A vaporiser assembly for an aerosol-generating system may include a capillary element made from porous glass. The capillary element has a first end and a second end. The vaporiser assembly further comprises a heater element. The first end of the capillary element is configured to be fluidically connected to a liquid storage portion. The heater element is provided at the second end of the capillary element. The pore size of the capillary element is configured to allow a liquid aerosol-forming substrate from the liquid storage portion to be conveyed from the first end to the second end of the capillary element by capillary action. The capillary element has a pore size gradient such that an average pore size of the capillary element transitions or varies from larger pores at the first end of the capillary element to smaller pores at the second end of the capillary element.
