Impact dynamics and deposition of pristine and graphene-doped PEDOT:PSS polymeric droplets on stationary and vibrating substrates

In this work, using high speed imaging, the impingement of water as the reference liquid, polymeric PEDOT:PSS and graphene-doped PEDOT:PSS droplets onto bare glass and FTO-coated glass substrates with varying contact angles (surface energy) is studied, in an attempt to understand the process of drop casting, spray coating and inkjet printing frequently used for the fabrication of polymeric thin films in thin film devices. The effect of imposing lateral and vertical ultrasonic vibration on the substrate on impinging droplets and deposition is also studied, and unprecedented results are obtained, and discussed. It is concluded that, when the substrate is fully cleaned and exposed to UV light, droplet contact angle significantly decreases because of an increase in the surface energy, resulting in an effective deposition process and suppression of receding. It is also demonstrated that imposing vertical ultrasonic vibration on the substrate increases spreading of PEDOT:PSS-based droplets. The enhanced spreading as a result of surface treatment and imposed vibration is of great importance for the fabrication of solution-processed thin films by droplet-based coating methods. It is also observed that imposed ultrasonic vibration results in a decrease in the roughness of the resulting thin solid films.