Introduction to Rheology Basics
📌 The webinar focuses on the basics of oscillatory rheology to quantitatively determine the viscoelastic behavior of samples, contrasting it with traditional rotational viscosity measurements.
💧 Viscosity ($\eta$) is defined by Newton's first law as the ratio between shear stress ($\tau$) and shear rate ($\dot{\gamma}$), but is never measured directly.
🔗 Viscoelastic behavior arises from structure in fluids, such as polymer chain entanglements or 3D networks in emulsions, allowing the fluid to store and relax energy elastically.

Creep and Recovery Tests
🔬 Creep and recovery tests apply an instantaneous shear stress jump to mimic low-stress conditions like gravitational sag or leveling behavior in coatings.
⚖️ For purely viscous samples, deformation increases constantly (viscous flow) and is not recoverable upon stress release.
🤸 Purely elastic samples show instantaneous deformation that is completely reversible upon stress release, storing all applied energy.
📊 Viscoelastic fluids show a mix: initial elastic response, transition, followed by viscous flow, allowing quantitative distinction between viscous and elastic components under small stresses.

Oscillatory Rheology: Amplitude Sweep and Frequency Sweep
🌊 Oscillatory tests involve sinusoidally oscillating strain or stress, allowing manipulation of both strain/deformation amplitude and speed (frequency).
🎯 The Amplitude Sweep determines the Linear Viscoelastic (LVE) Regime by increasing amplitude at a constant frequency, indicating the stress level before structural breakdown (yield stress).
⏱️ The length of the LVE range is frequency-dependent: higher speeds result in a longer LVE range because there is less time for relaxation.
📊 The Frequency Sweep is the main goal, measuring viscoelastic response as a function of characteristic speed ($\omega$), which correlates directly to shear rate.

Material Response and Moduli Separation
🌟 In oscillatory tests, the resulting deformation signal has a phase angle ($\delta$ or $\Delta$) relative to the input stress.
- A purely elastic body has $\delta = 0^{\circ}$ (in-phase).
- A purely viscous body has $\delta = 90^{\circ}$ (90 degrees phase shift).
- Viscoelastic fluids have a phase angle between $0^{\circ}$ and $90^{\circ}$.
📈 The complex modulus ($G^*$) separates into the Storage Modulus ($G'$ - elastic component) and the Loss Modulus ($G''$ - viscous component).
📉 The Loss Factor ($\tan \delta = G''/G'$ ) indicates brittleness; lower $\tan \delta$ means a stiffer, less deformable system.

Speed Dependence and Practical Implications
🔗 At low characteristic speeds (long timescales), the system behaves more viscous ($G'' > G'$); polymer chains can flow and dissipate energy.
🦴 At high characteristic speeds (short timescales), the system behaves more elastic ($G' > G''$) as relaxation mechanisms are suppressed, leading to the Dynamic Glass Transition.
🛑 The crossover point ($G' = G''$, or $\tan \delta = 1$) marks where problems begin, such as die swell, stringiness in shampoos, or poor mouthfeel in food products.
🧪 Molecular weight, concentration, and temperature severely influence this crossover point: higher molecular weight or lower temperature shifts the crossover to lower frequencies (more elastic behavior).

Time-Temperature Superposition (TTS)
🔄 TTS is used to expand the measurable frequency range by conducting frequency sweeps at different temperatures and then shifting the data onto a single master curve.
🌡️ This principle is valid because relaxation mechanisms at high temperatures mimic the response at low frequencies.

Key Points & Insights
➡️ Always perform an Amplitude Sweep at the lowest frequency of interest to ensure all subsequent Frequency Sweep measurements are conducted within the Linear Viscoelastic (LVE) Regime.
➡️ The crossover point ($G' = G''$) where the material transitions from predominantly viscous to predominantly elastic behavior is a critical parameter for predicting product performance (e.g., die swell, stringiness).
➡️ In rotational tests, measuring the elastic response ($G'$) is often prevented by the Weissenberg effect (sample climbing out of the gap), making oscillatory rheology superior for characterizing structure in many complex fluids.
➡️ For shear-thinning fluids where the zero shear viscosity ($\eta_0$) cannot be reached directly, the creep and recovery test can be used to calculate $\eta_0$ via the slope in the viscous flow regime.

📸 Video summarized with SummaryTube.com on Jan 12, 2026, 10:59 UTC