex geometries, where significant acceleration is created due to a modified channel geometry. 
Wednesday, March 16, 2022 4:36PM  4:48PM 
Q26.00007: Experimental study on the motion of a confined bubble in a nonNewtonian fluid Jie Feng When a confined long gas bubble translates in a capillary tube, a thin film of liquid separates the bubble surface and the tube inner wall, with its thickness determined by the interplay of viscous, inertial and capillary effects. Although the dynamics of a confined bubble in a Newtonian liquid has been the subject of quite a few studies since the pioneering works of Taylor and Bretherton, the case where the fluid exhibits a nonNewtonian behavior is much less understood, while in applications, such as enhanced oil recovery and drug delivery, the fluids are likely to exhibit nonNewtonian properties. Here we consider the classical Bretherton problem with a nonNewtonian fluid. We provide quantitative measurements of the thickness of deposited liquid film for carboxymethyl cellulose solutions with different concentrations in the range of small Capillary numbers. Depending on the concentration of CMC, the thickness of the liquid film shows distinguished behaviors compared with that of Newtonian case. We further compare our observations with the scaling law considering the effective viscosity to extend the classical Bretherton's correlations to nonNewtonian fluids. Our results may enrich the fundamental understanding of multiphase flows involving a nonNewtonian fluid. 
Wednesday, March 16, 2022 4:48PM  5:00PM 
Q26.00008: Dipole Alignment of Water Molecules Flowing Through a Carbon Nanotube Chandan Dasgupta, Hemant Kumar, Saheb Bera, Subhadeep Dasgupta, A K Sood, Prabal K Maiti Recent years have seen an upsurge of interest in exploring ultrafast transport of water in various nanochannels with potential applications in desalination, separation processes, nanomedicine and energy conversion. However, a quantitative understanding of flowinduced effects in nanochannels is still lacking. We have used molecular dynamics simulations to study pressureinduced flow of water through a (10,10) singlewalled carbon nanotube. We find that the dipole moments of water molecules inside the nanotube get aligned by flow, resulting in a net dipole moment in the flow direction. With increasing flow velocity, the net dipole moment first increases and eventually saturates to a constant value. This behavior is qualitatively similar to that in the Langevin theory of paraelectricity with the flow velocity acting as an effective aligning field. We show that the microscopic origin of this behavior is the entry of water molecules inside the nanotube with their dipole vectors preferentially pointing inward along the nanotube axis. 
Wednesday, March 16, 2022 5:00PM  5:12PM 
Q26.00009: Soft hydraulics of nonNewtonian fluids flows through compliant conduits Ivan C Christov The interplay between hydrodynamic forces and the confining boundaries of compliant conduits brings about rich physics involving fluidelastic structure interactions. For example, microfluidic devices for labonachip technologies are molded from soft polymeric materials. The mechanical compliance of channels in such devices is now recognized as an advantage to be exploited to enable, for example, new approaches to microrheological measurements and new modalities of micromixing. In this context, complex fluids exhibiting nonNewtonian rheological behavior arise. While hydraulics of Newtonian fluids in rigid conduits is now a textbook problem, a theory of soft hydraulics of nonNewtonian fluids remains elusive. In this talk, I will describe how to construct such a theory yielding reduced models that take into account sheardependent viscosity, hydrodynamic pressure gradients during flow, and the elastic response (bulging and deformation) of the soft conduits due to flow within. First, the relationship between volumetric flow rate and axial pressure gradient is needed. This relationship is challenging (or impossible) to obtain in closed analytical form, notable exceptions are the powerlaw and Ellis models for the sheardependent viscosity. Then, I will show how a perturbative approach justifies replacing the channel height or tube radius in the pressure gradientflow rate relation with an expression that accounts for the pressureinduced deformation of the conduit. The latter can be derived from the analysis of a suitable elasticity problem, for a given crosssectional conduit geometry. I will conclude with examples of how the proposed theoretical approach reduces 3D coupled, multiphysics problems to a single ODE, which can often be integrated. 
