Nanoparticle self-assembly in levitated drying droplets

How do nanoparticles organize as a liquid droplet dries? A new study uncovers the key role of surfactants in directing the formation of ordered plasmonic materials.

Scientists from LPS (CNRS/Paris-Saclay University), ICS (CNRS/University of Strasbourg), SOLEIL (SWING beamline) and CIC nanoGUNE (San Sebastian, Spain), have, thanks to Evaporation-induced self-assembly (EISA) in levitation, shed new light on this question by investigating the drying dynamics of silver nanorods in the presence of a common surfactant, cetyltrimethylammonium chloride (CTAC).

Using an innovative experimental approach based on levitated droplets, (developed between LPS and SWING), the team was able to monitor, in real time and without contact, the structural evolution of nanoparticles (NPs) throughout evaporation. Their findings challenge a widely accepted view of EISA: it is generally assumed that the progressive increase in concentration during drying drives the formation of ordered structures. But the study demonstrates that the initial surfactant concentration is in fact the key parameter controlling the emergence of NP superlattices. This effect outweighs the influence of NP concentration and even particle shape.

**Figure 1: a)** Schematic illustration of the setup to monitor EISA in levitated droplets. A droplet is kept in levitation from the ultrasound standing wave emitted from above and reflected below. The incident X-rays (pink beam) are diffracted by the droplet and give the 2D SAXS pattern on the sensor (colored shot). Meanwhile, two cameras monitor the droplet shape and size from a sideways position and from the incident X-rays direction using a slanted one-way mirror (following the grey beam). b) Images of the shrinking droplet (top) and the corresponding 2D SAXS patterns (bottom) acquired at two different times, close to the beginning and the end of the process.

The researchers show that surfactant micelles play a dual and dynamic role. At early stages, they act as depletants, i.e. inducing attractive interactions that trigger the nucleation and growth of ordered assemblies of nanorods. At later stages, as their concentration increases, these same micelles undergo a phase transition toward a gel-like state, which freezes the NPs assembly process. By combining both time-resolved small-angle X-ray scattering and microbeam SAXS analyses (at SWING beamline) with complementary electron microscopy (I2BC & NanoGUNE), the team also reveals that ordering occurs before complete drying and extends across the entire droplet, leading to a radial organization of nanorods at the mesoscale.

These results provide practical guidelines for designing reproducible nanoparticle superlattices and open new perspectives for the fabrication of plasmonic metamaterials, photonic structures, and functional nanodevices.