Is it permissible for a woman to show a small part of the hair at the front of her head?

It is not permissible for a woman to expose any part of her head's hair in front of non-mahram men (strangers), because it is considered part of her adornment which is forbidden to be displayed to them. This is based on the Almighty's saying: "And let them not expose their adornment except that which [necessarily] appears thereof; and let them draw their overgarments over their bosoms."