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Determination and Comparison of Critical Surface Tension of Glass and PTFE Surfaces Using the Zisman Method and ImageJ-Based Contact Angle Analysis

This study utilizes the Zisman method combined with ImageJ-based contact angle analysis to quantitatively determine the critical surface tension of glass and PTFE, successfully validating the approach for low-energy PTFE surfaces while identifying an apparent, lower-than-expected value for high-energy glass.

Original authors: Ansh Ansh, Aarav Jaiswal, Anurag Kumar

Published 2026-10-23

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Ansh Ansh, Aarav Jaiswal, Anurag Kumar

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Every day, we interact with surfaces that either welcome liquids or repel them. A raindrop beads up on a waxed car hood, while a spill soaks instantly into a paper towel. This behavior, known as wettability, is the measure of how easily a liquid spreads across a solid. It is a fundamental property that dictates whether a paint will stick to a wall, whether an inkjet printer will lay down a crisp line, or whether a self-cleaning window will actually shed dirt. Scientists describe this interaction using a single, observable number: the contact angle. This is the angle formed where a drop of liquid meets the solid surface. If the liquid spreads out flat, the angle is small, indicating the surface is eager to bond with the liquid. If the liquid sits up in a tight, round ball, the angle is large, showing the surface is resisting the liquid.

To understand why some materials are naturally wettable and others are not, researchers look for a specific threshold called the critical surface tension. Think of this as a tipping point for the material itself. If a liquid has a surface tension lower than this threshold, it will spread completely across the solid. If the liquid's tension is higher, it will bead up. By finding this number for a material, scientists can predict how it will behave with almost any liquid. While this concept has been known for decades, measuring it usually requires expensive, specialized machines that sit in high-tech labs. A new study by researchers at the Kalinga Institute of Industrial Technology in India asks a simple question: can we find this crucial number using nothing more than a camera, a computer program available to anyone for free, and a few common liquids?

The researchers set out to test two very different surfaces that represent opposite ends of the wettability spectrum. The first was a standard glass microscope slide, a material known for being high-energy and hydrophilic, meaning it loves water. The second was a sheet of polytetrafluoroethylene, commonly known as PTFE, the slippery, non-stick plastic found in cookware, which is low-energy and hydrophobic, meaning it hates water. To measure how these surfaces interact with liquids, the team did not use a high-end goniometer. Instead, they placed tiny drops of five different liquids onto each surface. These liquids ranged from ethanol, which is very thin and spreads easily, to glycerol and water, which are thicker and more resistant to spreading.

Once the drops were placed, the researchers captured side-view photographs of them. They then fed these images into ImageJ, a free, open-source software often used by scientists and students to analyze pictures. This program analyzed the shape of each droplet and calculated the precise angle where the liquid met the solid. They repeated this process three times for every liquid on every surface to ensure the results were consistent. With a table of contact angles in hand, the team plotted the data to see the relationship between the liquid's own tension and the angle it formed on the surface. By drawing a line through these points and extending it until it reached the point of complete wetting, they could calculate the critical surface tension for each material.

The results painted a clear picture of the two materials. For the PTFE sheet, the method yielded a critical surface tension of approximately 18.9 millinewtons per meter. This number aligns almost perfectly with values found in decades of scientific literature, confirming that the low-cost method works reliably for non-stick, hydrophobic surfaces. The glass slide told a slightly more complex story. The researchers calculated an apparent critical surface tension of about 39.8 millinewtons per meter. While this value is lower than the range typically cited for glass in textbooks, the authors explain that this is likely due to the specific cleaning method used and the composition of the slides they tested. Unlike the PTFE, which resisted all but the thinnest liquids, the glass allowed even water to spread out significantly, forming a much smaller contact angle.

The study concludes that combining the classical Zisman method with free image analysis software provides a powerful, accessible tool for understanding surface energy. It proves that you do not need a multi-thousand-dollar instrument to determine how a material will interact with liquids. For the PTFE, the results were definitive, matching established scientific expectations. For the glass, the results were specific to the conditions of the experiment, serving as a reminder that surface properties can change based on how a material is prepared. Ultimately, this work demonstrates that the tools to explore the invisible forces of adhesion and wetting are now within reach of any teaching laboratory or curious researcher, allowing for a deeper, more practical understanding of the materials that surround us.

