Uniform solute deposition of evaporable droplet in nanoliter wells

There have been many microdeposition processes that are based on the evaporation of nanoliter-sized droplets, such as inkjet printing, deoxyribonucleic acid/protein microarrays, or lithography direct writing. However, it is important but still difficult to control the uniformity of the solute deposition from a nanoliter sessile droplet on a plane substrate. This paper proposes a method for uniform solute deposition from evaporable droplet by confining the droplet with rib structures (wells) of specific surface properties. The hydrodynamic process was experimentally investigated and analyzed in detail. Surface wettability on the well surface is verified to be critical for controlling a droplet as a flat film inside a well during evaporation to minimize horizontal solute transfer for uniform solute deposition. Pure water and water/tracing particle mixture (2.57% solid latex, dyed blue) were employed for the test. The results demonstrated that a 97% uniformity is obtained for the solute deposited from a 37-nL droplet in a well with hydrophobic surface (contact angle of 100 degrees), whereas a 31% uniformity is obtained for a more hydrophilic surface (contact angle of 25 degrees). The higher hydrophobicity (contact angle above 90 degrees) on the well surface yields a Hatter profile of film during droplet evaporation inside a well and, thus, promotes a more uniform deposition of the solute.