Superparamagnetic colloid’s droplets evaporation under field

The evaporation of colloidal droplets is currently a topic of intensive research due to its wide range of potential applications, extending from blood analysis via inkjet printing, paint and polymers, up to and including nanotechnology. The main feature of colloidal droplets evaporation is the so-called coffee-ring effect. This effect implies that particles tend to be deposited at the edge of the final dried pattern. However, several studies have shown various mechanisms preventing this effect. Notably, capillary interactions of ellipsoidal particles and the presence of surface-adsorbed polymers allow a uniform coating of the particles. More recently, some researchers have shown that more complex deposits' structures can be achieved thanks to various concentration of liquid crystals. In our research, we demonstrate how magnetic interactions between superparamagnetic colloidal particles can be used to control the properties of colloidal droplets' deposits. This actually requires the right chemical composition of the suspension. But then, a single suspension can result in various dried patterns. The actual pattern is selected from the application of an external magnetic field during the evaporation. To our knowledge, this is the first time that such a tunable control of evaporative deposits is described in the scientific community.