SCARLET-1.0: SpheriCal Approximation for viRtuaL aggrEgaTes

Abstract. Aggregation of particles occurs in a large variety of settings, and, therefore, it is the focus of many disciplines, e.g. Earth and environmental sciences, astronomy, meteorology, pharmacy, food industry. It is thus very important to provide a full description of the outcome of an aggregation process, starting from its basic features such as the position in space of its components and the overall porosity of the final object. We present SCARLET-1.0, a Matlab package specifically created to provide a 3D virtual reconstruction of aggregates in central oriented collisions that can be later 3D printed. With aggregates in central oriented collisions we refer to aggregates that build up their own structure around the first particle (the core) acting as a seed. SCARLET-1.0 belongs to the class of sphere-composite algorithms, a family of algorithms that approximate 3D complex shapes in terms of not-overlapping spheres. The conversion of a 3D surface in its equivalent spherical approximation allows an analytical computation of their intersections. Thus, provided a vector of sizes and shapes, SCARLET-1.0 places each element in the vector around the core minimizing the distances between their centers of mass. The user can play with three main parameters that are in charge of controlling the minimization process, namely the solid angle of the cone of investigation (Ω), the number of rays per cone (Nr), and the number of orientations of the object (No). All the 3D shapes are described using the STL format, the nowadays standard for 3D printing. This is one of the key features of SCARLET-1.0, which results in an unlimited range of application of the package. The main outcome of the code is the virtual representation of the object, its size, porosity, density and the associated STL file. As an example, here SCARLET-1.0 has been applied to the investigation of ellipsoid-ellipsoid collisions and to a more specific analysis of volcanic ash aggregation. In the first application we show that the final porosity of two colliding ellipsoids is less than 20 % if flatness and elongation are greater than or equal to 0.5. Higher values of porosities (up to 40–50 %) can be, instead, found for ellipsoids with needle-like or extremely flat shapes. In the second application, we reconstruct the evolution in time of the porosity of two different aggregates characterized by different inner structures. We find that aggregates whose population of particles is characterized by a narrow distribution of sizes tend to rapidly reach a plateau in the porosity. In addition, to reproduce the observed densities, almost no minimization is necessary in SCARLET-1.0; a result that suggests how these objects are quite far from the maximum packing condition often investigated in literature.