Simplification of the derivative of the mean curvature

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I'm using the RK method to calculate the thin film profile. there is a complex interface curvature term, as shown in 1, κ. The final formula is a fourth-order ordinary differential equation which requires the first and second curvature derivative terms, they are also listed in 1. However, I don't understand how it was simplified. Specifically, I don't know where does ln(r) come from. Can anybody explain it to me?