Infrared thermography monitoring of the NaCl crystallisation process

• Published in: Infrared physics & technology 2015-07, Vol.71, p.198-207
• Main Authors: Vázquez, Patricia, Thomachot-Schneider, Céline, Mouhoubi, Kamel, Fronteau, Gilles, Gommeaux, Maxime, Benavente, David, Barbin, Vincent, Bodnar, Jean-Luc
• Format: Article
• Language: English
• Subjects: Creeping; Earth Sciences; Efflorescence; Emissivity; Infrared thermography; NaCl; Petrography; Salt crystallisation; Sciences of the Universe
• ISSN: 1350-4495; 1879-0275
• DOI: 10.1016/j.infrared.2015.03.013

•Quantitative investigation of the crystallisation of NaCl from evaporating droplets.•Infrared thermography revealed different crystal growth processes.•The thermosignal depended on variations in the emissivity and temperature.•Intermittent drops in the thermosignal revealed solution creeping processes.•Infrared thermography permitted, for the first time, the in-situ observation of the creeping phenomenon. In this work, we describe the growth of NaCl crystals by evaporating droplets of aqueous solution while monitoring them with infrared thermography. Over the course of the evaporation experiments, variations in the recorded signal were observed and interpreted as being the result of evaporation and crystallisation. In particular, we observed sharp and transient decreases in the thermosignal during the later stages of high-concentration drop evaporation. The number of such events per experiment, referred to as “pop-cold events”, varied from 1 to over 100 and had durations from 1 to 15s. These events are interpreted as a consequence from the top-supplied creeping (TSC) of the solution feeding the growth of efflorescence-like crystals. This phenomenon occurred when the solution was no longer macroscopically visible. In this case, efflorescence-like crystals with a spherulite shape grew around previously formed cubic crystals. Other crystal morphologies were also observed but were likely fed by mass diffusion or bottom-supplied creeping (BSC) and were not associated with “pop-cold events”; these morphologies included the cubic crystals at the centre, ring-shaped at the edge of droplets and fan-shaped crystals. After complete evaporation, an analysis of the numbers and sizes of the different types of crystals was performed using image processing. Clear differences in their sizes and distribution were observed in relation to the salt concentration. Infrared thermography permitted a level of quantification that previously was only possible using other techniques. As example, the intermittent efflorescence growth process was clearly observed and measured for the first time using infrared thermography.