# Evaporation of a Reactive Nanofluid Sessile Drop: Capturing Rapid Emergence of Surface Crystals with In Situ Synchrotron X‑ray Diffraction

## TL;DR

This study uses X-ray techniques to observe how surface crystals form as a ZnO nanofluid drop dries, revealing new insights into the process.

## Contribution

The study introduces a novel method using synchrotron X-ray diffraction to capture the rapid formation of surface crystals in reactive nanofluids.

## Key findings

- Zn(OH)2 surface crystals emerge rapidly from the onset of evaporation.
- Transient layered complexes form before Zn(OH)2, visible through X-ray diffraction peaks.
- The method provides new insights into evaporation-induced self-assembly of reactive nanofluids.

## Abstract

Mechanisms for surface
pattern formation from evaporation of a reactive nanofluid
sessile drop are not well understood.
In contrast to the coffee-ring effect from inert particles, rapid
chemical and morphological transformation of reactive nanoparticles
upon rapid evaporative drying are challenging to probe experimentally.
Here, using grazing-incidence X-ray surface scattering, the nanostructure
of nascent surface patterns has been probed as a ZnO nanofluid sessile
drop rapidly dries. The high temporal resolution enabled by the high
flux of synchrotron X-rays allows the observation of the emergence
of Zn­(OH)2 surface crystals from the onset of evaporation
and their rapid evolution into the final residual surface pattern, via transient layered complexes evident from the temporary
appearance of X-ray diffraction peaks preceding Zn­(OH)2 formation. The results offer mechanistic insights of morphogenesis
of surface patterns from evaporation-induced self-assembly and self-organization
of reactive nanofluids, previously untenable using other experimental
methods.