Derivation in Darcy's law
I hope it is okay, if I ask a question considering the lecture here. Please let me know, if I should ask Prof. Shih directly.
In Lecture 16 we discussed Darcy's law. In the beginning we had the formula <v> = \frac{\kappa}{\mu L} \cdot \Delta P_{eff} = \frac{\kappa}{\mu L} \cdot \left( \Delta P - \Delta \Pi_{eff} \right).
The final formula is then <v> = \frac{-\kappa}{\mu L} \cdot \left( \Delta P - \sigma k_B T \Delta c \right) as it is also written in Eq. (16.5) in the script.
Now I am wondering where this minus sign in the final formula is coming from.