If rubbing is obligatory for the Malikis, is it then obligatory to rub the cracks in the feet with the fingernails, or is it sufficient to ensure water reaches them?

Rubbing (dalk) is one of the obligatory acts of ablution (wudu') according to the Maliki school. One must rub the cracks of the feet while ensuring water reaches inside them. The person performing ablution should rub their heels and Achilles tendons, as well as any thick skin or cracks where water does not easily penetrate, while exaggerating in the rubbing and pouring water over these areas.