GATE 2024 MT – Question 33
A solid embryo contacts a perfectly flat mould wall as shown. Its wetting angle $\theta$ is ______ degrees (one decimal place). Surface tensions are: liquid–mould 0.35 J m$^{-2}$, solid–mould 0.02 J m$^{-2}$, and liquid–solid 0.40 J m$^{-2}$.
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Correct answer: 33.0 to 35.0
Explanation
Young’s balance at the wall gives γliquid–mould=γsolid–mould+γsolid–liquid cosθ.
Hence cosθ=(0.35−0.02)/0.40=0.825.
Taking the inverse cosine gives **34.4°** to one decimal place.