Figure 4: Efficiencies of oil collection on PDMS needle arrays.

(a) Efficiencies of needle arrays with different needle separations. The three inset SEM images are the arrays with needle separations of 0.50, 0.35, and 0.25 mm. A larger needle density provided a higher efficiency. (b) Efficiencies of needle arrays with different heights. The three inset SEM images are the arrays with heights of 0.0, 0.3, and 0.5 mm. Higher needles provided a larger efficiency. Each point represents the average efficiency of seven samples; the error bars indicate the standard deviation.