Evaporation limited spreading of ethanol on rectangular porous strips: an experimental and theoretical investigation

TL;DR

This study investigates ethanol wicking on porous strips, revealing evaporation limits height and introducing an improved model that accounts for nonlinear evaporation rates, supported by experimental and thermal imaging data.

Contribution

It provides the first detailed experimental and theoretical analysis of evaporation-limited wicking, introducing the Non-Constant Evaporation Model (NCEM) that improves upon previous models.

Findings

Evaporative mass loss limits the wicking height significantly.

Thermal imaging shows nonlinear temperature distribution and inversion.

The NCEM accurately captures the nonlinear evaporation behavior.

Abstract

Wicking is a widely studied process in both natural and artificial systems. In many industrial applications, such as heat pipes, the wicking liquid evaporates to regulate temperature effectively. This study focuses on a simpler scenario where liquid ethanol climbs a vertically oriented filter paper FP under laboratory conditions, facilitating mass loss through evaporation and inducing cooling. Three filter papers with different permeability values were used, and three diagnostic methods optical imaging, thermal imaging, and precision weighing were employed to understand the dynamics of the process. The results showed a steady state height Lc significantly lower than Jurins limit in all cases, indicating that evaporative mass loss, and not gravity, limits the process. For instance, the filter paper 1005FP, with a capillary radius of 59microm and an average pore size of 2.50microm, would reach a Jurins height of 9.6cm with ethanol if evaporation were not allowed. However, when evaporation occurred, the height reduced to 1.2cm, an eightfold decrease, a similar reduction by a factor of 3 was observed for 1004FP. Further, thermal imaging revealed a non constant temperature distribution along the filter paper, with an unusual temperature inversion near the middle of the wicking liquid. This observation led to an improvement of the Constant Evaporation Model CEM by Fries et al 2008 by accounting for the nonlinear behavior of evaporation rates varying with vertical position. This new model termed the Non-Constant Evaporation Model NCEM, tested two power-law relations for evaporation rates , both of which successfully captured the key features of the process.