Segregation by heating

Short time scale segregation by convection

Industrial applications of granular material research involve studies of mixing and demixing behavior of mixtures of grains of various sizes and properties. Completely mixed cement, sand and gravel in the right proportions is vital in construction, while segregation is important in handling chemicals and drugs.

This led to a surge in the number of granular materials research aiming to quantify the conditions needed for segregation and/or mixing. Rotating or vibrated tumblers have been shown to produce either phenomena for particular values of frequencies or under special configurations (e.g. baffled tumblers). But there is yet another possible driver of segregation or mixing that is relatively untouched: the convection cells of made by a fluid heated at the bottom and cooled at the top. The regularity in the vortex motion of the fluid may aid in separating components of a granular mixture suspended in it.

This is the main point of a recent paper by Felix Valenzuela and Christopher Monterola, physics researchers at the National Institute of Physics in the University of the Philippines Diliman. Valenzuela, who will be defending his MS Physics thesis this semester, wrote the paper titled "Convective flow-induced short timescale segregation in a dilute bidisperse particle suspension", accepted for publication in the International Journal of Modern Physics C [1].

When a fluid (say, water) is heated at the bottom and cooled at the top and it reaches a critical condition called the critical Rayleigh number, it forms a vortex motion because the warm fluid below rises and, upon cooling at the top, eventually sinks. Seen from the side, it forms symmetrical circular motion, one clockwise and the other counter-clockwise.

While the effect of the particles on the fluid - namely, to change the direction of motion of the vortex - is investigated in a similar paper [2], the segregating effect of the fluid motion on the particles have only been described in Valenzuela's work. For very short time scales, initially mixed particles at the bottom of the tank undergo separation, due to the faster motion of light particles as compared with the heavier ones.

The paper used two measures to describe the separation of the particles: the index of dissimilarity, first used in the social sciences in describing how mixed populations are spatially in terms of races; and the entropy, a general measure of "disorder" in a system. Used together, these measures better describe the dynamics of mixing and demixing in time.

The paper is a first approximation, primarily because the particles used in the simulation have no physical dimension. Moreover the fluid-particle interaction is one-way: only the fluid imparts momentum into the particle. But in real systems particles do have geometrical differences, and they give a "kick" to the liquid upon moving.

Additional simulations are underway. The next step is to conduct actual experiments to further strengthen their claims. Will there be a cycle of mixing-demixing, as their earlier plots suggest? Is total segregation possible? These are some of the unanswered questions that the researchers aim to answer. But with the way things are going, they are positive they can replicate their interesting results and provide better insights into this phenomenon.

1. Valenzuela, J.F. and Monterola, C. (2008). Convective flow-induced short timescale segregation in a dilute bidisperse particle suspension. Accepted for publication in International Journal of Modern Physics C.
2. Bin Liu and Jun Zhang (2006). Self-Induced Cyclic Reorganization of Free Bodies Through Thermal Convection. Phys. Rev. Lett. 100: 244501.