Evaporation-induced flow around a droplet in different gases
TL;DR: In this article, the influence of the ambient gas on the evaporation induced flow around a droplet at atmospheric conditions was investigated, and it was shown that the evapse-induced flow in these gases for different liquids was measured using particle image velocimetry.
Abstract: It is known from recent studies that evaporation induces flow around a droplet at atmospheric conditions. This flow is visible even for slowly evaporating liquids like water. In the present study, we investigate the influence of the ambient gas on the evaporating droplet. We observe from the experiments that the rate of evaporation at atmospheric temperature and pressure decreases in a heavier ambient gas. The evaporation-induced flow in these gases for different liquids is measured using particle image velocimetry and found to be very different from each other. However, the width of the disturbed zone around the droplet is seen to be independent of the evaporating liquid and the size of the needle (for the range of needle diameters studied), and only depends on the ambient gas used.It is known from recent studies that evaporation induces flow around a droplet at atmospheric conditions. This flow is visible even for slowly evaporating liquids like water. In the present study, we investigate the influence of the ambient gas on the evaporating droplet. We observe from the experiments that the rate of evaporation at atmospheric temperature and pressure decreases in a heavier ambient gas. The evaporation-induced flow in these gases for different liquids is measured using particle image velocimetry and found to be very different from each other. However, the width of the disturbed zone around the droplet is seen to be independent of the evaporating liquid and the size of the needle (for the range of needle diameters studied), and only depends on the ambient gas used.
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TL;DR: In this article, it is shown from the visualization inside the droplet that these liquids exhibit intense internal circulation during evaporation and the average velocity of the internal circulation is measured and is found to compare well with the velocity scale for Marangoni convection.
64 citations
TL;DR: Two key transport mechanisms besides vapor diffusion-evaporative cooling and natural convection in the surrounding gas-are investigated here as a function of the substrate wettability using an augmented droplet evaporation model, reconciling previous debate regarding the applicability of the classic vapor-diffusion model.
Abstract: Prediction and manipulation of the evaporation of small droplets is a fundamental problem with importance in a variety of microfluidic, microfabrication, and biomedical applications. A vapor-diffusion-based model has been widely employed to predict the interfacial evaporation rate; however, its scope of applicability is limited due to incorporation of a number of simplifying assumptions of the physical behavior. Two key transport mechanisms besides vapor diffusion—evaporative cooling and natural convection in the surrounding gas—are investigated here as a function of the substrate wettability using an augmented droplet evaporation model. Three regimes are distinguished by the instantaneous contact angle (CA). In Regime I (CA ≲ 60°), the flat droplet shape results in a small thermal resistance between the liquid–vapor interface and substrate, which mitigates the effect of evaporative cooling; upward gas-phase natural convection enhances evaporation. In Regime II (60 ≲ CA ≲ 90°), evaporative cooling at the ...
63 citations
TL;DR: In this paper, the viscosity of the three polyatomic gases, carbon dioxide, methane, and sulfur hexafluoride, in the limit of zero density was studied, and it was shown that a two-parameter law of corresponding states is inadequate for the representation of the data over these wide ranges of temperature.
Abstract: The paper contains accurate representations for the viscosity of the three polyatomic gases, carbon dioxide, methane, and sulfur hexafluoride, in the limit of zero density. These gases were studied because they possess permanent multipole moments of increasing order 4, 6, and 8, respectively. The correlations have associated uncertainties of ±0.3% around room temperature rising to ±1.5% at the low‐temperature extreme and to a maximum of ±2.0% at the high‐temperature extreme. The correlating equation for carbon dioxide is valid for the temperature range 200–1500 K, that for methane from 110–1050 K and that for sulfur hexafluoride from 220–900 K. It is shown that a two‐parameter law of corresponding states is inadequate for the representation of the data over these wide ranges of temperature.
62 citations
TL;DR: In this paper, the dissolution process of small (equivalent) radius R0 < 1 mm) long-chain alcohol sessile droplets in water is studied, disentangling diffusive and convective contributions.
Abstract: The dissolution process of small (initial (equivalent) radius R0 < 1 mm) long-chain alcohol (of various types) sessile droplets in water is studied, disentangling diffusive and convective contributions. The latter can arise for high solubilities of the alcohol, as the density of the alcohol–water mixture is then considerably less than that of pure water, giving rise to buoyancy-driven convection. The convective flow around the droplets is measured, using micro-particle image velocimetry (mPIV) and the schlieren technique. When non-dimensionalizing the system, we find a universal Sh Ra1=4 scaling relation for all alcohols (of different solubilities) and all droplets in the convective regime. Here Sh is the Sherwood number (dimensionless mass flux) and Ra is the Rayleigh number (dimensionless density difference between clean and alcohol-saturated water). This scaling implies the scaling relation c / R5=4 0 of the convective dissolution time c, which is found to agree with experimental data. We show that in the convective regime the plume Reynolds number (the dimensionless velocity) of the detaching alcohol-saturated plume follows Rep Sc
52 citations
TL;DR: The dissolution process of small (equivalent) radius $R_0 Ra_t, where $Ra_t = 12$ is the transition Ra-number as extracted from the data, was studied in this paper.
Abstract: The dissolution process of small (initial (equivalent) radius $R_0 Ra_t$, where $Ra_t = 12$ is the transition Ra-number as extracted from the data. For $Ra < Ra_t$ and smaller, convective transport is progressively overtaken by diffusion and the above scaling relations break down.
44 citations