Characterization of the surface tension and viscosity effects on the formation of nano-liter droplet arrays by an instant protein micro stamper

Rapid and parallel protein micro/nano array formation provides a powerful tool for protein chip fabrication and protein preservation. This paper presents the characterization of the surface tension and viscosity effects on the formation of nano-liter droplet arrays by a novel instant micro stamper, which can simultaneously immobilize hundreds of proteins on a chip. Capillary force is the major driving mechanism of the micro stamper, flowing protein solutions through the chip channel for array-registration and droplet-size control. Three important properties have been characterized in this paper, including uniformity of the parallel printing process, surface wettability and solution viscosity effect on micro droplet size and the dynamic sequence of micro droplet formation. Experimental results demonstrated the uniformity of the parallel stamping process for protein micro arrays from area to area and chip to chip. The effects of surface wettability of bioassay chips and solution viscosity on droplet-size variation have been investigated in detail by experiments and simulations. Both simulation and experimental results demonstrate that the spot size increases with increasing surface wettability and decreasing solution viscosity, and they showed similar tends. The dynamic process of droplet formation has also been observed and analyzed by high-speed images, demonstrating that the footprint reduction rate, formation time and necking of the printed micro droplet are lower on the more hydrophobic surface. This is due to the rapid shrinkage of the droplet footprint area on a hydrophobic surface, resulting in smaller droplet formation. The droplet size can shrink up to 50% when the contact angle of the bioassay chip surface increases from 30° to 80°. On the other hand, the variation of solution viscosity from 1.02 to 10.08 cp makes the droplet size shrink to 70%.