Europhys. Lett., 66 (2), pp. 212–218 (2004) DOI: 10.1209/epl/i2003-10157-4 EUROPHYSICS LETTERS 15 April 2004 Oscillatory granular segregation in a long drum mixer Z. S. Khan, W. A. Tokaruk and S. W. Morris Department of Physics, University of Toronto 60 St. George St., Toronto, Ontario, Canada M5S 1A7 (received 9 October 2003; accepted in final form 4 February 2004) PACS. 45.70.Mg – Granular flow: mixing, segregation and stratification. PACS. 45.70.Qj – Pattern formation. Abstract. – Heterogeneous granular mixtures tend to segregate when tumbled in a partially filled, horizontal rotating drum. The dynamical evolution of segregation can, under certain conditions, be oscillatory. Continuum, order parameter-style models of this process posit two coupled fields which oscillate out of phase with one another. Here we examine three candidate fields, the surface concentration, the local streaming angle and the projected concentration of the subsurface core. We find that all these quantities are in phase with one another, in contradiction to a recent order parameter model. One of the fascinating idiosyncrasies of dry granular materials is their tendency to seg- regate by size under various flow conditions [1,2]. Segregation is ubiquitous in natural and industrial processes, and can produce a startling degree of spontaneous order [3,4]. Probably thebestcontrolledexampleissegregationalongtheaxisofapartiallyfilled,horizontal“drum mixer”[5–14]. Aftermanyrotations,aninitiallymixedbinarydistributionsortsitselfintoal- mostperiodicbandsarrangedalongtheaxisofthecylinder. Theprocesscandisplaycomplex, oscillatorywavedynamicsduringthetransientbeforesegregationsaturates[9,10,13]. Similar oscillatory segregation has been observed in dense suspensions [12] and in multidisperse dry mixtures [14]. Accounting for this rich behaviour has been the objective of cellular-automata models [15], continuum theories [16–21], and direct molecular-dynamics simulations [22], yet the process remains poorly understood. Here we report experiments in the oscillatory regime and present measurements of three axial quantities; the surface concentration, the surface slope shape and the projected concentration. The latter is sensitive to the subsurface con- centration distribution. We find that all three quantities remain in phase during the course of an oscillation, a fact which is very difficult to reconcile with general features of continuum modelsofthisprocess[20,21]. The central difficulty in explaining any segregation process is to establish which are the relevant continuum quantities —the correct order parameters, in the language of condensed- matter physics— and how they are dynamically related to one another. In the case of axial segregation, we expect that the radially averaged surface concentration, the most obvious measurable order parameter, must be coupled to other axial fields to produce the effects ob- served. Inorderforsuchatheorytobesatisfactory,somephysicalunderstandingofthefields in terms of microscopic grain motions should be possible. There have been several attempts c  EDP Sciences