Combined effects of Marangoni, sedimentation and coffee-ring flows on evaporative deposits of superparamagnetic colloids

Abstract

Evaporation of sessile colloidal droplets is a way to organize suspended particles. It is already known that the composition of the surrounding fluid modifies the dried deposit. For superparamagnetic particles, recent studies showed that external magnetic fields can act as remote controls for those deposits. In this paper, we study the configuration space given by the interplay of such fields and a modification of the fluid composition by considering various concentrations of phosphate buffered saline (PBS). We show that the magnetic field modifies the morphological properties of the deposit, while the composition (i.e. PBS concentration) modifies the density profile of the deposit. We then present an explanation of these influences considering the competition between (i) sedimentation, (ii) coffee-ring and (iii) Marangoni flows. From these considerations, we propose a master curve which should be able to model the deposit densities of any system where the above mechanisms compete with each other.

De l’évaporation des colloïdes magnétiques

“…Moreover, the PBS has a small tensioactive eect. Indeed, surface tension of PBS slightly increases with the PBS concentration [90]. Since the outward ow due to the coee-ring creates a gradient of concentration, it also induces a higher surface tension near the edge of the droplet.…”

“…The attractive behavior occurs between the various particles but also between particles and substrate. This has then two eects on the system [49,90]. The rst one is that particles are more likely to form aggregates which can then sediment.…”

“…This drag then destroys the self-assembled structures, and the eventually dried deposit has a structure close to a typical coee-ring. However, when there is both an external magnetic eld and a high concentration of ions, the self-assembled structures can resist this stress and the eventual deposit keeps the structures created by the magnetic eld [90]. This then leads to a rich phase diagram, as represented in Figure 9.…”

“…The magnetic eld reinforces this prevention and also creates a detailed structure at a smaller scale. Figure reprinted from [90] (DOI : https://doi.org/10.1016/j.colcom.2019.100198), with permission from Elsevier. importance of sedimentation through agglomerating particles.…”

“…This also leads to various possible deposits for a single suspension of particles, chosen through the properties of the external eld. Moreover, it has already allowed to make signicant steps in the comprehension and modeling of the mechanisms governing the distribution of colloidal particles in evaporative deposits [59,90].…”