Ultrasonic monitoring of droplets’ evaporation: Application to human whole blood

• Human blood droplets were analyzed with shear reflectometry during evaporation.
• Specific evolution of shear reflection coefficient was linked to sol–gel transition.
• Specific times measured with ultrasounds were correlated to mass-versus time analysis.
• We therefore recommend this method to follow droplets during evaporation.

During a colloidal droplet evaporation, a sol–gel transition can be observed and is described by the desiccation time τD and the gelation time τG. These characteristic times, which can be linked to viscoelastic properties of the droplet and to its composition, are classically rated by analysis of mass droplet evolution during evaporation. Even if monitoring mass evolution versus time seems straightforward, this approach is very sensitive to environmental conditions (vibrations, air flow…) as mass has to be evaluated very accurately using ultra-sensitive weighing scales. In this study we investigated the potentialities of ultrasonic shear reflectometry to assess τD and τG in a simple and reliable manner. In order to validate this approach, our study has focused on blood droplets evaporation on which a great deal of work has recently been published. Desiccation and gelation times measured with shear ultrasonic reflectometry have been perfectly correlated to values obtained from mass versus time analysis. This ultrasonic method which is not very sensitive to environmental perturbations is therefore very interesting to monitor the drying of blood droplets in a simple manner and is more generally suitable for complex fluid droplets evaporation investigation.