Evaporation evolution of volatile liquid droplets in nanoliter wells

Abstract It is well known that solute usually carries out non-uniform deposition when deposited from evaporating droplets. However, droplets are commonly employed carriers for many applications requiring uniform deposition, such as ink-jet printing, pattern direct writing for color filter, protein/DNA micro arrays, or organic light emitting diode (OLED) fabrication, etc. In this study, the evaporation behavior of volatile micro droplets with nanoliter volume has been modeled and experimented in detail to insight this phenomenon for addressing the issue of non-uniform deposition by droplets. The results showed that there are three phases (I: constant baseline region, II: constant contact-angle region, III: shrinking region) existing during the evaporation of water droplets on a flat surface. Average volume rate of evaporation has been characterized to be −1.66 nl/s for initial baseline of 2.27 mm during the first 2160 s. The center part of droplet seemed to have more solute deposition than that of the border. To improve deposition uniformity, well-type microstructures were employed to compensate the center deposition effect. The evaporation experiment performed in circular nanoliter wells of height 65 μm and diameters ranging from 240 μm to 1270 μm has demonstrated that the moving speeds of thin water films during rupture process range from 21 μm/s to 25 μm/s. Circular wells carry out higher initial breakage percentage, thus reducing the non-uniform region for final deposition. This study will be beneficial to the design of microstructures for uniform solute deposition from the evaporation of liquid droplets.