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Hierarchical surface patterns from dissolution-reaction-crystallisation mediated evaporation controlled self-assembly (DRC-ECA) and its antimicrobial coating application

Periodic Reporting for period 1 - DRC-ECSA (Hierarchical surface patterns from dissolution-reaction-crystallisation mediated evaporation controlled self-assembly (DRC-ECA) and its antimicrobial coating application)

Reporting period: 2015-09-04 to 2017-09-03

From the mesmerizing intricacy of an ammonite shell to the chilled elegance of a snow flake, nature articulates langue of shape and geometry fluently and delivers complex patterns and structures on all length scales with ease and panache via self-assembly and self-organisation. In nanoscience, we aspire to harness such linguipotence of geometry to create hierarchical nanostructures with tailored geometry and enhanced functionalities. A widely studied system for spontaneous pattern formation is evaporative drying of a sessile drop containing non-volatile particles. The most familiar pattern is the “coffee ring”, due to an outward capillary flow that shuttles dispersed particles towards the peripheral contact line where they get trapped. Marangoni effects may counteract this capillary flow, and the residual pattern may be further influenced by instabilities triggered by a temperature gradient across the solvent layer that manifest in different convective patterns, e.g. the Bénard-Marangoni (BM) convection. By controlling parameters such as evaporation rate, substrate chemistry, particle shape, size and concentration, droplet confinement, and surfactant addition, a plethora of patterns can be obtained, such as concentric rings, polycrystalline dendrites, uniform deposits, and polygonal particle networks. The coffee ring effect has also been exploited in applications, e.g. inkject printing and fabrication of sensors and transparent conductors.

In these previous studies, the dispersed non-volatile particles were inert; mechanistically, the pattern formation resulted from a competition between inter-particle forces and capillary and convective solvent flows. It remains little understood how reactive particles may alter evaporation induced patterns, for in situ generated molecular and particulate species can affect the solvent flows and thus the residual pattern.

The overall objectives for this project are:
1) To elucidate a mechanism for the formation of complex patterns from the evaporation of a reactive ZnO nanofluid droplet
2) To study a plethora of physical parameters (such as particle size and morphology, substrate chemistry, evaporation rate, etc.) on the ultimate pattern formation
3) To explore potential functionalities of such surface patterns.
1) We studied the generation of polycrystalline residual patterns with dendritic micromorphologies upon fast evaporation of a mixed-solvent sessile drop containing reactive ZnO nanoparticles. The molecular and particulate species generated in situ upon evaporative drying collude with and modify the Marangoni solvent flows and Bénard-Marangoni instabilities, as they undergo self-assembly and self-organisation under conditions far from equilibrium, leading to the ultimate hierarchical central cellular patterns surrounded by a peripheral coffee ring upon drying. Here we show that, upon fast evaporation (several minutes) of a ZnO nanoparticle dispersion in chloroform/methanol/isobutylamine mixture (denoted as CM/iB), hierarchical polycrystalline patterns with dendritic micromorphologies were formed, comprising a central region of cellular patterns surrounded by a peripheral coffee ring. We elucidate a formation mechanism for such hierarchical surface patterns as follows. ZnO nanoparticles undergo rapid chemical transformation into isobutylamine-ZnOH molecular complexes (iZMCs) of sub-nanometer in size, which further self-assemble into nanoclusters. These surface active iZMCs and clusters accumulate at the drop surface and the peripheral contact line. As the drop thins, BM flows are triggered, and iZMC-cluster coalescence along multiple BM flows leads to the formation of the central cellular patterns. Concurrently, this dendritic cluster growth also occurs at the receding peripheral contact line. Further drying drives iZMC organisation into crystal lattices in the micro-dendrites in the final hierarchical polycrystalline surface structures. We also show that the micro-morphological details of the dendrites in the residual pattern depended on the solvent composition, evaporation rate and ZnO nanoparticle concentration, thus confirming the interplay between the in situ generated iZMC clusters and the dynamic solvent flows under fast evaporation.

2) Utilizing synchrotron X-ray scattering, we have studied the structural evolution of the surface pattern as a droplet dried. This was complemented by micro-focused GIXS on a pre-dried surface. These results shed further light on the mechanism of the pattern formation.

3) We have explored functionalities of nanotextured surfaces with ZnO patterns, such as anti-reflectance, wettability, and antibacterial efficacy.

Two manuscripts have been submitted, and are currently under review (to Phys. Rev. Materials and Langmuir respectively). 3 further manuscripts are under preparation. The results have been presented at the European Colloids and Interface Society annual conference (2016), the ACS Colloids Symposia (2015, 2016, and 2017). Initial test focusing on the functionalities of these surfaces have been tested, and it has also initiated collaborations with Indian Institute of Science (a Newton PhD placement 6 months).
1) Evaporation induced self-assembly from a reactive nanofluid droplet has been studied for the first time.
2) A mechanism has been proposed based on unprecedented observations.
3) Synchrotron scattering has been brought to bear to observe the structural evolution of the surface pattern.
4) Although the full exploitation of such complex patterns is in its infancy, we have demonstrated potential antimicrobial activity of such surfaces, which is relevant to the antimicrobial resistance challenge that faces EU and the world.