Investigation of the effect of air humidity on the condensation growth of levitating liquid microdroplets

Two-dimensional structured arrays of liquid microdroplets levitated over a hot liquid surface have been investigated in several recent papers, but the nature of this phenomenon has not yet been fully understood. In this work we investigate the effect of air humidity on the condensation growth of levitating liquid microdroplets. It was found that the higher the relative humidity of the surrounding air, the lower the rate of the droplet growth.

Development and Application of a Wide Dynamic Range and High Resolution Atmospheric Aerosol Water-Based Supersaturation Condensation Growth Measurement System

The supersaturated condensation of atmospheric aerosol is important in the study of mechanisms of cloud condensation and even heavy air pollution. The existing technology cannot realize accurate dynamic control of wide range supersaturation, so it is difficult to study condensation growth characteristics of nanoparticles through different levels of supersaturation. Here, a supersaturated condensation growth measurement system with three-stage microscope pipes was developed. The resolution of supersaturated condensation system is 0.14, within the range of 0.92 to 2.33 after calibration. Stabilization time is only about 80 s for saturation range 0.92–1.01, which helps to control saturation rapidly, and the control deviation of saturation is no more than 0.06. Measurement of different supersaturated condensation growth control conditions showed that, the particle size increased significantly compared with hygroscopic growth at high humidity. For single-component particles, the increase in size increased to a similar size at the same saturation, with a difference within 7.4%. The increase in size for ammonium sulfate (AS) increased by 13.4–30.2% relative to that of glucose. For the mixed-component, the increase in size decreased about 15.9–25.0% with the increase of the glucose. Because the glucose coating on the surface of AS have hindered particle growth. This also shows that atmospheric ultrafine particles, especially inorganic salt particles, will rapidly grow into larger particles under supersaturated conditions such as increased environmental humidity, thus having some impact on environmental pollution and climate change.

Impacts of coagulation on the appearance time method for new particle growth rate evaluation and their corrections

The growth rate of atmospheric new particles is a key parameter that determines their survival probability of becoming cloud condensation nuclei and hence their impact on the climate. There have been several methods to estimate the new particle growth rate. However, due to the impact of coagulation and measurement uncertainties, it is still challenging to estimate the initial growth rate of new particles, especially in polluted environments with high background aerosol concentrations. In this study, we explore the influences of coagulation on the appearance time method to estimate the growth rate of sub-3 nm particles. The principle of the appearance time method and the impacts of coagulation on the retrieved growth rate are clarified via derivations. New formulae in both discrete and continuous spaces are proposed to correct for the impacts of coagulation. Aerosol dynamic models are used to test the new formulae. New particle formation in urban Beijing is used to illustrate the importance of considering the impacts of coagulation on the sub-3 nm particle growth rate and its calculation. We show that the conventional appearance time method needs to be corrected when the impacts of coagulation sink, coagulation source, and particle coagulation growth are non-negligible compared to the condensation growth. Under the simulation conditions with a constant concentration of non-volatile vapors, the corrected growth rate agrees with the theoretical growth rates. However, the uncorrected parameters, e.g., vapor evaporation and the variation in vapor concentration, may impact the growth rate obtained with the appearance time method. Under the simulation conditions with a varying vapor concentration, the average bias in the corrected 1.5–3 nm particle growth rate ranges from 6 %–44 %, and the maximum bias in the size-dependent growth rate is 150 %. During the test new particle formation event in urban Beijing, the corrected condensation growth rate of sub-3 nm particles was in accordance with the growth rate contributed by sulfuric acid condensation, whereas the conventional appearance time method overestimated the condensation growth rate of 1.5 nm particles by 80 %.

Impacts of coagulation on the appearance time method for sub-3 nm particle growth rate evaluation and their corrections

The growth rate of atmospheric new particles is a key parameter that determines their survival probability to become cloud condensation nuclei and hence their impact on the climate. There have been several methods to estimate the new particle growth rate. However, due to the impact of coagulation and measurement uncertainties, it is still challenging to estimate the initial growth rate of sub-3 nm particles, especially in polluted environments with high background aerosol concentrations. In this study, we explore the feasibility of the appearance time method to estimate the growth rate of sub-3 nm particles. The principle of the appearance time method and the impacts of coagulation on the retrieved growth rate are clarified. New formulae in both discrete and continuous spaces are proposed to correct the impacts of coagulation. Aerosol dynamic models are used to test the new formulae. New particle formation in urban Beijing is used to illustrate the importance to consider the impacts of coagulation on sub-3 nm particle growth rate and its calculation. We show that the conventional appearance time method needs to be corrected when the impacts of coagulation sink, coagulation source, and particle coagulation growth are non-negligible compared to the condensation growth. Under the simulation conditions with a constant vapor concentration, the corrected growth rate agrees with the theoretical growth rates. The variation of vapor concentration is found to impact growth rate obtained with the appearance time method. Under the simulation conditions with a varying vapor concentration, the average bias of the corrected 1.5–3 nm particle growth rate range from 6–44 %. During the test new particle formation event in urban Beijing, the corrected condensation growth rate of sub-3 nm particles was in accordance with the growth rate contributed by sulfuric acid condensation, whereas the conventional appearance time method overestimated the condensation growth rate of 1.5 nm particles by 80 %.

