Body Forces Drive the Apparent Line Tension of Sessile Droplets.

The line tension of a three-phase contact line is implicated in a wide variety of interfacial phenomena, but there is ongoing controversy, with existing measurements spanning six orders of magnitude in both signs. Here, we show that computationally obtained magnitudes, sign changes, and nontrivial variations of apparent line tension can be faithfully reproduced in a parsimonious model that incorporates only liquid-substrate interactions. Our results suggest that the origin for the remarkable variation lies in the failure of a widely used estimation method to eliminate body forces, leading measured line tensions to behave like an extensive quantity.

Transient Solubility Gradients Mediate Oversaturation during Solvent Exchange.

Solvent exchange facilitates high-density nucleation of sessile nanodroplets or nanobubbles by successively wetting a surface with two solvents of contrasting solubility with respect to a target species. Yet the key physical mechanisms underlying its efficacy have yet to be theoretically explained. We develop a minimal model for solvent exchange, for the prototypical example of water and ethanol as the solvents and nitrogen as the target species. Our calculations show that solvent exchange is mediated by transient solubility gradients that dominate over the intrinsic concentration gradient of nitrogen in the incipient moments after exchange. Solubility gradients advect nitrogen toward the substrate during ethanol-water exchange but away from it in water-ethanol exchange, consistent with the directionality observed in experiments.

Direct Measurement of the Contents, Thickness, and Internal Pressure of Molybdenum Disulfide Nanoblisters.

Nanoblisters have attracted attention due to their ability to controllably modulate the properties of two-dimensional materials. The accurate measurement or estimation of their properties is nontrivial and largely based on Hencky's theory. However, these estimates require a priori knowledge of material properties and propagate large errors. Here we show, through a systematic atomic force microscopy study, several strategies that lead to vastly enhanced characterization of nanoblisters. First, we find that nanoblisters may contain both liquid and gas, resolving an ongoing debate in the literature. Second, we demonstrate how to definitively determine the membrane thickness of a nanoblister and show that Hencky's theory can only reliably predict membrane thicknesses for small aspect ratios and small membrane thicknesses. Third, we develop a novel technique to measure the internal pressures of nanoblisters, which quantitatively agrees with Hencky's theory but carries a 1 order smaller propagated error.

How Bulk Nanobubbles Might Survive.

The existence of bulk nanobubbles has long been regarded with scepticism, due to the limitations of experimental techniques and the widespread assumption that spherical bubbles cannot achieve stable equilibrium. We develop a model for the stability of bulk nanobubbles based on the experimental observation that the zeta potential of spherical bubbles abruptly diverges from the planar value below 10 µm. Our calculations recover three persistently reported-but disputed-properties of bulk nanobubbles: that they stabilize at a typical radius of ∼100 nm, that this radius is bounded below 1 µm, and that it increases with ionic concentration.

Stability, Dynamics, and Tolerance to Undersaturation of Surface Nanobubbles.

The theoretical understanding of surface nanobubbles-nanoscale gaseous domains on immersed substrates-revolves around two contrasting perspectives. One perspective, which considers gas transport in the nanobubbles' vicinity, explains numerous stability-related properties but systematically underestimates the dynamical response timescale by orders of magnitude. The other perspective, which considers gas transport as the bulk liquid equilibrates with the external environment, recovers the experimentally observed dynamical timescale but incorrectly predicts that nanobubbles progressively shrink until dissolution. We propose a model that couples both perspectives, which is capable of explaining the stability, dynamics, and unexpected tolerance of surface nanobubbles to undersaturated environments.

Surface Nanobubbles Are Stabilized by Hydrophobic Attraction.

The remarkably long lifetime of surface nanobubbles has perplexed researchers for two decades. The current understanding is that both contact line pinning and supersaturation of the ambient liquid are strictly required for the stability of nanobubbles, yet experiments show nanobubbles surviving in open systems and undersaturated environments. We find that this discrepancy can be addressed if the effects of an attractive hydrophobic potential at the solid substrate on the spatial distribution of the gas concentration is taken into account. We also show that, in our model, only substrate pinning is strictly required for stabilization; while hydrophobicity and supersaturation both aid stability, neither is mandatory-the absence of one can be compensated by an excess of the other.

Viscous field-aligned water exhibits cubic-ice-like structural motifs.