Technical Summary: Determination and Comparison of Critical Surface Tension of Glass and PTFE Surfaces Using the Zisman Method and ImageJ-Based Contact Angle Analysis

Problem Statement
Wettability is a fundamental surface property governing industrial processes such as adhesion, coating, printing, and the design of self-cleaning surfaces. While the Zisman method is a widely used empirical approach for characterizing solid wettability by determining the critical surface tension ( $\gamma_c$ ), traditional implementations often rely on specialized, costly goniometers. Furthermore, the applicability of the Zisman method to high-energy surfaces, such as glass, has been questioned in literature due to scattered data and composition-dependent variations. This study addresses the need for an accessible, low-cost alternative for surface energy screening and seeks to quantitatively characterize the wettability of contrasting reference surfaces: high-energy glass and low-energy polytetrafluoroethylene (PTFE).

Methodology
The researchers employed a combination of the classical Zisman method and open-source image analysis to determine the critical surface tension of glass microscope slides and flat PTFE sheets.

Materials: Five test liquids with known surface tensions ( $\gamma_{LV}$ ) were used: ethanol (22.1 mN/m), ethylene glycol (48.0 mN/m), formamide (58.0 mN/m), glycerol (63.4 mN/m), and deionized water (72.8 mN/m).
Procedure: Sessile droplets (5–10 µL) were deposited on substrates cleaned with ethanol. Side-profile images of the droplets were captured using a fixed camera setup.
Analysis: Contact angles ( $\theta$ ) were measured from the images using ImageJ software equipped with the LB-ADSA (Low-Bond Axisymmetric Drop Shape Analysis) plugin, which fits the Young–Laplace equation to the droplet boundary. Three replicate droplets were measured for each liquid-surface combination.
Zisman Plot Construction: The cosine of the contact angle ( $\cos \theta$ ) was plotted against the liquid surface tension ( $\gamma_{LV}$ ). Linear regression was applied to extrapolate the critical surface tension ( $\gamma_c$ ) where $\cos \theta = 1$ . Notably, the ethanol data point for glass (showing complete wetting) was excluded from the linear fit as it carries no angular information.

Key Results

PTFE (Hydrophobic): The Zisman plot for PTFE yielded a linear regression with an $R^2$ of approximately 0.99. The extrapolated critical surface tension ( $\gamma_c$ ) was 18.9 mN/m. This value aligns closely with literature values (18–20 mN/m) for PTFE, confirming its non-polar, low-energy nature.
Glass (Hydrophilic): The Zisman plot for glass (excluding the ethanol point) yielded an $R^2$ of approximately 1.00. The extrapolated apparent critical surface tension was 39.8 mN/m.
Comparison: The study observed that liquids with $\gamma_{LV}$ below the surface's $\gamma_c$ completely wetted the surface, while those above formed finite contact angles. The glass results showed lower contact angles (0–48°) compared to PTFE (23–109°), consistent with their respective hydrophilic and hydrophobic characters.

Significance and Claims
The paper claims that combining the Zisman method with ImageJ-based contact angle analysis provides a low-cost, accessible, and reproducible alternative to specialized goniometers, making it suitable for teaching laboratories and preliminary surface-energy screening.

The authors offer a modest interpretation of their findings:

Validation for Low-Energy Surfaces: The close agreement between the measured $\gamma_c$ for PTFE (18.9 mN/m) and established literature values validates the ImageJ-based methodology for low-energy surfaces.
Apparent Value for High-Energy Surfaces: The authors explicitly state that the $\gamma_c$ value for glass (39.8 mN/m) should be regarded as an apparent value specific to the slides and conditions used in this study. They acknowledge that this value is lower than the 43–73 mN/m range reported in literature for glass and discuss potential reasons, such as differences in glass composition and cleaning protocols (ethanol rinse vs. hexane/ethanol with desiccator storage).
Limitations: The study notes that uncertainty propagation was not performed due to the lack of tabulated standard deviations for the replicates. Consequently, the reported $\gamma_c$ values are approximate, and small deviations from literature values should not be over-interpreted.

In conclusion, the study successfully demonstrates a resource-efficient workflow for characterizing surface wettability, confirming the distinct hydrophilic and hydrophobic natures of glass and PTFE, while maintaining scientific rigor regarding the limitations of applying the Zisman method to high-energy surfaces without specialized equipment.

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