Shear thinning

Definition (what it is)

Shear thinning is a non-Newtonian flow behavior in which a material’s apparent viscosity decreases as the applied shear rate (or shear stress) increases. Many shear‑thinning fluids show a high, nearly constant “zero‑shear” viscosity at very low shear, a pronounced viscosity drop above a characteristic shear rate, and in some cases an approach to a lower “infinite‑shear” viscosity at very high shear. It is commonly described by rate‑dependent rheology models such as:

Power‑law (Ostwald–de Waele): τ = K (γ̇)^n, equivalently η = K (γ̇)^(n−1), with n < 1 indicating shear thinning (τ: shear stress; γ̇: shear rate; η: viscosity; K: consistency index).
Carreau–Yasuda and Cross models (to capture η0 and η∞ plateaus).
Herschel–Bulkley (adds a yield stress to a power‑law fluid when a static structure must first be overcome).

Shear thinning is instantaneous and rate‑dependent. It is distinct from thixotropy, which is time‑dependent (viscosity decreases over time under constant shear and recovers after flow stops), though both behaviors can coexist.

Key mechanisms (why it happens)

Polymer solutions and melts: shear‑induced orientation and partial disentanglement of long chains (coil‑to‑stretch transitions), reducing entanglement density and flow resistance.
Particle suspensions and pastes: disruption of flocculated or percolated networks; alignment of anisotropic particles (platelets, fibers); improved lubrication between particles; breakage of weak agglomerates.
Structured/complex fluids: alignment or reorganization of micelles, lamellae, or liquid‑crystalline domains; deformation of microgels and soft colloids.

Practical implications (what it enables)

Processability: Lower viscosity at high shear eases pumping, spraying, coating, dispensing, screen printing, extrusion, and injection molding; required pressure and energy can be reduced at a given throughput and shear heating may be mitigated.
Shape retention: Higher viscosity at rest or low shear helps prevent sagging, dripping, slump, and slosh; improves bead stability for adhesives and sealants; resists sedimentation of fillers.
Product quality: Influences flow front uniformity, die swell, surface finish, fiber wet‑out, void content, and layer fidelity in coatings and additive manufacturing.
Mixing and dispersion: High shear promotes deagglomeration and dispersion of fillers; at low shear, high viscosity can hinder homogenization and air release.

Measurement and modeling

Methods: Rotational rheometry (cone‑plate, parallel‑plate, concentric cylinder) for steady‑state flow curves; capillary rheometry for high shear rates; microfluidic or in‑line viscometry for process monitoring. Oscillatory and step‑shear tests are used to examine structure recovery and thixotropy.
Data treatment: Fit viscosity–shear‑rate data to power‑law, Carreau–Yasuda, Cross, or Herschel–Bulkley models; extract flow index n, consistency K, η0, η∞, and characteristic time scales. Use time–temperature superposition where applicable.
Good practice: Control temperature and shear‑history; check for wall slip, edge fracture, shear banding, and instrument limits; test across shear rates that bracket the intended process (e.g., <0.1 s⁻¹ at rest; 10–10³ s⁻¹ in coating gaps; 10³–10⁵ s⁻¹ in dies/nozzles).

Typical materials that exhibit shear thinning

Polymer melts: PE, PP, PS, PET, PA, PEEK (important in extrusion and injection molding).
Polymer solutions and binders: PVDF/NMP, SBR latex, acrylics, cellulose ethers, xanthan and associative thickeners (HEUR/HASE).
Filled systems and pastes: Epoxies and silicones loaded with Al2O3, AlN, BN, Ag; solder pastes; conductive inks; ceramic slurries; cementitious and clay suspensions (often with additional thixotropy).
Everyday and consumer products: Paints and coatings, shampoos, toothpaste, ketchup, yogurt.
Lubricants and greases: Multigrade oils with viscosity modifiers; soap‑thickened greases (reduced viscous drag at high shear).

Applications and sectors that exploit shear thinning

Coating and printing: Doctor‑blade, slot‑die, comma‑bar, gravure, spray, and screen printing rely on low high‑shear viscosity for application and higher low‑shear viscosity for anti‑sag and leveling.
Adhesives, sealants, and potting: Easy dispensing through narrow nozzles with good bead hold‑up after deposition.
Polymer processing and composites: Lower extrusion/injection pressures; improved fiber wet‑out in RTM and prepreg processing; controlled flow in automated fiber placement and 3D printing of pastes.
Energy and electronics (including EVs): Battery electrode slurries (graphite, LFP, NMC with binder and carbon additives) for mixing and slot‑die coating; highly filled thermal interface materials and gap fillers for pumpability with in‑situ stability; encapsulants and sealants in pack assembly.
Tribology: Shear‑thinning lubricants and greases reduce churning losses at operating speeds while maintaining film thickness under lower‑shear conditions.

Related terms and distinctions

Synonyms: Pseudoplastic flow, pseudoplasticity; shear‑thinning fluid; power‑law fluid (with n < 1).
Related: Thixotropy (time‑dependent thinning), shear thickening/dilatancy (viscosity increases with shear rate), yield‑stress fluids (e.g., Bingham, Herschel–Bulkley), Carreau/Cross fluids, rheopexy (rare time‑dependent thickening), viscoelasticity (elastic plus viscous response).

Design and formulation considerations

Balance low‑shear and high‑shear viscosity (and any yield stress) to meet stability, leveling, and pumpability targets.
Account for temperature dependence (Arrhenius/WLF) and possible shear‑ or thermally induced degradation of polymers or binders.
Tune microstructure and additives (dispersants, thickeners, particle shape/aspect ratio) to control flow index n and recovery time; manage thixotropy when open time, leveling, or print fidelity matter.
Match equipment to rheology: die/nozzle geometry, shear‑rate window, residence time, and allowable pressure; mitigate wall slip (surface roughness, particle loading).
Quality control: Specify viscosity at defined shear rates and temperatures; use up/down shear sweeps and hysteresis loops to monitor structure and batch‑to‑batch consistency; employ in‑line viscometry where possible.