Dynamic interplay between interfacial nanobubbles: oversaturation promotes anisotropic depinning and bubble coalescence.

Probing the dynamics of nanobubbles is essential to understand their longevity and behavior. Importantly, such an observation requires tools and techniques having high temporal resolutions to capture the intrinsic characteristics of the nanobubbles. In this work, we have used the in situ liquid-phase electron microscopy (LPEM) technique to gain insights into nanobubbles' behavior and their interfacial dynamics. Interestingly, we could observe a freely growing-shrinking nanobubble and a pinned nanobubble under the same experimental conditions, suggesting the possibility of multiple nanobubble stabilization theories and pathways. Remarkably, the study reveals that a freely growing-shrinking nanobubble induces anisotropic depinning in the three-phase contact line of a strongly pinned neighboring nanobubble. The anisotropic depinning is attributed to the differential local gas saturation levels, depending on the relative positioning of the freely growing-shrinking nanobubble. Furthermore, we also observed a unique pull-push phenomenon exhibited by the nanobubble's interfaces, which is attributed to the van der Waals interactions and the electric double layer collectively. The role of the electric double layer in suppressing and delaying the merging is also highlighted in this study. The present work aims to reveal the role of locally varying gas saturation in the depinning of nanobubbles, their longevity due to the electric double layer, and the consequent coalescence, which is crucial to understand the behavior of the nanobubbles. Our findings will essentially contribute to the understanding of these novel nanoscale gaseous domains and their dynamics.

Mechanistic Insights into Nanobubble Merging Studied Using In Situ Liquid-Phase Electron Microscopy.

Nanobubbles have attracted great interest in recent times because of their application in water treatment, surface cleaning, and targeted drug delivery, yet the challenge remains to gain thorough understanding of their unique behavior and dynamics for their utilization in numerous potential applications. In this work, we have used a liquid-phase electron microscopy technique to gain insights into the quasistatic merging of surface nanobubbles. The electron beam environment was controlled in order to suppress any new nucleation and slow down the merging process. The transmission electron microscopy study reveals that merging of closely positioned surface nanobubbles is initiated by gradual localized changes in the physical properties of the region between the adjoining nanobubble boundary. The observed phenomenon is then analyzed and discussed based on the different perceptions: localized liquid density gradient and bridge formation for gas exchange. In this study, it is estimated that the merging of the stable nanobubbles is initiated by the formation of a thin gas layer. This work not only enhances our understanding of the merging process of stable surface nanobubbles but will also lead to exploration of new domains for nanobubble applications.

Quantifying vapor transfer into evaporating ethanol drops in a humid atmosphere.

The effect of ambient temperature and relative humidity on the dynamics of ethanol drop evaporation is investigated. Although drop evaporation of mixtures and pure fluids has been extensively studied, very little is known about the transition from a pure fluid to a binary mixture following transfer of a second component present in the atmosphere. This is of importance for industrial, biological and medical applications where the purity of the solvent is paramount. Adsorption-absorption and/or condensation of water into ethanol drops during evaporation is presented through direct quantification of the drop composition in time. In particular, we combine drop profile measurements with Gas Injection Chromatography (GIC) to directly quantify the amount of ethanol evaporated and that of water intake over time. As expected, drops evaporate faster at higher temperatures since both the ethanol saturation concentration and the vapor diffusion coefficient are directly proportional to temperature. On the other hand, increases in the ethanol evaporation rate and in the water intake are observed at higher relative humidity. The increase in ethanol evaporation at higher relative humidity is interpreted by the greater diffusion coefficient of ethanol into humid air when compared to dry air. Moreover, as ethanol evaporates in a high humidity environment, the drop interfacial temperature falls below the dew point due to evaporative cooling and water condenses compared to lower humidity conditions. As a consequence of the heat released by adsorption-absorption and/or condensation, a greater temperature is reported at the liquid-vapor interface as confirmed by IR thermography, inducing a greater ethanol saturation concentration at the surface and hence a greater driving force for evaporation. By coupling the drop profile and the composition of ethanol and water within the drop, we propose a combined evaporation-adsorption/absorption and/or condensation empirical correlation. The proposed correlation accounts for: the decreases in ethanol concentration due to water adsorption-absorption and/or condensation, the diffusion coefficient dependence on relative humidity, and the amount of water intake during evaporation. The proposed empirical correlation agrees remarkably well with experimental observations.

Early Onset of Nucleate Boiling on Gas-covered Biphilic Surfaces.

For phase-change cooling schemes for electronics, quick activation of nucleate boiling helps safeguard the electronics components from thermal shocks associated with undesired surface superheating at boiling incipience, which is of great importance to the long-term system stability and reliability. Previous experimental studies show that bubble nucleation can occur surprisingly early on mixed-wettability surfaces. In this paper, we report unambiguous evidence that such unusual bubble generation at extremely low temperatures-even below the boiling point-is induced by a significant presence of incondensable gas retained by the hydrophobic surface, which exhibits exceptional stability even surviving extensive boiling deaeration. By means of high-speed imaging, it is revealed that the consequently gassy boiling leads to unique bubble behaviour that stands in sharp contrast with that of pure vapour bubbles. Such findings agree qualitatively well with numerical simulations based on a diffuse-interface method. Moreover, the simulations further demonstrate strong thermocapillary flows accompanying growing bubbles with considerable gas contents, which is associated with heat transfer enhancement on the biphilic surface in the low-superheat region.

Influence of Local Heating on Marangoni Flows and Evaporation Kinetics of Pure Water Drops.

The effect of localized heating on the evaporation of pure sessile water drops was probed experimentally by a combination of infrared thermography and optical imaging. In particular, we studied the effect of three different heating powers and two different locations, directly below the center and edge of the drop. In all cases, four distinct stages were identified according to the emerging thermal patterns. In particular, depending on heating location, recirculating vortices emerge that either remain pinned or move azimuthally within the drop. Eventually, these vortices oscillate in different modes depending on heating location. Infrared data allowed extraction of temperature distribution on each drop surface. In turn, the flow velocity in each case was calculated and was found to be higher for edge heating, due to the one-directional nature of the heating. Additionally, calculation of the dimensionless Marangoni and Rayleigh numbers yielded the prevalence of Marangoni convection. Heating the water drops also affected the evaporation kinetics by promoting the "stick-slip" regime. Moreover, both the total number of depinning events and the pinning strength were found to be highly dependent on heating location. Lastly, we report a higher than predicted relationship between evaporation rate and heating temperature, due to the added influence of the recirculating flows on temperature distribution and hence evaporation flux.

Thermal conductivity reduction of crystalline silicon by high-pressure torsion.

We report a dramatic and irreversible reduction in the lattice thermal conductivity of bulk crystalline silicon when subjected to intense plastic strain under a pressure of 24 GPa using high-pressure torsion (HPT). Thermal conductivity of the HPT-processed samples were measured using picosecond time domain thermoreflectance. Thermal conductivity measurements show that the HPT-processed samples have a lattice thermal conductivity reduction by a factor of approximately 20 (from intrinsic single crystalline value of 142 Wm(-1) K(-1) to approximately 7.6 Wm(-1) K(-1)). Thermal conductivity reduction in HPT-processed silicon is attributed to the formation of nanograin boundaries and metastable Si-III/XII phases which act as phonon scattering sites, and because of a large density of lattice defects introduced by HPT processing. Annealing the samples at 873 K increases the thermal conductivity due to the reduction in the density of secondary phases and lattice defects.