Experimental investigation of the impact, spreading, and drying of picolitre droplets onto substrates with a broad range of wettabilities

Abstract

The effect of the substrate wettability on the deposition of picolitre droplets by drop-on-demand technology is investigated experimentally. Substrates with a broad range of wettabilities are considered with contact angles ranging from 19∘ for water on glass to 118∘ for water on a super-hydrophobic plasma-treated glass slide. Droplet profiles are recorded using high-speed imaging. The effect of surface wettability on the drying of water droplets is investigated and compared with models and experiments from the literature. Results are presented for picolitre droplets and experimental conditions relevant to inkjet printing. The cases of the pinned contact line and the moving one with a constant contact angle are both addressed. For colloidal fluids, the formation of the deposit is further investigated using an inverted microscope coupled to a particletracking velocimetry system. This experimental technique allows us to track individual solute particles during the various stages of the deposition. Preliminary results show the promise of this technique for characterizing internal flows within the droplet.