Evaporation Behavior of Microliter- and Sub-nanoliter-Scale Water Droplets on Two Different Fluoroalkylsilane Coatings

The evaporation behavior of microliter (2.0 mu L) and subnanoliter (0.8 nL) scale water droplets was investigated on two smooth hydrophobic and hydrophilic fluoroalkylsilane coatings prepared using chemical vapor deposition. The contact angle was constant in the second stage of evaporation for a 2.0 mu L droplet on the hydrophobic coating, but it was slightly decreased in the case of a 0.8 nL droplet. The contact angle decreased gradually in the same stage of evaporation for a 2.0 mu L droplet on the hydrophilic coating, but it was almost constant for a 0.8 nL droplet. These differences in evaporation behavior are expected to originate from the differences of their magnitudes and signs of line tension.