The fabrication of Inverted Funnels (IF) required a thermal treatment at 250 °C and 500 °C to change the crystallographic phase of the alumina. The inverted funnels were labeled IF2 and IF3 according to their respective total number of layers (Figure 1c). IF2 consisted of a top layer of 15 µm, followed by a thermal treatment at 500 °C. A subsequent anodization step added a 15 µm thick bottom layer. Similarly, IF3 consisted of 3 layers, each with a thickness of 10 µm. After the anodization of the top and middle layer a thermal treatment of 500 °C and 250 °C was applied, respectively. In a final wet chemical etching step, the pores of IF2 and IF3 were widened for 2 h.