Aspherical particle orientation in the atmosphere

Abstract

534.214Phase states of aspherical particles in the atmosphere are examined. The orientation mechanisms at work in particles in the form of ellipsoids of revolution moving in a viscous medium on exposure to aerodynamic forces and in interactions of the induced dipole moment of such a particle with an external electric field are considered.It is common knowledge that polar molecules of water in the surface layer of a drop have an inwardly directed negative and outwardly directed positive end. For this reason, a subsurface double electrical layer with a potential jump ∆φ is formed. Notably, φ is higher within the drop than in the surrounding medium and is measured to be ∆φ = 0.26 V. The drop captures negative ions until the potential difference between the liquid and air vanishes [1]. The number N of absorbed charges is determined by the equality e·N/r = ∆φ, where r is the drop radius and e is the electron charge. For r = 10 -3 cm, N = 2·10 3 .The stability condition against small deformations for a charged spherical drop [2] is (e·N) 2 < 16πr 3 α, where α is the surface tension coefficient. The stability condition is violated for particles with r > ∆φ 2 /16πα. For ∆φ = 0.26V and α = 70·10 -3 n/m, r > 1.9 cm.Furthermore, experiments show that the charge of aerosol particles is very low in the absence of specific adsorption and is normally no more than 10 times as high as an elementary electric charge. A crucial distinction between aerosols and liquid disperse systems is the lack of electrical neutrality in the system as a whole. Even aerosols existing in abundance may have a fairly high static charge, and sedimentation is responsible for a nonuniform charge distribution over the system. As the result, the cloud base acquires a negative charge, whereas the cloud top is positively charged. The resulting electric field strength can be estimated on the assumption that a stationary state is established in sedimentation where convection current due to the charge transfer by falling drops is compensated by conduction current flowing in gas in the opposite direction according to the following relation:where m is the mass of a particle, g is the free fall acceleration, η is the dynamic viscosity coefficient, χ is the aerosol conductivity, and ν is the number of particles in unit volume. Assuming that φ = 0.25V, η = 1.7·10 -5 Pa·s, χ = 4·10 -4 s -1 , and r = 10 -5 m, m = 4.2·10 -12 kg for cloud water content νm = 10 -3 kg/m 3 , we get E = 200 V/cm. The order of magnitude of E approaches that of the observed value. The calculation is complicated in the case of actual conditions where sedimentation is enhanced by convection due to the wind or descending air currents. In this case, the field strength reaches a point (E > 30 kV/m) where thunderstorm phenomena develop.In the presence of a uniform external electric field E, the force that acts to elongate the drop is E·E/8π. The liquid particle changes its shape for E 2 > (πα/r). With r = 10 µm and α = 70 dyn/cm, E > MV/cm. The electric field strength in the atmosphere is inadequat...