An aerosol-generating device includes a housing having a first end defining a mouthpiece, a second end, and a cavity defined between the first end and the second end. The device also includes a reservoir within the cavity. The reservoir is configured to store a liquid aerosol-forming substrate. The device also includes an atomizer and a valve within the mouthpiece of the housing. The valve has a first side and an second side. The second side of the valve includes a hydrophilic coating, a hydrophobic coating, or both a hydrophilic coating and a hydrophobic coating.