Granular segregation [1] is of practical importance in applications ranging from the pharmaceutical industry to chemistry. In many cases, the segregation degrades the quality of the final product or causes safety problems. In order to avoid segregation, two methods could be used : (i) the optimization of the devices used to handle the granular material and (ii) the modification of the granular material itself. In both methods, a characterization of segregation is the necessary condition. We address in our last paper [2] the fundamental and practical problem of measuring segregation from a granular mixture. For that purpose, we propose a method for amplifying the segregation phenomenon and we provide the definition of a segregation index $p$, measuring the ability of a mixture to separate into its components.

Our method is based on the formation of a triangular heap in a closed cell. Segregation is known to take place in such geometry [3], as illustrated on the top picture. The trick is then to submit a part of the granular mixture to repeated flow cycles in order to separate the components, similarily to fractional distillation (see sketch below). We have experimentally demonstrated that weak segregation can be observed in such a “cascade of granular flows”. By measuring the component fractions at different steps of the cascade, it is possible to define a reliable segregation index !

Our system, allowing for quantification of granular segregation, is a step forward new fundamental and technological developments in powder/granular characterization. Our setup is already used for various powder characterizations in the APTIS platform [4].

[1] See the Focus of the New Journal of Physics on Granular Segregation
[2] G.Lumay et al., Powder Technology 234, 32 (2013) – PDF
[3] H.A.Makse, S.Havlin, P.R.King, and H.E.Stanley, Nature 386, 379 (1997)
[4] APTIS : http://www. aptis.be