Transport at the air/water interface is the reason for rings in protein microarrays.

Fabricating a protein microarray involves the deposition of nanoliter droplets of solutions on a solid surface. Instead of uniform spots, one often observes ring-like structures that add to the difficulty in quantification. We show that the accumulation of proteins at the air/water interface of the nanodroplet is the reason. Transformation to a uniform spot can be achieved via the addition of competitive surfactants or the control of surface reaction kinetics.