Remote-controlled deposit of superparamagnetic colloidal droplets

Abstract

Evaporation of sessile droplets is a way to organize suspended particles and create surface coating. Many studies have demonstrated that suspensions with various composition can give rise to qualitatively different dried patterns, often by focusing on the radial density profile of deposited particles. We demonstrate that a single suspension of superparamagnetic colloids can give rise to several dried patterns thanks to an external magnetic field applied during the evaporation process. We show the various patterns obtained with zero, constant, rotating and oscillating magnetic fields, and evidence the continuous control given by the intensity of a constant magnetic field. We also show this magnetic control has a substantial effect on the morphological details of the deposits.FIG. 1. Four deposits (four corners of the drop) obtained with different magnetic fields (null, homogeneous, rotating and oscillating fields, amplitude of non-zero magnetic fields are 22.5 G). Each picture has been numerically colorized, cropped as corner and positioned according to the corresponding field condition. The bottom picture is a zoom on the squared area. The global surface density of particles does not change very much. However, the morphological details of the deposit are radically different from one field to the other.

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

“…Interestingly, the length of these chains varies with the magnetic field intensity, but does not depend significantly on the angular position in the deposit. For high PBS concentration, the coffee-ring trend in the deposit is reduced as well but chains can be found even in the centre of the deposit, as already described in a previous publication [27].…”

“…More details on the preparation protocol can be found in Ref. [27]. The M1-070/60 particles are covered with carboxyl groups (-COOH), which induces a negative surface charge due to ionization (−COO − ) at pH 6.5 and higher.…”

“…In our case, the first feature is the background of homogeneous density. This is induced by both sedimentation of the particles (enhanced by the DLVO interaction between particles and the substrate [58] and the selfassembly of the particles [27]) and Marangoni instability (induced by transitional Marangoni bulk instability due to PBS [17]). It is worthwhile to notice there is a difference between Marangoni instability, creating several recirculation cells on a single radius length of the droplet, and Marangoni recirculation, creating a single recirculation cell on one radius length of the droplet.…”

“…Droplets containing magnetisable particles have also attracted attention due to additional easily controllable magnetic properties [24][25][26]. Regarding micron-sized superparamagnetic colloids, magnetic interactions between particles can be used to control the properties of colloidal droplets' deposits [27].…”

“…Those controllable assemblies open ways to new kinds of applications thanks to their unique optical properties and versatility to execute various tasks [50,51,54,57]. In the context of evaporating droplets, it has been shown recently that these assemblies, in the case of micron-sized particles, could be deposited in the dried deposit [27]. However, it has been highlighted that such a deposition requires the right concentration of PBS, and that this latter parameter also has an impact on the dried deposit [17].…”