Influence of initial distance between needle tip and substrate on contact dispensing of high-viscosity adhesive

In contact dispensing process including the extruding, stretching, and liquid-bridge breakup, the initial distance dn−sini between the needle tip and the substrate has great influence on the dispensed adhesive volume. Herein, the effect of dn−sini on the dispensing process in nano-liter level is studied for high-viscosity adhesive. Complete contact dispensing process is simulated using computational fluid dynamic technique. Moreover, the experimental platform including heating device, dispenser, and imaging unit etc. is built up to investigate the dispensing process. According to the results, with the increase of dn−sini, the variation of extruded adhesive volume can be divided into three regimes that are respectively dominated by different factors. As dn−sini is in the range of 45-75 μm, due to the spatial flow resistance decreasing, the extruded volume sharply increases. Then, when dn−sini is in the range of 75-115 μm, owing to the contact area sa-s between the extruded adhesive and the substrate continuing to decrease, the increase of extruded volume becomes slow. Once dn−sini reaches 115 μm, comparing to the decrease of sa-s, the contribution of spatial flow resistance can be neglected, thus the extruded volume will decrease until sa-s approaches to zero. Additionally, the following transfer process based on the liquid-bridge breakup is studied too. The variation of absolute transferred volume is consistent with that of the extruded volume. However, since sa-s continuously decreases with the increasing of dn−sini, the transfer ratio gradually decreases. The presented results can improve the precise contact dispensing process and other related techniques.