Squeezing Dynamic Mechanism of High-Viscosity Droplet and its Application for Adhesive Dispensing in Sub-Nanoliter Resolution.

The dispensing resolution of high-viscosity liquid is essential for adhesive micro-bonding. In comparison with the injection technique, the transfer printing method appears to be promising. Herein, an analytical model was developed to describe the dynamic mechanism of squeezing-and-deforming a viscous droplet between plates in a transfer printing process: as the distance between plates decreases, the main constituents of contact force between the droplet and substrate can be divided into three stages: surface tension force, surface tension force and viscous force, and viscous force. According to the above analysis, the transfer printing method was built up to dispense high-viscosity adhesives, which replaced the geometric parameters, utilized the critical contact force to monitor the adhesive droplet status, and served as the criterion to trigger the liquid-bridge stretching stage. With a home-made device and a simple needle-stamp, the minimum dispensed amount of 0.05 nL (93.93 Pa·s) was achieved. Moreover, both the volume and the contact area of adhesive droplet on the substrate were approximately linear to the critical contact force. The revealed mechanism and proposed method have great potential in micro-assembly and other applications of viscous microfluidics.