Randy H. Ewoldt - University Of Illinois At Urbana–Champaign

Designing complex fluidsRandy H. EwoldtAssociate ProfessorDepartment of Mechanical Science and EngineeringUniversity of Illinois at Urbana-Champaign

Dv g p DtA small step awayfrom Newtonian fluidcreates an explosionin the range of possibilities.Ewoldt Research GroupUniversity of Illinois at Urbana-ChampaignComplex Fluid:non-Newtonianconstitutive behavior (rheology)Design:1. to create, fashion, execute, orconstruct according to plan2. to conceive and plan out in the mind2

The 4 Key Phenomena of Rheology*1. Viscoelasticity2. Shear-Thinning‘yield-stress fluid’ extreme shear-thinning3. Extensional-Thickening*Macosko (1994)(and more! Thixotropy, Shear-Thickening, )Photos & Videos fromEwoldt Research Group YouTube Channel4. Rod-Climbing

The 4 Key Phenomena of Rheology*1. Viscoelasticity2. Shear-Thinning‘yield-stress fluid’ extreme shear-thinning2. Shear-Thinning3. Extensional-Thickening*Macosko (1994)(and more! Thixotropy, Shear-Thickening, )Photos & Videos fromEwoldt Research Group YouTube Channel4. Rod-Climbing

R.B. BirdR. C. ArmstrongO. HassagerThe 4 Key Phenomena of Rheology*1. Viscoelasticity2. Shear-Thinning‘yield-stress fluid’ extreme shear-thinningD.D. JosephE. GuazzelliJ. Morris3. Extensional-ThickeningM.D. Graham*Macosko (1994)(and more! Thixotropy, Shear-Thickening, )Photos & Videos fromEwoldt Research Group YouTube Channel4. Rod-Climbing

Design and optimization in fluid mechanicsDv g p DtDesigning the GeometryFluidCross-slot microchannels[F.J. Galindo-Rosales et al. 2014]KinematicsOptimal microswimmer stroke[Tam and Hosoi 2011]Ewoldt Research GroupUniversity of Illinois at Urbana-Champaign6

Complex fluids to achieve diverse objectivesEwoldt Research GroupUniversity of Illinois at Urbana-ChampaignImage credits (CW from top left): Avery et al. Sci Transl Med (2016); global-sei.com;Ewoldt et al. Soft Robotics (2014); EarthClean Corp7

Motivation: Design with yield-stress fluids8x Speed200μm diameter tipOil-in-water Emulsion [Rauzan and Nelson et al. Adv. Func. Matls. 2018]Arif Nelson Ewoldt Research GroupUniversity of Illinois at Urbana-Champaign8

Motivation: Design with yield-stress fluidsNasalCavity8x Speed200μm diameter tipFingering Instability - The Rheology ZooOil-in-water Emulsion [Rauzan and Nelson et al. Adv. Func. Matls. 2018][Curtis-Fisk et al. 2013][Nelson et al. Submitted]Hot IceCreamOrbitz DrinkHubba Bubba Bubble Tape [Nelson et al. JOR 2018]Arif Nelson Ewoldt Research GroupUniversity of Illinois at Urbana-Champaign9

Motivation: Design with yield-stress fluidsNasalCavity8x Speed200μm diameter tipFingering Instability - The Rheology ZooOil-in-water Emulsion [Rauzan and Nelson et al. Adv. Func. Matls. 2018][Curtis-Fisk et al. 2013][Nelson et al. Submitted]Hot Ice CreamOrbitz DrinkHubba Bubba Bubble Tape [Nelson et al. JOR 2018]Arif Nelson Ewoldt Research GroupMystic SmokeUniversity of Illinois at Urbana-ChampaignOctopalm10

Design perspective AnalysisDesignDv g p DtContinuum fluid physics What properties desired? Quantitative targets agnostic tomicrostructure/building blocks?Microstructural fluid physics How many ways to achieve thetarget properties? Quantitative prediction?Nelson, Schweizer, Rauzan, Nuzzo, Vermant, Ewoldt, Curr. Opin. Solid St M (2019)11

Design-based organization of knowledgeShear-ThinningNelson, Schweizer, Rauzan, Nuzzo, Vermant, Ewoldt, Curr. Opin. Solid St M (2019)12

How many ways to get a yield-stress fluid?Nelson, Schweizer, Rauzan, Nuzzo, Vermant, Ewoldt, Curr. Opin. Solid St M (2019)13

