Discover the key differences between the Du Noüy ring and Wilhelmy plate methods for measuring surface tension. Learn which technique offers better accuracy.
🧪 Du Noüy Ring Method
The Du Noüy ring method is a classical technique for measuring the surface tension (SFT) of a liquid and the interfacial tension (IFT) between two liquids.
🔬 How It Works
A platinum ring is submerged into a liquid and then slowly pulled upward. The maximum force needed to detach the ring from the surface determines the surface tension. This force acts on the wetted length of the ring as it lifts a liquid lamella between two phases.
⚠️ Key Considerations
Surface Disturbance: Pulling the ring disturbs the interface, preventing equilibrium—problematic for surfactants that need time to adsorb. This can lead to an overestimation of surface tension.
Correction Factors: Requires mathematical corrections (e.g., Harkins–Jordan) to account for meniscus curvature and capillary effects—omitting them may cause errors of up to 7%.
Mechanical Precision: The ring must remain perfectly circular and aligned. Deformations can significantly affect accuracy.
📏 Wilhelmy Plate Method
The Wilhelmy plate method is used to measure surface tension, interfacial tension, and contact angle between a liquid and a solid surface.
🔬 How It Works
A thin platinum plate is vertically immersed in the liquid. The wetting force acting on the plate is measured directly and used to calculate surface tension.
✅ Advantages
Equilibrium Measurement: Since the plate is stationary, it allows the liquid surface to reach equilibrium—ideal for surfactant solutions.
No Correction Needed: Assumes a contact angle of 0° under full wetting, eliminating the need for correction factors.
Higher Accuracy: Known for its precision and stability, it is widely preferred in modern research and industrial settings.
🔍 Conclusion
While the Du Noüy ring method is historically important and still relevant in some standards, the Wilhelmy plate method is typically more accurate, especially for surfactant-rich or viscous liquids. Its ability to operate under equilibrium conditions and avoid correction factors makes it the preferred choice in many laboratory and industrial applications.
📖 Read the full article: Ring or Plate? The Classic Tensiometer Question – Orbit Research
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