An experimental study on drop formation from a capillary tube

The dispersal of liquid into ambient air through a tube is a common method of identical drop formation. In the present work, the regimes and dynamics of droplet and/or jet formation during ejection from a vertical capillary nozzle are experimentally studied. The time evolutions of regimes of droplet and/or jet formation are recorded using a high-speed camera. The periodic dripping (PD), dripping faucet (DF), and jetting (J) regimes of deionized (DI) water and anhydrous ethanol are clearly observed as the Weber (We = ρV2D/σ) number increases. In the PD regime, drops periodically detach from the end of the capillary tube due to gravity overcoming the surface tension force. The mass of the detaching drops starts to vary from periodic to quasi-periodic in the DF regime, and a continuous jet with a smooth interface is formed in the J regime. The transition from PD to J occurs with an increase in the Weber number. The DF regime is difficult to identify and observe for anhydrous ethanol due to the lower surface tension. Over a complete period of dripping, a drop gradually changes from spherical- to pear-shaped. A liquid thread connects the top portion of the drop, and the remaining liquid at the end of tube gradually forms and pinches off over time. The period of drop emission of DI water is substantially longer than that of anhydrous ethanol for a selected tube due to the larger surface tension force. The dimensionless length Ztip/D0 continues to gradually increase with oscillations for DI water, while initially exhibiting very slight oscillations and subsequently suddenly increasing to its maximum value for ethanol. A linear relation between the period of drop emission and number of drops is identified in the PD regime. The dimensionless length of the drops increases and the dimensionless width varies minimally as the flow rate increases. The dimensionless limiting length (or thread) depends strongly on the flow rate and the tube diameter. Meanwhile, the liquid thread that connects the top portion of the droplet to the remainder of the liquid in the tube gradually thins as the tube diameter decreases, and faster narrowing was observed.