Microwetting of supported graphene on hydrophobic surfaces revealed by polymerized interfacial femtodroplets

In Situ Transmission Electron Microscope Liquid Cell 3D Profile Reconstruction and Analysis of Nanoscale Liquid Water Contact Line Movements

Static nanodroplets and dynamic contact line (CL) movements were visualized by an in situ transmission electron microscope (TEM) liquid cell technique at nanometer spatial resolution. Crawling and sliding movements of nanoscale CL were observed. The crawling happened at a capillary number (Ca) range of ∼10^-9 to ∼10^-8, and the sliding happened at a Ca range of ∼10^-8 to ∼10^-7. Three dimensional (3D) image construction had been employed to study static and dynamic contact angles (CAs) at nanoscale. CA hysteresis at nanoscale was observed in the sliding but not in the crawling. The energies associated with sliding were analyzed to investigate the CA hysteresis. An empirical model of the relationship between nanoscale CAs and Ca was developed. Both the experimental observation and the empirical analysis suggested that the competition among substrate defect, CL elastic, and molecular activation energies dictated different CL movements at nanoscale.

Surface Nanodroplets: Formation, Dissolution, and Applications

Droplets at solid-liquid interfaces play essential roles in a broad range of fields, such as compartmentalized chemical reactions and conversions, high-throughput analysis and sensing, and super-resolution near-field imaging. Our recent work has focused on understanding and controlling the nanodroplet formation on solid surfaces in ternary liquid mixtures. These surface nanodroplets resemble tiny liquid lenses with a typical height of <1 μm and a volume of subfemtoliters. The solvent exchange is based on the process of displacing a droplet liquid solution by a poor solvent to create a transient oversaturation for droplet formation. A quantitative understanding of growth dynamics of surface nanodroplets in ternary liquid mixtures not only provides insight into the liquid-liquid phase separation induced by solvent addition in general but also has made it possible to control the droplet size well. This review article will summarize our findings in the last ∼5 years from the research with our collaborators. The first part will explain the fundamental aspects that are key to the formation and stability of surface nanodroplets. In the second part, we will highlight the applications of nanodroplets in chemical analysis and functional surface fabrication and finally point out future directions in droplet-based applications.

Morphological Transformation of Surface Femtodroplets upon Dissolution

Constructing controllable liquid patterns with high resolution and accuracy is of great importance in droplet depositions for a range of applications. Simple surface chemical micropatterns have been popularly used to regulate the shape of liquid droplets and the final structure of deposited materials. In this work, we study the morphological evolution of a dissolving femtoliter droplet pinned on multiple microdomains. On the basis of minimization of interfacial energy, the numerical simulations predict various symmetric droplet profiles in equilibrium at different liquid volumes. However, our experimental results show both symmetric and asymmetric shapes of droplets due to contact line pinning and symmetry breaking during droplet dissolution. Upon slow volume reduction, the deposited microdroplet arrays on one single type of simple surface prepatterns spontaneously morphed into a series of complex regular 3D shapes. The findings in this work offer insights into design and prepararion of the rich and complex morphology of liquid patterns via simple surface premicropatterns.

Microwetting of pH-Sensitive Surface and Anisotropic MoS2 Surfaces Revealed by Femtoliter Sessile Droplets

Understanding the microwettability of anisotropic molybdenum disulfide crystal is critically important in separation and processing of this material in liquid. In this work, static microwetting properties of MoS₂ face (MF) and MoS₂ edge (ME) surfaces in water are revealed by the morphology of femtoliter interfacial droplets. The oil droplets with different size distribution were produced from heterogeneous nucleation and growth of nanodroplets during the solvent exchange under controlled flow and solution conditions, and were polymerized for droplet morphology characterization to reveal the relative wettability of the droplets on surfaces. We first demonstrate that the shape of the nanodroplets is responsive to the surface charges on a model pH sensitive substrate of gold coated with a self-assembled monolayer of two types of thiol. The experimental results on MoS₂ substrates indicate that (1) oil contact angle of the droplets on ME surface is much larger than that on MF surface at pH 3.0, suggesting that the ME surface is more hydrophilic than MF; (2) the droplets are pinned by the layered nanostructure on MoS₂ edge. The fundamental understanding of microwettability elucidated in this study may allow for an improved control of the interaction between anisotropic MoS₂ surfaces and the surrounding liquid environment, which is critically important for many industrial applications such as flotation and catalysis systems.

Effects of the Molecular Structure of a Self-Assembled Monolayer on the Formation and Morphology of Surface Nanodroplets

The formation and morphology of microscopic droplets on a chemically modified surface are important for many droplet-related applications. In this study, we examined the formation and morphological characteristics of nanodroplets produced in the same process of solvent exchange on a gold surface coated with a methyl-terminated alkanethiol monolayer. From atomic force microscopy images, we obtained the contact angles of polymerized nanodroplets in 12 combinations of the length of a straight alkyl chain and the type of droplet liquid. Our results show a significant decrease in the number density of the droplets as the number of methyl groups extends from 8 to 12 or 14. The contact angle of the droplets on octanethiol is significantly larger than that on dodecanethiol or tetradecanethiol, possibly because of the screening effect from the monolayer. Our results demonstrate that under the same solution conditions the morphology of surface nanodroplets is sensitive to the detailed molecular structures of the monolayer on the substrate. This finding has important implications for understanding static wetting on the microscopic scale and the origin of three-phase contact line pinning.

