Study on pL-scale fluid dispensing behavior and transfer ratio variation based on a stretching liquid bridge

pL-scale fluid dispensing technology has broad applications in biomedicine, microelectronic encapsulation, and advanced manufacturing. However, existing methods still struggle to achieve stable, precise dispensing at the pL scale. To address this issue, the paper proposes a pL-scale fluid-dispensing method based on a stretching liquid bridge. This method requires no electric field, heating, or pneumatic assistance; it solely utilizes the up-and-down motion of a micrometer-scale transfer needle to dispense fluid onto a substrate surface. To identify the roles of the key parameters, this study combines theoretical analysis, VOF simulations, and experiments to investigate the dynamic behavior of the stretching liquid bridge, analyze the effects of transfer needle radius and surface separation on fluid dispensing behavior, and establish the corresponding empirical equations. The results indicate that, within the scope of this study, the capillary number ( Ca ) reflects the relative importance of viscous forces and surface tension during stretching and can characterize variations in the transfer ratio for different transfer needle radii and surface separations. A comparison of the experimental and simulation results shows that the Rheology, Polynomial, and Power models can describe the relationships between the capillary number and the transfer ratio, the transfer needle radius and the initial droplet volume, and the surface separation and the transfer ratio, respectively. These findings can serve as a reference for parameter configuration in pL-scale fluid dispensing.