Diffusion-limited evaporation of polymer solutions

Abstract

In many processes in nature and industry, water evaporates from polymer solutions. When the evaporation is fast, a polymer layer can form at the water-air interface resulting in a diffusive mass transfer resistance inside the solution, in a process termed diffusion-limited evaporation (DLE). DLE has been observed in solutions containing lipids or microgels, that often have a specific structural complexity. To understand the generality of the physical mechanisms in DLE, it is highly relevant to study DLE of polymer solutions.

In this thesis, we study the evaporation of water from solutions containing structurally simple polymers, with the aim of improving the fundamental understanding of DLE. We first present experimental data, where a polymerwater solution evaporates unidirectionally from rectangular capillaries. The evaporation rate settles according to the predicted diffusive scaling m˙ (t) ∼ t−1/2, but surprisingly does not settle with m˙ (t) → 0 at late times, which would be expected at thermodynamic equilibrium with the environment. The steady state evaporation dynamics in our experiments is explained as resulting from the buildup of a tensile stress in a glassy polymer layer at the interface, resulting from the compression by the evaporation flux, that offsets the water activity at the interface ai from thermodynamic equilibrium with the environment. We furthermore show how DLE can result in evaporation from polymer solutions that is insensitive to the ambient humidity, which sets the driving force for evaporation. Importantly, we find that for certain polymers evaporation is also humidity-insensitive at early times, without becoming diffusion-limited. This effect is caused by the rapid adsorption of polymers at the solution-air interface, with the interfacial polymer layer setting the early-times evaporation rate.

In the second part of this thesis we show DLE from polyelectrolyte solutions, that is similar to the neutral polymer at late times as a result of increased counterion condensation at high polyelectrolyte concentrations. The polymers we tested also approach a similar non-zero steady state evaporation rate m˙ (t), which suggests similar contributions from the mixing energy and mechanical response to the evaporative driving force.

The final part of this thesis presents a phase field model which allows us to establish the ubiquitous nature of DLE, independent of polymer properties. For a one-dimensional geometry, results from the model clearly show how DLE with m˙ (t) ∼ t−1/2 occupies a large region in the evaporation phase space, which also includes pure solvent-like (m˙ (t) ∼ const.) and arrested (m˙ (t) ∼ 0) evaporative regions. Finally, we show how the model can be extended to more complicated systems, like droplets or materials with heterogeneous internal structuring, which is important to translating our results to many practical applications.