Can we predict properties from formulation and structure?Nelson & Ewoldt, Soft Matter (2017)Nelson, Schweizer, Rauzan, Nuzzo, Vermant, Ewoldt, Curr. Opin. Solid St M (2019)14

Design-based organization of knowledge Organizes research efforts across diversematerial classes Reviews current microstructure fluid physicsknowledge to support the design process Similar ‘rheology-to-structure’ inverseorganization for other rheologicalphenomena?Shear-Thinning Microstructure-level fluid physics:more needed beyond scaling laws Continuum-level fluid physics:target properties, agnostic to structure?Nelson, Schweizer, Rauzan, Nuzzo, Vermant, Ewoldt, Curr. Opin. Solid St M (2019)15

MICROSTRUCTURE-AGNOSTIC RHEOLOGY TARGETS?VISCOPLASTIC IMPACT:Blackwell et al., JNNFM (2016)Blackwell et al., Phys Fl. (2015)Blackwell, Ph.D. thesis (2017)Sen et al., JFM (2019)Sen et al. arXiv (2020)

Droplet coating: microstructure-agnostic rheology target?DFD Z03.08103Blackwell et al., JNNFM (2016)Blackwell et al., Phys Fl. (2015)Blackwell, Ph.D. thesis (2017)Sen et al., JFM (2019)Sen et al. arXiv (2020)What is target rheology for enhancedfire suppression? or droplet coating? yield stress fluid(function of dynamic conditions) low thixotropy V 2/( y V/t)102SplashBroken sheetIntact sheetCraterLump10110010-1 -21010-1100t/D DFiner V 2 const Fdiss y V / t t

Drop impact tests for (unaged) Laponiteintact sheetbroken sheetsplashClassify each impact:Samya SenEwoldt Research GroupBlackwell et al., Phys. Fluids (2015)Sen et al., J. Fluid Mech. (2020)18

Microstructure does not matter (sort of)A single dimensionless group tounderstand splash regimes:This scaling with IF worksfor both Carbopol and(unaged) Laponite, but theC are differentSamya SenEwoldt Research GroupSen et al., J. Fluid Mech. (2020)19

Microstructure-agnostic targeting Enables design targets for ‘stick’ versus ‘splash’,with fluid physics revealed in a dimensionless group Target rheology, even with complex formulations More opportunities Some dependence on microstructure, or more complex rheological propertiesAnalysis may fail for very thin or thick coatingsAdditional effects from surface tension with very small droplet sizesThixotropic effects not previously studied (until now! see Z03.08)DFD Z03.08EarthClean Corp. (2015)What if properties more complex?(e.g. shapes of rheologicalmaterial functions)Samya SenEwoldt Research Group20Shear-Thinning

ViscoelasticityLinear viscoelastic design targets Dv g p Dt (t ) t G ( t t ) ( t ) dt Optimization techniques for shapes of functions Design freedom is NOT G', G" Models may be independent of microstructure (in a good way)8x Speed200μm diameter tipG ( t ) Gi exp t i i 1 MG ( t ) Ge H ( ) exp ( t ) d ln continuousrelaxation spectraCorman et al., J. Mech. Design (2016)Corman, PhD thesis (2019)1 mm

Surface topography design with Newtonian fluid Dv g p Dt( v ( v )T)Viscous lubrication Optimization of arbitrary shapes Manufacturing constraints Newtonian Stokes flow(lubrication flow) solverLee, Schuh, Ewoldt, Allison, J. Mech. Design (2017)

Co-design of surfaces and rheological functions Dv g p DtCEF:orGiesekus:(multi-mode) Sliding friction reduction with nonlinear viscoelasticity, e.g. polymer added to oil Optimization (inverse) problem is computationally expensive use surrogate modeling Identify a family of optimal combinations of surface topography and rheologyShear-ThinningNormal stress in shearLee, Schuh, et al. Structural and Multidisciplinary Optimization (2019)Objective function spacebetterperformance23

Co-design of surfaces and rheological functions Dv g p DtCEF:orGiesekus:(multi-mode) Sliding friction reduction with nonlinear viscoelasticity, e.g. polymer added to oil Optimization (inverse) problem is computationally expensive use surrogate modeling Identify a family of optimal combinations of surface topography and rheologyShear-ThinningNormal stress in shearObjective function spacebetterperformanceDifficult questions with modeling: Too free/unachievable? (but it starts with targets) Too restrictive?Yes! e.g. there exists behavior outside of currentmodeling paradigms Lee, Schuh, et al. Structural and Multidisciplinary Optimization (2019)24

