Comparison of Squeezing Dynamics with Contactless/Contact-loaded High Viscous Droplets Based on a Transfer Printing

For microdispensing of high-viscosity adhesives, the transfer printing by a needle-stamp with loaded microdroplets holds great potential, which is not limited by in-tube flow resistance and needle-stamp structures. Here, the squeezing dynamics of high viscous droplets were respectively described by the flat needle-stamp with contactless-loaded droplets and the conical/spherical needle-stamp with contact-loaded droplets in a transfer printing process. The expressions of the squeeze force between the loaded droplets and the substrate were established separately, and the main components of the force are compared and analyzed. Under the slit condition, the squeeze force was mainly composed of surface tension force and viscous force, and the viscous force dominated the squeezing and deforming of loaded droplets. By comparison, as the working radii of the conical/spherical needle-stamp increased gradually, the dominant effect of viscous force enhanced accordingly until the same effect as the flat needle-stamp. With the increase of the viscous force, the resolution of the contact radius between the deformed droplet and the substrate increased exponentially, improving the resolution of final transferred droplets. From the above analysis, transfer printing with contactless-loaded viscous droplets has more advantages for the improvement of dispensing resolution. Moreover, the squeezing, stretching, and breaking of contactless- loaded droplets between the flat needle-stamp and the substrate were also simulated by Fluent to verify its advantages.