Dynamic study of nanodroplet nucleation and growth on self-supported nanothick liquid films

In Situ Electron Microscopy Study of the Dynamics of Liquid Flow in Confined Cells

Confined liquid has attracted great attention due to its potential applications in nanofluidic devices. With the development of liquid-cell transmission electron microscopy (LC-TEM), investigating the behaviors of confined liquid can be realized in real time. However, the dynamics of the liquid layer in liquid cells have not been fully understood. Here, nanoparticles (NPs) adhered to the cell window membranes are used as reference objects to study the flow regime of the liquid layer, which causes cooperative motion of the membranes and the NPs. Two categories of motion behaviors are investigated. One is the contraction of NPs toward the interior viewing area which results from the spreading out of the liquid to the surrounding region, with the bending of the membranes increasing with the loss of liquid in the viewing area. The other motion behavior is the occasional movement of all the NPs in the same direction with the directional movement of the liquid layer. This work offers a new method to study the dynamics of liquids by LC-TEM, the discoveries of which are valuable for understanding the confined liquid dynamics.

From Contact Line Structures to Wetting Dynamics

An important reason for the century-long debate concerning wetting dynamics is the lack of decisive information about the contact line. The contact line cannot be treated as a geometric line but is rather a region with complex structures. The contact line regions have been intensively explored in recent years by utilizing advanced nanoscopic experimental and modeling methods. This feature article summarizes the primary observation results and related modeling progress. A framework is then proposed for understanding the wetting dynamics. Basic questions are raised for future research on the partial wetting of nonvolatile as well as volatile liquids.

Direct Imaging of Superwetting Behavior on Solid-Liquid-Vapor Triphase Interfaces

A solid-liquid-vapor interface dominated by a three-phase contact line usually serves as an active area for interfacial reactions and provides a vital clue to surface behavior. Recently, direct imaging of the triphase interface of superwetting interfaces on the microscale/nanoscale has attracted broad scientific attention for both theoretical research and practical applications, and has gradually become an efficient and intuitive approach to explore the wetting behaviors of various multiphase interfaces. Here, recent progress on characterizing the solid-liquid-vapor triphase interface on the microscale/nanoscale with diverse types of imaging apparatus is summarized. Moreover, the accurate, visible, and quantitative information that can be obtained shows the real interfacial morphology of the wetting behaviors of multiphase interfaces. On the basis of fundamental research, technical innovations in imaging and complicated multiphase interfaces of the superwetting surface are also briefly presented.

Paradoxical Long-Timespan Opening of the Hole in Self-Supported Water Films of Nanometer Thickness

The opening of holes in self-supported thin (nanoscaled) water films has been investigated in situ with the environmental scanning electron microscope. The opening of a hole occurs within a two-stage process. In the first stage, the rim surrounding a hole is formed, resembling the process that is observed under the puncturing of soap bubbles. In the second stage, the exponential growth of the hole is observed, with a characteristic time of a dozen seconds. We explain the exponential kinetics of hole growth by the balance between inertia (gravity) and viscous dissipation. The kinetics of opening a microscaled hole is governed by the processes taking place in the nanothick bulk of the self-supported liquid film. Nanoparticles provide markers for the visualization of the processes occurring in self-supported thin nanoscale liquid films.

Dynamic Behaviors of Condensing Clusters Based on Rayleigh Scattering Experiment

Condensation is a common physical process which widely exists in natural phenomena and thermal energy systems. In a condensation process, cluster is considered as the important bridge between vapor body and condensates. However, limited by the minimum imaging dimension of traditional measurements, early experimental studies about initial stages of condensation process are not sufficient. This paper provides a powerful optical platform for the study of dynamic clusters process. Based on the Rayleigh law, optical experiments were firstly introduced to investigate the clusters spatial distribution close to and far from condensation surface. The results show that clusters are mainly generated in the vicinity of the condensation surface within the thickness of 200 μm. When they move away from the condensation surface, clusters progressively vanish and they have a life cycle of a fraction of a millisecond. Though scattering intensity is proportional to the 6th power of cluster radius r and cluster number density N _c theoretically, the scattering intensity does not increase sharply with the increase of subcooling degree from the experimental results, so we can infer that the cluster number density plays a dominate role in this process and the effect of cluster radius almost can be ignored.Zhong Lan and Di Wang contributed equally to this work.

