DOI: 10.1594/pangaea.951092
title: Laboratory particle reflectivity measurements of hollow glass microsphere products 'Potters 25P45' and '3M K1'

description: We commissioned laboratory evaluations of several available hollow glass microsphere products. Covalent Metrology made reflectivity measurements for 3M™ K1 and Potters Sphericel™ 25P45 hollow glass microspheres (HGMs). The measurements were made from August 25 to September 11, 2020 at Covalent Metrology in Sunnyvale, CA. The purpose of the measurements was to evaluate widely available hollow glass microsphere materials obtainable from multiple sources for effectiveness in reducing ice melt. A further purpose was to select materials for evaluation in field experiments and compare laboratory results with results from field experiments. A further purpose was to determine the effect of water on reflectivity. While the particles are known to have high reflectivity in their dry state, it was unknown how much the reflectivity might decrease if particles are embedded in the ice or on the surface of water, since the refractive index of water (n=1.33 at 589 nm) is closer to the refractive index of glass (n ~1.52, depending on glass composition) than the refractive index of air (n=1.00).The reflectivity of the HGM materials was measured with a Perkin-Elmer 1050 UV-Vis-NIR Spectrometer at 5 nm resolution using a 150mm integrating sphere to collect the diffuse reflectance. Samples were placed on the reflectance sample plate inside a Spectrosil® Far UV quartz cuvette of fixed pathlength (Starna Type 20 short path length, demountable). To measure the change in reflectivity with varying layer thicknesses of hollow glass microsphere materials, HGM were loaded into cuvettes with pathlengths varying from 0.2mm to 1 mm in thickness.Measurements of the reflectivity of the cuvette were taken on just the cuvette alone, then measurements were made with the cuvette loaded with material. The reflectivity of the cuvette alone was subtracted from these measurements to provide measurements of the material reflectivity. The silica material was loaded into the cuvette and a doctor blade method was used to remove excess material in the well. The cuvette was sealed and placed onto the integrating sphere port with the cuvette well facing the incident light. The HGM material was loaded into the cuvette and a doctor blade method was used to remove excess material in the well. The cuvette was sealed and placed onto the integrating sphere port with the cuvette well facing the incident light. To measure the reflectivity of HGM in water, a few droplets of water were deposited onto the sample after doctor blading, taking care not to disturb or overly saturate the layer. Due to the hydrophilic nature of these materials, the surfaces appeared to be fully wet. The UV-Vis reflectivity of the K1 and 25P45 hollow glass microspheres was measured as a function of optical path length and of the surrounding medium. While the particles are known to have high reflectivity in their dry state, it was unknown how much the reflectivity might decrease if particles are embedded in the ice or on the surface of water as the ice begins to melt, since the refractive index of water (n=1.33 at 589 nm) is closer to the refractive index of glass (n ~1.52, depending on glass composition) than the refractive index of air (n=1.00).