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.

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.

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