Diffuse reflectance of foams
There have been few studies of the optical properties of foams but the results of these investigations are of particular relevance to current discussions about climate restoration by means of oceanic foam because they indicate the influence of foam raft number on albedo. The diffuse reflectance of foams prepared from eight surfactants was measured in a 0°/45° geometry with a collimated, solar simulation light source and a photodiode by reference to barium sulphate white standards. Reflectance, or albedo, in the range of 0.5 to 0.59 was recorded for foams prepared with sodium dodecyl sulphate (SDS), methyl cellulose, and TWEEN 20TM. Perfluoro-surfactants produced more stable foams with reflectance over 60%. The apparatus was adjusted to the 45°/45° configuration (specular reflection) and used to test the suggestion that a foam can be treated, for purpose of approximation, as a series of horizontal planar reflective layers in which reflectance reaches a limiting value. The resulting geometric series, when summed over an infinite number of layers yields an albedo of 0.62 to 0.65 when the reflectance of individual layers of which the model is comprised was varied over the range 0.01 to 0.1 respectively. This was tested experimentally using eight sheets of 500 μm thick glass separated by 18 μm thick glass cover slides in 45°/45° mode and the results agreed to within less than 9% with the model calculation. The insensitivity to layer reflectance and the fact that limiting reflectance for the assembly is reached after about 20 layers provides a simple working model to assess the dependence of reflectance on thickness and cell diameter.
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Document Type: Research Article
Publication date: January 1, 2014
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