Thermal and dynamic scaling of spontaneous imbibition in plasma-treated paper

Spontaneous imbibition in paper-based materials generates heat release at the wetting front, with its magnitude and evolution governed by capillary dynamics and surface energy. Plasma activation of the paper substrate is expected to selectively modify surface wettability without changing the bulk properties of the fibers, and hence to tune the capillary thermodynamics in a controlled manner.

A systematic study of water imbibition in purely cellulosic paper was conducted using low-pressure O₂ and H₂ treatments over a range of exposure times. Vertically mounted strips were fed from a water reservoir, while simultaneous optical and infrared thermal imaging tracked front propagation and the temperature field. Front trajectories, velocities, and characteristics of the transient thermal signal were extracted to construct a scaling analysis that examines the validity of Lucas-Washburn dynamics and relates heat release to capillary dynamics and surface wettability.

Wetting front propagation remains Lucas-Washburn-like, with exponents clustered around 0.5 for all plasma conditions, while the thermal response features a localized temperature peak that travels one-to-one with the optical front. Plasma gas type and exposure time primarily modify the Washburn prefactor through changes in wettability, thereby altering the front speed and the spatiotemporal structure of the thermal peak. Expressed in terms of an Eckert-like number, capillary number, and effective wettability, all measurements collapse onto a single correlation, demonstrating that capillary-driven heat release in plasma-activated paper is quantitatively predictable and can be exploited in the design of thermographically readable paper-based devices.