Rheology Fundamentals and Applications
📌 Rheology is essential for characterizing materials across various industries, including cosmetics, food, clothing, and inks.
⚙️ Rheological properties are crucial throughout a material's lifecycle: before, during, and after production (raw materials, processing, and end-use).
🔬 The first step in a rheological measurement is selecting the correct measuring geometry (rotor/tool) appropriate for the material type and properties being characterized.

Geometry Selection Guide
💧 For low-viscosity materials (like water or oils), coaxial cylinders offer the best option due to their high surface area and suitability for large sample volumes.
🍦 For medium to high-viscosity materials, or those with particles or gels, parallel plate or cone-and-plate setups are ideal, using much smaller sample volumes (1–2 mL).
📏 The gap height in parallel plate setups is adjustable to accommodate lower volumes or interrogate materials at higher shear rates.

Sample Handling and Measurement Integrity
🥄 For structured or fragile samples, avoid shaking, stirring, or squeezing through pipettes/syringes; instead, gently scoop material onto the bottom plate and lower the top plate very slowly.
📉 Rapid head closure during loading can disturb sample structure, leading to up to a 50% difference in measured yield stress (e.g., lotion sample).
📊 To avoid measurement artifacts like sample ejection or drying, use solvent traps or low-viscosity oil around the rim when using plate geometries for long tests or elevated temperatures.

Improving Measurement Methods
🔢 A single-point measurement at a constant shear rate only provides absolute viscosity for simple Newtonian materials (like water or oil) and misses crucial shear rate dependence for non-Newtonian fluids.
🔄 For complex materials, use a rotational ramp to generate a "fingerprint" (viscosity vs. shear rate plot), but ensure the step profile method is used, which waits for the material to reach equilibrium (e.g., change $<5\%$) at each shear rate step for high-quality data.
🔬 Oscillatory (Oscillatory Shear) measurements are preferred over rotational tests for studying structure stability (measuring elastic modulus $G'$ and viscous modulus $G''$) because they do not damage the sample network.

Key Points & Insights
➡️ There is no universal measuring geometry; the test method and geometry must be specifically designed for the material and the parameter of interest.
➡️ Use the Range Calculator in software (like RheoWin) to determine the operating window (shear rate/stress range) for a specific geometry and known viscosity, significantly reducing guesswork.
➡️ When using plate geometries, always overfill the sample volume slightly and then use the trimming position to remove excess, as under-filling leads to larger measurement errors (e.g., 20% under-fill caused a 30% viscosity error).

📸 Video summarized with SummaryTube.com on Jan 12, 2026, 11:14 UTC