Atomic Scale Imaging of Nucleation and Growth Trajectories of an Interfacial Bismuth Nanodroplet

Because of the lack of experimental evidence, much confusion still exists on the nucleation and growth dynamics of a nanostructure, particularly of metal. The situation is even worse for nanodroplets because it is more difficult to induce the formation of a nanodroplet while imaging the dynamic process with atomic resolution. Here, taking advantage of an electron beam to induce the growth of Bi nanodroplets on a SrBi2Ta2O9 platelet under a high resolution transmission electron microscope (HRTEM), we directly observed the detailed growth pathways of Bi nanodroplets from the earliest stage of nucleation that were previously inaccessible. Atomic scale imaging reveals that the dynamics of nucleation involves a much more complex trajectory than previously predicted based on classical nucleation theory (CNT). The monatomic Bi layer was first formed in the nucleation process, which induced the formation of the prenucleated clusters. Following that, critical nuclei for the nanodroplets formed both directly from the addition of atoms to the prenucleated clusters by the classical growth process and indirectly through transformation of an intermediate liquid film based on the Stranski-Krastanov growth mode, in which the liquid film was induced by the self-assembly of the prenucleated clusters. Finally, the growth of the Bi nanodroplets advanced through the classical pathway and sudden droplet coalescence. This study allows us to visualize the critical steps in the nucleation process of an interfacial nanodroplet, which suggests a revision of the perspective of CNT.

Nanodroplet Depinning from Nanoparticles

Nanoscale defects on a substrate affect the sliding motion of water droplets. Using in situ transmission electron microscopy imaging, we visualized the depinning dynamics of water nanodroplets from gold nanoparticles on a flat SiNx surface. Our observations showed that nanoscale pinning effects of the gold nanoparticle oppose the lateral forces, resulting in stretching, even breakup, of the water nanodroplet. Using continuum long wave theory, we modeled the dynamics of a nanodroplet depinning from a nanoparticle of comparable length scales, and the model results are consistent with experimental findings and show formation of a capillary bridge prior to nanodroplet depinning. Our findings have important implications on surface cleaning at the nanoscale.

Numerical study of homogeneous nanodroplet growth

We investigate the axisymmetric homogeneous growth of 10-100 nm water nanodroplets on a substrate surface. The main mechanism of droplet growth is attributed to the accumulation of laterally diffusing water monomers, formed by the absorption of water vapour in the environment onto the substrate. Under assumptions of quasi-steady thermodynamic equilibrium, the nanodroplet evolves according to the augmented Young-Laplace equation. Using continuum theory, we model the dynamics of nanodroplet growth including the coupled effects of disjoining pressure, contact angle and monomer diffusion. Our numerical results show that the initial droplet growth is dominated by monomer diffusion, and the steady late growth rate of droplet radius follows a power law of 1/3, which is unaffected by the substrate disjoining pressure. Instead, the disjoining pressure modifies the growth rate of the droplet height, which then follows a power law of 1/4. We demonstrate how spatial depletion of monomers could lead to a growth arrest of the nanodroplet, as observed experimentally. This work has further implications on the growth kinetics, transport and phase transition of liquids at the nanoscale.

Droplet nucleation on a well-defined hydrophilic-hydrophobic surface of 10 nm order resolution

Water condensation on a hybrid hydrophilic-hydrophobic surface was investigated to reveal nucleation mechanisms at the microscale. Focused ion beam (FIB) irradiation was used to change the wettability of the hydrophobic surface with 10 nm order spatial resolution. Condensation experiments were conducted using environmental scanning electron microscopy; droplets, with a minimum diameter of 800 nm, lined up on the FIB-irradiated hydrophilic lines. The heterogeneous nucleation theory was extended to consider the water molecules attracted to the hydrophilic area, thereby enabling explanation of the nucleation mechanism under unsaturated conditions. Our results showed that the effective surface coverage of the water molecules on the hydrophilic region was 0.1-1.1 at 0.0 °C and 560 Pa and was dependent on the width of the FIB-irradiated hydrophilic lines and hydrophobic area. The droplet nucleation mechanism unveiled in this work would enable the design of new surfaces with enhanced dropwise condensation heat transfer.

