N and mu

The equivalent hieroglyphic alphabet of the letter N is represented by the pictorial surface or ripple of water. "en" can mean "of" "to" or "for" where it can be seen joining long texts as an important preposition. Very rarely used as an ideogram (Gardiner refers to the absence of an ideogram altogether), it should mean water or the surface of water. The letter is more commonly used as a phonetic complement rather than an ideogram, to guide the pronunciation of other hieroglyphs in many instances. 

A similar sign with the phonetic value of mu will also have the same pictorial representation but with three repeated ripples instead. The ideograph of a water surface is no longer valid here and we will either be referring to a whole body of water or a watery mass of the sky. This symbol can be seen frequently as a determinative such as at the end of "itrw" (river) where it does not have any phonetic value but is only there to signify water. Phonetically, it is simply mw and is a bileteral sign accordingly.

Sources:

Alan Gardiner, Egyptian Grammar