Precursor film dynamics during imbibition in capillaries with variable cross sections

Understanding imbibition and the associated microscopic wetting dynamics is essential for advances in microfluidics, energy-related applications, and surface material design. In this work, we numerically investigate imbibition in straight, converging, and diverging capillaries under various pressure differences using a three-dimensional (3D) Shan–Chen lattice Boltzmann method. In straight capillaries, the meniscus advances steadily with a dynamic contact angle that remains consistently larger than the static value. In converging capillaries, imbibition initiates slowly due to the wide inlet and the resulting weak capillary pressure but accelerates as the interface propagates downstream. In contrast, imbibition in diverging capillaries is initially rapid and progressively decelerates as the channel expands. The imbibition rate is found to strongly regulate precursor film formation, with slower meniscus advancement favoring earlier film propagation. Under small pressure differences, precursor films emerge at early stages in converging capillaries, whereas in diverging geometries they appear only at intermediate or late stages. We further examine precursor film dynamics in rapidly diverging capillaries subjected to adverse pressure gradients. With increasing adverse pressure, the main meniscus progressively slows and may eventually arrest, while the precursor film continues to spread and can fully coat the capillary wall. Beyond a critical pressure threshold, both imbibition and precursor film propagation are completely suppressed. These results highlight that by tuning the axial capillary geometry and the driving pressure difference, the imbibition velocity can be effectively regulated, enabling the controlled manipulation of precursor film dynamics in capillary-driven microfluidic systems.