Convex nanobending at a moving contact line: the missing mesoscopic link in dynamic wetting

The morphological information on the very front of a spreading liquid is fundamental to our understanding of dynamic wetting. Debate has lasted for years concerning the nanoscopic local angles and the transition from them to the macroscopic counterpart, θ(D). This study of nonvolatile liquids analyzes the interface profile near the advancing contact line using an advanced atomic force microscopy. The interface is found following the macroscopic profile until bending in a convex profile around 20 nm from the substrate. This shoe-tip-like feature is common in partially wetting while absent for completely wetting, and its curvature varies with advancing speed. The observation ends the long-standing debate about the nanoscopic contact angles and their speed dependency. The convex nanobending provides a mesoscopic link and effectively complicates the dynamic wetting behaviors.

Nucleation dynamics of water nanodroplets

The origin of the condensation of water begins at the nanoscale, a length-scale that is challenging to probe for liquids. In this work we directly image heterogeneous nucleation of water nanodroplets by in situ transmission electron microscopy. Using gold nanoparticles bound to a flat surface as heterogeneous nucleation sites, we observe nucleation and growth of water nanodroplets. The growth of nanodroplet radii follows the power law: R(t)~(t-t 0) β , where β~0.2-0.3.

Revisiting the fine structure of the triple line

The fine structure of the triple line for water droplets deposited on porous polymer substrates was investigated. Substrates were obtained with the breath-figures self-assembly. Water droplets demonstrated the pronounced Cassie-Baxter wetting regime. The triple line was imaged with environmental scanning electron microscopy. The roughness of a triple line was characterized with its averaged root-mean-square (rms) width w(L), and its scaling experimental dependence upon the length L of the triple line w(L) is proportional to L(ζ) was analyzed. The values of exponents in the range of 0.60-063 were established. The deduced values of ζ evidence the local nature of the triple-line elasticity and support the idea that the elastic potential of the triple line includes only even powers of the displacement.

Direct imaging of complex nano- to microscale interfaces involving solid, liquid, and gas phases

Surfaces with special wetting properties not only can efficiently repel or attract liquids such as water and oils but also can prevent formation of biofilms, ice, and clathrate hydrates. Predicting the wetting properties of these special surfaces requires detailed knowledge of the composition and geometry of the interfacial region between the droplet and the underlying substrate. In this work we introduce a 3D quantitative method for direct nanoscale visualization of such interfaces. Specifically, we demonstrate direct nano- to microscale imaging of complex fluidic interfaces using cryostabilization in combination with cryogenic focused ion beam milling and SEM imaging. We show that application of this method yields quantitative information about the interfacial geometry of water condensate on superhydrophilic, superhydrophobic, and lubricant-impregnated surfaces with previously unattainable nanoscale resolution. This type of information is crucial to a fundamental understanding as well as the design of surfaces with special wetting properties.

Methodology for imaging nano-to-microscale water condensation dynamics on complex nanostructures

A better understanding of the role that nanoscale surface chemical heterogeneities and topographical features play in water droplet formation is necessary to improve design and robustness of nanostructured superhydrophobic surfaces as to make them fit for industrial applications. Lack of an imaging method capable of capturing the water condensation process on complex nanostructures with required magnification has thus far hindered experimental progress in this area. In this work, we demonstrate that by transferring a small part of a macroscale sample to a novel thermally insulated sample platform we are able to mitigate flooding and electron heating problems typically associated with environmental scanning electron microscopy of water condensation. We image condensation dynamics on individual complex particles and a superhydrophobic network of nanostructures fabricated from low thermal conductivity materials with an unobstructed 90° perspective of the surface-to-water interface with field of view as small as 1 μm(2). We clearly observe the three-stage drop growth process and demonstrate that even during late stages of the droplet growth the nearly spherical drop remains in a partially wetting Wenzel state.