Intrusive and Impact Modes of a Falling Drop Coalescence with a Target Fluid at Rest

Abstract

The evolution of the falling drop substance transfer in a target fluid at rest was traced by high-speed video techniques. Two flow modes were studied: slow intrusive flow, when the KE of the drop was comparable or less than the available potential energy (APSE), and a fast impact flow, at a relatively high drop contact velocity. For the substance transfer visualization, a drop of alizarin ink solution at various concentrations was used. The use of transparent partially colored fluid allows tracing the drop matter motion in the bulk and on the fluid free surface. The traditional side and frontal view of flow patterns were registered and analyzed. In both flow modes, the substance of the drop partially remained on the free surface and partially went into the target fluid bulk, where it was distributed non-uniformly. In the intrusive mode, the drop substance partially remained on the surface, while the main mass of the drop flowed into the thickness of the target fluid, forming the lenticular colored domain. The intrusion was gradually transformed into an annular vortex. In the impact mode, the drop broke up into individual fibers during the coalescence, creating linear and reticular structures on the surface of the cavity and the crown. The flow patterns composed of individual fibers were rapidly rebuilt as the flow evolved and the splash emerged and decayed. The sizes of cavities and colored fluid domains were compared in different flow regimes as well. The total energy transfer and transformation impact on the flow structure formation and dynamics was revealed.

Fine Flow Structure at the Miscible Fluids Contact Domain Boundary in the Impact Mode of Free-Falling Drop Coalescence

“…Traditionally, the technique of "back-light" illumination with a bright continuous [81] or pulsed [82] light source with an arc, gas-discharge or stroboscopic lamp is used in the droplet flows observation experiments. Recently, LED [44], fiber-optic [62] and laser [83] sources have become widespread. The emitted light is directly projected on to the studied flow domain, and sometimes, it passes through a semitransparent screen (for The drop formed in a dispenser 8 with a replaceable capillary of a diameter 0.8 < d o < 4 mm with a flat cross-section broke off under the gravity and fell on the surface of partially degassed tap water in a target cuvette 1 of the size 10 × 10 × 7 or 30 × 30 × 5 cm 3 .…”

“…Traditionally, the technique of "back-light" illumination with a bright continuous [81] or pulsed [82] light source with an arc, gas-discharge or stroboscopic lamp is used in the droplet flows observation experiments. Recently, LED [44], fiber-optic [62] and laser [83] sources have become widespread. The emitted light is directly projected on to the studied flow domain, and sometimes, it passes through a semitransparent screen (for example, tracing paper [47]) for more uniform illumination of the field of view.…”

“…An uneven section of the boundary in the range of angles "10-12 o'clock" allows us to consider the visible boundary as the actual edge of the spreading drop. The boundary is radially stretched by thin-layer flows localized in the contact domain of the cavity shell and the near-surface layer of the target fluid [62].…”

“…Here, the coefficient of surface tension at the interface of the liquid droplet with the air environment is σ a d . The intrusive mode is observed at R En < 1 [62]. In the impact mode, when the drop kinetic energy exceeds ASPE and R En > 1, thin jets are formed in the domain where a drop merges with a thin layer of liquid on a solid substrate [63].…”

“…When the free surfaces are eliminated during the coalescence of liquids, the ASPE quickly transforms into other forms (at a coalescence velocity of about 1 m/s per time 10 −10 < τ < 10 −8 s). The processes of ASPE conversion form ligaments (fast thin jets [70]), colored traces of which are observed when a free-falling drop merged with a fluid at rest in the impact mode at En k > En σ [62].…”