Effect of impurities in the description of surface nanobubbles: Role of nonidealities in the surface layer\ud

In a recent study [ S. Das, J. H. Snoeijer and D. Lohse Phys. Rev. E 82 056310 (2010)], we provided quantitative demonstration of the conjecture [ W. A. Ducker Langmuir 25 8907 (2009)] that the presence of impurities at the surface layer (or the air-water interface) of surface nanobubbles can substantially lower the gas-side contact angle and the Laplace pressure of the nanobubbles. Through an analytical model for any general air-water interface without nonideality effects, we showed that a large concentration of soluble impurities at the air-water interface of the nanobubbles ensures significantly small contact angles (matching well with the experimental results) and Laplace pressure (though large enough to forbid stability). In this paper this general model is extended to incorporate the effect of nonidealities at the air-water interface in impurity-induced alteration of surface nanobubble properties. Such nonideality effects arise from finite enthalpy or entropy of mixing or finite ionic interactions of the impurity molecules at the nanobubble air-water interface and ensure significant lowering of the nanobubble contact angle and Laplace pressure even at relatively small impurity coverage. In fact for impurity molecules that show enhanced tendency to get adsorbed at the nanobubble air-water interface from the bulk phase, impurity-induced lowering of the nanobubble contact angle is witnessed for extremely small bulk concentration. Surface nanobubble experiments being typically performed in an ultraclean environment, the bulk concentration of impurities is inevitably very small, and in this light the present calculations can be viewed as a satisfactory explanation of the conjecture that impurities, even in trace concentration, have significant impact on surface nanobubble

Explosive Formation and Dynamics of Vapor Nanobubbles around a Continuously Heated Gold Nanosphere

We form sub-micrometer-sized vapor bubbles around a single laser heating gold nanoparticle in a liquid and monitor them through optical scattering of a probe laser. The fast, inertia-governed expansion is followed by a slower contraction and disappearance after some tens of nanoseconds. In a narrow range of illumination powers, bubble time traces show a clear echo signature. We attribute it to sound waves released upon the initial explosion and reflected by flat interfaces, hundreds of microns away from the particle. Echoes can trigger new explosions. A steady state of nanobubble with a vapor shell surrounding the heated nanoparticle can be reached by a proper time profile of the heating intensity. Stable nanobubbles could have original applications for light modulation and for enhanced optical-acoustic coupling in photoacoustic microscopy

Effect of added salt on preformed surface nanobubbles: A scaling estimate\ud

In this paper we propose a scaling argument to quantify the role of added electrolyte salt in affecting the stability and the morphology of preformed surface nanobubbles on hydrophobic substrates like the water-OTS-silicon or the water-HOPG interfaces. The added salt controls the electric double layer formation as well as affects the zeta (ζ) potential at the air-water and solid-water interfaces. The resulting electrostatic wetting tension acts in conjunction with the air-water surface tension (analogous to electrowetting scenarios), thereby affecting the nanobubble morphologies. Weak ζ potential of the water-HOPG interface or the water-OTS-silicon interface at acidic pH ensures that the added salt will have imperceptible effect on the corresponding preformed surface nanobubbles, validating the experimental observations. However, at alkaline buffer pH for the OTS-silicon substrate, under certain system conditions, salt-induced ζ potential can be substantially high so that the properties of preformed surface nanobubbles will be affected. This paper will thus readdress the long-held universal notion that added salt, no matter in what concentration, will not influence the properties of preformed surface nanobubble

Quantum-dot-like states in molybdenum disulfide nanostructures due to the interplay of local surface wrinkling, strain, and dielectric confinement

The observation of quantum light emission from atomically thin transition metal dichalcogenides has opened a new field of applications for these material systems. The corresponding excited charge-carrier localization has been linked to defects and strain, while open questions remain regarding the microscopic origin. We demonstrate that the bending rigidity of these materials leads to wrinkling of the two-dimensional layer. The resulting strain field facilitates strong carrier localization due to its pronounced influence on the band gap. Additionally, we consider charge carrier confinement due to local changes of the dielectric environment and show that both effects contribute to modified electronic states and optical properties. The interplay of surface wrinkling, strain-induced confinement, and local changes of the dielectric environment is demonstrated for the example of nanobubbles that form when monolayers are deposited on substrates or other two-dimensional materials

Knudsen gas provides nanobubble stability

We provide a model for the remarkable stability of surface nanobubbles to bulk dissolution. The key to the solution is that the gas in a nanobubble is of Knudsen type. This leads to the generation of a bulk liquid flow which effectively forces the diffusive gas to remain local. Our model predicts the presence of a vertical water jet immediately above a nanobubble, with an estimated speed of $\sim3.3\,\mathrm{m/s}$, in good agreement with our experimental atomic force microscopy measurement of $\sim2.7\,\mathrm{m/s}$. In addition, our model also predicts an upper bound for the size of nanobubbles, which is consistent with the available experimental data

Diffusive shielding stabilizes bulk nanobubble clusters

Using molecular dynamics, we study the nucleation and stability of bulk nanobubble clusters. We study the formation, growth, and final size of bulk nanobubbles. We find that, as long as the bubble-bubble interspacing is small enough, bulk nanobubbles are stable against dissolution. Simple diffusion calculations provide an excellent match with the simulation results, giving insight into the reason for the stability: nanobubbles in a cluster of bulk nanobubbles "protect" each other from diffusion by a shielding effect