A committed Muslim woman is required to cover her `Awrah (parts of the body to be concealed from marriageable persons), and to keep her Muslim identity in public.

Referring to the aforementioned fact, Allah Almighty says: “Tell the believing men to lower their gaze and be modest. That is purer for them. Lo! Allah is Aware of what they do. And tell the believing women to lower their gaze and be modest, and to display of their adornment only that which is apparent, and to draw their veils over their bosoms, and not to reveal their adornment save to their own husbands or fathers or husbands fathers, or their sons or their husbands’ sons, or their brothers or their brothers’ sons or sisters sons, or their women, or their slaves, or male attendants who lack vigor, or children who know naught of women’s nakedness. And let them not stamp their feet so as to reveal what they hide of their adornment. And turn unto Allah together, O believers, in order that ye may succeed.” (An-Nur: 31-32)
Focusing more on your question regarding women wearing sandals that may expose their feet, we’d like to state that there is a difference of opinion among Muslim jurists whether woman’s feet are considered as `Awrah or not. Some scholars are of the view that all of a woman’s body is `Awrah, including the feet, except the face and hands. Others hold that it is permissible for a woman to uncover her feet.
So, according to the first opinion, it is not permissible for a woman to wear sandals or shoes that will expose her feet, but it is permissible for her to do so according to the second opinion.
We think, it is better for a woman to cover her feet if she can, but if not, she may uncover her feet.
however, uncovering her feet does not render looking at the feet lawful. It is not permissible for a man to look at a woman’s feet.

Having stated the above, it becomes clear that covering woman’s feet is a controversial issue among Muslim jurists and that she is permitted to follow what is more convenient to her in this regard. However, it is preferable to wear sandals or shoes that do not expose her feet.