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Comment trier des nanoparticules en utilisant l’évaporation ?

We studied the evaporation of sessile droplets containing mixtures of colloids and found that small colloidal particles always segregate at the contact line (Figure 1a). The mechanism is the following : evaporation is very fast at the contact line (shown by Deegan and coworkers in 2000) so a compensating hydrodynamical flow drives particles from the center of the drop to its periphery -it is known as the coffee-stain effect. In the case of mixtures of colloids, the large particles are stopped further away from the contact line than the small ones (Figure 1b). The typical size, , of the segregated zone is simply controled by the contact angle and the diameter of the particles by the relation tan = Dp/

Figure 1a. MEB image of the edge of a dried drop containing a mixture of nanoparticles and micrometric particles. The nanoparticles segregate at the contact line.

Figure 1b. .Schematic diagram of the edge of the drop.

In this Langmuir paper we also provide a simple method to track colloids as they flow toward the contact line. We obtain the velocity of the particles as a function of time and provide for the first time an complete validation of the theoretical model developped by Deegan in 2000. In particular, we find that the particles strongly accelerate at the end of the drying process and quantitatively relate this phenomenon to the decrease of the contact angle in the last stage of the evaporation process.

Paru dans “Packing and sorting colloids at the contact line of an evaporating drop”

C. Monteux and F. Lequeux, Langmuir, ASAP, DOI : 10.1021/la104055j