To pattern the silicon substrates, a positive photo resist, S1811 (Rohm and Haas), was first spin coated at 4000 RPM for 45 seconds onto a clean silicon wafer (<100> orientation, Silicon Quest International Inc., Santa Clara, CA). After prebaking at 110°C for 2 minutes, micro patterns were transferred onto the substrate using a chrome photo mask and a UV mask aligner. The exposed photo resist was developed using a 1:1 mixture of DI water: MF 312 developer (Rohm and Haas) for 30 seconds. Development was stopped by rinsing with DI water and drying with a gentle stream of nitrogen gas. A deep reactive ion etch (DRIE) was performed at 25°C for 4 minutes on an Oxford Instruments Plasma lab 100 (Oxfords hire, UK) until a depth of 2 µm was achieved. The remaining unexposed resist was removed with acetone, followed by DI water rinse and drying with nitrogen.