A study on the droplet-jet electrospinning modes: Dynamic behavior and control theory

The steady droplet-jet electrospinning mode is a preferred mode for the production of fine nanofibers. However, it can only be achieved within a narrow range of operating parameters. The experimental findings on the range are not universally applicable due to the coupled interactions among solution properties, electric field configurations, and operating parameters. Additionally, the dynamic behavior of the meniscus complicates the identification of stable operating parameters. The shape parameters of the meniscus, such as volume and contact angle, offer a unified and easily measurable evaluation of the dynamic stability, yet a comprehensive investigation is lacking. In this paper, we identified various droplet-jet electrospinning modes and dynamic transitions between them from captured images of droplet-jet electrospinning process. Subsequently, the meniscus volumes and electrical contact angles in one-jet modes were measured by an image processing method. Based on the relationship between inflow and outflow rates, characterized by the fluctuations of meniscus volume, the operating parameters were classified into oversupply, equilibrium, and undersupply regimes. We employed the Gibbs inequality condition to explain the principles governing the stable range of electrical contact angles in these regimes. The alignment between experimental results and the predicted values supports the assumption that the range of electrical contact angles in steady droplet-jet electrospinning modes only depends on the intrinsic properties of the liquid solution and the needle geometry. Furthermore, we established a voltage-control law between the electrical contact angle and applied voltage, inspired by the electrowetting theory. These findings provide a theoretical foundation for controlling droplet-jet electrospinning processes.