Measurement of the Contact Angle of the PDMS Block

PDMS blocks used for the measurement of the contact angle were fabricated as follows. The base of SILPOT 184 W/C and its curing agent were mixed in a 10:1 weight ratio (Thinky, Inc., AR-100). The mixture was heated at 75 °C for more than 2 h. For the surface modification of the PDMS block, we further adopted the following procedures. We exposed the PDMS block to a single-step oxygen plasma treatment: 25 W, 20 cc/min, 5 s (SAMCO, FA-1). After being heated at 75 °C for 30 min, the PDMS block was dipped into an approximately 10 wt % PFS/methanol solution containing an aliquot of 6 M hydrochloric acid for 60 min. Then, the PDMS block was washed with an excess amount of methanol and dried at room temperature (approximately 25 °C). The fabricated PDMS block was placed on a horizontal table, and 10 μL of Milli-Q water or hexadecane was placed on the PDMS block. The image of the droplet was taken by a commercially available contact angle meter (Excimer Inc., SImage Entry 5). The measurement proceeded independently five times for each experimental condition. The results were presented as mean values ± standard deviations.