The petri dish (Fisher Scientific, Waltham, MA, USA) was firstly treated with 20 mL of 2% (3-aminopropyl) triethoxysilane (APTES) (Sigma–Aldrich, St. Louis, MO, USA) in ethanol for 30 min on 50 °C hot plate. The heated petri dish was then rinsed with ethanol, dried with N2 gas, and placed in a 65 °C oven for 30 min. After 30 min, the surface-treated petri dish was kept at room temperature before immediate usage. During the use, the petri dish was firstly filled with the droplet carrier oil, FC-40 oil (3M, Two Harbors, MN, USA), to reduce the droplet evaporation issues during the printing.
The printed array density was characterized by controlling the oil injection time in between the droplets. More specifically, food dye was used to continuously generate and print 75-nL droplets on a 10-cm diameter petri dish with different oil injection time of 5 and 15 s. With the robotic arm set to move at the minimum speed, the array density increased with shorter oil injection time. The contact printing of the droplet array was further tested to its maximum capability on a 15-cm diameter petri dish with around 2700 75-nL droplets composed of alternating blue and green food dyes.
The printed array density was characterized by controlling the oil injection time in between the droplets. More specifically, food dye was used to continuously generate and print 75-nL droplets on a 10-cm diameter petri dish with different oil injection time of 5 and 15 s. With the robotic arm set to move at the minimum speed, the array density increased with shorter oil injection time. The contact printing of the droplet array was further tested to its maximum capability on a 15-cm diameter petri dish with around 2700 75-nL droplets composed of alternating blue and green food dyes.