Morphogenesis of polycrystalline dendritic patterns from evaporation of a reactive nanofluid sessile drop

Hua Wu and Wuge H. Briscoe

Phys. Rev. Materials 2, 045601 – Published 4 April 2018

Abstract

We report 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-organization under conditions far from equilibrium, leading to the ultimate hierarchical central cellular patterns surrounded by a peripheral coffee ring upon drying.

Received 15 September 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.2.045601

Physics Subject Headings (PhySH)

Evaporation Marangoni convection Reacting flows

Nanoparticles

Polymers & Soft Matter

Authors & Affiliations

Hua Wu^* and Wuge H. Briscoe^†

School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom

^*chzhw@bristol.ac.uk
^†wuge.briscoe@bristol.ac.uk

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Vol. 2, Iss. 4 — April 2018

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Images

Figure 1
Residual surface pattern and SEM images from evaporative drying of a ZnO nanofluid droplet on a glass coverslip (24 × 24 mm). (a) Photographic image of a residual surface pattern. (b), (c) Enlarged view of regions 1 and 2 in (a). (d) Enlarged view of region 3 in (a). (e) Enlarged view of region 4 in (d). The nanofluid droplet was ∼400 μL in volume, containing 1 mg/mL ZnO nanoparticles (Supplemental Material Fig. S1 [13]) in a mixture of chloroform and methanol (denoted CM; in a volume ratio of $V_{C} : V_{M} = 10 : 3)$ and also isobutylamine in an overall volume ratio $V_{CM} : V_{iB} = 5 : 1$ . Typically this evaporative drying process was completed in ∼20 min at 20 °C and a relative humidity (RH) ∼65%.
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Figure 2
HRTEM images of the constituent nanostructures of a ZnO nanofluid droplet on a glass cover slip at different time intervals: (a) 2 min, (b) 4 min, (c) 12 min, and (d) 20 min (complete drying). [Experimental protocol for HRTEM sample preparation of Figs. 2–2 in Supplemental Material SM 3 and Fig. 2 in SM 4.] (e) The two-dimensional hexagonal lattices (as indicated by the red hexagon) in the white frame in (d). (f) SAED pattern of the 2D lattices in (d) assigned to different lattice planes. (g) HRTEM image of a single crystallite from completely dried deposits dispersed into ethanol, showing four lattice layers (as indicated with four colored lines) all with iZMCs in a 2D hexagonal lattice structure. The dotted white circles in (a)–(c), (e) indicate iZMCs.
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Figure 3
Cryo-TEM images of nanoconstituents in an evaporating droplet at different time intervals: 2 min (a), (b), 4 min (d), (e), 6 min (g), (h), and 8 min (j), (k); and corresponding schematic of flows and cluster formation (c), (f), (i) and (l). Images (b), (e), (h), (k) are enlarged views of the white frames in (a), (d), (g), (j), respectively. The insets in (h), (k) show enlarged views of the white frames in (h), (k), respectively. The scale bars in (a), (d), (g), (j) are 200 nm. The red frames (numbered 1–3) in Fig. 3 highlight branched or linear aggregates of coalesced secondary clusters [red dashed circles in Fig. 3].
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Figure 4
Schematic images of capillary ripples and following BM cell development via cluster coalescence (a), (b), and a series of images (c)–(h) extracted from Video 2 at time sequence of 0, 0.3, 0.75, 1.65, 2.25, and 2.7 s respectively: (a) BM convection flows induced capillary ripples (schematically); (b) BM cell formation via cluster coalescence in DLA process; (i) the diameter of BM cells at ten site as a function of time, with the growth of the BM cell at site 1 highlighted with red circles from (b) to (f) as an example; and (j)–(m) the magnified images of site 1 in (c), (d), (e), (f), respectively. For the video microscopy experiment, a 100-µL droplet containing 1 mg/mL ZnO nanoparticles (SM Fig. 1) was dropped on the glass coverslip (1 mm × 1 mm). The scale bars are 200 µm in (c)–(h) and 50 µm in (j)–(m).
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Figure 5
Photos and SEM images of residual patterns under different solvent compositions and slow evaporation. (a)–(c) The solvent compositions are $V_{CM} : V_{iB} = 50 : 1$ and (d)–(f) pure isobutylamine, with drying time ∼20 min for both. For slow evaporation of a droplet (particle concentration 1 mg/mL; $V_{CM} : V_{iB} = 5 : 1$ ; 20 °C; $RH \approx 65 %)$ , with evaporation times of (g) 4 h, (h) 8 h, (i) 24 h, the residual patterns appeared as interconnected fiber networks, with enlarged views in (j), (k), (l) corresponding to the white frames in (g), (h), (i), respectively. Note that the coffee ring had just begun to merge at evaporation time of 4 h (g) but was absent for slower evaporation (h), (l).
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