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Article Dans Une Revue Langmuir Année : 2024

Convective Drying of Films of Polymer Solutions: Front Propagation Revealed by Thermal Measurements

Résumé

We report on the drying of films of polymer solutions under a controlled laminar air flow. Temperature measurements reveal that a drying front propagates in the film at constant velocity. Using thermal calibration, we are able to quantitatively determine the local drying rate of the film, and we find it agrees with conservation arguments. We further show that a simple mass balance allows us to relate the front velocity to the drying rate.

Introduction

Evaporation of water is a slow process because, in usual conditions, it is limited by diffusion of vapor in the atmosphere. Techniques to accelerate drying have been developed in industrial lines. Submitting the sample from which water or solvent must be extracted to a gas flow is particularly efficient and convective drying is widely used, for instance in the food [13] and coating [8] industries. When a thin liquid film is dried in a gas flow, as in a coating process, a front forms at the upstream edge, which propagates and separates a dry region from a liquid one. During the so-called directional drying, the microscopic structure of the sample may be modified by the passage of the front ; for instance, ordering [6] or aggregation [12] have been reported in colloidal suspensions.

The kinetics of the front in unidirectional drying has been the object of less attention, although it could provide crucial information on the local drying rate. Measurements of global drying kinetics by techniques such as gravimetry [2], were conducted but they do not provide information on the front propagation. Recently, the volume fraction distribution of drying colloidal suspensions obtained from transmitted light intensity in presence of and added dye [9] was reported but the authors did not discuss the front kinetics. Other systems dried in air flows were studied with spatially resolved techniques, such as Fourier Transform Infrared Microscopy [19] or thickness measurements based on laser reflectometry [20, 21], but the front velocity was not inferred from those measurements, although the effect of the front passage on local shrinking was emphasised.

In contrast, the kinetics of spontaneously forming drying fronts in evaporating films has been considered in several studies. Without air convection, lateral fronts form because drying is faster at the edges of the film. In the literature, the mechanism of faster drying is under debate and is attributed either to a larger evaporation flux at the edges [3] or to the film being thinner at the edges in the case of a small contact

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Dates et versions

hal-04689085 , version 1 (05-09-2024)

Identifiants

Citer

Apolline Faidherbe, Emmanuel Garre, Maxence Wilmet, Jérémie Teisseire, François Lequeux, et al.. Convective Drying of Films of Polymer Solutions: Front Propagation Revealed by Thermal Measurements. Langmuir, 2024, ⟨10.1021/acs.langmuir.4c02617⟩. ⟨hal-04689085⟩
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