The invention is related to a vaporiser assembly for an aerosol-generating system. The vaporiser assembly comprises a capillary element (2), which is made from porous glass. The capillary element has a first end (4) and a second end (6). The vaporiser assembly further comprises a heater element (8). The first end of the capillary element is configured to be fluidly connected to a liquid storage portion (10). 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 of the capillary element to the second end of the capillary element by capillary action. The average pore size of the capillary element varies from large pores at the first end of the capillary element to small pores at the second end of the capillary element such that a pore size gradient from the first end of the capillary element to the second end of the capillary element is provided.