How a Surface Nanodroplet Sits on the Rim of a Microcap

The location and morphology of femtoliter nanodroplets that nucleate and grow on a microcap-decorated substrate in contact with a liquid phase were investigated. We experimentally examined four different wetting combinations of the flat area and the microcaps. The results show that depending on the relative wettability, the droplets sit either on the plain surface or on the top of the microcap or on the rim of the microcap. The contact angle and, for the last case, the radial positions of the nanodroplets relative to the microcap center were characterized, in reasonable agreement with our theoretical analysis, which is based on an interfacial energy minimization argument. However, the experimental data show considerable scatter around the theoretical equilibrium curves, reflecting pinning and thus nonequilibrium effects. We also provide the theoretical phase diagram in parameter space of the contact angles, revealing under which conditions the nanodroplet will nucleate on the rim of the microcap.

Wettability of partially suspended graphene

The dependence of the wettability of graphene on the nature of the underlying substrate remains only partially understood. Here, we systematically investigate the role of liquid-substrate interactions on the wettability of graphene by varying the area fraction of suspended graphene from 0 to 95% by means of nanotextured substrates. We find that completely suspended graphene exhibits the highest water contact angle (85° ± 5°) compared to partially suspended or supported graphene, regardless of the hydrophobicity (hydrophilicity) of the substrate. Further, 80% of the long-range water-substrate interactions are screened by the graphene monolayer, the wettability of which is primarily determined by short-range graphene-liquid interactions. By its well-defined chemical and geometrical properties, supported graphene therefore provides a model system to elucidate the relative contribution of short and long range interactions to the macroscopic contact angle.

Influence of Solution Composition on the Formation of Surface Nanodroplets by Solvent Exchange

Solvent exchange is a simple process of forming surface nanodroplets on an immersed substrate. In this process, the droplets nucleate and grow in response to transient oversaturation when a good solvent of the droplet liquid is displaced by a poor solvent. Here we will show how the final droplet size is influenced by solution composition in the solvent exchange. To do this, we produced water droplets on a hydrophilic substrate and cyclohexane droplets on a hydrophobic substrate by using a tertiary system of cyclohexane, ethanol, and water. The composition of the good solvent was varied systematically in the one-phase region on the phase diagram. We found that the key feature closely related to the droplet size is the area (A) in the phase diagram defined by the phase boundary and the concentration ratio between the good solvent and the droplet liquid. This area reflects the excessive amount of droplet liquid in the tertiary mixture, which can be complicated by bulk droplet formation during solvent exchange. We will also show that the droplet volume per unit surface area also increases with A. The findings from this work will provide guideline for the selection of solution conditions to achieve a desirable droplet size and number density on the surface.

Solvent Effects on the Formation of Surface Nanodroplets by Solvent Exchange

Solvent exchange is a simple process to form oil nanodroplets at solid-liquid interfaces with well-defined location and morphology. In this process, a good solvent of the oil is displaced by a poor solvent, leading to the nucleation and growth of oil droplets from a transient oversaturation at the mixing front. Our recent work has shown that the final volume of the droplets is related to the flow conditions. In this work, we investigate the effects of the type and the composition of solvents on the droplet formation under the same flow conditions. Water nanodroplets were produced by ethanol/cyclohexane (solution A) and cyclohexane (solution B) on a hydrophilic substrate. We found that the droplet size increases first and then decreases with an increase of the initial ethanol concentration in solution A. This is attributed to the phase separation of ethanol-cyclohexane-water; in particular, the composition of solution A on the phase boundary above the Ouzo region. The same reason also contributes to the lower efficiency in droplet formation for a longer alkane. The important implication from this work is that the maximal droplet volume is limited by the phase separation of the solvents used in the solvent exchange.

Mixed mode of dissolving immersed nanodroplets at a solid-water interface

The dissolution dynamics of microscopic oil droplets (less than 1 μm in height, i.e. nanodroplets) on a hydrophobilized silicon surface in water was experimentally studied. The lateral diameter was monitored using confocal microscopy, whereas the contact angle was measured by (disruptive) droplet polymerisation of the droplet. In general, we observed the droplets to dissolve in a mixed mode, i.e., neither in the constant contact angle mode nor in the constant contact radius mode. This means that both the lateral diameter and the contact angle of the nanodroplets decrease during the dissolution process. On average, the dissolution rate is faster for droplets with larger initial size. Droplets with the same initial size can, however, possess different dissolution rates. We ascribe the non-universal dissolution rates to chemical and geometric surface heterogeneities (that lead to contact line pinning) and cooperative effects from the mass exchange among neighbouring droplets.