Evaporation of picoliter droplets on surfaces with a range of wettabilities and thermal conductivities

The evaporation of picoliter water and ethanol droplets generated by drop-on-demand inkjet printing was investigated on substrates with apparent contact angles between 10 degrees and 135 degrees and thermal conductivities between 0.25 and 149 Wm(-1)K(-1). Drying times were calculated from a diffusion- limited model for droplets with both pinned and moving contact lines as a function of droplet diameter and apparent contact angle. Droplets with a moving contact line take longer to dry on hydrophilic substrates than pinned droplets. The difference in drying times between evaporative modes vanishes at large apparent contact angles. Hence similar drying times are obtained for both modes on hydrophobic substrates. The predicted drying times for glass and silicon substrates were in good quantitative agreement with experimental data, suggesting that thermal effects are negligible for substrates of these base materials. However, on a PTFE substrate which has a lower thermal conductivity more relevant to inkjet printing, evaporative cooling reduces the evaporation rate causing drying times to be underpredicted by isothermal models.