Researchers have provided the first quantitative characterization of coalescence between a sessile droplet (attached to a fiber) and a pendant droplet (suspended in the continuous phase) – an asymmetric configuration prevalent in industrial fiber coalescers for oily wastewater treatment. Using high‑speed imaging and a Mask R‑CNN deep‑learning model, the study identified three distinct stages. The liquid bridge expansion (Stage I) is governed by capillary pressure difference, insensitive to droplet size ratio. In the oscillation decay stage (Stage II), fiber adhesion rapidly dissipates energy. Crucially, increasing the sessile‑to‑pendant radius ratio to 1.5 significantly reduces the size of secondary droplets generated during neck rupture, offering a direct strategy to improve separation efficiency.
Fiber coalescers are widely used to separate oil from water in petrochemical and other industries. Their performance depends on how dispersed oil droplets merge on fiber surfaces. Most previous studies focused on symmetric coalescence of two sessile droplets sitting on a fiber. In real industrial conditions, however, a more common event is the interaction between a sessile droplet already attached to a fiber and a pendant droplet suspended in the flowing continuous phase. This asymmetric configuration introduces gravity orientation and different kinematic freedom, yet it has remained largely unexplored.
In a study published in ENGINEERING Chemical Engineering, researchers from East China University of Science and Technology used a high speed camera at 20,000 frames per second and trained a Mask R CNN neural network to automatically segment droplet boundaries and extract morphological parameters with high precision (mean relative error for liquid bridge width only 1.12 %). They systematically investigated coalescence for sessile to pendant radius ratios of 1, 1.25, and 1.5, with water droplets in isooctane under conditions where inertia dominates over viscosity.
The coalescence process comprises three stages. In Stage I (liquid bridge formation), upon contact a liquid bridge expands. The bridge width grows in proportion to the square root of time, confirming the classical capillary inertial scaling. The driving capillary pressure difference is nearly independent of the droplet size ratio, showing that the initial dynamics are locally controlled by curvature rather than global dimensions.
Stage II (oscillation decay) shows unique behavior. In contrast to symmetric sessile sessile systems where periodic oscillations persist over many cycles, the pendant sessile configuration exhibits rapid energy dissipation because the fiber strongly suppresses oscillations through contact line damping. The amplitude of the capillary wave on the pendant droplet side increases with the size ratio. Neck rupture at the needle tip and fiber junction causes abrupt energy loss, eliminating distinct periodicity and leading to quick equilibrium.
Stage III (stable morphology formation) involves secondary droplet generation from pinch off of neck filaments. The upper neck (needle side) evolves through hourglass, conical, and droplet formation stages, producing multiple secondary droplets. The lower neck (fiber side) is suppressed by fiber adhesion, generating only a single tiny droplet. Increasing the radius ratio to 1.5 systematically reduces secondary droplet sizes and completely eliminates fiber side breakup.
These findings provide direct guidance for controlling polydisperse droplet emissions in industrial fiber coalescers, enhancing oil water separation efficiency.
