Measuring physical characteristics of particles: a new method of simultaneous measurement for size, settling velocity and density of constituent matter

Measurements of physical properties of particle aggregates, including size, density and shape, are important for understanding the dynamics of suspended particulate material in the ocean. Of these physical properties, particle density (specific gravity) is the least well known and has been considered problematic for understanding particle settling behavior. However, two types of density must be considered. The bulk density, or density of the aggregate including interstitial sea water, is a determinant of aggregate settling rate. A second density, the constituent matter density, represents the density of the aggregated mass and is essential for understanding the settling behavior of aggregates in a pycnocline. Both types of density are important in studies of the distribution and flux of particulate matter in the ocean.

We report here the development of a linear density gradient column, made from a solution of Metrizamide in sea water, that permits the direct measurement of the constituent matter density. The density gradient column has been integrated into a system that also allows independent measurements of settling velocity, and aggregate size and shape. The measurements reported here are the first direct density measurements of any type, and although the constituent matter density does not relate directly to settling velocity, these measurements will help reduce errors in estimates of other physical parameters, e.g. porosity. Aggregates produced in the laboratory from diatom culture and from particles collected in coastal waters have been used to test the system.