Strong electric fields are known to greatly accelerate the freezing of water in molecular dynamics simulations, and have also been shown to affect the thermodynamics of the phase transition. In this work, a mechanistic explanation for field-induced crystallization of water is presented. Due to the coupling between the rotational and the translational degrees of freedom of individual water molecules, an applied field can directly drive the formation of cubic-ice like local motifs in water. Analysis of the angular distributions of water molecules in TIP4P-2005 water at field strengths between 0.0 and 0.32 V Å-1 demonstrates the existence of such motifs in the field-aligned liquid phase that is observed prior to the onset of the freezing transition. The dynamic properties of this field-aligned liquid phase are also studied, and its viscosity is shown to be within a factor of two of that of regular liquid water using the Green-Kubo method as well as mean squared displacements. The choice between the NPT and the NVT ensembles is shown to have a strong impact on the evolution of molecular dynamics trajectories at field strengths close to the threshold for the freezing transition, and the importance of properly accounting for the electric field terms in the pressure virial is emphasized.

Graphene Nanobubbles Produced by Water Splitting.

Graphene nanobubbles are of significant interest due to their ability to trap mesoscopic volumes of gas for various applications in nanoscale engineering. However, conventional protocols to produce such bubbles are relatively elaborate and require specialized equipment to subject graphite samples to high temperatures or pressures. Here, we demonstrate the formation of graphene nanobubbles between layers of highly oriented pyrolytic graphite (HOPG) with electrolysis. Although this process can also lead to the formation of gaseous surface nanobubbles on top of the substrate, the two types of bubbles can easily be distinguished using atomic force microscopy. We estimated the Young's modulus, internal pressure, and the thickness of the top membrane of the graphene nanobubbles. The hydrogen storage capacity can reach ∼5 wt % for a graphene nanobubble with a membrane that is four layers thick. The simplicity of our protocol paves the way for such graphitic nanobubbles to be utilized for energy storage and industrial applications on a wide scale.

Robust Whispering-Gallery-Mode Microbubble Lasers from Colloidal Quantum Dots.

Microlasers hold great promise for the development of photonics and optoelectronics. Among the discovered optical gain materials, colloidal quantum dots (CQDs) have been recognized as the most appealing candidate due to the facile emission tunability and solution processability. However, to date, it is still challenging to develop CQD-based microlasers with low cost yet high performance. Moreover, the poor long-term stability of CQDs remains to be the most critical issue, which may block their laser aspirations. Herein, we developed a unique but generic approach to forming a novel type of a whispering-gallery-mode (WGM) microbubble laser from the hybrid CQD/poly(methyl methacrylate) (PMMA) nanocomposites. The formation mechanism of the microbubbles was unraveled by recording the drying process of the nanocomposite droplets. Interestingly, these microbubbles naturally serve as the high-quality WGM laser resonators. By simply changing the CQDs, the lasing emission can be tuned across the whole visible spectral range. Importantly, these microbubble lasers exhibit unprecedented long-term stability (over one year), sufficient for practical applications. As a proof-of-concept, the potential of water vapor sensing was demonstrated. Our results represent a significant advance in microlasers based on the advantageous CQDs and may offer new possibilities for photonics and optoelectronics.

Resolving the Pinning Force of Nanobubbles with Optical Microscopy.

Many of the remarkable properties of surface nanobubbles, such as unusually small contact angles and long lifetimes, are related to the force that pins them onto their substrates. This pinning force is yet to be quantified experimentally. Here, surface-attached nanobubbles are pulled with an atomic force microscope tip while their mechanical responses are observed with total internal reflection fluorescence microscopy. We estimate that a pinning force on the order of 0.1µN is required to unpin a nanobubble from its substrate. The maximum force that the tip can exert on the nanobubble is limited by the stability of the neck pulled from the bubble and is enhanced by the hydrophobicity of the tip.

Stability of Nanobubbles Formed at the Interface between Cold Water and Hot Highly Oriented Pyrolytic Graphite.

For the wider application of nanobubbles, a simple and reproducible nucleation process is not readily available. Here we describe a method for nucleating nanobubbles using only the most basic of conditions: depositing cold water at 4 °C on heated highly oriented pyrolytic graphite substrates. This method thus avoids the need, as in previous studies, to use secondary liquids, salts, or electrolysis to nucleate the nanobubbles and provides a pure system in which the properties of nanobubbles can be studied. The nanobubbles generated with this method are observed to survive for at least 5 days, barely changing their contact angles or heights after the first few hours. The stability of the nanobubbles in our system is discussed within the framework of some recently published theories.

