Interface shape and wetting transition during fluid-fluid displacement in a capillary tube: laboratory experiments and bifurcation analysis

The displacement of one fluid by another immiscible fluid in a confined geometry is a key physical process in many natural and industrial settings, from geological CO₂ sequestration to microfluidics. One fundamental aspect of fluid-fluid displacement on a solid surface is the shape of the moving interface, characterized by the dynamic contact angle θ_d.

Despite its importance, the current literature presents conflicting views: one shows that θ_d increases monotonically to 180° as the flow rate increases, beyond which the fluid interface destabilizes and a trailing film of defending fluid is formed on the solid surface (Zhao et al., PRL 2018); another suggests that θ_d decreases to 0°, and the wetting transition occurs by the formation of pilot film of invading fluid (Levache & Bartolo, PRL 2014).



Here we experimentally study the fluid-fluid displacement in a prewetted capillary tube to establish a unified description of θ_d and the shape of the moving interface in strong imbibition and for different viscosity ratios. Remarkably, we observe a sharp wetting transition where θ_d jumps to 180° abruptly at a critical flow rate; beyond this critical flow rate, the invading fluid forms a finger that leaves a trailing film of defending fluid behind. We rationalize the emergence of this sharp, trailing-film-type of wetting transition by means of a minimal-ingredients hydrodynamic theory that exhibits bifurcated solutions. We further show that the pilot-film-type of wetting transition is caused by surface roughness.