Wednesday, March 16, 2022 5:12PM  5:24PM 
Q26.00010: Flow ratepressure drop relation for viscoelastic fluids in narrow and confined nonuniform geometries Evgeniy Boyko, Howard A Stone Pressuredriven flows of viscoelastic polymer solutions in narrow nonuniform geometries are ubiquitous in nature and various applications. One of the key interests for such flows is understanding the relationship between the flow rate and pressure drop, which, to date, is studied primarily using numerical simulations. Here, we aim to rationalize the longstanding contradiction between simulations using the "simple" OldroydB and FENECR models and experiments for the flow ratepressure drop relation of viscoelastic fluids in some geometries. To this end, we provide a theoretical framework for calculating the flow ratepressure drop relation in arbitrarily shaped, narrow channels. Through the combination of the lubrication approximation and the Lorentz reciprocal theorem, our theory allows deriving analytical expressions for the flow ratepressure drop relation for a wide variety of continuumlevel constitutive viscoelastic models in the weakly viscoelastic limit. Furthermore, we discuss the shortcomings of the OldroydB and FENECR models and suggest their modification through accounting for additional microscopic features of polymer flows. 
Wednesday, March 16, 2022 5:24PM  5:36PM 
Q26.00011: Peeling of elastic sheet using complex fluids at low Reynolds numbers Vishal Anand, Anirudh Venkatesh, Vivek Narsimhan We investigate the transient fluid structure interactions (FSIs) of a nonNewtonian fluid peeling a elastic sheet at low Reynolds numbers (Re) .We express the rheology of the fluid through the simplified PhanThienTanner (sPTT) model. Invoking the lubrication approximation for fluid flow and modeling the structure as series of Hookean springs, we reduce the problem to that of a partial differential equation for the evolution of the deformed height in time and space. An order of magnitude analysis reveals two distinct regimes of peeling, based on the relative magnitude of the viscoelasticity and FSI parameters, further aided by similarity solutions. On inspecting the numerical solution, we infer that the nonNewtonian nature brings the system to a steady state faster in comparison to a Newtonian fluid. We explore this idea by investigating the dynamics of peeling actuated by purely shear thinning fluids. And therefore, we present the results for the peeling actuated by such generalized Newtonian fluids as well. To conclude, this study aims to afford to the experimentalist a system of knowledge to a priori delineate the peeling characteristics of a certain class of complex fluids. 
Wednesday, March 16, 2022 5:36PM  5:48PM Withdrawn 
Q26.00012: Simultaneous stress, pressure, and 3D velocity measurements of viscoelastic instabilities in a crossslot. Gerardo E Pradillo, Paul Salipante, Steven D Hudson Turbulent flow and the hydrodynamic instabilities which may appear, are generally observed when inertia dominates the fluid motion. However, polymeric fluids with viscoelastic properties do not require an inertial flow regime to exhibit such instabilities or even turbulentlike behavior. Instead, these types of fluids need only the correct flow gradients and curvature to develop what has been classified as purely elastic turbulence. Using holographic particle tracking, we elucidate the 3D flow structures that underly the transition and early stages of purely elastic turbulence in a model mixed flow type system. Furthermore, via measurements of the pressure fluctuations upstream of the crosschannel middle section, and using birefringence polarization to visualize the fluid stresses, we correlate the changes in retardance with both pressure and velocity measurements to show how stress disturbances propagate upstream even in the presence of viscous dissipation. Finally, we further study the spatiotemporal dynamics of the pressure and velocity measurements to fully describe the different regimes in the elastic turbulent regime. 
Wednesday, March 16, 2022 5:48PM  6:00PM 
Q26.00013: Timedependence of local boundary conditions and global throughput for confined, semidilute polymer solutions Gabriel R Guyard, Alexandre Vilquin, Nicolas Sanson, Stéphane Jouenne, Frédéric Restagno, Joshua D McGraw Confined polymer solution flows are often found, for example, in biological systems and porous media. In such systems, for which the surface to volume ratio is high, interfacial effect are key. Specifically, the flow throughput is highly impacted by boundary effects such as slip or chain adsorption at the wall, and these latter interfacial effects may be timedependent. In this study, we use evanescent wave microscopy and particle tracking velocimetry to map the flow field within a onemicron layer close to the wall in a 5micronthick microfluidic chip. While this technique has been shown very efficient to characterize the nearwall shear rate and hydrodynamic boundary condition, we additionally use stateoftheart flow sensors to measure the flow rate and pressure drop across the chip simultaneously. Live monitoring of the local flow profile and global flow rate allows a description of the adsorption dynamics of hydrolyzed polyacrylamide chains onto a glass surface, under shear flow. The dependence in the shear rate, chain concentration and electrolyte concentration is studied. 
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