The interaction effects between droplets condensing from moist air using a distributed point sink method

In modelling dropwise condensation, growth of the droplet is frequently assumed in an isolated form. However, for dropwise condensation of moist air the vapor diffusion from the surrounding to the droplet surface will be tremendously influenced by the blocking effect of the neighboring droplets. The influenced spatial distribution of water vapor totally determines a different condensation rate comparing with that by the isolated droplet model. In this work, a distributed point method (DPSM) as the methodof Green function is developed to capture the interaction effects of droplets without requiring solution of the diffusion equation and the numerical discretization. Due to its nature, the automaticallysatisfied boundary conditions make sure the solution accuracy based on the uniqueness theorem. The significant characteristics for the interaction effects between droplets are investigated by handling a series of droplet arrays. During dropwise condensation, a typical droplet array containing up to 1000 droplets is considered. The results indicate that the interaction effect between droplets is critical in accurately predicting the condensation behavior for dropwise condensation in the presence of non-condensable gas.

Gas Sensing with Iridium Oxide Nanoparticle Decorated Carbon Nanotubes

The properties of multi-wall carbon nanotubes decorated with iridium oxide nanoparticles (IrOx-MWCNTs) are studied to detect harmful gases such as nitrogen dioxide and ammonia. IrOx nanoparticles were synthetized using a two-step method, based on a hydrolysis and acid condensation growth mechanism. The metal oxide nanoparticles obtained were employed for decorating the sidewalls of carbon nanotubes. Iridium-oxide nanoparticle decorated carbon nanotube material showed higher and more stable responses towards NH3 and NO2 than bare carbon nanotubes under different experimental conditions, establishing the optimal operating temperatures and estimating the limits of detection and quantification. Furthermore, the nanomaterials employed were studied using different morphological and compositional characterization techniques and a gas sensing mechanism is proposed.

Gas Sensing Properties of Carbon Nanotubes Decorated with Iridium Oxide Nanoparticles

The properties of Iridium oxide (IrO2) decorated Multi-Wall Carbon Nanotubes (IrO2-MWCNTs) are studied for detecting nitrogen dioxide and ammonia vapors. IrO2 nanoparticles were synthetized using a hydrolysis and acid condensation growth mechanism, and subsequently employed for decorating the sidewalls of carbon nanotubes. Decorated MWCNTs films were deposited onto SiO2/Si substrates for achieving chemoresistive gas sensors. NO2 and NH3 gases were detected under different experimental conditions. Higher and more stable responses towards NH3 and NO2 were observed for iridium-oxide nanoparticle decorated MWCNT material, compared to bare MWCNT material. Raman Spectroscopy was employed to study the nanomaterials and the optimal operating temperatures were determined.

Trapping a Hot Drop on a Superhydrophobic Surface with Rapid Condensation or Microtexture Melting

A water drop can bounce upon impacting a superhydrophobic surface. However, on certain superhydrophobic surfaces, a water drop will stick rather than bounce if it is sufficiently hot. Here, we aim to better understand the mechanisms that can lead to this bouncing-sticking transition. Specifically, we model two potential mechanisms in which a superhydrophobic surface could trap a sufficiently hot drop within milliseconds: melting of microtextured wax and condensation of the vapor within the superhydrophobic texture. We then test these mechanisms through systematic drop impact experiments in which we independently vary the substrate and drop temperatures on a waxy superhydrophobic Nasturtium leaf. We find that, whenever the surface or the drop is above a microtexture-melting temperature, the drop sticks. Below this temperature, a critical temperature threshold for bouncing can be predicted and controlled by considering the relative timescales between condensation growth and drop residence time. We envision that these results can provide insight into the design of a new class of superhydrophobic surfaces to act as a rapid thermal fuse to prevent drops that exceed a critical temperature from bouncing onto a thermally sensitive target.