When water is added to flour, why is the hydration process so rapid?
Dough mixing is essentially a hydration process. When water is added to the flour, the small flour particles hydrate rapidly. This is due to the presence of gluten protein fibrils in a mixture of water and dough. The flour consists of starch granules and protein molecules. When flour particles and water are mixed, gluten protein fibrils are spontaneously formed and they extend into the water. This results in mobility of flour particles and interactions of neighboring gluten fibrils, which ultimately results in dough formation.
Initially when the mixing begins, the fibrils are wiped away upon contact with surfaces (such as mixer blade, etc.). This exposes new flour particles to rapid hydration and development of gluten fibrils. This continuous process creates a continuous system of hydrated protein polymers with starch granules dispersed throughout. Over time, as the mixing progresses, free water quantity decreases and system's resistance to mobility increases and we finally end up with dough.
hope this helps.