Nanopipette dynamic microscopy unveils nano coffee ring

Abstract

Liquid-phase electron microscopy (LP-EM) imaging has revolutionized our understanding of nanosynthesis and assembly. However, the current closed geometry limits its application for open systems. The ubiquitous physical process of the coffee-ring phenomenon that underpins materials and engineering science remains elusive at the nanoscale due to the lack of experimental tools. We introduce a quartz nanopipette liquid cell with a tunable dimension that requires only standard microscopes. Depending on the imaging condition, the open geometry of the nanopipette allows the imaging of evaporation-induced pattern formation, but it can also function as an ordinary closed-geometry liquid cell where evaporation is negligible despite the nano opening. The nano coffee-ring phenomenon was observed by tracking individual nanoparticles in an evaporating nanodroplet created from a thin liquid film by interfacial instability. Nanoflows drive the assembly and disruption of a ring pattern with the absence of particle-particle correlations. With surface effects, nanoflows override thermal fluctuations at tens of nanometers, in which nanoparticles displayed a "drunken man trajectory" and performed work at a value much smaller than k_BT.

Keywords: coffee-ring effect; directed assembly; liquid-phase electron microscopy; nanopipette.

Fig. 1.

Quartz nanopipette for LP-EM. (A) Schematic diagram of a quartz nanopipette (Left), a SiN liquid cell (Right-Top), and a GLC (Right-Bottom). (B) The bright-field optical image (Left) and EM images (Right) of nanopipettes on a standard EM grid. (C) A plot of window (wall) thickness as a function of diameter for a nanopipette. Inset: comparison of nanopipettes’ success rate (Left axis) and mean sample search time (Right axis) with GLCs. (D) Comparisons of images obtained from nanopipette and GLC. Sample solutions: single-stranded DNA (Left, Bottom panel adapted from ref. 31), 100 mM lipoate in H₂O (Middle, Bottom panel adapted from ref. 36), Fe₃O₄ nanocube in oDCB (Right). (E) Liquid surface curvature at the tip of the nanopipette, and the definition of α, β and d. (F) A HR image and corresponding pseudocolor image of a GNR were obtained from a nanopipette showing a lattice structure. Zoomed (Scale bar: 2 1/nm) shows corresponding Fast Fourier Transformation (FFT) analysis. Imaging condition and statistics in SI Appendix, Text.

Fig. 2.

The formation of a nano coffee ring in a pinned nanodroplet (Movie S10). (A–C) Diagram (Top row) and time-lapse electron micrographs (Bottom row) of evaporating droplets: an evaporating plain droplet in A and its side view (highlighted by yellow arrows) in B; and a nanoparticle-containing droplet (C). Time zero denotes when the electron beam was on. The scale bar is 10 nm—details in SI Appendix, Text. In (C), the center-of-mass position of nanoparticles was tracked and superpositioned on the image of 1,228 s, and the color code indicates the time. (D) Quantifying changes in droplet diameter and mean intensity for A (squares and stars) and C (circles). Arrows point to the corresponding axis. (E) The step size distribution of nanoparticles of C. Insets: traces for individual nanoparticles, matching 1 to 7 in B. The solid black line is the guide to the eyes.

Fig. 3.

The reverse of a nano coffee ring in a “stick–slip” nanodroplet (Movie S12). (A and B) The schematic diagram (Top) of a droplet undergoes repetitive “stick–slip” transitions and the time-lapse LP-TEM images (Bottom) of a plain droplet (A) and a droplet containing nanoparticles (B). Color coding indicates the time: yellow for the initial and purple for the final. Time zero denotes when the electron beam was on. The scale bar is 10 nm. (C) The time-dependent changes of droplet diameters for data in A (circles) and B (stars: nanoparticles included, squares: nanoparticles excluded) are plotted as a function of time. (D) The step size distribution of individual particles in B. Insets: the traces of individual nanoparticles, matching 1 to 7 in B. The solid black line is the guide to the eyes.

Fig. 4.

Comparisons of nano coffee ring to micro coffee ring. (A) (a) Schematic depiction of a particle motion without or with thermal fluctuation and definition of longitudinal and lateral direction. (b–d) Particle trajectories for the cases (i), (ii), and (iv) as described in the text. (B) Comparisons of tortuosity (blue symbols), the variance of cosθ (red filled symbols), and the variance of sinθ (red grid symbols) of the particles for the four cases (i–iv). (C) Distributions for the four cases (i–iv) in blue, yellow, red, and purple. (D) Cross-correlation of speed for every two particles in (ii) the nano coffee ring formation, n = 7.