Effect of Droplet Size and Microstructure on Contact Angle of Ductile Iron with Water Swaroop Behera, Neil Dogra, Pradeep Rohatgi Department of Material Science and Engineering, UW-Milwaukee

Figure 1. Contact angle of a droplet, as seen through goniometer. Overview Introduction Methods Results Microstructure Contact Angle Discussion Theoretical Calculation Conclusion Future Scope θ Figure 1. Contact angle of a droplet, as seen through goniometer.

Introduction Ductile iron, a mild steel alloy, is widely used for piping in the water industry due to its flexibility, durability, and mechanical properties. However, this piping often corrodes, leading to the accumulation of unwanted buildup, which is costly and difficult to remove. The wastewater industry has relied on hydrophobic alloy coatings for their pipes, in the attempt to reduce corrosion. However, these coatings are expensive and prone to damage. With the ultimate goal of finding a corrosion resistant ductile iron alloy, experimental work was done to study the contact angle of ductile iron with water, since higher contact angles imply reduced corrosion. Factors which could influence the hydrophobicity of a surface were studied. It was hypothesized that an increase in droplet size would result in an increase in contact angle. Furthermore, it was hypothesized that samples containing higher amounts of graphite would have a higher contact angle.

Methods For this experiment, Four different Ductile Iron samples were tested with varying phase percentages of graphite . Sample designations: DI 1, DI 2, DI 3, DI 4 Samples were polished to the appropriate grit with sand paper of different grit sizes. A Ramé-Hart goniometer was used to measure the contact angle of the water droplet on the samples. The contact angle of a sessile droplet of water with varying volumes on samples of ductile iron was measured. Droplet Size: 2 µl, 4 µl, 6 µl, & 8 µl Roughness: 400 grit, 800 grit, 1200 grit Figure 2. Ramé-Hart goniometer, used to take contact angle measurements.

Results: Microstructure Figure 3. Micrographs of unetched samples showing the distribution of graphite nodules (a) DI 1, (b) DI 2, (c) DI 3, (d) DI 4.

Microstructure Analysis Samples were fully polished with 1 micrometer of Alumina prior to taking microstructural images. Using an optical microscope, 3 images were taken at each magnification for every sample. Afterwards, each image was analyzed using an image processing program for phase percentages of graphite, and averaged to find the amount of graphite in each sample. Sample % of Graphite Nodules DI 1 10.13% DI 2 12.68% DI 3 16.34% DI 4 25.84% Table 1. Phase percentage of graphite nodules.

Contact Angle Figure 4. Variation of Contact Angle with Droplet Volume

Contact Angle Figure 5. Variation of Contact Angle with Phase percent of Graphite

Discussion Previous studies have concluded with droplets small enough to disregard the effect of gravity, an increase in contact angle is seen as droplet size increases. The Young equation for quantifying the wettability of a solid surface is stated as: where γsv is the surface tension of the solid against the vapor, γls is the surface tension of the liquid against the solid, and γlv is the surface tension of the liquid against the vapor. Young’s equation shows when γsv > γls, the contact angle is less than 90º. As the droplet size increases, γls, or the surface tension of liquid against the solid increases. Thus, by Young’s equation, the difference between γsv and γls decreases, and the contact angle increases. For DI 1 and DI 2, a clear increase in contact angle is seen as droplet size increases. Graphite is estimated to have a contact angle of 90º. The mild steel matrix is estimated to have a contact angle of 40º, which is significantly lower than that of graphite. Thus, as graphite levels increase, the overall hydrophobicity of the composite increases, and a clear increase in contact angle is expected and observed.

Theoretical Calculations Cassie’s law for calculating the contact angle of a liquid on a composite surface is stated as: cos θc = f1 cos θ1 + f2 cos θ2 where f1 is the fraction of component 1 in the composite, θ1 is the theoretical contact angle of component 1, f2 is the fraction of component 2 in the composite, and θ2 is the theoretical contact angle of component 2. Ductile iron is composed of graphite and a mild steel matrix. The theoretical contact angle of graphite is 90º. The theoretical contact angle of mild steel is 40º. So by Cassie’s law, cos θDI 1 = [(.8987)*cos (40º) +(.1013) cos (90º)] θDI1 = 46.81º cos θDI 2 = [(.8642)*cos (40º) +(.1268) cos (90º)] θDI2 = 48.55º cos θDI 3 = [(.8376)*cos (40º) +(.1634) cos (90º)] θDI3 = 50.09º cos θDI 4 = [(.7426)*cos (40º) +(.2584) cos (90º)] θDI4 = 55.33º Contact angles measured in the experiments tended to be larger than the theoretical calculations, because the samples were polished at a higher roughness, which generally increases contact angle. However, the theory predicts an increase in contact angle as graphite amounts increase, as observed.

Conclusions As droplet size increases, the contact angle increases for both samples of Ductile Iron 1 and 2, tested at 3 different roughness. As the amount of graphite in the Ductile Iron microstructure increases, the contact angle of the sample with water increases. To develop a more hydrophobic and corrosion-resistant Ductile Iron alloy, the effects of microstructure on the contact angle of Ductile Iron with water was studied, because high contact angles imply reduced corrosion. It was found that Ductile Iron containing higher amounts of graphite, such as DI 4, appears to be preferable for use in water industry pipes, since it has a higher contact angle, suggesting corrosion resistance. As the droplet size of water increased, contact angle increased. Thus, water of higher droplet sizes will most likely corrode the Ductile Iron piping less.

Future Work Investigating advancing and receding angles of Ductile Iron. Modifying surfaces of the microstructures to create contact angles ≥ 90º, and studying reduced fouling under flowing water.

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