ramé-hart Technical Document

Notes on the ADSA Method for Measuring Surface Tension using DROPimage

Surface tension, a fundamental property of liquids, governs phenomena such as wetting, spreading, and capillarity. Measuring surface tension accurately is critical in fields ranging from material science to biology. Among various methods, the Axisymmetric Drop Shape Analysis (ADSA) technique which is employed in both DROPimage Advanced and DROPimage Pro software using pendant drops stands out due to its precision and theoretical rigor. This document explores the principles, procedure, and significance of using DROPimage's ADSA for surface tension measurement.

Principles of ADSA

The ADSA method is based on analyzing the shape of a liquid drop suspended from a needle or capillary. When a drop hangs under the influence of gravity, its shape is determined by a balance of forces:

Surface Tension: Acts along the drop's surface, minimizing its surface area.
Gravity: Causes deformation, pulling the drop downward.

The shape of the drop can be described mathematically using the Young-Laplace equation, which relates the pressure difference across the liquid interface to its curvature and surface tension. ADSA leverages this equation to extract surface tension by comparing the actual shape of the drop with a theoretical model. For a more technically detailed explanation including theoretical background, please see this document.

Procedure for Measuring Surface Tension

Setup and Drop Formation
- A small volume of liquid is dispensed from the needle of the ramé-hart tensiometer in a controlled environment.
- The volume is carefully adjusted to create a stable pendant drop without detachment. The ramé-hart Automated Dispensing System can be employed to ensure precise drop volume.
Imaging
- A high-resolution digital SuperSpeed camera such as ramé-hart's U Series camera captures the side profile of the pendant drop.
- The image must be clear and free from distortions, with precise illumination to highlight the drop's boundaries.
Edge Detection
- The edges of the pendant drop are identified from the captured image.
- Accurate detection is essential for reliable analysis.
Shape Fitting
- DROPimage then compares the detected shape to theoretical models derived from the Young-Laplace equation.
- Numerical algorithms iteratively adjust parameters such as surface tension to minimize deviations between the experimental and theoretical shapes.
Calculation of Surface Tension
- Once the best fit is achieved, DROPimage calculates the surface tension based on the fitted parameters.
- Factors such as drop density and gravity are accounted for in the calculations.

Advantages of the ramé-hart ADSA Method

High Precision
- ADSA accounts for gravitational and hydrostatic effects, making it highly accurate for a wide range of liquids.
Versatility
- It can measure low surface tensions (e.g., surfactant solutions) and high tensions (e.g., pure liquids) with equal reliability.
Non-Invasive
- The method requires minimal liquid volume and avoids altering the liquid properties during measurement.
Insights into Interfacial Phenomena
- By observing shape changes over time, DROPimage ADSA method can analyze dynamic surface tension and adsorption kinetics.

Challenges and Limitations

Despite its accuracy, the ADSA method has some challenges to be aware of:

Experimental Conditions
- Variations in temperature, vibrations, or imperfections in the needle tip can introduce errors.
Instrument Performance
- The ramé-hart instrument must be properly calibrated.

Applications

The ADSA method using pendant drops is widely used across industries and research fields:

Pharmaceuticals: For studying drug delivery systems and emulsions.
Oil and Gas: To analyze interfacial tensions critical in enhanced oil recovery.
Material Science: To evaluate coatings, adhesives, and nanomaterials.

Conclusion

The ramé-hart ADSA method with pendant drops exemplifies the elegance of combining physical principles with computational analysis to measure surface tension. Its accuracy and versatility make it an invaluable tool for understanding liquid behavior at interfaces. ramé-hart instruments have been set the standard for contact angle goniometry for over sixty years. And for nearly three decades, ramé-hart's DROPimage software has been the world leader in ADSA surface tension analysis.