Distinguishing Nanobubbles from Nanodroplets with AFM: The Influence of Vertical and Lateral Imaging Forces.

The widespread application of surface-attached nanobubbles and nanodroplets in biomedical engineering and nanotechnology is limited by numerous experimental challenges, in particular, the possibility of contamination in nucleation experiments. These challenges are complicated by recent reports that it can be difficult to distinguish between nanoscale drops and bubbles. Here we identify clear differences in the mechanical responses of nanobubbles and nanodroplets under various modes of AFM imaging that subject the objects to predominantly vertical or lateral forces. This allows us to distinguish among nanodroplets, nanobubbles, and oil-covered nanobubbles in water.

Surface Cleaning of Oil Contaminants Using Bulk Nanobubbles.

The removal of oil from solid surfaces, such as textiles and plates, remains a challenge due to the strong binding affinity of the oil. Conventional methods for surface cleaning often require surfactants and mechanical abrasion to enhance the cleaning process. However, in excess, these can pose adverse effects on the environment and to the material. This study investigated how bulk nanobubble water can clean oil microdroplets deposited on surfaces like glass coverslips and dishes. Microscopy imaging and further image analysis clearly revealed that these microdroplets detached from both hydrophobic and hydrophilic surfaces when washed with bulk nanobubble water within a fluidic microchannel. Oil contaminant cleaning was also conducted in water as mobile phase to mimic the circumstances that occur in a dishwasher and washing machine. Cleaning on a larger scale also proved very successful in the removal of oil from a porcelain bowl. These results indicate that nanobubble water can easily remove oil contaminants from glass and porcelain surfaces without the assistance of surfactants. This is in stark contrast to negligible results obtained with a control solution without nanobubbles. This study indicates that nanobubble technology is an innovative, low-cost, eco-friendly approach for oil removal, demonstrating its potential for broad practical applications.

The interplay among gas, liquid and solid interactions determines the stability of surface nanobubbles.

Surface nanobubbles are gaseous domains found at immersed substrates, whose remarkable persistence is still not fully understood. Recently, it has been observed that the formation of nanobubbles is often associated with a local high gas oversaturation at the liquid-solid interface. Tan, An and Ohl have postulated the existence of an effective potential attracting the dissolved gas to the substrate and producing a local oversaturation within 1 nm from it that can stabilize nanobubbles by preventing outgassing in the region where gas flow would be maximum. It is this effective solid-gas potential - which is not the intrinsic, mechanical interaction between solid and gas atoms - its dependence on chemical and physical characteristics of the substrate, gas and liquid, that controls the stability and the other characteristics of surface nanobubbles. Here, we perform free energy atomistic calculations to determine, for the first time, the effective solid-gas interaction that allows us to identify the molecular origin of the stability and other properties of surface nanobubbles. By combining the Tan-An-Ohl model and the present results, we provide a comprehensive theoretical framework allowing, among others, the interpretation of recent unexplained experimental results, such as the stability of surface nanobubbles in degassed liquids, the very high gas concentration in the liquid surrounding nanobubbles, and nanobubble instability in organic solvents with high gas solubility.

Growth and wetting of water droplet condensed between micron-sized particles and substrate.

We study heterogeneous condensation growth of water droplets on micron-sized particles resting on a level substrate. Through numerical simulations on equilibrium droplet profiles, we find multiple wetting states towards complete wetting of the particle. Specifically, a partially wetting droplet could undergo a spontaneous transition to complete wetting during condensation growth, for contact angles above a threshold minimum. In addition, we find a competitive wetting behavior between the particle and the substrate, and interestingly, a reversal of the wetting dependence on contact angles during late stages of droplet growth. Using quasi-steady assumption, we simulate a growing droplet under a constant condensation flux, and the results are in good agreement with our experimental observations. As a geometric approximation for particle clusters, we propose and validate a pancake model, and with it, show that a particle cluster has greater wetting tendency compared to a single particle. Together, our results indicate a strong interplay between contact angle, capillarity and geometry during condensation growth.