DESIGN FOR DIRECT-WRITE 3D PRINTINGB. Rauzan*, A. Z. Nelson*, R. H. Ewoldt, and R. G. Nuzzo,“Particle-free emulsions for 3D printing elastomers,”Advanced Functional Materials (2018)

Rheological Design of Direct-Write 3D Printing Inks8x Speed200μm diameter tipWhat is the rheology for a“good” 3D Printing Ink?1 mmYield-stress keningRutz et al. 2015 Shepherd et al. 2011Blackwell et al. 2015 Dinec et al. in PreparationArif Nelson Ewoldt Research Group26

University of Illinoisat Urbana-ChampaignExtensible yield-stress fluids(new paradigm, new model material)ExtensibilityArchitecture concept:silicone oil-in-water emulsion (yield stress) poly(vinyl alcohol) network transientlycrosslinked with borate ions (extensibility)8x Speed200μm diameter tip5mm8mmBentonit Printing Bubblee 11% ResinGumShear Yield Stress[Nelson et al., J. Rheol. (2018)]PVA molecular weight 85,000 – 124,000Emulsified on stand mixer at 300rpmMixed with sodium tetraborate solutionModelMaterial

Model 3D Printing Ink MaterialSilicone oil-in-water emulsion with varying molecular weight of poly(ethylene oxide)5MExtension600K400K4M103ShearModulus (Pa)104G', Storage ModulusStress (Pa)No PEO1M900K8M103G'', Loss Modulus10290%wt oil emulsionMixed with 1% PEO SolutionArif Nelson Ewoldt Research Group10-210-1100101102Oscillation Stress (Pa)10310210-210-1100101-1Shear Rate (s )University of Illinois at Urbana-Champaign28

Faster printing with extensible yield-stress fluids20 mm/s20 mm/sNo PEOPrint Speed (mm/sec)100 μm nozzle20158M PEO1050012345678Molecular Weight of PEO (x106 g/mol)5 mm1 mm/sArif Nelson Ewoldt Research GroupDesign Target:εSTB 500% to achieve max available print speedUniversity of Illinois at Urbana-Champaign1 mm/s29

Chemical transformation may be required3D PrintedArif Nelson Ewoldt Research GroupTransformedUniversity of Illinois at Urbana-Champaign30

Success here, but there’s a bigger picture.Continuum-level: target properties may be non-obvious (not yet in constitutive models) High extensibility allows higher print speeds and increased gap spanningMaterial-level: we formulated a model direct-write 3D printing ink Particle-free emulsions allow for patterning of a buildable material below 10μm Post-printing transformations provide a method for greatly improving mechanicalpropertiesArif Nelson Ewoldt Research Group Dv g p DtUniversity of Illinois at Urbana-Champaign31

Vision for Designing Complex Fluids Many inspiring materials: yield-stress fluids, thixotropic fluids, flow battery working fluids, Design engineers need intuition for this complex behaviorMany open questions for the “engineering toolbox” for design & inverse problemsWill require scientific understanding and predictive equations:structure-to-rheology (material-level fluid physics) & rheology-to-flow (continuum-level fluid physics)32

University of Illinoisat Urbana-Champaign Example: drag on sphere Sensitivity to parametersat every location in space inspires design (local change?Heating? Other?)[Freund, Kim, Ewoldt, JNNFM (2018)]Inverse methods:adjoint-based sensitivity

University of Illinoisat Urbana-ChampaignViscoelastic design intuition8x Speed200μm diameter tipModel materials Feel modulus? viscosity? timescale? Pipkin maps Low-dimensional descriptions Ashby-style property maps[Corman & Ewoldt, Appl Rheol (2019)]1 mm

Flow battery working fluidsWhat happens at high concentration (1 M)?propertiesViscosity increasesdramaticallyDFD G03.17Conductivity decreasesEnergy density increasesconcentrationWhat are limits to energy density andconductivity?Other fluids based on polymeric orcolloidal structures have more complexrheology/conductivity/energy densitydesign trade offs

Vision for Designing Complex Fluids Many inspiring materials: yield-stress fluids, thixotropic fluids, flow battery working fluids, Design engineers need intuition for this complex behaviorMany open questions for the “engineering toolbox” for design & inverse problemsWill require scientific understanding and predictive equations:structure-to-rheology (material-level fluid physics) & rheology-to-flow (continuum-level fluid physics)36

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