Controlled addition of new liquid component into surface droplet arrays by solvent exchange

Microscopic droplets integrating multiple functionalities are essential in the microcompartmentalized related reaction and applications. Solvent exchange is a simple approach for producing femtoliter surface droplets by the transit oversaturation created at the mixing front of a solution by a poor solvent. But it remains challenging to control the compositions in nanodroplets. Our hypothesis is the new liquid component can be added to the pre-formed surface droplets at certain ratios controlled by solvent exchange.

In this work we investigate the growth of the droplets during addition of a new component by solvent exchange. Two-component droplets were formed on a microdomain patterned substrate as highly ordered arrays. The physical and chemical parameters that control droplet composition and a possible application of the binary droplet arrays were presented in this work.

The added amount of the second component in the binary droplet can be quantitatively controlled by the solution and flow conditions. The theoretical prediction of the component ratio based on the droplet diffusive growth dynamics mode shows a good agreement with the experimental results. The results show that the solvent exchange on the surface with pre-formed droplets provides a highly efficient method to tune the droplet compositions to desired ratio. The unique feature of this approach enables a gradient structure of droplet composition over the surface, demonstrated by an application of different microlens performance on a surface.