Coherent light transmission by a dew pattern

Abstract

We study both theoretically and experimentally the transmission of coherent light by a drop pattern (dew). The theory is based on the Kirchhoff scalar approach to diffraction. The polarization of the diffracted wave in the zero diffraction order is analyzed separately. The intensity in the zero diffraction order in the far zone is an oscillatory function of the drop size. These oscillations are observed with a pattern of water drops growing on glass. The model allows the evolution of the important parameters of the drop pattern (average radius and surface coverage) to be obtained from the light intensity in the zero diffraction order. IntroductionDepending on the wettability of the surface on which vapor condenses, dew forms a transparent film or a diffuse assembly of droplets. The knowledge of the properties of dew, "breath figures" or, generally speaking, dropwise condensation, opens a vast field of applications ranging from high-technology processes of film growth [1] to soil desinfection in agronomy [2], sterilization in pharmacology [3] and water recovery in dew condensers [4]. The optical properties of dew have received considerable attention from the scientists who studied the natural physical effects (see, e.g. [5,6]) and from those who are interested in industrial applications [7,8].The morphology and kinetics of dew formation were investigated extensively both theoretically and experimentally (see e.g. [9,10] and refs. therein). Its growth can be characterized by several physical values, the most important of which are the mean radius a of the drops and the surface coverage ε 2 , which is the fraction of surface area covered by the projections of the drops on the surface.Two regimes of growth can be identified. At the beginning of the condensation process the drops grow independently, a follows a power law a ∼ t µ 0 , where t is the elapsed time, and the surface coverage increases. When the temperature of the substrate is kept constant, µ 0 = 1 3 . When the drop radius becomes large enough, coalescences between drops occur and the exponent changes to another value µ = 3µ 0 . The growth is then self-similar and the surface coverage reaches a saturation value Email address: vadim.nikolayev@cea.fr (Vadim S. Nikolayev) ε 2 ∞ as well as all the statistical characteristics of the drop pattern, except a . When the surface is ideally smooth and clean ε 2 ∞ ≈ 0.55 independently of the wetting properties (characterized by the contact angle φ, see Fig. 1). For a nonideal surface, the pinning of the contact lines by the surface heterogeneities leads to a hysteresis of the contact angle, i.e. to a significant difference between the advancing and receding contact angles. The shape of the drop is no more a spherical cap. In this case ε 2 ∞ becomes dependent on the hysteresis, and since the hysteresis effects are stronger for small contact angles [11], ε 2 ∞ becomes higher as φ is smaller. This dependence will be used in section 3 to determine the contact angle. In the analysis below, the drops are considered ...

Measurements and ecological implications of non‐rainfall water in desert ecosystems—A review

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Infrared Spectral Emissivity Dynamics of Surfaces Under Water Condensation

“…Spectral measurements in the atmospheric window have already shown that the average emissivity of a plastic substrate increases when wet. [20] In the visible spectrum, the impact of dropwise condensation on the trajectory and polarization of light modified by an assembly of droplets, covering windows, [22,23] solar panels, or greenhouses [24] have been previously studied. A lack of knowledge remains regarding the spectrally-resolved effective emissivity of a wet surface in the MWIR spectral range.…”

The effect of the orientation and the height of periodic sub-micrometric texturing on dropwise condensation

“…Breath figures and droplet growth rates during the condensation process are relevant in the context of a number of applications, notably for water harvesting in desert areas, for heat transfer processes and for the formation of dew on transparent surfaces [21,22,23,24]. The impact of dew on transparency can be problematic when it impacts the efficacy of helmet visors, diving masks or car headlights where the light emission is affected by the presence of dew.…”